What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
On 22/01/2026 09:29, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which its reactance first swung through purely resistive. From this I calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Was this a semi air-spaced coax?
On 22/01/2026 8:29 pm, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which its reactance first swung through purely resistive. From this I calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Some coax relies on a foamed dielectric, which would have a lower
dielectric constant than solid plastic.
https://en.wikipedia.org/wiki/Coaxial_cable
There are more complicated ways of getting much the same effect.
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for >'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
On 22/01/2026 09:29, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax?ÿ Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive.ÿÿ From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Was this a semi air-spaced coax?
John
On 1/22/26 12:04, John R Walliker wrote:
On 22/01/2026 09:29, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax?ÿ Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which >> its reactance first swung through purely resistive.ÿÿ From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was >> not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Was this a semi air-spaced coax?
John
The signal propagation velocity depends entirely on the material
and consistency of the dielectric. A velocity factor of 78%
does not shock me. I've used cables with a foamed polyethylene
dielectric with a velocity factor of 0.84. Some cables use
plastic spiral wire or discrete spacers and can reach velocity
factors pretty close to unity.
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Jeroen Belleman <jeroen@nospam.please> wrote:
On 1/22/26 12:04, John R Walliker wrote:
On 22/01/2026 09:29, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax??ÿ Most >>>> of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about >>>> 6 metres), open circuit at the far end, and found the frequency at which >>>> its reactance first swung through purely resistive.?ÿ?ÿ From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was >>>> not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Was this a semi air-spaced coax?
John
The signal propagation velocity depends entirely on the material
and consistency of the dielectric. A velocity factor of 78%
does not shock me. I've used cables with a foamed polyethylene
dielectric with a velocity factor of 0.84. Some cables use
plastic spiral wire or discrete spacers and can reach velocity
factors pretty close to unity.
In that case it seems quite likely my cable is rather better than I anticipated. I shall start chopping it up into lengths to make a 145
Mc/s vertical co-linear array with reversals every 88.85 cm.
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 8:29 pm, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which >>> its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was >>> not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Some coax relies on a foamed dielectric, which would have a lower
dielectric constant than solid plastic.
https://en.wikipedia.org/wiki/Coaxial_cable
There are more complicated ways of getting much the same effect.
What are they?
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
ITYF capacitance/meter dominates.
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 8:29 pm, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which >> > its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was >> > not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Some coax relies on a foamed dielectric, which would have a lower
dielectric constant than solid plastic.
https://en.wikipedia.org/wiki/Coaxial_cable
There are more complicated ways of getting much the same effect.
What are they?
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for >'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Your VNA measurement may be suspect.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
On 22/01/2026 16:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Your VNA measurement may be suspect.
I think it would be difficult to get it wrong when it is so
easy to check the calibration with a simple 50 ohm terminator.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which >>> its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was >>> not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
On Thu, 22 Jan 2026 16:28:49 +0000, John R Walliker
<jrwalliker@gmail.com> wrote:
On 22/01/2026 16:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>> of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Your VNA measurement may be suspect.
I think it would be difficult to get it wrong when it is so
easy to check the calibration with a simple 50 ohm terminator.
You really should use a proper calibration set: open, short and load
and maybe a through too if circumstances require it. The quality of
that cal kit will be crucial to the accurate of subsequent
measurements. If you're doing a lot of VNA stuff, you can expect to
spend a significant portion of the day calibrating and recalibrating.
It's a real time thief.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about >>>> 6 metres), open circuit at the far end, and found the frequency at which >>>> its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was >>>> not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
On 22/01/2026 10:41 pm, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 8:29 pm, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax? Most >>> of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about >>> 6 metres), open circuit at the far end, and found the frequency at which >>> its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was >>> not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Some coax relies on a foamed dielectric, which would have a lower
dielectric constant than solid plastic.
https://en.wikipedia.org/wiki/Coaxial_cable
There are more complicated ways of getting much the same effect.
What are they?
Read the link...
Jeroen listed most of them. It's all about getting more
air into the space tween the inner and outer conductor
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
made from a metallised plastic tape and a loosely-woven copper braid.
Using a VNA I measured the reflected impedance of a known length (about
6 metres), open circuit at the far end, and found the frequency at which its reactance first swung through purely resistive. From this I calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
I think 93 ohm RG62 has a vf of about 0.85 - did your measurements confirm the Zo of your cable was actually close to 50 ohms? Sorry RF new to me.
On Thu, 22 Jan 2026 16:28:49 +0000, John R Walliker
<jrwalliker@gmail.com> wrote:
On 22/01/2026 16:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>> of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Your VNA measurement may be suspect.
I think it would be difficult to get it wrong when it is so
easy to check the calibration with a simple 50 ohm terminator.
You really should use a proper calibration set: open, short and load
and maybe a through too if circumstances require it. The quality of
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for >'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Your VNA measurement may be suspect.
Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 16:28:49 +0000, John R Walliker
<jrwalliker@gmail.com> wrote:
On 22/01/2026 16:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >> >>> of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Your VNA measurement may be suspect.
I think it would be difficult to get it wrong when it is so
easy to check the calibration with a simple 50 ohm terminator.
You really should use a proper calibration set: open, short and load
and maybe a through too if circumstances require it. The quality of
I did.
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Solid polyethylene isn't too bad but foamed has a nasty habit of moving
under the influence of its own 'memory'. You solder the end of a
slightly bent centre conductor and, as the heat travels down it, the
foam springs back to the straight position, leaving you with a slot in
the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the >electrical length of the cable that is independent of the terminating >impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive'
impedance point was at exactly 15.000 Mc/s. (That's another reason I
was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Your VNA measurement may be suspect.
On 1/22/26 17:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to
the start of the cable, which is probably not at the same place as the
VNA output connector. At lowish frequencies, it probably doesn't matter,
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 10:41 pm, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 8:29 pm, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>> of the amateur radio books give around 60% as the velocity factor for >>>>> 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen >>>>> made from a metallised plastic tape and a loosely-woven copper braid. >>>>> Using a VNA I measured the reflected impedance of a known length (about >>>>> 6 metres), open circuit at the far end, and found the frequency at which >>>>> its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor,
which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was >>>>> not in common use when the original 'words of wisdom' were written.
Are there any physical properties of the co-ax could I check, which
might explain my measured velocity factor?
Some coax relies on a foamed dielectric, which would have a lower
dielectric constant than solid plastic.
https://en.wikipedia.org/wiki/Coaxial_cable
There are more complicated ways of getting much the same effect.
What are they?
Read the link...
Did you read it before suggesting it?
The link you gave me contained 29 screenfuls of information.
None of
them said anything helpful about the velocity factor; it was mentioned
once, with no explanation of what it was or what caused it. There was
just a formula with two variables, neither of which was defined.
Jeroen listed most of them. It's all about getting more
air into the space tween the inner and outer conductor
Jeroen's information was helpful because it confirmed that the results
I obtained were not necessarily an indication of a faulty measurement technique.
john larkin <jl@glen--canyon.com>wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for >>'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
On 23/01/2026 4:20 am, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 10:41 pm, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 8:29 pm, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>> of the amateur radio books give around 60% as the velocity factor for >>>>> 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen >>>>> made from a metallised plastic tape and a loosely-woven copper braid. >>>>> Using a VNA I measured the reflected impedance of a known length (about >>>>> 6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I
calculated its effective electrical length and the velocity factor, >>>>> which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written. >>>>>
Are there any physical properties of the co-ax could I check, which >>>>> might explain my measured velocity factor?
Some coax relies on a foamed dielectric, which would have a lower
dielectric constant than solid plastic.
https://en.wikipedia.org/wiki/Coaxial_cable
There are more complicated ways of getting much the same effect.
What are they?
Read the link...
Did you read it before suggesting it?
I always do. Wikipedia pages get edited from time to time, and it's wise
to check for changes.
The link you gave me contained 29 screenfuls of information.
There was more than one screen, but 29 sounds a bit high. I read faster
than most people do, but not that fast.
None of
them said anything helpful about the velocity factor; it was mentioned once, with no explanation of what it was or what caused it. There was
just a formula with two variables, neither of which was defined.
It did list a number of ways of making coaxial cable, some of which
included schemes where the dielectric was mostly air.
Jeroen listed most of them. It's all about getting more
air into the space tween the inner and outer conductor
Jeroen's information was helpful because it confirmed that the results
I obtained were not necessarily an indication of a faulty measurement technique.
That's always a potential explanation for odd results, but it's well
known that the propagation delay through regular cables is a bit slower
than the speed of light in a vacuum.
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 16:28:49 +0000, John R Walliker
<jrwalliker@gmail.com> wrote:
On 22/01/2026 16:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >> >>> of the amateur radio books give around 60% as the velocity factor for >> >>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Your VNA measurement may be suspect.
I think it would be difficult to get it wrong when it is so
easy to check the calibration with a simple 50 ohm terminator.
You really should use a proper calibration set: open, short and load
and maybe a through too if circumstances require it. The quality of
I did.
Can you not do it by TDR? Use some fast switching logic, tee piece to
a scope input and a dead short at the far end of the cable.
On 1/22/26 17:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to
the start of the cable, which is probably not at the same place as the
VNA output connector. At lowish frequencies, it probably doesn't matter,
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Solid polyethylene isn't too bad but foamed has a nasty habit of moving >under the influence of its own 'memory'. You solder the end of a
slightly bent centre conductor and, as the heat travels down it, the
foam springs back to the straight position, leaving you with a slot in
the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the >electrical length of the cable that is independent of the terminating >impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive' >impedance point was at exactly 15.000 Mc/s. (That's another reason I
was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
Looking for the max resistance may not be ideal.
If you jack up the frequency and get multiple wavelengths in the
cable, resolution will improve.
15.000 MHz seems suspicious.
I don't have a VNA. I use TDR to measure time delays.
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>> of the amateur radio books give around 60% as the velocity factor for >>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Solid polyethylene isn't too bad but foamed has a nasty habit of moving
under the influence of its own 'memory'. You solder the end of a
slightly bent centre conductor and, as the heat travels down it, the
foam springs back to the straight position, leaving you with a slot in
the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the
electrical length of the cable that is independent of the terminating
impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive'
impedance point was at exactly 15.000 Mc/s. (That's another reason I
was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of
maximum resistance, it was the point where the capacitive reactance
swung through zero to become an inductive reactance; it was quite
sharply defined. At that point the impedance was purely resistive but
it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the
cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An
accuarcy of around 1% would be good enough for the present purposes -
after all, where exactly is the 'end' of a piece of co-ax that is
splayed out for connection to something else? I also wouldn't expect a length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and
an oscilloscope will work too.
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>> of the amateur radio books give around 60% as the velocity factor for >>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Solid polyethylene isn't too bad but foamed has a nasty habit of moving >>> under the influence of its own 'memory'. You solder the end of a
slightly bent centre conductor and, as the heat travels down it, the
foam springs back to the straight position, leaving you with a slot in >>> the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the
electrical length of the cable that is independent of the terminating
impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive'
impedance point was at exactly 15.000 Mc/s. (That's another reason I
was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of
maximum resistance, it was the point where the capacitive reactance
swung through zero to become an inductive reactance; it was quite
sharply defined. At that point the impedance was purely resistive but
it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the
cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An accuarcy of around 1% would be good enough for the present purposes -
after all, where exactly is the 'end' of a piece of co-ax that is
splayed out for connection to something else? I also wouldn't expect a length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and
an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've[...]
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Solid polyethylene isn't too bad but foamed has a nasty habit of moving >>>> under the influence of its own 'memory'. You solder the end of a
slightly bent centre conductor and, as the heat travels down it, the
foam springs back to the straight position, leaving you with a slot in >>>> the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the
electrical length of the cable that is independent of the terminating
impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive'
impedance point was at exactly 15.000 Mc/s. (That's another reason I
was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of
maximum resistance, it was the point where the capacitive reactance
swung through zero to become an inductive reactance; it was quite
sharply defined. At that point the impedance was purely resistive but
it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the
cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An
accuarcy of around 1% would be good enough for the present purposes -
after all, where exactly is the 'end' of a piece of co-ax that is
splayed out for connection to something else? I also wouldn't expect a
length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and
an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
I've got a picture of a TDR and a VNA-derived measurement of the
same setup here. Note how much cleaner is the trace from the
VNA. You can't beat a VNA for S/N.
<https://cern.ch/jeroen/WCMIV/journal.php?entry=20140910>
I made these measurements in the context of constructing a wide
band beam transformer for the Proton Synchrotron at CERN.
Jeroen Belleman
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>> of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to
the start of the cable, which is probably not at the same place as the
VNA output connector. At lowish frequencies, it probably doesn't matter,
True, but the OP also wants to measure the length of the cable AIUI,
and for that, you want as high a frequency as possible for greatest
accuracy. It's a trade-off (as ever).
Bill Sloman <bill.sloman@ieee.org> wrote:
On 23/01/2026 4:20 am, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 10:41 pm, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 22/01/2026 8:29 pm, Liz Tuddenham wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>>> 'common' types of 50-ohm co-ax.
I recently bought a drum of fairly cheap 50-ohm co-ax with the screen >>>>>>> made from a metallised plastic tape and a loosely-woven copper braid. >>>>>>> Using a VNA I measured the reflected impedance of a known length (about >>>>>>> 6 metres), open circuit at the far end, and found the frequency at which
its reactance first swung through purely resistive. From this I >>>>>>> calculated its effective electrical length and the velocity factor, >>>>>>> which turned out to be 78%.
This seems so different from the 'conventional' value that I am
suspicious of my measurements - but this type of screen construction was
not in common use when the original 'words of wisdom' were written. >>>>>>>
Are there any physical properties of the co-ax could I check, which >>>>>>> might explain my measured velocity factor?
Some coax relies on a foamed dielectric, which would have a lower
dielectric constant than solid plastic.
https://en.wikipedia.org/wiki/Coaxial_cable
There are more complicated ways of getting much the same effect.
What are they?
Read the link...
Did you read it before suggesting it?
I always do. Wikipedia pages get edited from time to time, and it's wise
to check for changes.
The link you gave me contained 29 screenfuls of information.
There was more than one screen, but 29 sounds a bit high. I read faster
than most people do, but not that fast.
So you didn't read all of it, but you expected me to.
Where did you find the information on that webpage about air dielectric affecting the Velocity Ratio, which was the reason you referred me to
the page? If you can give me an unique quoted phrase, I can soon find
it.
None of
them said anything helpful about the velocity factor; it was mentioned
once, with no explanation of what it was or what caused it. There was
just a formula with two variables, neither of which was defined.
It did list a number of ways of making coaxial cable, some of which
included schemes where the dielectric was mostly air.
There was no mention of this affecting the Velocity Ratio. I wasn't
asking for advice on making co-ax cables, I was asking how the
construction affected the Velocity Ratio.
Jeroen listed most of them. It's all about getting more
air into the space tween the inner and outer conductor
Jeroen's information was helpful because it confirmed that the results
I obtained were not necessarily an indication of a faulty measurement
technique.
That's always a potential explanation for odd results, but it's well
known that the propagation delay through regular cables is a bit slower
than the speed of light in a vacuum.
Yes, that is called the Velocity Ratio. I would have though it was
obvious from my question that I realised what the VR was - my question
was about the physical properties of the cable that affected it.
Jeroen Belleman wrote:
Liz Tuddenham wrote:
[...]My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and >> > an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
My VNA is a relatively cheap hand-held device but it is good enought for
the measurements I want to make and a lot better than the previous rule-of-thumb and guesswork method.
Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 16:28:49 +0000, John R Walliker
<jrwalliker@gmail.com> wrote:
On 22/01/2026 16:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >> >> >> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for >> >> >>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >> >> >>
Your VNA measurement may be suspect.
I think it would be difficult to get it wrong when it is so
easy to check the calibration with a simple 50 ohm terminator.
You really should use a proper calibration set: open, short and load
and maybe a through too if circumstances require it. The quality of
I did.
Can you not do it by TDR? Use some fast switching logic, tee piece to
a scope input and a dead short at the far end of the cable.
I could; and some time ago I did try this with some cable from the same
drum. The results were similiar, around 78%.
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Solid polyethylene isn't too bad but foamed has a nasty habit of moving >>>> under the influence of its own 'memory'. You solder the end of a
slightly bent centre conductor and, as the heat travels down it, the
foam springs back to the straight position, leaving you with a slot in >>>> the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the
electrical length of the cable that is independent of the terminating
impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive'
impedance point was at exactly 15.000 Mc/s. (That's another reason I
was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of
maximum resistance, it was the point where the capacitive reactance
swung through zero to become an inductive reactance; it was quite
sharply defined. At that point the impedance was purely resistive but
it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the
cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An
accuarcy of around 1% would be good enough for the present purposes -
after all, where exactly is the 'end' of a piece of co-ax that is
splayed out for connection to something else? I also wouldn't expect a
length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and
an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
I've got a picture of a TDR and a VNA-derived measurement of the
same setup here. Note how much cleaner is the trace from the
VNA. You can't beat a VNA for S/N.
<https://cern.ch/jeroen/WCMIV/journal.php?entry=20140910>
I made these measurements in the context of constructing a wide
band beam transformer for the Proton Synchrotron at CERN.
Jeroen Belleman
On Thu, 22 Jan 2026 23:27:14 +0000, Cursitor Doom <cd@notformail.com>
wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>> of the amateur radio books give around 60% as the velocity factor for >>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to
the start of the cable, which is probably not at the same place as the >>>VNA output connector. At lowish frequencies, it probably doesn't matter,
True, but the OP also wants to measure the length of the cable AIUI,
and for that, you want as high a frequency as possible for greatest >>accuracy. It's a trade-off (as ever).
The problem becomes "where does the cable actually start and end" ?
With my TDR I can resolve within, say, a transitiion from an
edge-launch connector to a PCB trace, so one has to decide where
inside the connector the handover happens.
On Fri, 23 Jan 2026 13:59:18 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>> (Liz Tuddenham) wrote:Solid polyethylene isn't too bad but foamed has a nasty habit of moving >>>>> under the influence of its own 'memory'. You solder the end of a
What physical properties determine the velocity factor of co-ax? Most >>>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>
slightly bent centre conductor and, as the heat travels down it, the >>>>> foam springs back to the straight position, leaving you with a slot in >>>>> the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the
electrical length of the cable that is independent of the terminating >>>>> impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive'
impedance point was at exactly 15.000 Mc/s. (That's another reason I >>>>> was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of
maximum resistance, it was the point where the capacitive reactance
swung through zero to become an inductive reactance; it was quite
sharply defined. At that point the impedance was purely resistive but
it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the
cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An
accuarcy of around 1% would be good enough for the present purposes -
after all, where exactly is the 'end' of a piece of co-ax that is
splayed out for connection to something else? I also wouldn't expect a
length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and >>> an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
Snap! Got the same one here, Jeroen. I've expanded mine to 6Ghz and am >contemplating the colour LCD conversion for it. Have you done this
with yours? My screen was getting pretty dim!
I've got a picture of a TDR and a VNA-derived measurement of the
same setup here. Note how much cleaner is the trace from the
VNA. You can't beat a VNA for S/N.
<https://cern.ch/jeroen/WCMIV/journal.php?entry=20140910>
I made these measurements in the context of constructing a wide
band beam transformer for the Proton Synchrotron at CERN.
Jeroen Belleman
On Fri, 23 Jan 2026 06:00:26 -0800, john larkin <jl@glen--canyon.com>
wrote:
On Thu, 22 Jan 2026 23:27:14 +0000, Cursitor Doom <cd@notformail.com> >>wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman >>><jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI,
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to >>>>the start of the cable, which is probably not at the same place as the >>>>VNA output connector. At lowish frequencies, it probably doesn't matter, >>>
and for that, you want as high a frequency as possible for greatest >>>accuracy. It's a trade-off (as ever).
The problem becomes "where does the cable actually start and end" ?
With my TDR I can resolve within, say, a transitiion from an
edge-launch connector to a PCB trace, so one has to decide where
inside the connector the handover happens.
If you have a high enough frequency and can resolve it, you ought to
be able to 'see' this clearly enough? I know for a fact it's do-able.
Are you *sure* it's standard RG58? No suffixes/prefixes? Does it
physically (I mean from visual inspection) conform to the published
specs if you peel back the layers and have a good look? And preferably
a measure-up.
Am 23.01.26 um 19:50 schrieb Cursitor Doom:
Are you *sure* it's standard RG58? No suffixes/prefixes? Does it
physically (I mean from visual inspection) conform to the published
specs if you peel back the layers and have a good look? And preferably
a measure-up.
There is no standard for RG58. There was one defined in WWII but it
has been withdrawn some eons ago. Nowadays you can write RG58 on just
about everything. The first thing to get worse was the amount of
copper in the shield. See-thru now.
On Fri, 23 Jan 2026 11:34:17 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 16:28:49 +0000, John R Walliker
<jrwalliker@gmail.com> wrote:
On 22/01/2026 16:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >> >> >> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax?
Most of the amateur radio books give around 60% as the velocity
factor for 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >> >> >>
Your VNA measurement may be suspect.
I think it would be difficult to get it wrong when it is so
easy to check the calibration with a simple 50 ohm terminator.
You really should use a proper calibration set: open, short and load
and maybe a through too if circumstances require it. The quality of
I did.
Can you not do it by TDR? Use some fast switching logic, tee piece to
a scope input and a dead short at the far end of the cable.
I could; and some time ago I did try this with some cable from the same >drum. The results were similiar, around 78%.
Are you *sure* it's standard RG58? No suffixes/prefixes?
Does it
physically (I mean from visual inspection) conform to the published
specs if you peel back the layers and have a good look? And preferably
a measure-up.
On Fri, 23 Jan 2026 19:08:50 +0000, Cursitor Doom <cd@notformail.com>
wrote:
On Fri, 23 Jan 2026 13:59:18 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid >>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>> (Liz Tuddenham) wrote:Solid polyethylene isn't too bad but foamed has a nasty habit of moving >>>>> under the influence of its own 'memory'. You solder the end of a
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for >>>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>
slightly bent centre conductor and, as the heat travels down it, the >>>>> foam springs back to the straight position, leaving you with a slot in >>>>> the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the >>>>> electrical length of the cable that is independent of the terminating >>>>> impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive' >>>>> impedance point was at exactly 15.000 Mc/s. (That's another reason I >>>>> was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of
maximum resistance, it was the point where the capacitive reactance
swung through zero to become an inductive reactance; it was quite
sharply defined. At that point the impedance was purely resistive but >>> it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the
cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An
accuarcy of around 1% would be good enough for the present purposes -
after all, where exactly is the 'end' of a piece of co-ax that is
splayed out for connection to something else? I also wouldn't expect a >>> length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and >>> an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
Snap! Got the same one here, Jeroen. I've expanded mine to 6Ghz and am >contemplating the colour LCD conversion for it. Have you done this
with yours? My screen was getting pretty dim!
I've got a picture of a TDR and a VNA-derived measurement of the
same setup here. Note how much cleaner is the trace from the
VNA. You can't beat a VNA for S/N.
<https://cern.ch/jeroen/WCMIV/journal.php?entry=20140910>
I made these measurements in the context of constructing a wide
band beam transformer for the Proton Synchrotron at CERN.
Jeroen Belleman
There are lots of very cheap VNAs around now.
Do any come with the software to transform TDRs?
Am 23.01.26 um 19:50 schrieb Cursitor Doom:
Are you *sure* it's standard RG58? No suffixes/prefixes? Does it
physically (I mean from visual inspection) conform to the published
specs if you peel back the layers and have a good look? And preferably
a measure-up.
There is no standard for RG58. There was one defined in WWII but it
has been withdrawn some eons ago. Nowadays you can write RG58 on just
about everything. The first thing to get worse was the amount of
copper in the shield. See-thru now.
On Fri, 23 Jan 2026 19:12:15 +0000, Cursitor Doom <cd@notformail.com>
wrote:
On Fri, 23 Jan 2026 06:00:26 -0800, john larkin <jl@glen--canyon.com> >>wrote:
On Thu, 22 Jan 2026 23:27:14 +0000, Cursitor Doom <cd@notformail.com> >>>wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman >>>><jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI, >>>>and for that, you want as high a frequency as possible for greatest >>>>accuracy. It's a trade-off (as ever).
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to >>>>>the start of the cable, which is probably not at the same place as the >>>>>VNA output connector. At lowish frequencies, it probably doesn't matter, >>>>
The problem becomes "where does the cable actually start and end" ?
With my TDR I can resolve within, say, a transitiion from an
edge-launch connector to a PCB trace, so one has to decide where
inside the connector the handover happens.
If you have a high enough frequency and can resolve it, you ought to
be able to 'see' this clearly enough? I know for a fact it's do-able.
It's an arbitrary decision as to where the thing actually starts.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
"Don" <g@crcomp.net>wrote:
Liz Tuddenham wrote:
Jeroen Belleman wrote:
Liz Tuddenham wrote:
<snip>
[...]My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and >>> > an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
My VNA is a relatively cheap hand-held device but it is good enought for
the measurements I want to make and a lot better than the previous
rule-of-thumb and guesswork method.
My VNA collection also includes a cheap hand-held device. Because ...
why not?
One of the hams in my club uses his VNA exclusively for SWR
measurements. So he simply skips Smith charting altogether.
On the other hand, it's all about the Smith chart for me. The VNA's
sort of a shortwave Swiss Army knife you can use as you see fit.
Your combo square-wave signal generator and oscilloscope as a TDR is >intriguing. Thank you for sharing.
Also got a marine radio license, for when I sail away,,, when the trumpape invades my country.
Jan Panteltje <alien@comet.invalid> wrote:
[...]
Also got a marine radio license, for when I sail away,,, when the trumpape invades my country.
He will declare that the the Antillies have to be part of the USA first, >because that's where the oil seems to be coming from.
If he does decide to invade 'Holland', he will probably finish up in >Lincolnshire.
Jan Panteltje <alien@comet.invalid> wrote:
[...]
Also got a marine radio license, for when I sail away,,, when the trumpape invades my country.
He will declare that the the Antillies have to be part of the USA first, because that's where the oil seems to be coming from.
If he does decide to invade 'Holland', he will probably finish up in Lincolnshire.
On Sat, 24 Jan 2026 10:51:22 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
Jan Panteltje <alien@comet.invalid> wrote:
[...]
Also got a marine radio license, for when I sail away,,, when the trumpape invades my country.
He will declare that the the Antillies have to be part of the USA first,
because that's where the oil seems to be coming from.
If he does decide to invade 'Holland', he will probably finish up in
Lincolnshire.
If you've seen the state of Boston lately, that would probably be a
good thing.
Don wrote:
Liz Tuddenham wrote:
Jeroen Belleman wrote:
Liz Tuddenham wrote:
<snip>
[...]My VNA will also function as a TDR and I have a telephone-testing TDR >>>> > for longer lines. If all else fails, a square-wave signal generator and >>>> > an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
My VNA is a relatively cheap hand-held device but it is good enought for >>> the measurements I want to make and a lot better than the previous
rule-of-thumb and guesswork method.
My VNA collection also includes a cheap hand-held device. Because ...
why not?
One of the hams in my club uses his VNA exclusively for SWR >>measurements. So he simply skips Smith charting altogether.
On the other hand, it's all about the Smith chart for me. The VNA's >>sort of a shortwave Swiss Army knife you can use as you see fit.
Your combo square-wave signal generator and oscilloscope as a TDR is >>intriguing. Thank you for sharing.
I once designed and build this for my 27 MHz CB transmitter:
https://panteltje.nl/pub/swr_box_LEDs.jpg
And I have one of these:
https://www.ebay.com/itm/389512220032
And build a very small one for Giggle Hartz:
https://panteltje.nl/pub/SWR_meter_IMG_6147.JPG
https://panteltje.nl/pub/2.4GHz_SWR_reflected_with_directional_coupler_IMG_5102.JPG
My analog scope goes to 10 MHz, 20 MHz a bit attenuated,
bought it in 1979 or there about
still in use!
Designed and build my own scope in the sixties, Vidicon camera too.
And shortwave 250 W transmitter 1967!
Then I was the pirate :-)
Now I have a license and we have our own satellite QO100
Thinking about dropping the license, it costs more every year here.
https://duckduckgo.com/?q=qo100+website
Not using it now, have to adjust the satellite dish again (bit higher),
maybe in the summer.
Not active with ham radio now.
john larkin <jl@glen--canyon.com> wrote:
On Fri, 23 Jan 2026 19:08:50 +0000, Cursitor Doom <cd@notformail.com>
wrote:
On Fri, 23 Jan 2026 13:59:18 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >> >>>>>> (Liz Tuddenham) wrote:Solid polyethylene isn't too bad but foamed has a nasty habit of moving
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >> >>>>>
under the influence of its own 'memory'. You solder the end of a
slightly bent centre conductor and, as the heat travels down it, the >> >>>>> foam springs back to the straight position, leaving you with a slot in >> >>>>> the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement.
However... my method of finding the first reactance swing in the
reflection from an open circuit should give me a measurement of the
electrical length of the cable that is independent of the terminating >> >>>>> impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive'
impedance point was at exactly 15.000 Mc/s. (That's another reason I >> >>>>> was suspicious, it really was spot-on 15.000 Mc/s, give or take
nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of
maximum resistance, it was the point where the capacitive reactance
swung through zero to become an inductive reactance; it was quite
sharply defined. At that point the impedance was purely resistive but >> >>> it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the
cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An
accuarcy of around 1% would be good enough for the present purposes -
after all, where exactly is the 'end' of a piece of co-ax that is
splayed out for connection to something else? I also wouldn't expect a >> >>> length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR
for longer lines. If all else fails, a square-wave signal generator and >> >>> an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
Snap! Got the same one here, Jeroen. I've expanded mine to 6Ghz and am
contemplating the colour LCD conversion for it. Have you done this
with yours? My screen was getting pretty dim!
I've got a picture of a TDR and a VNA-derived measurement of the
same setup here. Note how much cleaner is the trace from the
VNA. You can't beat a VNA for S/N.
<https://cern.ch/jeroen/WCMIV/journal.php?entry=20140910>
I made these measurements in the context of constructing a wide
band beam transformer for the Proton Synchrotron at CERN.
Jeroen Belleman
There are lots of very cheap VNAs around now.
Do any come with the software to transform TDRs?
Mine does.
I think sine waves are boring - they just go around in circles - but
we may be doing some RF switch modules and I guess people will expect
some sort of S-parameter things. Or at least a spec of some Sxx at
some max frequency, which will be 3 GHz in our case.
I guess we can buy a cheap Amazon VNA and measure a couple of points.
We could check its calibration with a couple of hanks of hardline or something.
Sounds like an intern project.
Can anyone suggest a suitable small inexpensive VNA? We might use it
10 times in its lifetime.
The generic Amazons are about $150 for 3 GHz and twice that for 6.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
On Fri, 23 Jan 2026 20:21:42 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Fri, 23 Jan 2026 19:08:50 +0000, Cursitor Doom <cd@notformail.com>
wrote:
On Fri, 23 Jan 2026 13:59:18 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid >>>>>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>>>>> (Liz Tuddenham) wrote:Solid polyethylene isn't too bad but foamed has a nasty habit of moving
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>>>>
under the influence of its own 'memory'. You solder the end of a >>>>>>>> slightly bent centre conductor and, as the heat travels down it, the >>>>>>>> foam springs back to the straight position, leaving you with a slot in >>>>>>>> the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement. >>>>>>>>
However... my method of finding the first reactance swing in the >>>>>>>> reflection from an open circuit should give me a measurement of the >>>>>>>> electrical length of the cable that is independent of the terminating >>>>>>>> impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive' >>>>>>>> impedance point was at exactly 15.000 Mc/s. (That's another reason I >>>>>>>> was suspicious, it really was spot-on 15.000 Mc/s, give or take >>>>>>>> nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of >>>>>> maximum resistance, it was the point where the capacitive reactance >>>>>> swung through zero to become an inductive reactance; it was quite
sharply defined. At that point the impedance was purely resistive but >>>>>> it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the >>>>>>> cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An >>>>>> accuarcy of around 1% would be good enough for the present purposes - >>>>>> after all, where exactly is the 'end' of a piece of co-ax that is
splayed out for connection to something else? I also wouldn't expect a >>>>>> length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR >>>>>> for longer lines. If all else fails, a square-wave signal generator and >>>>>> an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
Snap! Got the same one here, Jeroen. I've expanded mine to 6Ghz and am >>>> contemplating the colour LCD conversion for it. Have you done this
with yours? My screen was getting pretty dim!
I've got a picture of a TDR and a VNA-derived measurement of the
same setup here. Note how much cleaner is the trace from the
VNA. You can't beat a VNA for S/N.
<https://cern.ch/jeroen/WCMIV/journal.php?entry=20140910>
I made these measurements in the context of constructing a wide
band beam transformer for the Proton Synchrotron at CERN.
Jeroen Belleman
There are lots of very cheap VNAs around now.
Do any come with the software to transform TDRs?
Mine does.
I think sine waves are boring - they just go around in circles - but
we may be doing some RF switch modules and I guess people will expect
some sort of S-parameter things. Or at least a spec of some Sxx at
some max frequency, which will be 3 GHz in our case.
I guess we can buy a cheap Amazon VNA and measure a couple of points.
We could check its calibration with a couple of hanks of hardline or something.
Sounds like an intern project.
Can anyone suggest a suitable small inexpensive VNA? We might use it
10 times in its lifetime.
The generic Amazons are about $150 for 3 GHz and twice that for 6.
john larkin <jl@glen--canyon.com> wrote:
On Fri, 23 Jan 2026 20:21:42 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Fri, 23 Jan 2026 19:08:50 +0000, Cursitor Doom <cd@notformail.com>
wrote:
On Fri, 23 Jan 2026 13:59:18 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid >>>>>>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>>>>>> (Liz Tuddenham) wrote:Solid polyethylene isn't too bad but foamed has a nasty habit of moving
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>>>>>
under the influence of its own 'memory'. You solder the end of a >>>>>>>>> slightly bent centre conductor and, as the heat travels down it, the >>>>>>>>> foam springs back to the straight position, leaving you with a slot in
the foam and a bare centre conductor shorted to the screen.
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement. >>>>>>>>>
However... my method of finding the first reactance swing in the >>>>>>>>> reflection from an open circuit should give me a measurement of the >>>>>>>>> electrical length of the cable that is independent of the terminating >>>>>>>>> impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive' >>>>>>>>> impedance point was at exactly 15.000 Mc/s. (That's another reason I >>>>>>>>> was suspicious, it really was spot-on 15.000 Mc/s, give or take >>>>>>>>> nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of >>>>>>> maximum resistance, it was the point where the capacitive reactance >>>>>>> swung through zero to become an inductive reactance; it was quite >>>>>>> sharply defined. At that point the impedance was purely resistive but >>>>>>> it was the reactance that I was measuring, not the resistance.
If you jack up the frequency and get multiple wavelengths in the >>>>>>>> cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An >>>>>>> accuarcy of around 1% would be good enough for the present purposes - >>>>>>> after all, where exactly is the 'end' of a piece of co-ax that is >>>>>>> splayed out for connection to something else? I also wouldn't expect a >>>>>>> length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR >>>>>>> for longer lines. If all else fails, a square-wave signal generator and
an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've >>>>>> always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
Snap! Got the same one here, Jeroen. I've expanded mine to 6Ghz and am >>>>> contemplating the colour LCD conversion for it. Have you done this
with yours? My screen was getting pretty dim!
I've got a picture of a TDR and a VNA-derived measurement of the
same setup here. Note how much cleaner is the trace from the
VNA. You can't beat a VNA for S/N.
<https://cern.ch/jeroen/WCMIV/journal.php?entry=20140910>
I made these measurements in the context of constructing a wide
band beam transformer for the Proton Synchrotron at CERN.
Jeroen Belleman
There are lots of very cheap VNAs around now.
Do any come with the software to transform TDRs?
Mine does.
I think sine waves are boring - they just go around in circles - but
we may be doing some RF switch modules and I guess people will expect
some sort of S-parameter things. Or at least a spec of some Sxx at
some max frequency, which will be 3 GHz in our case.
I guess we can buy a cheap Amazon VNA and measure a couple of points.
We could check its calibration with a couple of hanks of hardline or
something.
Sounds like an intern project.
Can anyone suggest a suitable small inexpensive VNA? We might use it
10 times in its lifetime.
The generic Amazons are about $150 for 3 GHz and twice that for 6.
Didn?t you folks get a NanoVNA a year or so back?
You want the genuine one, not the fleabay or jungle ones.
https://www.tindie.com/products/hcxqsgroup/nanovna-v2-plus4/
Cheers
Phil Hobbs
On 2026-01-24 17:15, john larkin wrote:
I think sine waves are boring - they just go around in circles - but
we may be doing some RF switch modules and I guess people will expect
some sort of S-parameter things. Or at least a spec of some Sxx at
some max frequency, which will be 3 GHz in our case.
I guess we can buy a cheap Amazon VNA and measure a couple of points.
We could check its calibration with a couple of hanks of hardline or something.
Sounds like an intern project.
Can anyone suggest a suitable small inexpensive VNA? We might use it
10 times in its lifetime.
The generic Amazons are about $150 for 3 GHz and twice that for 6.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
The best one I know of is the NanoVNA NanoVNA | Very tiny handheld Vector Network Analyzer <https://nanovna.com/> Discussion group and more info at nanovna-users@groups.io | Wiki
On Sat, 24 Jan 2026 18:50:17 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
john larkin <jl@glen--canyon.com> wrote:
On Fri, 23 Jan 2026 20:21:42 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Fri, 23 Jan 2026 19:08:50 +0000, Cursitor Doom <cd@notformail.com> >>>>> wrote:
On Fri, 23 Jan 2026 13:59:18 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/23/26 12:34, Liz Tuddenham wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid >>>>>>>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most
of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
Solid polyethylene isn't too bad but foamed has a nasty habit of moving
under the influence of its own 'memory'. You solder the end of a >>>>>>>>>> slightly bent centre conductor and, as the heat travels down it, the >>>>>>>>>> foam springs back to the straight position, leaving you with a slot in
the foam and a bare centre conductor shorted to the screen. >>>>>>>>>>
Your VNA measurement may be suspect.
That was why I asked about it here, I suspected the measurement. >>>>>>>>>>
However... my method of finding the first reactance swing in the >>>>>>>>>> reflection from an open circuit should give me a measurement of the >>>>>>>>>> electrical length of the cable that is independent of the terminating
impedances, calibration etc
The cable was physically 6.39 metres long and the first 'resistive' >>>>>>>>>> impedance point was at exactly 15.000 Mc/s. (That's another reason I
was suspicious, it really was spot-on 15.000 Mc/s, give or take >>>>>>>>>> nothing.)
Looking for the max resistance may not be ideal.
Perhaps I didn't explain that very clearly. It wasn't the point of >>>>>>>> maximum resistance, it was the point where the capacitive reactance >>>>>>>> swung through zero to become an inductive reactance; it was quite >>>>>>>> sharply defined. At that point the impedance was purely resistive but >>>>>>>> it was the reactance that I was measuring, not the resistance. >>>>>>>>
If you jack up the frequency and get multiple wavelengths in the >>>>>>>>> cable, resolution will improve.
True, but the electrical errors in measurement may increase too. An >>>>>>>> accuarcy of around 1% would be good enough for the present purposes - >>>>>>>> after all, where exactly is the 'end' of a piece of co-ax that is >>>>>>>> splayed out for connection to something else? I also wouldn't expect a
length of cheap co-ax to be particularly homogenous.
15.000 MHz seems suspicious.
Yes, that worried me.
I don't have a VNA. I use TDR to measure time delays.
My VNA will also function as a TDR and I have a telephone-testing TDR >>>>>>>> for longer lines. If all else fails, a square-wave signal generator and
an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've >>>>>>> always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
Snap! Got the same one here, Jeroen. I've expanded mine to 6Ghz and am >>>>>> contemplating the colour LCD conversion for it. Have you done this >>>>>> with yours? My screen was getting pretty dim!
I've got a picture of a TDR and a VNA-derived measurement of the >>>>>>> same setup here. Note how much cleaner is the trace from the
VNA. You can't beat a VNA for S/N.
<https://cern.ch/jeroen/WCMIV/journal.php?entry=20140910>
I made these measurements in the context of constructing a wide
band beam transformer for the Proton Synchrotron at CERN.
Jeroen Belleman
There are lots of very cheap VNAs around now.
Do any come with the software to transform TDRs?
Mine does.
I think sine waves are boring - they just go around in circles - but
we may be doing some RF switch modules and I guess people will expect
some sort of S-parameter things. Or at least a spec of some Sxx at
some max frequency, which will be 3 GHz in our case.
I guess we can buy a cheap Amazon VNA and measure a couple of points.
We could check its calibration with a couple of hanks of hardline or
something.
Sounds like an intern project.
Can anyone suggest a suitable small inexpensive VNA? We might use it
10 times in its lifetime.
The generic Amazons are about $150 for 3 GHz and twice that for 6.
Didn?t you folks get a NanoVNA a year or so back?
I can't recall. Maybe one of my guys did.
You want the genuine one, not the fleabay or jungle ones.
https://www.tindie.com/products/hcxqsgroup/nanovna-v2-plus4/
Cheers
Phil Hobbs
We should probably go to 6 GHz, especially since these things are so
cheap.
"Don" <g@crcomp.net>wrote:
Jan Panteltje wrote:
Don wrote:
Liz Tuddenham wrote:
Jeroen Belleman wrote:
Liz Tuddenham wrote:
<snip>
[...]My VNA will also function as a TDR and I have a telephone-testing TDR >>>>> > for longer lines. If all else fails, a square-wave signal generator and
an oscilloscope will work too.
Having used both VNAs and TDRs for cable length measurements, I've
always found the VNA measurements much superior to those of a
TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
corresponds to a little under 3ps. TDRs are much too noisy to
do that.
My VNA is a relatively cheap hand-held device but it is good enought for >>>> the measurements I want to make and a lot better than the previous
rule-of-thumb and guesswork method.
My VNA collection also includes a cheap hand-held device. Because ...
why not?
One of the hams in my club uses his VNA exclusively for SWR >>>measurements. So he simply skips Smith charting altogether.
On the other hand, it's all about the Smith chart for me. The VNA's >>>sort of a shortwave Swiss Army knife you can use as you see fit.
Your combo square-wave signal generator and oscilloscope as a TDR is >>>intriguing. Thank you for sharing.
I once designed and build this for my 27 MHz CB transmitter:
https://panteltje.nl/pub/swr_box_LEDs.jpg
And I have one of these:
https://www.ebay.com/itm/389512220032
And build a very small one for Giggle Hartz:
https://panteltje.nl/pub/SWR_meter_IMG_6147.JPG
https://panteltje.nl/pub/2.4GHz_SWR_reflected_with_directional_coupler_IMG_5102.JPG
My analog scope goes to 10 MHz, 20 MHz a bit attenuated,
bought it in 1979 or there about
still in use!
<snip>
Designed and build my own scope in the sixties, Vidicon camera too.
And shortwave 250 W transmitter 1967!
Then I was the pirate :-)
Now I have a license and we have our own satellite QO100
Thinking about dropping the license, it costs more every year here.
https://duckduckgo.com/?q=qo100+website
Not using it now, have to adjust the satellite dish again (bit higher),
maybe in the summer.
Not active with ham radio now.
This is Ham radio's "year of the club" (at least in the USA). Casper's >hosting the 2026 Ham Convention for the Rocky Mountain Division
(Colorado, New Mexico, Utah, & Wyoming). The website's not ready for
prime time yet: wyohamcon.org
During our meeting last Wednesday, the SWR VNA Ham mentioned
earlier, showed off an amateur mesh network. He uploaded Linux firmware
into MicroTik omnidirectional and parabolic antennas.
The omnidirectional is slated for the county's Casper Mountain shack.
We're in the process of moving radio infrastructure from the old
dilapidated shack into a new sea can sitting nearby.
The parabolic antennas, in turn, are housed at each ham's home. The
most intriguing aspect about this network is its utilization of network >cable. There's no coax. Instead, network cable dangles down from the >omnidirectional anchored up in the tower.
Our sister club in Cheyenne hosted the 2022 Convention. On Friday
they held an all day long satellite class. The President said it was
one of the more popular events.
Long story short, the QO-100 may prove popular for our own
Convention. Thank you for sharing.
73, Don, KB7RPU veritas _|_
liberabit |
https://www.qsl.net/kb7rpu vos |
Don wrote:
https://www.qsl.net/kb7rpu
Fun site, I have the same small mp3 player, but am using a Creative Muvo with a 1.5 V eneloop rechargable AAA cell
lasts 10 hours on a charge...
As to QO100, I dunno where exactly you are, but it is not accesable from
the US, see 'coverage':
https://www.sarts.org.sg/wp/wp-content/uploads/2025/09/QO-100-Presentation-1.pdf
But I think there are many small Amsat satellites?
I got into digital transmission systems because I worked many years here in broadcasting when all was analog,
needed deeper understanding to keep up, so hands on, designed my own and wrote the DVB-S encoder code myself:
https://panteltje.nl/panteltje/raspberry_pi_dvb-s_transmitter/
No PCB needed, GHz out..
Jan Panteltje wrote:
Don wrote:
<snip>
1. Why is a parabolic antenna used instead of a couple of yagis (two to
compensate for tumble)?
2. Why is the parabolic aimed slightly above the horizon? Doesn't this
orientation attenuate the signal when a satellite's overhead?
"Don" <g@crcomp.net>wrote:
Jan Panteltje wrote:
Don wrote:
<snip>
https://www.qsl.net/kb7rpu
Fun site, I have the same small mp3 player, but am using a Creative Muvo
with a 1.5 V eneloop rechargable AAA cell
lasts 10 hours on a charge...
As to QO100, I dunno where exactly you are, but it is not accesable from
the US, see 'coverage':
https://www.sarts.org.sg/wp/wp-content/uploads/2025/09/QO-100-Presentation-1.pdf
But I think there are many small Amsat satellites?
I got into digital transmission systems because I worked many years here in >> broadcasting when all was analog,
needed deeper understanding to keep up, so hands on, designed my own and
wrote the DVB-S encoder code myself:
https://panteltje.nl/panteltje/raspberry_pi_dvb-s_transmitter/
No PCB needed, GHz out..
Thank you for your kind words of encouragement. My site almost needs as
much work as the Ham Con site: <https://wyohamcon.org>. My location is
in the USA, so it looks like QO100 is a "no go."
Allow me to jog your memory. Last year we posted to a thread about
the Serenity satellite:
<https://tis.org/serenity-satellite/>
<https://www.orbtrack.org/#/?satSCN=60208>
This Danish ham operates QO-100:
<https://moonbounce.dk/hamradio/ham-radio-current-systems/qo-100-satellite-setup>
A few open questions to the group:
1. Why is a parabolic antenna used instead of a couple of yagis (two to
compensate for tumble)?
2. Why is the parabolic aimed slightly above the horizon? Doesn't this
orientation attenuate the signal when a satellite's overhead?
73, Don, KB7RPU veritas _|_
liberabit |
https://www.qsl.net/kb7rpu vos |
Jan Panteltje wrote:
Don wrote:
<snip>
https://www.qsl.net/kb7rpu
Fun site, I have the same small mp3 player, but am using a Creative Muvo
with a 1.5 V eneloop rechargable AAA cell
lasts 10 hours on a charge...
As to QO100, I dunno where exactly you are, but it is not accesable from
the US, see 'coverage':
https://www.sarts.org.sg/wp/wp-content/uploads/2025/09/QO-100-Presentation-1.pdf
But I think there are many small Amsat satellites?
I got into digital transmission systems because I worked many years here in >> broadcasting when all was analog,
needed deeper understanding to keep up, so hands on, designed my own and
wrote the DVB-S encoder code myself:
https://panteltje.nl/panteltje/raspberry_pi_dvb-s_transmitter/
No PCB needed, GHz out..
Thank you for your kind words of encouragement. My site almost needs as
much work as the Ham Con site: <https://wyohamcon.org>. My location is
in the USA, so it looks like QO100 is a "no go."
Allow me to jog your memory. Last year we posted to a thread about
the Serenity satellite:
<https://tis.org/serenity-satellite/>
<https://www.orbtrack.org/#/?satSCN=60208>
This Danish ham operates QO-100:
<https://moonbounce.dk/hamradio/ham-radio-current-systems/qo-100-satellite-setup>
A few open questions to the group:
1. Why is a parabolic antenna used instead of a couple of yagis (two to
compensate for tumble)?
2. Why is the parabolic aimed slightly above the horizon? Doesn't this
orientation attenuate the signal when a satellite's overhead?
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>> of the amateur radio books give around 60% as the velocity factor for
'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to
the start of the cable, which is probably not at the same place as the
VNA output connector. At lowish frequencies, it probably doesn't matter,
True, but the OP also wants to measure the length of the cable AIUI,
and for that, you want as high a frequency as possible for greatest
accuracy. It's a trade-off (as ever).
On 2026-01-22, Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>> of the amateur radio books give around 60% as the velocity factor for >>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse.
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to
the start of the cable, which is probably not at the same place as the >>>VNA output connector. At lowish frequencies, it probably doesn't matter,
True, but the OP also wants to measure the length of the cable AIUI,
and for that, you want as high a frequency as possible for greatest
accuracy. It's a trade-off (as ever).
you probably want to do wavelengths shorter than the cable for best results >but crazy high frequencies aren't needed.
You run a frequency sweep and record the reflected amplitude and phase,
then do a Fourier transform, (which is what the NanoVNA does in TDR
mode) This will give you good answers if the cable has a linear
response.
On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts ><usenet@revmaps.no-ip.org> wrote:
On 2026-01-22, Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman >>><jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI,
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to >>>>the start of the cable, which is probably not at the same place as the >>>>VNA output connector. At lowish frequencies, it probably doesn't matter, >>>
and for that, you want as high a frequency as possible for greatest
accuracy. It's a trade-off (as ever).
you probably want to do wavelengths shorter than the cable for best results >>but crazy high frequencies aren't needed.
You run a frequency sweep and record the reflected amplitude and phase, >>then do a Fourier transform, (which is what the NanoVNA does in TDR
mode) This will give you good answers if the cable has a linear
response.
NanoVNA?? ROTFLMAO!!!!!!!!!!!!
On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts <usenet@revmaps.no-ip.org> wrote:
On 2026-01-22, Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI,
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to
the start of the cable, which is probably not at the same place as the >>>> VNA output connector. At lowish frequencies, it probably doesn't matter, >>>
and for that, you want as high a frequency as possible for greatest
accuracy. It's a trade-off (as ever).
you probably want to do wavelengths shorter than the cable for best results >> but crazy high frequencies aren't needed.
You run a frequency sweep and record the reflected amplitude and phase,
then do a Fourier transform, (which is what the NanoVNA does in TDR
mode) This will give you good answers if the cable has a linear
response.
NanoVNA?? ROTFLMAO!!!!!!!!!!!!
On Sun, 01 Feb 2026 17:33:37 +0000, Cursitor Doom <cd@notformail.com>
wrote:
On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:
On 2026-01-22, Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI,
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to >>>>> the start of the cable, which is probably not at the same place as the >>>>> VNA output connector. At lowish frequencies, it probably doesn't matter, >>>>
and for that, you want as high a frequency as possible for greatest
accuracy. It's a trade-off (as ever).
you probably want to do wavelengths shorter than the cable for best results >>> but crazy high frequencies aren't needed.
You run a frequency sweep and record the reflected amplitude and phase,
then do a Fourier transform, (which is what the NanoVNA does in TDR
mode) This will give you good answers if the cable has a linear
response.
NanoVNA?? ROTFLMAO!!!!!!!!!!!!
I just bought the 6 GHz version from Amazon.
Does it not work?
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
On 1.2.2026 20.41, john larkin wrote:
On Sun, 01 Feb 2026 17:33:37 +0000, Cursitor Doom <cd@notformail.com>
wrote:
On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:
On 2026-01-22, Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI, >>>>> and for that, you want as high a frequency as possible for greatest
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to >>>>>> the start of the cable, which is probably not at the same place as the >>>>>> VNA output connector. At lowish frequencies, it probably doesn't matter, >>>>>
accuracy. It's a trade-off (as ever).
you probably want to do wavelengths shorter than the cable for best results
but crazy high frequencies aren't needed.
You run a frequency sweep and record the reflected amplitude and phase, >>>> then do a Fourier transform, (which is what the NanoVNA does in TDR
mode) This will give you good answers if the cable has a linear
response.
NanoVNA?? ROTFLMAO!!!!!!!!!!!!
I just bought the 6 GHz version from Amazon.
Does it not work?
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
It is not a precision thing, but more than adequate for the problem
of Liz.
The TDR mode measured the supplied 20 cm (8 in) cable to within
a few per cent, which is astonishingly good, considering the
velocity factor guess I used.
On 2026-02-01 12:33, Cursitor Doom wrote:
On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:
On 2026-01-22, Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI,
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to >>>>> the start of the cable, which is probably not at the same place as the >>>>> VNA output connector. At lowish frequencies, it probably doesn't matter, >>>>
and for that, you want as high a frequency as possible for greatest
accuracy. It's a trade-off (as ever).
you probably want to do wavelengths shorter than the cable for best results >>> but crazy high frequencies aren't needed.
You run a frequency sweep and record the reflected amplitude and phase,
then do a Fourier transform, (which is what the NanoVNA does in TDR
mode) This will give you good answers if the cable has a linear
response.
NanoVNA?? ROTFLMAO!!!!!!!!!!!!
I'm quite fond of mine. What don't you like about yours?
Cheers
Phil Hobbs
On Sun, 1 Feb 2026 14:03:03 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2026-02-01 12:33, Cursitor Doom wrote:
On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:
On 2026-01-22, Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI, >>>>> and for that, you want as high a frequency as possible for greatest
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to >>>>>> the start of the cable, which is probably not at the same place as the >>>>>> VNA output connector. At lowish frequencies, it probably doesn't matter, >>>>>
accuracy. It's a trade-off (as ever).
you probably want to do wavelengths shorter than the cable for best results
but crazy high frequencies aren't needed.
You run a frequency sweep and record the reflected amplitude and phase, >>>> then do a Fourier transform, (which is what the NanoVNA does in TDR
mode) This will give you good answers if the cable has a linear
response.
NanoVNA?? ROTFLMAO!!!!!!!!!!!!
I'm quite fond of mine. What don't you like about yours?
Cheers
Phil Hobbs
Don't get me wrong. I've said here before that what they've done for
the money is truly exceptional. But you don't get lab grade accuracy
from those things and (the ones I've seen at any rate) give very
misleading readings at anything remotely close to their claimed
limits. A pro designer couldn't possibly rely on one to ensure the
specs a prototype meets comply with a customer's (or regulatory) requirements.
On Sun, 1 Feb 2026 14:03:03 -0500, Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2026-02-01 12:33, Cursitor Doom wrote:
On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:
On 2026-01-22, Cursitor Doom <cd@notformail.com> wrote:
On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 1/22/26 17:16, john larkin wrote:True, but the OP also wants to measure the length of the cable AIUI, >>>>> and for that, you want as high a frequency as possible for greatest
On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid >>>>>>> (Liz Tuddenham) wrote:
What physical properties determine the velocity factor of co-ax? Most >>>>>>>> of the amateur radio books give around 60% as the velocity factor for >>>>>>>> 'common' types of 50-ohm co-ax.
V = c/(sqrt(Er))
Solid polyethylene has Er around 2.3.
Foamed stuff is lower.
Polyethylene is awful. It melts when you solder it. Foamed is worse. >>>>>>>
That's why we have crimped connectors.
Your VNA measurement may be suspect.
Maybe. The VNA needs to be calibrated to move the reference plane to >>>>>> the start of the cable, which is probably not at the same place as the >>>>>> VNA output connector. At lowish frequencies, it probably doesn't matter, >>>>>
accuracy. It's a trade-off (as ever).
you probably want to do wavelengths shorter than the cable for best results
but crazy high frequencies aren't needed.
You run a frequency sweep and record the reflected amplitude and phase, >>>> then do a Fourier transform, (which is what the NanoVNA does in TDR
mode) This will give you good answers if the cable has a linear
response.
NanoVNA?? ROTFLMAO!!!!!!!!!!!!
I'm quite fond of mine. What don't you like about yours?
Cheers
Phil Hobbs
Don't get me wrong. I've said here before that what they've done for
the money is truly exceptional. But you don't get lab grade accuracy
from those things and (the ones I've seen at any rate) give very
misleading readings at anything remotely close to their claimed
limits. A pro designer couldn't possibly rely on one to ensure the
specs a prototype meets comply with a customer's (or regulatory) >requirements.
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