From: jl@glen--canyon.com
On Sun, 01 Feb 2026 17:33:37 +0000, Cursitor Doom
wrote:
>On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts
> wrote:
>
>>On 2026-01-22, Cursitor Doom wrote:
>>> On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
>>> 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.
>
>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
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