From: jl@glen--canyon.com
On Fri, 23 Jan 2026 19:12:15 +0000, Cursitor Doom
wrote:
>On Fri, 23 Jan 2026 06:00:26 -0800, john larkin
>wrote:
>
>>On Thu, 22 Jan 2026 23:27:14 +0000, 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).
>>
>>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
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