From: cd@notformail.com
On Fri, 23 Jan 2026 13:59:18 +0100, Jeroen Belleman
wrote:
>On 1/23/26 12:34, Liz Tuddenham wrote:
>> john larkin wrote:
>>
>>> On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
>>> (Liz Tuddenham) wrote:
>>>
>>>> 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.
>>>>
>>>> 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.
>
>
>
>I made these measurements in the context of constructing a wide
>band beam transformer for the Proton Synchrotron at CERN.
>
>Jeroen Belleman
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