From: pcdhSpamMeSenseless@electrooptical.net
john larkin wrote:
> On Fri, 23 Jan 2026 20:21:42 +0000, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
>
>> john larkin wrote:
>>
>>> On Fri, 23 Jan 2026 19:08:50 +0000, Cursitor Doom
>>> wrote:
>>>
>>>> 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
>>>
>>> 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
>
--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
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