From: pcdhSpamMeSenseless@electrooptical.net
john larkin wrote:
> On Sat, 24 Jan 2026 18:50:17 -0000 (UTC), Phil Hobbs
> wrote:
>
>> 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.
>>>
>>
>> Didnt 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.
>
I’d probably be spending a few bucks on a calibration kit, then. Cheap,
uncalibrated VNAs can produce lots of reasonable-looking wrong answers.
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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