From: jl@glen--canyon.com   
      
   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.   
      
      
      
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
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