From: liz@poppyrecords.invalid.invalid   
      
   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.   
      
   --   
   ~ Liz Tuddenham ~   
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