From: jeroen@nospam.please   
      
   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.   
      
   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   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)