From: liz@poppyrecords.invalid.invalid   
      
   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.   
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)