From: jl@glen--canyon.com   
      
   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.   
      
   If you jack up the frequency and get multiple wavelengths in the   
   cable, resolution will improve.   
      
   15.000 MHz seems suspicious.   
      
   I don't have a VNA. I use TDR to measure time delays.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)