From: liz@poppyrecords.invalid.invalid   
      
   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.)   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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