From: liz@poppyrecords.invalid.invalid   
      
   John R Walliker  wrote:   
      
   > On 22/01/2026 09:29, 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.   
   > >   
   > > I recently bought a drum of fairly cheap 50-ohm co-ax with the screen   
   > > made from a metallised plastic tape and a loosely-woven copper braid.   
   > > Using a VNA I measured the reflected impedance of a known length (about   
   > > 6 metres), open circuit at the far end, and found the frequency at which   
   > > its reactance first swung through purely resistive.   From this I   
   > > calculated its effective electrical length and the velocity factor,   
   > > which turned out to be 78%.   
   > >   
   > > This seems so different from the 'conventional' value that I am   
   > > suspicious of my measurements - but this type of screen construction was   
   > > not in common use when the original 'words of wisdom' were written.   
   > >   
   > > Are there any physical properties of the co-ax could I check, which   
   > > might explain my measured velocity factor?   
   > >   
   >   
   > Was this a semi air-spaced coax?   
      
   The dielectric looks like solid polythene - almost transparent and   
   definitely hard and solid.   
      
      
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   ~ Liz Tuddenham ~   
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