From: g@crcomp.net   
      
   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?   
      
   78% is closer to the typical 82% of foamed polyethylene dielectric and   
   further from the 66% of solid polyethylene dielectric. An open wire   
   ladder line typically has a 95% to 99% velocity factor.   
      
   The greater the dielectric air, the higher the velocity factor. The   
   higher the velocity factor, the closer the coax's electrical length   
   is to its physical length.   
      
   --   
   73, Don, KB7RPU                                           veritas    _|_   
                                                             liberabit   |   
   https://www.qsl.net/kb7rpu                                vos         |   
      
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