From: jeroen@nospam.please   
      
   On 1/22/26 12:04, 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?   
   > John   
   >   
      
   The signal propagation velocity depends entirely on the material   
   and consistency of the dielectric. A velocity factor of 78%   
   does not shock me. I've used cables with a foamed polyethylene   
   dielectric with a velocity factor of 0.84. Some cables use   
   plastic spiral wire or discrete spacers and can reach velocity   
   factors pretty close to unity.   
      
   Jeroen Belleman   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)