From: bill.sloman@ieee.org   
      
   On 22/01/2026 8:29 pm, 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?   
      
   Some coax relies on a foamed dielectric, which would have a lower   
   dielectric constant than solid plastic.   
      
   https://en.wikipedia.org/wiki/Coaxial_cable   
      
   There are more complicated ways of getting much the same effect.   
      
   --   
   Bill sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)