From: cd@notformail.com   
      
   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.   
   >   
   >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?   
      
   ITYF capacitance/meter dominates.   
      
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