From: bill.sloman@ieee.org   
      
   On 22/01/2026 11:19 pm, Cursitor Doom wrote:   
   > 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.   
      
   It's ironic to find Cursitor Doom saying "I think you will find" when he   
   clearly isn't fond of thinking.   
      
   The capacitance between the inner and out conductors of a specific   
   length of coaxial cable probably is diagnostic, but a lower dielectric   
   constant dielectric spacer has to be compensated by a thinner inner   
   conductor for a given characteristic impedance, so it isn't going to be   
   all that simple.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)