From: jrwalliker@gmail.com   
      
   On 22/01/2026 16:16, john larkin 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.   
   >>   
   >   
   >   
   > V = c/(sqrt(Er))   
   >   
   > Solid polyethylene has Er around 2.3.   
   >   
   > Foamed stuff is lower.   
   >   
   > Polyethylene is awful. It melts when you solder it. Foamed is worse.   
   >   
   > Your VNA measurement may be suspect.   
      
   I think it would be difficult to get it wrong when it is so   
   easy to check the calibration with a simple 50 ohm terminator.   
      
   >   
   >> 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?   
   >   
   > John Larkin   
   > Highland Tech Glen Canyon Design Center   
   > Lunatic Fringe Electronics   
      
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