From: jeroen@nospam.please   
      
   On 1/22/26 14:33, Liz Tuddenham wrote:   
   > Jeroen Belleman  wrote:   
   >   
   >> 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.   
   >   
   > In that case it seems quite likely my cable is rather better than I   
   > anticipated.  I shall start chopping it up into lengths to make a 145   
   > Mc/s vertical co-linear array with reversals every 88.85 cm.   
   >   
   >   
      
   The velocity factor isn't really a determinant of the cable quality.   
   There are also such things as loss, shielding effectiveness, accuracy   
   of Z0 and lots of other lesser considerations. Your description of   
   the cable construction does not suggest a high-quality cable.   
      
   Jeroen Belleman   
      
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