From: liz@poppyrecords.invalid.invalid   
      
   Bill Sloman  wrote:   
      
   > On 22/01/2026 10:41 pm, Liz Tuddenham wrote:   
   > > Bill Sloman  wrote:   
   > >   
   > >> 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.   
   > >   
   > > What are they?   
   >   
   > Read the link...   
      
   Did you read it before suggesting it?   
      
   The link you gave me contained 29 screenfuls of information.  None of   
   them said anything helpful about the velocity factor; it was mentioned   
   once, with no explanation of what it was or what caused it.  There was   
   just a formula with two variables, neither of which was defined.   
      
      
   >Jeroen listed most of them. It's all about getting more   
   > air into the space tween the inner and outer conductor   
      
   Jerogen's information was helpful because it confirmed that the results   
   I obtained were not necessarily an indication of a faulty measurement   
   technique.   
      
      
   --   
   ~ Liz Tuddenham ~   
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