From: bill.sloman@ieee.org   
      
   On 23/01/2026 4:20 am, Liz Tuddenham wrote:   
   > 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?   
      
   I always do. Wikipedia pages get edited from time to time, and it's wise   
   to check for changes.   
      
   > The link you gave me contained 29 screenfuls of information.   
      
   There was more than one screen, but 29 sounds a bit high. I read faster   
   than most people do, but not that fast.   
      
   > 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.   
      
   It did list a number of ways of making coaxial cable, some of which   
   included schemes where the dielectric was mostly air.   
      
   >> Jeroen listed most of them. It's all about getting more   
   >> air into the space tween the inner and outer conductor   
   >   
   > Jeroen's information was helpful because it confirmed that the results   
   > I obtained were not necessarily an indication of a faulty measurement   
   > technique.   
      
   That's always a potential explanation for odd results, but it's well   
   known that the propagation delay through regular cables is a bit slower   
   than the speed of light in a vacuum.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)