From: bill.sloman@ieee.org   
      
   On 23/01/2026 10:34 pm, Liz Tuddenham wrote:   
   > Bill Sloman  wrote:   
   >   
   >> 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.   
   >   
   > So you didn't read all of it, but you expected me to.   
      
   I read through all of it. Most of it was familiar and predictable.   
   You may well have needed to read it more carefully, and you clearly didn't   
      
   > Where did you find the information on that webpage about air dielectric   
   > affecting the Velocity Ratio, which was the reason you referred me to   
   > the page?  If you can give me an unique quoted phrase, I can soon find   
   > it.   
      
   You have to read down to the section labelled "important parameters",   
   then work through that to the section "derived electrical parameters"   
   where you will find "velocity of propagation"   
      
   "The velocity of propagation depends on the dielectric constant and   
   permeability (which is usually 1)" and they give the formula   
      
   v= one over the square root of the product of the permeability and the   
   dielectric constant.   
      
   >>> 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.   
      
   The permeability of free space,and the electric constant of the spacer   
   aren't "variables" but parameters. and they should be familiar concepts   
   to any body who has had any kind of education in physics or elementary   
   electronics. It is remarkably basic stuff.   
   >> It did list a number of ways of making coaxial cable, some of which   
   >> included schemes where the dielectric was mostly air.   
   >   
   > There was no mention of this affecting the Velocity Ratio.  I wasn't   
   > asking for advice on making co-ax cables, I was asking how the   
   > construction affected the Velocity Ratio.   
      
   You were actually asking how the construction affected the velocity of   
   propagation. Your "velocity ratio" is actually the ratio of the velocity   
   of propagation to the speed of light. A "ratio" compares two numbers and   
   you didn't mention the speed of light.   
      
   The formula tells you exactly how the parameters affect the velocity of   
   propagation. Telling you that in a way you can follow hasn't been easy   
      
   >>>> 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.   
   >   
   > Yes, that is called the Velocity Ratio.  I would have though it was   
   > obvious from my question that I realised what the VR was - my question   
   > was about the physical properties of the cable that affected it.   
      
   And you got told, repeatedly. Sadly, we didn't realise that you hadn't   
   had much formal education in the area.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)