From: jrwalliker@gmail.com   
      
   On 22/01/2026 16:42, Cursitor Doom wrote:   
   > On Thu, 22 Jan 2026 16:28:49 +0000, John R Walliker   
   >  wrote:   
   >   
   >> 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.   
   >   
   > You really should use a proper calibration set: open, short and load   
   > and maybe a through too if circumstances require it. The quality of   
   > that cal kit will be crucial to the accurate of subsequent   
   > measurements. If you're doing a lot of VNA stuff, you can expect to   
   > spend a significant portion of the day calibrating and recalibrating.   
   > It's a real time thief.   
      
   Yes, but if all you are doing is looking for 50 ohm resistive   
   then verifying with a 50 ohm terminator is all you need to do   
   to verify your result.   
      
   >   
   >>   
   >>>   
   >>>> 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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