From: liz@poppyrecords.invalid.invalid   
      
   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   
      
   I did.   
      
      
   --   
   ~ Liz Tuddenham ~   
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