From: liz@poppyrecords.invalid.invalid   
      
   Jeroen Belleman  wrote:   
      
   > On 1/22/26 17:16, 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.   
   > >   
   >   
   > That's why we have crimped connectors.   
   >   
   > > Your VNA measurement may be suspect.   
   >   
   > Maybe. The VNA needs to be calibrated to move the reference plane to   
   > the start of the cable, which is probably not at the same place as the   
   > VNA output connector. At lowish frequencies, it probably doesn't matter,   
      
   Yes, I did that., the maximum error after that would have been the   
   length of an adaptor, around 1 cm in 6 metres of cable.   
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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