From: cd@notformail.com   
      
   On Thu, 22 Jan 2026 23:31:15 +0100, 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,   
      
   True, but the OP also wants to measure the length of the cable AIUI,   
   and for that, you want as high a frequency as possible for greatest   
   accuracy. It's a trade-off (as ever).   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)