From: cd@notformail.com   
      
   On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts   
    wrote:   
      
   >On 2026-01-22, Cursitor Doom  wrote:   
   >> 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).   
   >   
   >you probably want to do wavelengths shorter than the cable for best results   
   >but crazy high frequencies aren't needed.   
   >   
   >You run a frequency sweep and record the reflected amplitude and phase,   
   >then do a Fourier transform, (which is what the NanoVNA does in TDR   
   >mode) This will give you good answers if the cable has a linear   
   >response.   
      
   NanoVNA?? ROTFLMAO!!!!!!!!!!!!   
      
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