From: cd@notformail.com   
      
   On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid   
   (Liz Tuddenham) wrote:   
      
   >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.   
      
   Can you not do it by TDR? Use some fast switching logic, tee piece to   
   a scope input and a dead short at the far end of the cable.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)