From: liz@poppyrecords.invalid.invalid   
      
   Cursitor Doom  wrote:   
      
   > 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.   
      
   I could; and some time ago I did try this with some cable from the same   
   drum.  The results were similiar, around 78%.   
      
      
      
   --   
   ~ Liz Tuddenham ~   
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