From: cd@notformail.com   
      
   On Fri, 23 Jan 2026 11:29:13 -0800, john larkin    
   wrote:   
      
   >On Fri, 23 Jan 2026 19:12:15 +0000, Cursitor Doom    
   >wrote:   
   >   
   >>On Fri, 23 Jan 2026 06:00:26 -0800, john larkin    
   >>wrote:   
   >>   
   >>>On Thu, 22 Jan 2026 23:27:14 +0000, 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).   
   >>>   
   >>>The problem becomes "where does the cable actually start and end" ?   
   >>>With my TDR I can resolve within, say, a transitiion from an   
   >>>edge-launch connector to a PCB trace, so one has to decide where   
   >>>inside the connector the handover happens.   
   >>   
   >>If you have a high enough frequency and can resolve it, you ought to   
   >>be able to 'see' this clearly enough? I know for a fact it's do-able.   
   >   
   >It's an arbitrary decision as to where the thing actually starts.   
      
   Some of 'em have an internal taper which really bluntens the effect. I   
   believe the idea is to minimize reflections during dimensional shifts.   
      
   >   
   >   
   >John Larkin   
   >Highland Tech Glen Canyon Design Center   
   >Lunatic Fringe Electronics   
      
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