From: jl@glen--canyon.com   
      
   On Sun, 1 Feb 2026 21:49:08 +0200, Tauno Voipio   
    wrote:   
      
   >On 1.2.2026 20.41, john larkin wrote:   
   >> On Sun, 01 Feb 2026 17:33:37 +0000, Cursitor Doom    
   >> wrote:   
   >>   
   >>> 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!!!!!!!!!!!!   
   >>   
   >> I just bought the 6 GHz version from Amazon.   
   >>   
   >> Does it not work?   
   >>   
   >>   
   >> John Larkin   
   >> Highland Tech Glen Canyon Design Center   
   >> Lunatic Fringe Electronics   
   >   
   >   
   >It is not a precision thing, but more than adequate for the problem   
   >of Liz.   
   >   
   >The TDR mode measured the supplied 20 cm (8 in) cable to within   
   >a few per cent, which is astonishingly good, considering the   
   >velocity factor guess I used.   
      
   We're designing a small family of modular RF switches and   
   multiplexers, and figured that we should have some S-param graphs in   
   the manuals.   
      
   $280 is a real deal compared to 10 passes through some test lab.   
      
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
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