From: tauno.voipio@notused.fi.invalid   
      
   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.   
      
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