From: cd@notformail.com   
      
   On Sun, 1 Feb 2026 14:03:03 -0500, Phil Hobbs   
    wrote:   
      
   >On 2026-02-01 12:33, 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'm quite fond of mine.  What don't you like about yours?   
   >   
   >Cheers   
   >   
   >Phil Hobbs   
      
   Don't get me wrong. I've said here before that what they've done for   
   the money is truly exceptional. But you don't get lab grade accuracy   
   from those things and (the ones I've seen at any rate) give very   
   misleading readings at anything remotely close to their claimed   
   limits. A pro designer couldn't possibly rely on one to ensure the   
   specs a prototype meets comply with a customer's (or regulatory)   
   requirements.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)