From: jl@glen--canyon.com   
      
   On Sun, 01 Feb 2026 23:47:11 +0000, Cursitor Doom    
   wrote:   
      
   >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.   
      
   Are there regulatory requirements for s-params?   
      
   I think there simple ways to calibrate a nanovna.   
      
   I wouldn't expect many production units to have precisely the same   
   s-params anyhow.   
      
   Most s-param tables or graphs have no specified min/max, just   
   typicals.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)