From: liz@poppyrecords.invalid.invalid
Jeroen Belleman wrote:
> On 1/23/26 12:34, Liz Tuddenham wrote:
> > john larkin wrote:
> >
> >> On Thu, 22 Jan 2026 17:38:41 +0000, liz@poppyrecords.invalid.invalid
> >> (Liz Tuddenham) wrote:
> >>
> >>> 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.
> >>>
> >>> Solid polyethylene isn't too bad but foamed has a nasty habit of moving
> >>> under the influence of its own 'memory'. You solder the end of a
> >>> slightly bent centre conductor and, as the heat travels down it, the
> >>> foam springs back to the straight position, leaving you with a slot in
> >>> the foam and a bare centre conductor shorted to the screen.
> >>>
> >>>>
> >>>> Your VNA measurement may be suspect.
> >>>
> >>> That was why I asked about it here, I suspected the measurement.
> >>>
> >>> However... my method of finding the first reactance swing in the
> >>> reflection from an open circuit should give me a measurement of the
> >>> electrical length of the cable that is independent of the terminating
> >>> impedances, calibration etc
> >>>
> >>> The cable was physically 6.39 metres long and the first 'resistive'
> >>> impedance point was at exactly 15.000 Mc/s. (That's another reason I
> >>> was suspicious, it really was spot-on 15.000 Mc/s, give or take
> >>> nothing.)
> >>
> >> Looking for the max resistance may not be ideal.
> >
> > Perhaps I didn't explain that very clearly. It wasn't the point of
> > maximum resistance, it was the point where the capacitive reactance
> > swung through zero to become an inductive reactance; it was quite
> > sharply defined. At that point the impedance was purely resistive but
> > it was the reactance that I was measuring, not the resistance.
> >
> >
> >> If you jack up the frequency and get multiple wavelengths in the
> >> cable, resolution will improve.
> >
> > True, but the electrical errors in measurement may increase too. An
> > accuarcy of around 1% would be good enough for the present purposes -
> > after all, where exactly is the 'end' of a piece of co-ax that is
> > splayed out for connection to something else? I also wouldn't expect a
> > length of cheap co-ax to be particularly homogenous.
> >
> >
> >> 15.000 MHz seems suspicious.
> >
> > Yes, that worried me.
> >
> >
> >> I don't have a VNA. I use TDR to measure time delays.
> >
> > My VNA will also function as a TDR and I have a telephone-testing TDR
> > for longer lines. If all else fails, a square-wave signal generator and
> > an oscilloscope will work too.
> >
>
> Having used both VNAs and TDRs for cable length measurements, I've
> always found the VNA measurements much superior to those of a
> TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That
> corresponds to a little under 3ps. TDRs are much too noisy to
> do that.
[...]
My VNA is a relatively cheap hand-held device but it is good enought for
the measurements I want to make and a lot better than the previous
rule-of-thumb and guesswork method.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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