From: liz@poppyrecords.invalid.invalid
Bill Sloman wrote:
> On 23/01/2026 4:20 am, Liz Tuddenham wrote:
> > Bill Sloman wrote:
> >
> >> On 22/01/2026 10:41 pm, Liz Tuddenham wrote:
> >>> Bill Sloman wrote:
> >>>
> >>>> On 22/01/2026 8:29 pm, 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.
> >>>>>
> >>>>> I recently bought a drum of fairly cheap 50-ohm co-ax with the screen
> >>>>> made from a metallised plastic tape and a loosely-woven copper braid.
> >>>>> Using a VNA I measured the reflected impedance of a known length (about
> >>>>> 6 metres), open circuit at the far end, and found the frequency at
which
> >>>>> its reactance first swung through purely resistive. From this I
> >>>>> calculated its effective electrical length and the velocity factor,
> >>>>> which turned out to be 78%.
> >>>>>
> >>>>> This seems so different from the 'conventional' value that I am
> >>>>> suspicious of my measurements - but this type of screen construction
was
> >>>>> not in common use when the original 'words of wisdom' were written.
> >>>>>
> >>>>> Are there any physical properties of the co-ax could I check, which
> >>>>> might explain my measured velocity factor?
> >>>>
> >>>> Some coax relies on a foamed dielectric, which would have a lower
> >>>> dielectric constant than solid plastic.
> >>>>
> >>>> https://en.wikipedia.org/wiki/Coaxial_cable
> >>>>
> >>>> There are more complicated ways of getting much the same effect.
> >>>
> >>> What are they?
> >>
> >> Read the link...
> >
> > Did you read it before suggesting it?
>
> I always do. Wikipedia pages get edited from time to time, and it's wise
> to check for changes.
>
> > The link you gave me contained 29 screenfuls of information.
>
> There was more than one screen, but 29 sounds a bit high. I read faster
> than most people do, but not that fast.
So you didn't read all of it, but you expected me to.
Where did you find the information on that webpage about air dielectric
affecting the Velocity Ratio, which was the reason you referred me to
the page? If you can give me an unique quoted phrase, I can soon find
it.
>
> > None of
> > them said anything helpful about the velocity factor; it was mentioned
> > once, with no explanation of what it was or what caused it. There was
> > just a formula with two variables, neither of which was defined.
>
> It did list a number of ways of making coaxial cable, some of which
> included schemes where the dielectric was mostly air.
There was no mention of this affecting the Velocity Ratio. I wasn't
asking for advice on making co-ax cables, I was asking how the
construction affected the Velocity Ratio.
> >> Jeroen listed most of them. It's all about getting more
> >> air into the space tween the inner and outer conductor
> >
> > Jeroen's information was helpful because it confirmed that the results
> > I obtained were not necessarily an indication of a faulty measurement
> > technique.
>
> That's always a potential explanation for odd results, but it's well
> known that the propagation delay through regular cables is a bit slower
> than the speed of light in a vacuum.
Yes, that is called the Velocity Ratio. I would have though it was
obvious from my question that I realised what the VR was - my question
was about the physical properties of the cable that affected it.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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