Contrary to as stated by the other poster; "transfer impedance" does NOT
remain constant with frequency.
The transfer impedance of a cable is how a lightning pulse gets onto the
center conductor of a cable when there is a dc short circuit on the antenna
end. It would seem that there would be no way for it to penetrate the
shield. In this case the skin effect starts to fall apart at the low
frequencies and the same current flows on the inside of the cable that flows
on the outside of the shield.
But all shields have a transfer impedance, even solid shields, where the
current on one side of the shield causes a voltage drop over some length of
the cable and that voltage drop is seen on the other side of the shield.
The less dense the shielding is generally the higher the transfer impedance.
A lower value of transfer impedance means less leakage.
Here is a link to belden's paper on transfer impedance. Note the charts at
the bottom of the page comparing different amounts of shielding and
frequency.
Also a google on "transfer impedance" will show several other interesting
articles.
http://www.belden.com/pdfs/TechInfo/TechTransfer.htm
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of jeremy-ca
> Sent: Friday, June 15, 2007 12:43 PM
> To: ve3zi@rac.ca; towertalk@contesting.com
> Subject: Re: [TowerTalk] Balun question shield leakage
>
> Thank you for that explanation Roger. Since there was no question about
> the
> connector integrity and the long lengths laying in cable trays were next
> to
> all types of other cables, RF and power, your explanation makes sense.
>
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Roger Parsons" <ve3zi@yahoo.com>
> To: <towertalk@contesting.com>
> Sent: Friday, June 15, 2007 9:55 AM
> Subject: [TowerTalk] Balun question shield leakage
>
>
> > The leakage from a coaxial cable is only indirectly
> > related to the braid coverage and/or 'hole size'. It
> > does not leak out of the holes like water through a
> > sieve! What happens is that the coaxial mode signal is
> > continually transferred to a single wire mode signal
> > on the outer of the coax over a considerable distance
> > - the so-called G-wave. This happens even when the
> > coax has a 100% shield that is 100% conductive.
> >
> > The standard test for measuring transfer impedance is
> > highly flawed as it uses a 1m length of cable with a
> > closely spaced probe. At less than high vhf
> > frequencies this is far too short for the single wire
> > mode signal to emerge and be properly measured.
> >
> > The only method of which I am aware to correctly
> > measure transfer impedance (and hence leakage and
> > coupling to another cable) is to set up a long length
> > of cable in a tunnel with non-conductive walls and
> > take a very large number of fs measurements. Even this
> > test is complicated when the frequency is above the
> > waveguide mode cut off frequency of the tunnel -
> > perfect measurements could only be done in space!
> >
> > Some quite interesting facts emerge:
> >
> > The relationship between braid coverage and leakage is
> > by no means simplistic. It is not at all true to say
> > that 'less braid = more leakage'. Some of the very
> > cheaply made cables with low braid coverage have quite
> > good shielding, although most don't. Some cables with
> > high braid coverage have relatively high leakage;
> >
> > It is quite possible to design a cable with less
> > longitudinal attenuation and more leakage. The
> > flexible leaky feeder developed by British Coal
> > (T3529), which is pretty much the world standard for
> > this lype of cable, was originally based on URM57
> > (RG11), and has slightly lower longitudinal
> > attenuation and about 60dB more leakage than that
> > cable;
> >
> > Discontinuities in the coaxial mode do not in
> > themselves create more leakage, but set up reflected
> > waves that make a complex situation more complex. Of
> > course, as others have mentioned, a bad connection in
> > the cable outer will change the cable into an antenna;
> >
> > Discontinuities in the single wire mode have a
> > dramatic effect, just as they do with a wire antenna.
> >
> > Transfer impedance remains constant with frequency
> > within the limits of the cable design.
> >
> > A short length of badly shielded cable at hf will not
> > cause measurable leakage.
> >
> > It is possible to deterministically calculate transfer
> > impedance for any given cable, but it is quite a
> > complicated sum involved multiple differential
> > equations.
> >
> > 73 Roger
> > VE3ZI
> >
> > Former Head of Communications Group, British Coal
> > Mining Research and Development Establishment.
> >
> >
> >
> >
> >
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> >
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