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.
----- Original Message -----
From: "Roger Parsons" <firstname.lastname@example.org>
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
> 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
> 73 Roger
> Former Head of Communications Group, British Coal
> Mining Research and Development Establishment.
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