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[TowerTalk] Shields

Subject: [TowerTalk] Shields
From: Roger Parsons <>
Date: Wed, 13 Oct 2004 13:02:28 +0100 (BST)
List-post: <>
Sorry, but I just have to stick my oar in here!

Even cable with a perfectly conducting 100% outer
shield will radiate to some extent. This is because
the diameter of the INSIDE of the shield is less than
the diameter of the OUTSIDE of the shield so there
must be a field across it. This field will set up a
single wire mode transmission mechanism on the outside
of the coax. This effect is quite independent of poor
matching/lack of a balun, etc.

The amount of radiation is related to many factors,
but particularly to the weave of the braiding and the
cable velocity factor. 

In my previous incarnation with British Coal we tested
many different cables for leakage, admittedly in a
tunnel! We hung a long length (500m) of cable from
insulated hangers near the wall of the tunnel (which
was of brick construction and had no conductors other
than the cable), measured the field at many different
points in the tunnel and computed mean coupling
losses. There was little difference between any cables
designed not to 'leak' - good quality cables generally
had a coupling loss of between 100dB and 120dB at
80MHz. Cheap cables such as 'TV coax' had a coupling
loss of about 80dB (I've not seen an equivalent cable
in North America, but this is a foam dielectric
loosely braided coax. Its longitudinal attenuation is
surprisingly low, and its leakage surprisingly small
considering that it only has about 30% braid
coverage.) Cables designed to operate as leaky feeders
had coupling losses ranging from 50dB-80dB.

[Coupling loss is defined as the attenuation measured
between a 1m length of cable to a 1m pickup wire
spaced 1m. Unfortunately, trying to measure it using
that test setup gives totally erroneous results!]

I personally think that double shielded cable is a
waste of money for just about any purpose...

73 Roger

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