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Re: [TowerTalk] Double shield

Subject: Re: [TowerTalk] Double shield
From: VR2BrettGraham <>
Date: Wed, 13 Oct 2004 10:29:52 +0000
List-post: <>
W6RMK (I think it was) continued:

> The "holes" in the braid of a piece of coaxial cable I see are often
> bigger than that in a mesh surface of a dish receiving C-band, if
> not Ku-band.  The mesh is like waveguide beyond cutoff & so are
> the holes in the braid.

Big difference between woven braid and perforated surfaces as far as EM
propagation goes...
Yes, the holes in the mesh are like WG past cutoff, but there's no conductor
penetrating the hole.  In a braid, there's conductors weaving in and out
through the holes.

There is no conductor, just like with waveguide - the mechanism looks the same to me, but I don't really know much about this stuff & could be completely wrong...

It's like building screen rooms.. woven mesh bad: welded mesh good.

But woven mesh would work, at least to begin with. Over time, the corrosion turns a woven mesh into just a grid of perpendicular woven wires, that no longer look anything like waveguide beyond cutoff & the screen room is no more. Electrically, the welds just ensure the connection where the wires cross each other.

However, Tom's comments relating to the numerical difference are well taken.
In most applications, the difference between 60 and 100 dB isolation is

40 dB? That does make a difference & the conclusion from Tom's measurements is that as bad as it may seem to us, there is not that much of a detriment from doing the cabling in a duplexer with something like RG8X.

Moot is the right word, though. ;^)

73, VR2BrettGraham


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