Re: [TowerTalk] Double shield

[VR2BrettGraham <vr2bg@harts.org.hk>]

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
> moot.

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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