[RFI] RF shielding using aluminum foil tape-Fabricating Aluminum Foil Enclosures

Tom Rauch w8ji at contesting.com
Thu Jan 13 10:04:57 EST 2005

> > My project is to construct an antenna-switching unit for
a multiple radio
> > station using vacuum relays.
> > In the frequency range of 1.8 to 30 MHz and at the power
level of 1500
> > watts

Hi Rich,

When you go to reflectors seeking out answers you'll get
specific detailed technical answers from everyone even
though almost no one fully understands what you are doing.
On towertalk, you received an answer that in my opinion
over-exaggerated the amount of isolation that would be
USEFUL (that's the key word) in your system. The answers
sometimes border on being silly because the respondents
visualize the most extreme cases.

In virtually all cases the antenna system, NOT the relays,
will set the ultimate isolation in a system like you
propose. My problems here at my house, for example, are
largely determined by antenna spacing and path loss (and I
have some antennas 1/2 mile apart).

Quite frankly systems in use by many people that are sold by
primary manufacturers of systems like this are not that as
good as your modestly constructed vacuum relay setup could
be without any special shielding.

You are, in my opinion anyway, worrying far too much about
shielding. As an example the DX Engineering RR8, despite not
having an "on board" groundplane or internal closed shield,
surpasses all the competing switches for board radiation
(even when out of the housing) and isolation by a pretty
large margin because of layout and attention to ground loops
and relay to relay coupling. The Ameritron RCS-8V is another
example of superior port isolation and minimal radiation
without "on board" groundplane or shielding. As a matter of
fact BOTH are vastly better than two competing products that
have on board groundplanes and metal housings. RF systems
are nearly 100% how things are implemented and laid out, not
what the system "looks like" by electrical description.
Grounding your coax shields to the shield housing, and even
using shielded cables, might actually make your system worse
if you don't fully understand ground loops and current flow
in the system. It happens time and time again commercially,
and it certainly happens in one-off construction.

My experience designing systems like this is your biggest
worry is component or wiring failures that might connect one
radio to another. You need to very carefully be sure that
one or two components can't fail in a way that connects two
radios together, and can't possibly hot switch relays.
That's number 1.

Everything else is secondary.

73 Tom

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