Rich,
To the reply below, and other excellent ones, including W8JI, I add:
Everything that has been said about the working isolation value being
controlled by the relays and/or antennas is correct. Let's look at some
numbers:
Xmit power: 1000 watts = +60 dBM
Relay isolation (typical): 50 to 60 dB
Antenna-to-antenna isolation (coupling factor): 40 to 60 dB
(approximately), depending on spacing and orientation (least coupling with
orthogonal orientations)
If you have 60 dB of relay isolation and 60 dB of antenna isolation (an
optimistic value), the net isolation for those 2 factors is 57 dB.
Isolation of typical braided shield coax (in the HF range): 70 to 90 dB
(varies with construction and type of braid)
Isolation of hard line: 100 to 120+ dB (except for leakage at connectors)
Isolation of aluminum foil shield (as you propose): 10 to 30 dB, depending
on quality of bonding at the seams, and accounting for penetration of the
coax cable (which will carry significant coupled RF on the outside of the
braid if the transmit field is near/strong enough)
Isolation of commercial shielding "boxes" or enclosures: 100+ dB, except
for any penetrations
I would expect that a receiver input would see around +5 to +10 dBm input
from any other xmitter, a level which should not toast out anything, but it
will blast the heck out the filters and possibly result in cross-mod and
other undesired effects.
Incidentally, I happen to have a "canned" vacuum relay in my junque box.
It came from an FAA ground-to-air transmitter made by Wilcox Electric. It
is a copper can (round - the size of a Campbell soup can) fitted with 3
Type-N connectors and using #10 solid copper wire to go from each connector
to the glass-enclosed relay. I might add that the connecting #10 wires
cross-over each other in 2 places at a spacing of about 0.2" - any guesses
what that did for isolation?
Best luck on your project.
73, Dale
WA9ENA
(former RF shielding engineer)
k1ttt@arrl.net
Sent by:
rfi-bounces@conte To
sting.com "RCARIELLO" <RCARIELLO@si.rr.com>
cc
rfi@contesting.com
01/13/2005 06:38 Subject
AM Re: [RFI] RF shielding using
aluminum foil tape-Fabricating
Aluminum Foil Enclosures
you might want to look at some existing solutions and see why they don't
all use multiple vaccuum relays and separate shielded enclosures and all
that stuff.
that being said, aluminum tape is probably not the best thing for
something like this. not that it won't shield well based on skin depth,
but that joints are hard to properly bond. it is very important when
making shielded enclosures to be sure that the edges and corners have as
good a joint as possible. you can not count on the adhesive being
conductive which forces you to have surface to surface pressure contacts,
which would likely require special cleaning of the surface of the tape to
be sure it isn't coated with something or just oxidized, and some kind of
conductive sealent... just overlapping joints is not that great of a
method if you want it really tight.
oh, and there is no such thing as 'full' isolation. isolation between
ports in something like that is specified in db. you probably want to
read something like this:
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=1856 to get an idea of
realistic goals and what is available off the shelf.
> RF shielding using aluminum foil tape
> Fabricating Aluminum Foil Enclosures
>
> Hello to all RFI members,
> I believe this topic comes within the subject matter of this list.
> This is my first posting.
> Please excuse me if it dose not.
>
> 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
> I am hopping to achieve full isolation between each port of the unit
> without
> any bandpass filtering.
> I am considering to place each vacuum relay used within it's own small
box
> made out of heavy construction cardboard.
> The cardboard box would then be covered with aluminum foil tape.
> The aluminum foil tape with the glue backing normally found at local Home
> Depot stores.
> The RF connections for each vacuum relay would be made with Coax. The
> center
> conductor going to the vacuum relay and the shield connected to the
> aluminum
> foil covering the box. Another thought would be to mount SO-239's onto
the
> side of the box and use coax jumpers between each relay.
>
> How much shielding can I expect from this type of enclosure?
> Will I need to use and if so how many multiple layers of the aluminum
> tape?
> Should both the inside and outside of the box be covered with the
aluminum
> tape?
> I understand the skin depth of copper to be 0.0026 inch @1 MHz and
0.00047
> inch @ 30 MHz.
> What would the skin depth be for aluminum and dose it apply to this
> application?
>
> I am trying to reach the full isolation advantage offered by the use of
> the
> vacuum relays.
> Should I consider putting a shielded baffle inside the box to isolate the
> RF
> contacts of the relay from each other?
> Should the shield on the Coax run up to the relay contact to eliminate
the
> stray RF pickup within the box?
>
> I suspect that I will have to connect two or three vacuum relays in
series
> to obtain the wanted isolation.
> The end filter assembly will probably look like a very large IF section
> from
> an old tube radio receiver.
>
> Another question for comments:
> If I substitute the cardboard with a tin can.
> Covering the inside of the tin can with the aluminum foil tape.
> I know I have seen copper plated steel chassis on old AM transmitters.
>
> With the availability of today's building materials, PVC and copper pipe,
> plastics, aluminum and copper foils the projects are endless. The
> traditional hard enclosures and chassis construction can be so much
> improved.
>
> Thanks for your help,
> Rich AA2MF
> rcariello@si.rr.com
>
>
>
>
>
>
>
>
>
> _______________________________________________
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> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>
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