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

k1ttt at arrl.net k1ttt at arrl.net
Thu Jan 13 07:38:42 EST 2005

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 at si.rr.com
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