Shielded rooms
Mark Lee
I want to build an electromagnetically shielded room in my
laboratory. Something like 6' by 8' or somewhat larger, and 8'
tall, made of either copper (sheet or mesh) or galvanized steel
and including a power supply filter and ground line.
Can anyone suggest a place to get one of these things, new or
used, preferably without spending a fortune? The commercial
outfits I know about want about 2x more than I am willing to
spend.
Please email me if you know anything about this.
Thanks,
Mark Lee
Bill Mayhew
tight your specifications are. On a previous job, we had a
laboratory with five screen rooms. We were shooting for a noise
floor that would give us almost 100 dB of dynamic range for
testing. Preventing stuff from getting into the room is not easy.
We had to periodically remove the joints holding the panels of the
wall to clean and shine the connections. Several of the rooms were
made by Keene; the walls were something like particle board with
galvanized steel on either side. The panels were joined with
channels shapped like the letter H and tightened with many screws.
The door is also problematic. Our rooms had sliver plated
phosphor-bronze finger stock around the door that needed regular
cleaning. The other problem is lining the rooms with RF
echo-absorbing material. It is pretty difficult to get thick
enough cone material to be effective at HF ranges in a 6 foot * 8
foot room. Our audio test room had 1/2 inch thick welded steel
plate walls; even with that, 60 Hz still measurably penetrates.
The best place to check for info would be to look through trade
magazines aimed at the military/microwave industry at your favorite
local tech library. I am fairly sure that some manufacturers have
rooms in knocked-down "kit" form that you can put together
yourself. I don't have any current references since I haven't been
in that business for about 12 years.
--
Bill Mayhew NEOUCOM Computer Services Department
Rootstown, OH 44272-9995 USA phone: 216-325-2511
w...@uhura.neoucom.edu amateur radio 146.58: N8WED/AA
John F. Woods
>How easy it is to construct your own RF screen room depends on how
>tight your specifications are. On a previous job, we had a
>laboratory with five screen rooms. We were shooting for a noise
>floor that would give us almost 100 dB of dynamic range for
>testing. Preventing stuff from getting into the room is not easy.
Conversely, if your requirements are more like 60dB or so, RF Design a few
months back had an article on how to build a cheap shielded room from common
building materials (in particular, a brand of aluminum-foil covered wallboard).
I think it cost about $1000 for a 10x10 room, but again, the shielding was
nowhere near perfect. It was, however, good enough that you could get a
reasonable idea of whether equipment was quiet enough to warrant renting time
in a real shielded room.
If this sounds useful, I'll look up the article.
Dave Spring
RF emissions before the expensive process of getting FCC approval.
You don't want to try changing components while someone is charging you by
the hour for gaining approval at their sanctioned facilities.
Basically the setup was, a wide band aerial and the Equipment under test
are put in a sheilded room and the output of the aerial is fed to a spectrum
analyser.
3 solutions were used to varying effectiveness:
1. Take the equipment and a *long* mains extension cable into the
middle of a large field away from power pylons etc. (One of the
employees lived in a rural area).
2. We got loads of the metal mesh (about 1cm grid in about 1m sheets)
which was readily available from hardware stores and lined a room
6' x8' with it. Overlapping by about 4" and soldering every
couple of inches. The door jamb was lined with berylium(?) copper
fingers designed for RF screening (we had enough samples). and the
analyser was kept outside the room, with the aerial cable going
through a hole in the wall. For our purposes this was fine, you
could take a portable radio or TV in, shut the door, and the radio
would go silent.
3. Hire a cargo container - you know - a large metal box about 25'x8'x8' used
for shipping goods around the world. This was great, apart from
lighting and getting a reliable door seal ( we had the cable running
out the door to the analyser)
These were good enough for us to get an idea of what emission problems we might
have, but I can't remember what sort of numbers we were getting for the
shielding aspects.
Dave Spring
--
d...@oasis.icl.co.uk
John F. Woods
OK, the article is from RF Design July 1992, pp 55-59: "Construction of a
Low-Cost Shielded Room." First you build a frame from 2x4s, using standard
carpentry techniques. You line the walls with aluminum foil-coated
insulated building sheathing (priced, in the article, at $7.77 (4x8 panel)).
You have to get one where the foil has a conductive finish (use an ohmmeter
to check, using pennies as pads to avoid damaging the surface).
Nail it to the frame with plastic-cap siding nails, with a slight gap between
sheets (usual carpentry methods). Seams may be sealed with either conductive
EMI tape or a combination of aluminum flashing and ordinary metal tape with a
nonconductive adhesive (glue down the flashing over the gap, then use aluminum
tape to make an ohmic bond; it turns out the capacitance between the flashing
and the siding is a substantial part of the RF sealing, anyway). Walls and
ceiling are joined in the same manner (the ceiling is treated with the same
sheathing). The simplest flooring is galvanized steel sheets, overlapped and
taped together, joined to the walls with the same kind of tape.
Having sealed the room, you now need to get back out :-). Their approach
was to cut a removable 2x4.5 foot doorway. Line the doorway with aluminum
plate over aluminum flashing (the plate protects the weaker flashing), then
make a door that is a 2x4 frame with metal sheeting of sufficient size to
provide an overlap between the door and the doorway. Metal-to-metal contact
is achieved by metal weather stripping on the door frame. The door is held
in place by the friction of the weather stripping against the shimmed door
frame. Simple, but it works.
Cable access: either stuffing tubes or bulkhead panels. Bulkhead panels must
be attaches to boxes made of 2x4s that are built into the wall framing, as
the foil clad foam siding can't support the weight. Stuffing tubes can be
made from aluminum ductwork, or steel pipe (handy for welding said tube to
a steel bulkhead panel). Stuffing tubes can be filled with steel wool.
Remember to properly ground coax going through the wall; you may want to use
bulkhead feedthrough connectors to ensure a good ground for coax. AC power is
filtered with ordinary AC line filters. Ventilation can be routed into the
enclosure through a stuffing tube or through aluminum screening bonded to the
foil-backed siding with metal tape. A screened opening on the ceiling and
a complementary opening near the floor (on the wall) with appropriate fans
should be enough (depending on how much heat gets generated inside).
USE A SINGLE POINT GROUND. Have one grounded work surface, which is grounded
to an outside single-point ground by a heavy copper strap. Ground all interior
equipment to that work surface. The foil-covered walls are NOT adequate for
an AC safety ground.
Cost and results: they said their 16x8x8' room cost $1100 and 40 hours of
labor. Looking over their graph of attentuation between .014MHz and 1000MHz,
the average is about 20-30dB; 30-60dB in the AM broadcast band, 30dB in the
FM broadcast band, ~20dB in the HF spectrum, below 10dB above about 200MHz.
(actually, the performance is probably better than this; the "ambient outside
room" graph has lots and lots of spikes, and the "room ambient" is pretty
smooth, making a single figure hard to come up with)
If you want to make careful scientific measurements requiring absolute radio
quiet, this won't cut it. If you want to avoid embarassment at the local FCC
testing lab when your completely-untested equipment proves to be a better
transmitter than WRKO, this is a cheap way to roll your own RFI screening room.
I think back issues can be gotten from either (303) 220-0600 or (708) 647-0756
(I got bounced around from one to another the last time I called them). They
don't make a point of stocking back issues, but they keep a reference copy and
will photocopy articles if they've run out of a given back issue. (They're
published by Cardiff Publishing Company.)
(Does anyone have January and February 1989 RF Design copies? Someone a long
time ago mentioned an article of interest in one or the other of those, but
by the time I got around to calling RF Design, (a) they'd run out of the
issues, and (b) I'd forgotten *what* I was looking for and couldn't remember
which article to ask them to photocopy...)
Dave Medin
If anyone wants a good Cornell Dubilier RF screen room line filter
(the BIG ones that mount on the wall outside the room), I've got
several to sell for probably $40 each. They'd have to ship by truck.
--
--------------------------------------------------------------------
Dave Medin--SSD Networking Phone: (205) 730-3169 (w)
Intergraph Corp. (205) 837-1174 (h)
M/S GD3004 Internet: dtm...@ingr.com
Huntsville, AL 35894 UUCP: ...uunet!ingr!b30!catbyte!dtmedin
* The opinions expressed here are mine (or those of my machine)