Optimal Faraday cage

[Edward Quinn]

I need a quiet environment for operating test instruments. Needs to be
as close to full spectrum as practical, and about 1.5 metres square or
equivalent internal space.

From what I have read, this requires a combination of shielding types.
With cost in mind, what is state-of-the-art homebrew for this?

A few ideas:

1. Aluminum foil on plywood box. Edges sealed with self-adhesive
aluminum tape. I assume a solid surface is more effective than screen
or mesh.

2. Corrugated steel water tank. Thick metal may be better for lower
frequencies(?) Corrugations may provide deflection(?)

Of course, the unit would be earthed. For vents, etc. I would fit a
curved section of flexible aluminum ducting a used in domestic
heating/air con installations, with a metal screen over the opening.

In addition, I would apply a thick paint with iron filings to the
outer surface.

Any feedback or analysis regarding the above would be appreciated.

Ed Quinn

 

[Joerg]

I need a quiet environment for operating test instruments. Needs to be
as close to full spectrum as practical, and about 1.5 metres square or
equivalent internal space.

From what I have read, this requires a combination of shielding types.
With cost in mind, what is state-of-the-art homebrew for this?

A few ideas:

1. Aluminum foil on plywood box. Edges sealed with self-adhesive
aluminum tape. I assume a solid surface is more effective than screen
or mesh.

2. Corrugated steel water tank. Thick metal may be better for lower
frequencies(?) Corrugations may provide deflection(?)

Of course, the unit would be earthed. For vents, etc. I would fit a
curved section of flexible aluminum ducting a used in domestic
heating/air con installations, with a metal screen over the opening.

In addition, I would apply a thick paint with iron filings to the
outer surface.

Any feedback or analysis regarding the above would be appreciated.

For similar purposes I have a large thick corrugated carton that is
covered with copper foil. Draped around the edges so the lid makes a
tight seal. So far any sort of radio goes totally silent in there.

A word of caution: In certain weather conditions removing and closing
the lid can generate an impressive blue spark, often accompanied by a
succinct *BANG*, so be careful. Maybe keep it grounded together.

 

[Phil Allison]

"Greegor"

Wouldn't the aluminum foil be passive to the
magnetic component?


** Aluminium or other non-magnetic foils will have no effect on a static or
slowly varying ( ie 50 /60Hz) magnetic field.

A static electric field is defeated because the charges on the surface of
the foil become re-arranged so as to cancel any static field INSIDE the
enclosure.

A radio frequency wave is very much attenuated inside the enclosure too -
the surface charges respond to the wave's electric field and MOVE in synch
with the wave thereby creating a magnetic field that opposes the magnetic
part of the arriving wave.


...... Phil

 

[Paul Keinanen]

The most difficult part is RF sealed door. It must have perfect contact
over all perimeter; otherwise the chamber will leak badly.

Fingerstock will help a lot, but the door and the frame needs to be
tightly matched with each other, so if the door i damaged, you can not
just replace the door, but also the frame must be replaced that fits
tightly with the new door.

 

[Glen Walpert]

Iron filings are of little use; suggest Nickel Carbonyl. Better yet,
use carbon loaded foam like what Jameco carries.

The carbon loaded foam can be a useful absorber inside the chamber.

For a cost effective functional shielded chamber you should start with
Ralph Morrison "Grounding and Shielding" and Henry Ott "Noise Reduction
Techniques In Electronic Systems". Ralph specifically addresses shielded
chambers and Henry provides a good complementary discussion of shielding.

One of the first requirements is conductive seams, so as others have
noted aluminum is out (unless you can weld all the seams). Steel sheet
metal is fairly cheap, easily cut, bent and nailed to a wooden frame, and
then the seams can be soldered. But the devil is in the details and the
details best found in the above books and perhaps a look at shielding
gasket suppliers literature.

 

[GregS]

I need a quiet environment for operating test instruments. Needs to be
as close to full spectrum as practical, and about 1.5 metres square or
equivalent internal space.

From what I have read, this requires a combination of shielding types.
With cost in mind, what is state-of-the-art homebrew for this?

A few ideas:

1. Aluminum foil on plywood box. Edges sealed with self-adhesive
aluminum tape. I assume a solid surface is more effective than screen
or mesh.

2. Corrugated steel water tank. Thick metal may be better for lower
frequencies(?) Corrugations may provide deflection(?)

Only for magnetic interference. All you need is low Z for electrostatic.

Of course, the unit would be earthed. For vents, etc. I would fit a
curved section of flexible aluminum ducting a used in domestic
heating/air con installations, with a metal screen over the opening.

In addition, I would apply a thick paint with iron filings to the
outer surface.

Any feedback or analysis regarding the above would be appreciated.

Ed Quinn

The most common are home brewed aluminum screening with aluminum frame.
Sometimes steel is used but aluminum is better for the frame. Most I see are for
lower frequencies.

greg

 

[GregS]

I've always wondered how well a 55 gallon steel drum would work for
this sort of thing. You'd want to remove the paint where the
removable top fits onto the drum... and then the thing would start to
rust...sigh. One nice thing is that you could have different tops
with different fed throughs.

George H.

A space withing a room is nice. You can build a good sized outer room
using aluminum backed drywall with steel studs. I have seen a very large room
fitted with 1/4 inch steel plate on the bottom and edges, to reduce
magnetic field from a MRI below. I could get a steel nail to move around
on a string pretty good while walking above that small MRI unit.
Of course and small vibration of electronic devices could induce
electrical noise at the vibration frequency.

greg

 

[Capt. Cave Man]

Make whatever box you (he) want(s), and remember to fill each seam,
during mating, with silver filled, electrically conductive epoxy like
what they use to attach IC chip dies to their substrate like: EPOTEK
H20E

http://www.epotek.com/SSCDocs/datasheets/H20E.PDF

 

[Capt. Cave Man]

Entire rooms have
been
made into 3 layer shielded rooms.

'They' have chambers big enough to put an Osprey or even a C-17 into.
The 'cones' are like 3.5 feet tall, and it is like the area of a couple
football fields. NOTHING gets in OR out. Every signal read within there
originated in there. And it is nowhere near Vegas.

 

[Adrian Jansen]

Skin depth is the smaller of the two effects governing low frequency
shielding. The bigger one is that the continuous conductor suppresses
the B field in its neighbourhood. The leakage due to finite skin depth
has to be calculated _after_ the field suppression.

Also it's hard to get the full effect of mu metal in a large
fabrication, because it's so delicate--the mu drops like a rock if you
bang it or bend it, and it has to be re-annealed. It's tough to anneal a
shield room!


Cheers

Phil Hobbs

Reminds me of a l-o-o-o-n-g time ago when we at Philips had the only
large kiln around capable of holding a mu-metal shield about 1.5 m
diameter and 1 m high, wanted for some scientific experiment by a
defence department. They provided the shield, and told us the
temperature profile they wanted for annealing, in pure nitrogen. No
problem, it got done on night shift, when we were not using that kiln
for production. Only problem was that someone got the instructions
mixed up, and fired in pure oxygen. Was a bit of a surprise when they
opened the kiln, and found a small pile of powder.

 

[Tim Williams]

Reminds me of a l-o-o-o-n-g time ago when we at Philips had the only
large kiln around capable of holding a mu-metal shield about 1.5 m
diameter and 1 m high, wanted for some scientific experiment by a
defence department. They provided the shield, and told us the
temperature profile they wanted for annealing, in pure nitrogen. No
problem, it got done on night shift, when we were not using that kiln
for production. Only problem was that someone got the instructions
mixed up, and fired in pure oxygen. Was a bit of a surprise when they
opened the kiln, and found a small pile of powder.

Wow, I hope that person got...fired (use your imagination as to how)!

Tim

 

[I AM THAT I AM]

Reminds me of a l-o-o-o-n-g time ago when we at Philips had the only
large kiln around capable of holding a mu-metal shield about 1.5 m
diameter and 1 m high, wanted for some scientific experiment by a
defence department. They provided the shield, and told us the
temperature profile they wanted for annealing, in pure nitrogen. No
problem, it got done on night shift, when we were not using that kiln
for production. Only problem was that someone got the instructions
mixed up, and fired in pure oxygen. Was a bit of a surprise when they
opened the kiln, and found a small pile of powder.

Pobably cost them the price of the item plus a penalty as well.

Several tens of $ks sounds likely.