How not to make a Faraday cage
Today a thought floated around in my head of those little mesh screens they put on the door of microwave ovens. The holes in the screen are spaced smaller than the wavelength of the microwaves and that’s why it makes an effective shield. At least that’s how I understand it.
Then I thought of all the 60 Hz electromagnetic interference I’ve noticed on my oscilloscope while tinkering around in the lab. The two thoughts collided and I was wondering, hey, the wavelength on 60 Hz is something around 5,000 km, huge. So does that mean if I surround something with, say, just a wireframe cube of wire which is somewhat smaller than 5,000 km, would that be enough to shield it from interference?
Only one way to find out!
I took some breadboard wire and made a little cube out of it. I then hooked a scope probe up to the end of a spool of wire (my antenna) and put it inside the cube.
The results, was that an effective shield? Nope. Not a sausage. Didn’t do anything. Maybe it was because the wire was insulated. Maybe it’s because I still have a long way to go before I understand electromagnetic radiation. Loooong way to go.
I did however happen to find where the majority of the EMI (interference) was coming from which was something that had been puzzling me. Turns out it comes from the charging circuitry within my laptop and it’s leaking through one of the vents along the side.
Vents through which EMI is leaking:
Before and after EMI readings with charger on and off:
March 3rd, 2008 at 3:27 pm
The size of screen you need to make a cage depends on not only the wave length, but the magnitude of the waves as well. The idea is to force any incoming wave through a “tunnel” in order to get to the inside of the cage. By making the hole small enough and the wall thick enough, you can force a wave to “crash into the tunnel wall” and be absorbed.
If that doesn’t make much sense, try this. Take a spare piece of wire and bend it into a sine wave. Then take a stack of washers (all the same size) and try to feed the wire through the center of the stack. Notice that if you make the hole smaller, it’s more difficult to thread the wire without losing the wave. If you make the stack of washers larger, the same thing happens.
Practically speaking, a sheet of shielding material with holes of 0 diameter is 100% effective (assuming the shielding is thick enough to absorb the energy of the wave. Tin Foil doesn’t stop nuclear blasts and all…) The only purpose of adding all the holes is reducing the amount of material used. That makes it cheaper to construct and easy to manipulate the shield material (think window screen vs. a sheet of solid copper).
March 3rd, 2008 at 5:49 pm
That makes… a whole lot of sense actually.
I think your “stack of washers” concept is particularly illuminating. In particular it reveals how I’ve been stuck thinking about EM in only two-dimensions… the idea of considering amplitude never crossed my mind. Looking back I think that has been a key missing piece to the puzzle for me.
You have my sincere thanks.
June 22nd, 2008 at 5:00 am
On a wristwatch forum someone says a Faraday shield or cage has to be the same material throughout. Therefore, it was said that a brass cover on the back of a steel body is merely a dust cover. Any chance of an experiment or any comments?
Ted.