Shielding

At high frequencies "best conductors" are "best shielding". However, Iron and steel should not be under estimated they are not that bad a conductor compared to air. The electrical conductivity of iron is much much closer to Copper than air. So the shielding at high frequencies is not that bad for Iron. If you look into your TV or Radio you will often see iron or steel used as shielding for the RF frequencies. Note the tin plated iron box around the RF tuner section. Why, because it's cheaper and easier to use and works almost as well at high frequencies for the same thickness.

Note: Copper and brass screens are used in laboratories to build Faraday's cages to shield for electromagnetic nose. Why is this used? Usually the main interest is in shielding the higher frequencies and budget and availably are not an issue. Why use a screen over a solid plate - this is so the room can be made small and the lab technician doesn't get claustrophobia inside it. A thicker iron screen or plate would work just as well. Any conductor would work at higher frequencies as long as you adjusted the thickness to simulate what a good conductor like gold, silver, and copper can do.

Shielding results of testing Aluminum compared to Iron at 60 Hz:

I used a "bulk cassette tape eraser" that puts out a strong 60 cycle/sec electromagnetic field as a source transmitter. I took a coil from a transformer and hooked it to a volt meter as a receiver. Cardboard and several .2" thick plastic sheets were used as spacers on both sides of the test sample (centered) so that the distance from the source to receiver was the same (.5") for all tests. The voltage was measured for each test shielding material. The positioning of the receiver was adjusted for maximum voltage for each test. This turned out to be the centered on the source.

Measured voltage for each sample:

Air (cardboard) - measured 19.4 Volts AC

Aluminum (.100" thick) - measured 16.5 Volts AC

Iron ammo box (.032" thick) - measured 6.5 Volts AC

Note: The higher the voltage the more the amount of electromagnetic field that got through the sample.

Analysis of results: The Aluminum being 3 times thicker than iron only dropped the electromagnetic field by 2.9 volts as compared to iron dropping it 12.9 volts. This gives a net result of (12.9/2.9) * 3 = 13.3 times thicker aluminum would be needed to shield the same amount as the Iron (or steel) ammo box material. I didn't have any copper or bass available to test, however the result should be some what similar. Summary: Conservatively speaking, Iron is more than 10 times more effective shielding than Aluminum at 60 cycles/sec. This will get even better at lower frequencies.

The point is, the lower the frequency the better the shielding properties of Iron as compared to other non-ferrous conductors. At low frequencies magnetic shielding is more effective. At high frequencies electrical conductive shielding is more effective. The audio industry for years used iron shielding over tubes when needed. A further test will confirm this for you if you take a permeant magnet and wave it (1-10 cycles/sec) close to a iron plate with a floppy on the other side. Then do the same test with Gold, Silver, Copper, Brass, or Aluminum plate of the same thickness using another floppy. Then try and read each floppy. During core movement, possibly generating strong localized magnetic fields, I believe the lower frequencies are what needs to be shielded against. I believe the bigger the event, the stronger the field, and the lower the frequency. We should experience lots of random noise spikes getting stronger and stronger as one goes below 10 and even 1 cycle/sec.

Now with all factors taken into account a good solution needs to be cost effective, available to all, and technologically sound. I believe Iron or steel satisfies all these conditions much better than the alternatives. Annealed cold rolled Iron plate would work slightly better than steel but not enough to worry about. The only thing I don't know for sure is how thick does it need to be to effectively work for us in all cases. I suspect one box thickness of .032" will be enough to knock the edge off any strong electromagnetic bursts, however, if you really want to be safe use one box within another for those really critical magnetic medium. If one has a Iron or steel safe or lock box that would work also. Just make sure it is fully covered with Iron or steel the thicker the better.

Offered by Mike.