Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: You talked of metal planes. How does the metal material affect E and H shielding as a function of frequency? 50 and 60 Hz are problem areas. Is iron the best shield for that frequency?
Answer: Yes, the type of metal can be very important for shielding, especially at low frequencies.
Above 1MHz or so the skin depth is so small that any metal thicker than 1mm gives wonderful shielding – usually severely limited by its gaps, holes, joints, and conductor penetrations.
But at low frequencies where the skin depth is more than half the thickness of the metal, ferromagnetism becomes very important for shielding magnetic fields. Iron is certainly a good material for 50 and 60Hz, but even so will need to be quite thick (several mm) to be very effective.
Of course, iron quickly rusts and isn’t very strong, so we generally use mild steel with a protective layer of zinc or tin, which isn’t as good as pure iron for shielding low frequencies – but it’s always a trade-off in the end, and achieving high levels of ‘passive’ shielding at such low frequencies requires large thicknesses of steel or iron.
However, special alloys, such as Mumetal™ and RadioMetal™ have been developed specifically for good low-frequency magnetic shielding without requiring large thicknesses. They have relative permeabilities measured in the thousands, sometimes tens of thousands (mild steel is a few hundred, depending on its composition). Some types can require special handling techniques.
Strong fields can easily saturate these special alloys, so it is not unusual to find a layer of steel (which is hard to saturate) followed by a layer of MuMetal™. Some specialist magnetic field shielding companies use proprietary triple-layer shielding, each layer being a different metal or alloy.
At low frequencies, electric fields are easily shielded even by thin metal foil, as long as it has a very highly conductive surface, such as tin plate.
-Keith Armstrong
