Microwave Ovens Are NOT Good Faraday Cages

May 12, 2015

in Contingency Plan

The theory that microwave ovens make good Faraday cages has been proposed many times and debunked many times. Here is the reasoning:

A Faraday cage works by shielding against Radio Frequency (RF) signals. The shielding material does not have to be solid metal and it can have gaps in it, however, the gaps must be smaller than a fraction of the wavelength in order to be effective. A large radio wave hitting a small gap in metal shielding gets bent (diffracted) around the edges in the gap or hole. If the hole is large enough, the wave is somewhat diminished (attenuated) by the diffraction, but if the hole is small enough, the diffraction causes enough induced interference that the wave is countered and does not pass through. A standard rule is that mesh holes at a ratio of 1/10 of the wavelength size, or smaller, will provide solid shielding against penetration.

Microwave ovens use a frequency of 2.45 Ghz. That gives them a wavelength of about 12 cm and a mesh hole shielding requirement of 1.2 cm or smaller. We’ve all seen the mesh shielding material embedded in the glass door of a microwave oven to protect us from the energy of the radio waves. It makes intuitive sense that since a microwave has built in Faraday shielding to prevent the radio waves from escaping, that it would also work in reverse to protect anything inside it. And basically, that is true. Microwave ovens do offer protective shielding against incoming radio waves.

However, this theory breaks down because microwave ovens are not required to prevent ALL emissions, just the 2.45 Ghz wavelength that they radiate on. Further, they are not required to block the energy entirely, just to weaken the power level of the emissions to a point where it is not dangerous to humans nearby. The US FDA sets a regulated limit to power leakage levels from microwave ovens.

One of the problems with designing shielding for a microwave oven is creating a good seal at the door, which does not have metal to metal contact all around, leaving a gaping hole for RF to leak through. This problem is handled by using a “choke” or “ditch” that resonates with harmonics of the 2.45 Ghz wavelength and cancels out any radio waves that might leak out.

SUMMARY
A microwave oven does offer some protection against incoming radio waves but it is most effective near 2.45 Ghz, less effective at other frequencies and not absolute. It will not completely protect electronic devices placed inside it from the E-1 wave of an EMP. Testing will show that some radio based devices close to 2 Ghz may not work well inside a microwave, other devices operating at other frequencies operate almost normally.

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