Can an old microwave oven be used as a Faraday cage?
YES, to an extent, an old microwave oven may be re-purposed as a Faraday cage against EMP (electro magnetic pulse). In fact, its design is very much similar to a basic Faraday cage. A Faraday cage is an enclosure formed by conducting material or by a mesh of such material. A microwave oven’s very design is to enclose the electro-magnetic radiation of microwaves, and keep them from getting out. The reverse will also be true – they can’t get in.
Think of a Faraday cage as a reflector. A reflector of electro-magnetic waves. It reflects waves on the outside from getting in and waves on the inside from getting out.
A Faraday cage by its very definition does not have to be grounded to reflect or keep out electro-magnetic waves (they normally are not grounded). From inside the cage, it makes no difference if the conductive shell is grounded or not. The inside ‘doesn’t know’ about the outside with regards to electro magnetic radiation.
The effectiveness of the ‘reflection’ properties of a Faraday cage depends upon the wavelength of the electro-magnetic radiation in question, the diameter of the holes in the cage’s conductive material, and the conductivity of the material itself. Aluminum, or even steel window screen is “good enough” to prevent any significant electro-magnetic radiation.
Most purpose built “Faraday cages” that you buy are made out of copper screen instead of solid metal. As long as the holes in the screen are smaller than the wavelength of the frequencies you are trying to protect against, screen works just as well as a solid piece of metal.
An EMP is a broadband, high-intensity, short-duration burst of electromagnetic energy. In the case of a nuclear detonation, the electromagnetic pulse consists of a continuous frequency spectrum. Most of the energy is distributed throughout the lower frequencies between 3 Hz and 30 kHz. However the first effects of nuclear detonation are the very-high-frequency pulses, in the microwave range, and can work their way into Faraday cages if there are cracks, seams, or vents.
The frequency of a microwave oven is 2.45 GHz (gigahertz) and has a wavelength of 4.82 inches. Since the holes of the screen mesh of a microwave oven are small compared to the wavelength of the microwave itself, little radiation can leak out. There are also mesh screens on the sides of the oven cavity, one to protect the oven light while allowing it to shine into the cavity, the other to permit ventilation.
A microwave will indeed protect your electronic gadgets during an EMP, so long as you don’t press ‘START’…