Minimize stray emissions from your microwave oven

Article By : John Dunn

A microwave oven may look innocuous, but it can be dangerous. A simple examination of the unit you are thinking of buying would be very much worth your while.

Incidental exposure of a user to microwave radiation from a microwave oven is of some concern. The appliance generates energy at a frequency of 2.45 GHz. It is important that measures be taken to minimize any stray emissions at 2.45 GHz that might escape from the chamber. One such measure is sometimes called a “choke flange.”

diagram of how a microwave choke flange worksFigure 1 When the microwave oven door is closed, we have a half-wave, 2.45 GHz stub.

The above is a simple sketch showing that when the microwave oven door is closed, we have a half-wave, 2.45 GHz stub. The “zero” impedance seen at the one end of the stub reflects to the other end of the stub at the interface to the outside world. That other end will also be at “zero” ohms.

Such microwave energy as gets past the metal to metal interface of the door with the chassis is very much attenuated at the interface to the outside world where the “zero” reflected impedance is located. Radiation to the outside world is thereby minimized.

Unfortunately, this safety provision is quite easily compromised. Loosely-made door hinges that allow the door to wiggle even a little bit will corrupt the stub structure. Also, if there is ever any damage to the door itself so that it does not properly close, the stub structure will be compromised.

This article was originally published on EDN.

If you go shopping for a microwave oven, be sure to examine your choice for the sturdiness and mechanical stability of its door.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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