Your microwave oven requires a throttle flange –


The accidental exposure of a user to microwave radiation from a microwave oven is of some concern. The device generates energy at a frequency of 2.45 GHz. It is important that measures are taken to minimize stray emissions at 2.45 GHz that could escape from the chamber. Such a measure is sometimes referred to as a “throttle flange”.

illustration 1 When the microwave oven door is closed, we have a 2.45 GHz half-wave stub.

The above is a simple sketch that shows that with the microwave door closed, we have a half-wave 2.45GHz stub. The “zero” impedance at one end of the stub line is reflected at the other end of the stub line at the interface with the outside world. This other end is also at “zero” ohms.

Such microwave energy, which passes the metal-metal interface between the door and the chassis, is very strongly attenuated at the interface with the outside world, where the reflected impedance “zero” is located. The radiation to the outside is thereby minimized.

Unfortunately, this security can easily be compromised. Loosely made door hinges that allow the door to wobble even a little damage the stub structure. Should the door itself ever be damaged so that it does not close properly, the stub structure will be compromised.

When buying a microwave oven, you should check your choice for the robustness and mechanical stability of the door.

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

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