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.”

Figure 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.

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).

A reader of the above essay seems, if I may paraphrase, to disagree on LinkedIn regarding potential hazards from personal exposure to microwave oven radiation leakage. Clearly that reader and I disagree.

I suggest that one look at the following URL:

https://www.fda.gov/radiation-emitting-products/resources-you-radiation-emitting-products/microwave-oven-radiation

from which we read the following excerpt:

“Microwave radiation can heat body tissue the same way it heats food. Exposure to high levels of microwaves can cause a painful burn. Two areas of the body, the eyes and the testes, are particularly vulnerable to RF heating because there is relatively little blood flow in them to carry away excess heat. Additionally, the lens of the eye is particularly sensitive to intense heat, and exposure to high levels of microwaves can cause cataracts. But these types of injuries – burns and cataracts – can only be caused by exposure to large amounts of microwave radiation.”

I most earnestly advise caution.

Just to add, “large amounts” is an imprecise term. It is not clear to me that a kitchen-top appliance can be taken with assurance not to emit at such levels.

How do the ferrite strips around the edge of the door come into play with this 1/2 wave stub?

There are certain ferrite materials which are characterized for their RF/microwave absorption properties. The last time I looked at those, there was a lower frequency limit specification of 30 MHz. How those would be combined with a stub structure, I do not know, but I would love to find out..

When used according to manufacturers’ instructions, microwave ovens are safe and convenient for heating and cooking a variety of foods. However, several precautions need to be taken, specifically with regards to potential exposure to microwaves, thermal burns, and food handling.

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