Obscure but Interesting: Why Your Microwave Zaps Your Satellite Radio

(Please see update below.) Last week, in the latest roundup on "the Kindle menace" -- whether electronic devices actually create any kind of danger in flight -- I quoted a reader who gave this real-world example of the way unknowable problems could crop up:

I have a microwave over that knocks out my satellite internet when cooking at high power. No doubt FCC approved, but it clearly impacts communications devices. I have no idea if the oven is non-conforming, the satellite system is non-conforming, or if the standards aren't tight enough.

Another reader, who works in the medical-devices industry, says this interference actually demonstrates a different point:

This is down in the weeds but interesting. The guy who commented that his microwave interferes with his satellite radio and thinks it is a sign that something is wrong is missing a crucial piece of information. It's not really knocking out the satellite radio, which is far, far away, it's knocking out his 2.4 GHz router, which is probably underneath his desk.

His router is 2.4GHz because that is one of the few unlicensed bands. (Ham radio requires a license, police radio requires a license, FM 103.7 Golden Oldies needs a license, but you don't need a license for your wireless router or anything else (cordless phone, bluetooth keyboard, etc) that operates at that frequency.)

The reason the FCC made it an unlicensed frequency is because there was something already there that messed up reliable transmission. It seems that 2.4 GHz is the natural frequency of the water molecule, and microwave ovens work by transmitting high levels of energy at that frequency so as to vibrate water molecules fast enough to cause them to heat. So... Messing up routers is exactly what to expect from microwave ovens. No one needs to fix anything.

A long dispatch just in from another electronic engineer. Stay tuned (bada-bing!).

Update. A reader in the UK writes back to correct the corrector:

Microwave heating is sometimes explained as a resonance of water molecules, but this is incorrect: such resonance only occurs in water vapour at much higher frequencies, at about 20 GHz. Moreover, large industrial/commercial microwave ovens operating at the common large industrial-oven microwave heating frequency of 915 MHz--wavelength 328 millimetres (12.9 in)--also heat water and food perfectly well.

The wavelength of microwave oven radiation is say in the range 4" to 8" which is far, far too long to resonate with a water molecule! Also if you did use radiation that was short enough to resonate with water molecules the energy would be absorbed in the surface of the item being cooked.

I know when we've reached a point beyond which I am incompetent to referee a discussion or disagreement. Ladies and gentleman, that point has arrived! I will use this as an incentive for improvement in my own grasp of the physics of microwave heating, and I leave further exploration of these issues to the actual experts.

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James Fallows is a national correspondent for The Atlantic and has written for the magazine since the late 1970s. He has reported extensively from outside the United States and once worked as President Carter's chief speechwriter. His latest book is China Airborne. More

James Fallows is based in Washington as a national correspondent for The Atlantic. He has worked for the magazine for nearly 30 years and in that time has also lived in Seattle, Berkeley, Austin, Tokyo, Kuala Lumpur, Shanghai, and Beijing. He was raised in Redlands, California, received his undergraduate degree in American history and literature from Harvard, and received a graduate degree in economics from Oxford as a Rhodes scholar. In addition to working for The Atlantic, he has spent two years as chief White House speechwriter for Jimmy Carter, two years as the editor of US News & World Report, and six months as a program designer at Microsoft. He is an instrument-rated private pilot. He is also now the chair in U.S. media at the U.S. Studies Centre at the University of Sydney, in Australia.

Fallows has been a finalist for the National Magazine Award five times and has won once; he has also won the American Book Award for nonfiction and a N.Y. Emmy award for the documentary series Doing Business in China. He was the founding chairman of the New America Foundation. His recent books Blind Into Baghdad (2006) and Postcards From Tomorrow Square (2009) are based on his writings for The Atlantic. His latest book is China Airborne. He is married to Deborah Fallows, author of the recent book Dreaming in Chinese. They have two married sons.

Fallows welcomes and frequently quotes from reader mail sent via the "Email" button below. Unless you specify otherwise, we consider any incoming mail available for possible quotation -- but not with the sender's real name unless you explicitly state that it may be used. If you are wondering why Fallows does not use a "Comments" field below his posts, please see previous explanations here and here.


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