A Staff Report from the Straight Dope Science Advisory Board

How are the microwaves in ovens different from those in cell phones?

August 28, 2003

Dear Straight Dope:

What is the difference in physical terms between the microwaves that power my microwave oven, and those that power my cell phone, or portable phone for that matter? How come they function differently and don't, say, cook my ear off or ring my Swanson's (TM) TV dinner?

Cell phones, cordless phones and microwave ovens all use radio-frequency (RF) energy, which is a portion of the electromagnetic spectrum. This spectrum runs the gamut from the longest radio waves to visible light to the highest-energy gamma rays. Radio occupies a huge slice of the spectrum from the shortest microwaves at wavelengths of a few millimeters, all the way down to the ELF (extremely low frequency) band used by submarines with wavelengths in the hundreds of meters and longer. Much of our modern technology is dependent on radio, including television, wireless phones, WiFi networks, and other applications too numerous to mention. It's possibly the most used but least understood technology we have.

One of the most important characteristics of a radio wave is its frequency. Frequency refers to how rapidly the wave oscillates, or changes polarity, and is measured in hertz (Hz). One hertz = one complete cycle per second. An oscillation rate of 1 Hz is too slow to be of any use in radio, so units like kilohertz (kHz), megahertz (MHz) or even gigahertz (GHz) are more commonly used.

The frequency of a radio wave is tied to its wavelength. Wavelength is the linear distance between two peaks of a radio wave. This distance is usually expressed in meters. Wavelength is related to frequency by the speed of light, c, which is easy to see by visualizing a wave with a frequency of 1 Hz. The distance the wave can travel before the next cycle begins is one second, and therefore the distance between the cycles will be how far the wave can travel in one second--at c, about 300 million meters. For frequencies other than 1 Hz, divide 300 million by the the frequency in hertz and you'll get the the wavelength in meters.

Why is frequency so important? Many characteristics of a radio wave are dependent upon its frequency. For example, submarines use the ELF band (300 Hz to 3 kHz) because these low frequencies penetrate water to a greater depth than higher frequencies do. In everyday communications, frequency partially determines how far a transmission can propagate. Ham radio operators used low-frequency bands, such as 80 and 160 meters, to send transmissions around the world, whereas higher frequencies, like those used in many handheld two-way walkie-talkies, are limited to line-of-sight, which means they cannot reach beyond the horizon (except under certain atmospheric conditions).

Cell phones don't generally approach the frequency used by microwave ovens, which is normally 2450 MHz. The closest that I'm aware of is the 1900 MHz PCS band in the U.S. Some cordless phones do operate near or even in excess of this range--the 2.8 GHz phones come closest, and now there are phones operating at 5.6 GHz. There is no fundamental physical difference between the radiation emitted by cell phones and cordless phones and that used by microwave ovens, as all are radio waves in the electromagnetic spectrum. Other than frequency, the main difference is power. Phones emit on the order of a few hundred milliwatts to a few watts maximum, while ovens use power levels in the neighborhood of 700-1000 watts. That's the difference between a cool conversation with your best friend and a hot roast beef sandwich.

The other difference between microwave oven radiation and cell phone emissions is information. The oven radiation carries none; it is unmodulated. The cell phone signal on the other hand carries a wealth of information, including the data that comprises your voice, the phone's electronic serial number and other signals and data needed to maintain a connection to the network. It isn't merely the presence of a radio signal that makes your phone ring. Without going into a great deal of detail about the cellular network: your phone sends out periodic signals to tell the network where it is, and when someone calls you, the network sends a signal through the nearest tower to you that tells your phone that someone is calling, and your phone rings and acknowledges to the network that it has received the ring signal. No matter how much power you throw at your TV dinner, it's not going to ring. If it does, I'd stay out of the kitchen for a while.

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