Why Won't 'Microwave Wi-Fi' Cook People?

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Microwave radiation can be used to cook chicken and also to cook people. WiMAX is a wireless internet technology which shoots microwaves across distances of several miles to provide service. Call it "Super-WiFi."

But what is the difference between kitchen microwaves and WiMAX microwaves, and does this new technology present a public health risk?

A typical kitchen unit directs roughly 650 watts (W) of microwaves into the center of the oven. Drawing 1000 W from the wall, a 65% efficient microwave is one of the most effective uses of electricity in your home. (You can bake a potato with 650 W in 10 minutes or so. A traditional oven requires roughly 3000-9000 watts intermittently for up to an hour.)

If you press up close to the window, will your nose get cooked? No. Microwave ovens contain their radiation by employing a physics concept called a Faraday cage. The core physics here is that electric fields (microwaves are fluctuations of electric fields) inside of a conducting metal box cannot produce electric fields outside of the enclosure. The reverse is also true: even a very strong electric field outside of a metal container cannot generate a field inside. This is why the inside of a car is safe from lightning strikes (not because of the rubber tires!), and it's also why your cell phone won't work in a closed elevator.

The inside of a microwave is a metal box, which reflects the microwaves back into itself and acts as a Faraday cage, shown roughly 40 seconds in to this video:

It is completely safe to say that unless there is a gaping hole in your microwave, or it is actively running with its door open, almost no radiation is escaping. The shape of the box is unimportant, as long as the windows in it are smaller than the waves. This is why there is a metal mesh over the inside of the window.

650 watts of microwave power is enough to rotate the water molecules in food and tissue back and forth. This causes them to slap into the surrounding tissue molecules, creating motion we know as heat. Microwave ovens must be specifically engineered to keep this power from leaking out and cooking people in the vicinity.

So, how can WiMAX get away with shooting microwaves through the atmosphere in all directions without cooking people?

The answer lies in the diminishing strength of electromagnetic fields as they spread. At the very source heart of a WiMAX transmission tower, a maximum of roughly 30 W of microwave power is produced. Even though this is a relatively small amount of power, it would be dangerous to a human inches away from the transmitting antenna. (And because our electronics are very sensitive, they can pick up these WiMAX signals from long distances away.)

However, as a pulse of EM energy comes out of the transmitter, it spreads through the atmosphere in a pattern called dipole radiation due to the antenna design. This is a particular shape of electric field emitted and absorbed by long rod-shaped antennas.

MIT dipole radiation.png
Dipole Radiation: The gray line is the antenna, the radiation pattern is in yellow. The donut shape comes from rotating this image around the circumference of the antenna. (Image: MIT)

Dipole radiation is shaped roughly like a big donut, shooting out in all directions from the antenna. Employing some fairly formidable math, you can calculate just how much a pulse of microwaves has thinned out and weakened when it has traveled some distance from the tower. The total power being spread out across that entire donut means that as you get further away, the radiation in any one spot decreases by a factor of the distance away cubed squared.

For example, the power at two miles away is only about 1/8th 1/4th the power at one mile away. Even by the time it has reached the ground beneath the antenna, the total power of transmission is less than one-thousandth of a watt across the entire area of your body. This is so little energy that there is no danger whatsoever. The minute microwave leakage from your oven and even the transmissions of cordless telephones in your home may be stronger than WiMax waves by the time they reach you.

In other words, you won't be cooked by a WiMAX tower.

But, if this device becomes popular, rest assured that there will be plenty of scaremongering over WiMAX causing cancer or autism.

Post updated to reflect correction in radiated power formula for dipole radiation and subsequent calculation.

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