Garmin pulsed radar
The microwaves that reheat your coffee are very much the same as those transmitted by marine radars, so it isn’t too far-fetched to believe that they could “cook” anyone who gets in their way. And we all know that some types of radiation can produce deformities in unborn children or trigger the runaway growth of cells within the body—i.e., cancer. But is it valid to make the connection between pleasureboat radars and health hazards? To answer that you need to understand the science of electromagnetic radiation.
“Microwave” is a general term for a particular part of the wide spectrum of electromagnetic radiation. At about 600 THz (600 million MHz), visible light occupies a narrow band of this spectrum; gamma and X-rays have higher frequencies and shorter wavelengths. By contrast, VHF is fairly low on the spectrum, with a frequency of about 157 MHz (157 million cycles per second). Pleasureboat radars operate at a frequency of about 9.4 GHz (9,400 MHz).
For health and safety purposes, the huge electromagnetic spectrum is divided into two main groups: ionizing and nonionizing radiation. The dividing line between the two is the ultraviolet part of the spectrum with a frequency that’s more than ten times higher than visible light—about 8,000 THz.
Ionizing radiation can literally shake molecules to pieces, corrupting the chemical blueprints that control the creation and growth of the body’s cells. That’s why the ultraviolet radiation in sunlight can produce skin cancer. Fortunately, radar is well down into the nonionizing part of the spectrum, at frequencies a billion times lower than the dividing line. So there’s no reason to believe that it might cause cancer or genetic defects. But what about the cooking part?
We all know that microwaves can cook meat. The question is, how much heat can we stand? Actually, different parts of our bodies have different “safe working temperatures” and so vary widely in their ability to rid themselves of excess heat. For example, the cornea is particularly vulnerable because it has to be transparent, so it has no blood flow to remove heat.
Experts say, however, that generally speaking, the body can withstand a heat load of about 4 watts per kilogram (kg). Erring on the side of caution, the FCC set a limit of 0.4 watts per kg for “occupational” exposure, and 0.08 watts per kg for the general public. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has set exactly the same limits as the FCC, but added some practical guidelines expressed in terms of power density. According to ICNIRP, the FCC’s “general public” limit corresponds to a power density of 10 watts per square meter, while its “occupational” limit equates to 50 watts per square meter.
But that’s for continuous exposure. Shipboard radars transmit in pulses, each pulse lasting less than a millionth of a second and only a few hundred pulses per second. So a typical yacht radar actually transmits only about 0.05 percent of the time, reducing the average power output of a 2-kW radar to a much more docile 1 watt. That power is concentrated into a beam—but not the well-defined, fan-shaped beam you see in navigation textbooks. It actually has blurred edges and stray waves escaping at all sorts of odd angles. And it’s not really fan-shaped, either: For the few inches closest to the antenna, the sides of the beam are almost parallel to each other. That means that at the surface of the radome, the beam of a typical 2-kW radar covers an area of about 0.06 square meters. The one-watt average output of a 2-kW radar, spread over that area, works out at a power density of about 22 watts per square meter.
This article originally appeared in the June 2009 issue of Power & Motoryacht magazine.