Published: 2007-09-26 14:31:25
Updated: 2007-09-26 14:31:25
Posted September 26, 2007
By John Beavin
MIKE MOSS SAYS: John, Health impacts associated with radio and microwave frequency transmissions depend on proximity to the source, frequency (wavelength), average power transmitted, and duration of exposure. Weather radars have design features that are intended to ensure that they meet strict standards that will result in safe operation, both for the public and for station employees who maintain the system. For the most part, these features are actually intrinsic to the purpose of the radars themselves.
To give you some background information regarding our newly installed unit, it is a C-band pulsed doppler radar, meaning it operates at a wavelength of about 5 centimeters (equivalent to a frequency of around 5.6 GHz), a beam width of 1 degree, and with a peak transmitted power of 1 million watts. This peak power value implies a much more imposing radiation signature than is actually encountered even in areas relatively close to the system, however, because of the manner in which pulsed weather radars are operated.
In our case, there are two primary operational modes for the radar, the first being Long range mode, the other HCLASS (Hydrometeor Classification) mode. I'll call them L and H for short. In L mode, the radar sends out 300 pulses of energy per second, each pulse lasting 2 microseconds (2x10-6 seconds). This means that in L mode, the radar is actually radiating for about 2.2 minutes out of every hour - the rest of the time, it is in receive mode listening for returns from precipitation areas. This yields an average transmitted power of 600 watts. In H mode, the transmitter sends out 900 1 microsecond pulses per second at 500,000 watts in each of two polarizations. A vertically polarized beam and a horizontally polarized beam component each go out with an average power of 450 watts. In this mode there is radiation actually emitted from the radar 3.2 minutes per hour, or about 5% of the time. By comparison, a lot of us have microwave ovens in our kitchen that operate (on HIGH) at a frequency of 2.45 GHz (12 cm wavelength) with powers ranging from about 750 to 2000 watts, with that energy nominally sealed inside a metal casing and the wave guide directing energy into the oven only a few inches from the food to be heated - imagine trying to heat the food in there by pulsing the microwave on for less than 4 hundredths of each second that you set the timer for. I'm not sure it would ever warm at all, or if it's the best analogy, but perhaps that helps illustrate the difference in effect between a pulsed system and a continuous wave transmission.
Beyond those basic numbers, some other factors play into the safety of the system. First, as with any radiated energy originating at essentially a point source, the intensity (power density) decreases with the square of the distance from the transmitter (called the Inverse Square Law) so that the amount of radiant energy passing through any given area falls off rapidy with distance (intensity of X at distance A becomes X/4 at distance 2A and X/9 at distance 3A, and so on). Second, the radar sits atop a 100 foot tall tower and is operated with a minimum elevation angle of .5 degree. This initial upward angle combined with the downward curvature of the earth's surface with increasing range means that the peak power in the center of the beam is already around 200-250 feet above the ground as it passes over the northwest part of Clayton, for example. Finally, the beam (keeping in mind that "beam" actually refers to a space that is only filled with a passing pulse of energy around 3-5% of the time) is not stationary and focused on one location but sweeps continuously at a rate of anywhere from about 1 to as many as 6 revolutions per minute, for minimal dwell time (i.e. duration of exposure) in any direction.
For all these reasons, health professionals have deemed most radar systems as quite safe to the public and to industry workers. The World Health Organization has a fact sheet on radar transmission and health that may do a better job than I have and also covers a broader array of the various types of radars around. In particular, note the summary points at the very bottom of the page. Here's the address: