NEXRAD weather radars are expected to operate in a variety of patterns. Nearly all of them will involve bursts, or pulses, of high-energy, narrow-focus electrical beams rotating around a vertical axis. With the beam pointed just slightly above the horizon, the radar will usually make two complete rotations. When the beam is scanning the higher reaches of the skies (up to about 60 degrees), it will make just one complete rotation at each selected elevation. When it completes a cycle, the beam elevation drops down to where it began, and the pattern begins anew.
The duration of the pulses will usually be extremely short measured in microseconds. The electromagnetic frequency of the emitted electrical beam is very high, in the range of 2,700103,000 megahertz. (For comparison, the frequency of the wave associated with overhead power lines is orders of magnitude lower – about 60 hertz, or 60 cycles per second. One megahertz, abbreviated as MHz, is a million hertz. Thus, a beam whose frequency is 2,700 to 3,000 MHz has a frequency several billion times that typical of overhead power lines. The frequency of the NEXRAD radar beam is, in fact, higher than the frequencies used in many microwave ovens.)
Finally, the power density of the wave is measured in waves per square centimeter. The pulse associated with NEXRAD is incredibly strong, as these things go, radiating a maximum pulse power of 475 kilowatts. Pulse strength diminishes rapidly over distance, however, so if a person stood in front of the beam at a distance of about 600 feet from the antenna, he or she would be exposed to 2.2 waves per square centimeter (equal to 2200 milliwatts, or mW). At 3,800 feet from the antenna, the maximum power density will be 100 mW/cm2. More than two miles distant, the maximum pulse power density is expected to be 10 mW/cm2 – which, however, is still five times the limit for exposure recommended by the Institute of Electrical and Electronics Engineers.
Volume 4, Number 4 October 1993
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