Session II - Field intensities generated by microwave radiating systems energy densities of microwave radiating systems

Abstract

rFJ1 HE development during World War II of powerful by body fluids. Generally this type of radiation is produced microwave generators has caused some concern as to in narrow bands of the spectrum, and our discussion will whether the power radiated from devices using such be confined to equipment operating at frequencies of generators reaches levels that may be harmful to persons from 200 to 10,090 megacycles. located in the vicinity of the apparatus. It would be We can divide microwave equipment into two catehelpful therefore to review the types of equipment using gories, pulsed equipment such as radar and continuous high power microwave oscillators, and estimate in each wave generators such as communicationi and TV transcase the power density radiated as well as possible future mitters. Although radar equipment may be designed to trends in total radiated power.produce extremely high peak powers, since the eniergy High power generators of electromagnetic wvaves are comes in short bursts spaced by long intervals of zero not new. In the 1920's, AT&T inistalled a 150 kilowatt energy, the average power, which is the important contransmitter at Rocky Point, Lonig Island, for transatlantic sideration for tissue heating, is not too great. Table I communication. However this transmitter operated at 57 lists the properties of representative high power radars kilocycles and the energy was fed into a mile-long antenna; used by the U.S. Navy.' thus the energy density even near the antenna was not It can be seen that, although the peak power of one of too great, so that although the field streingth near the the sets is as high as one million watts, the average power antenna (of the order of 10 volts/meter) was enough to is only about 400 watts. However, this is not a negligible require precautions about grounding metal objects like amouint of energy. Since S-Band waveguide has a cross automobiles approaching the transmitter, the power section of about 25 cm2, the energy density of radiation density in the field was small (about 10-0 w\latts/ cm2). leaving an open ended wave guide (assuming no losses) The essential difference between microwaves and the averages about 16 watts/cm2 or about 100 times the broadcast band is not that we generate more power in energy density of solar radiation striking the earth. the microwave region (in fact the opposite is true) butThe avTerage power may be increased by increasing any that we can concentrate the electromagnetic energy in the one of the three factors: peak power, pulse width, or pulse relatively small volume of a waveguide or coaxial cable repetition rate. The pulse repetition rate in radar applicaand radiate it from extremely high gain antennas. tions is generally set by the maximum range at which We can, of course, generate waves even shorter than targets are expected to be detected. Since it takes 0.002 the microwave region by going into the visible and infrared second for a pulse to travel out and return from a target part of the spectrum. A very powerful source of these at 300...