Practical_Antenna_Handbook_0071639586
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CHAPTER 20 Microwave Waveguides and Antennas Caution Microwave RF energy is dangerous to your health! Anything that can cook meat can also cook you! The U.S. government sets the safety limit for microwave exposure at 10 mW/ cm 2 averaged over 6 min; some other countries use a level 1 ⁄10 of the U.S. standard. The principal problem is tissue heating, and eyes seem especially sensitive to microwave energy. Some authorities believe that cataracts form from prolonged exposure. Some authorities also believe that genetic damage to offspring is possible as well as other long-term effects as a result of cumulative exposure. Because of their relatively high gain, microwave antennas can produce hazardous field strengths in close proximity—even at relatively low RF input power levels. At least one technician in a TV satellite earth station suffered abdominal adhesions, solid matter in his urine, and genital dysfunction after servicing a 45-m-diameter 3.5-GHz antenna with RF power applied. Be very careful around microwave antennas. Do not service a radiating antenna. When servicing nonradiating antennas, be sure to stow them in a position that prevents the inadvertent exposure of humans should power accidentally be applied. A Radiation Hazard sign should be prominently displayed on the antenna. Good design practice requires an interlock system that prevents radiation in such situations. “Hot” transmitter service should be performed with a shielded dummy load replacing the antenna. The microwave portion of the radio spectrum covers frequencies from about 900 MHz to 300 GHz, with wavelengths in free space ranging from 33 cm down to 1 mm. At these wavelengths we find that conventional antennas derived from the basic wire dipole are not the only efficient radiating structures available to the designer or user. Similarly, conventional transmission lines—which exhibit a variety of losses that increase with frequency—become impractical above about 5 GHz, except for very short runs. Instead, at microwave frequencies a number of alternative devices, including waveguides, horns, and other components reminiscent of a plumber’s parts bin, dominate antenna system design. Waveguides Conventional transmission lines exhibit three kinds of losses: ohmic, dielectric, and radiation. Ohmic losses are the result of skin effect, which increases a conductor’s apparent 447
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