3-18 The current distribution of the dipole resembles the voltage distribution of the slot. The edges on the slot have a high voltage concentration and relatively low current distribution; the complementary dipole has a high current concentration and relatively low voltage. Slot antennas are adaptable for the vhf and uhf ranges. One of their practical advantages is that the feed section which energizes the slot may be placed below the large metal surface in which the slot is cut. Thus, nothing needs to extend from the surface. In addition, the slot itself may be covered by a section of insulating material to provide a seal so that the antenna can be pressurized with dry air. Dry air pressurization reduces moisture in the waveguide and prevents arcing. Many of the new radar systems reaching the fleet over the next few years will use frequency- or phase-sensitive antennas. Some of the new radars will use antennas that electronically scan the azimuth as well as elevation, eliminating the moving antenna. Q-15. What is a set of antenna elements called? Q-16. What type of antenna has all elements connected to the same energy source? Q-17. What determines the beam elevation angle of an antenna that is electronically scanned in elevation? Q-18. What is the polarization of the energy radiated by a vertical slot? SUMMARY This chapter has presented information on the characteristics of microwave antennas. The information that follows summarizes the important points of this chapter. The ANTENNA CHARACTERISTICS of microwave and low-frequency antennas are essentially the same. The efficiency of an antenna is expressed as a POWER GAIN or POWER RATIO as compared to a standard reference antenna. The STANDING WAVE RATIO (swr) is a measurement of the impedance mismatch between a transmission line and its load and is an indicator of overall system efficiency. DIRECTIVITYrefers to the direction in which an antenna radiates and the narrowness of the radiated beam in DIRECTIONAL ANTENNAS. OMNIDIRECTIONAL ANTENNASradiate and receive in all directions at once. RECIPROCITYis the ability of an antenna to both transmit and receive electromagnetic energy. REFLECTOR ANTENNASare antennas that use a reflector to focus electromagnetic energy into a beam that is directional in either the vertical plane, the horizontal plane, or both planes at once. The basic PARABOLIC REFLECTOR shown in the illustration, or one of its variations, is most often used.
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