08-22-1994 Council Packet - Laserfiche WebLink

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antenna from 35 to 70 feet Even greater heights would provide still greater communications distances unoer the same conditions The radiation angle of the lowest lobe for a hori zontal antenna above the ground may be Determined mathematically. s sin where h 0 25 the wave angle or radiation angle ; the antenna height above ground in wavelengths (see Table 2) In short, the higher the horizontal antenna, the lower IS the lowest looe of the pattern Therefore, tne higher an HF antenna can be placed, the farther it will provide effective communications because of the resulting lower radiation angle This is true for any horizontal antenna Electrical Characteristics of HF Antennas As mentioned previously, a dipole antenna, when viewed from one end of the conductor, radiates an ecual amount of power in all directions. The plotted radiation pattern is a perfect circle. If the dipole is vertical, this circle represents the azimuth coverage, with an equal amount of power radiated m every compass direction For point-to-point communications, however, it is bene ficial to concentrate the radiated energy into a beam which IS aimed toward a single distant point. An analogy can be made by comparing the light emanating from a bare electric lamp to that from an automobile headlight. For illuminating a distant point, the headlight is far more effective. Antennas designed to concentrate the radiated energy into a beam are called beam antennas. For a fixed amount of transmitter power fed to the transmitting antenna, beam antennas provide increased signal strength at a distant receiver. In radio communications, the use of a beam antenna is also beneficial during reception, because the antenna pattern for transmission IS the same as for reception A beam antenna helps to reject signals f'om unwanted directions, and in effect boosts the strength of signals received from the desired direction Tne increase m signal or field strength which a beam antenna offers is frequently referenced to a dipole antenna by a term called gam. Gam is commonly expressed m oecioels One type of beam antenna is a Yagi, named after one of its Japanese inventors. Different varieties of Yagi antennas exist, each having somewhat different characteristics. Many antennas erected for television reception are a form of multi element Yagi beam. Subsequent discussions m this paper refer to a 3-element Yagi array, one having a driven element, a parasitic director and a parasitic reflector The parasitic elements are not fed any power directly; instead they are excited through mutual coupling to the driven element, and in turn also radiate power Each parasitic element is spaced 0.2 wavelength from the driven element, and Fig 6—Computer-calculated azimuth-plane radiation pattern tor a horizontal 3-element Yagi array (solid line), and tor a comparison dipole (bioken line). Fig 7—Computer-calculated elevation-plane or vertical- profile pattern of a horizontal Yagi array, solid line, and for a comparison dipole, broken line. These patterns are those seen from the ends of the elements with the antennas in free space. the theoretical gam of this array, determined through computer analysis, is 7.2 dB over a dipole. This means that for the same transmitter power, the effective radiated power of the Yagi in its favored direction is 5.2 times that of the dipole in the same direction The theoretical front- to-back ratio of the Yagi antenna is 12 dB. meaning the power radiated in its forward direction is. 15.7 times that radiated m the opposite direction. Values for a practical antenna of this type are quite close to theoretical. Computer-calculated radiation patterns for the horizontal Yagi array are presented m Figs 6 and 7. Fig 6 sho .s the azimuth plane, the response m various