01-18-1994 Planning Packet - Laserfiche WebLink

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r antenna from 35 to 70 feet Even greater ne^ghts Aiouid provide still create' communications distances unoer the same conaitions The radiation angle of the lowest icoe tor a hori zontal antenna above tne grouno may be oete'mmed maLhematicaliy sm 0 25 where h the wave angle or radiation ar'gie the antenna he'ght above grouno m waveengttis (see Table 2i In short, the higher the horizontal antenna the lower IS the lowest (ooe of the pattern Therefc-e tne higher an HF antenna can be placed the fame' it wiii p'ovioe effective communications because of the resuitmc lowe' radiation angle This is true tor any honzcnta' antenna. Electrical Characteristics of HF Antennas As mentioned previously, a dipoie antenna when viewed from one end cf the conouctor. lac.aies an eoual amount of powe- m all directions The plotted rad ation pattern is a perfect circle If the dipole is ventica'. this circle represents tne azimuth coverage, with an equal amount of power radiated m every compass direction. For point-to-poinf communications, however it is bene ficial to concentrate the radiated energy mto a beam which IS aimed toward a single distant point. An ariaiogy 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 ‘a' more effective. Antennas designed to concentrate the rad.ated energy into a beam are called beam antennas For a fixed amount of transmitter power fed to me transmitting antenna. Deam antennas provide increased signal strength at a distant receiver In racio communications, the use of a beam antenna is also benefic.ai curing reception, because tne antenna pattern fo- transmission IS the same as for reception A beam antenna hebs to reject signals from unvi/anted directions ana m effect boosts the strength of signals received from t.ne desired direction Tne increase in signal or field strength wmch a ream antenna offers is frecuentiy re'erencec to a dioole antenna by a term called gam Gam is comimonly expressed in oecibels One type of beam antenna is a Yagi named after one of its Japanese inventors. Diffe'ent varieties of Yagi antennas exist each having somewhat different characteristics. Manv antennas erected for television reception are a form of multi element Yagi beam Subsequent ciscussions m this paper refer to a 3-element Yagi array, one having a driven element, a parasitic director ano a parasitic reflector The parasitic elements are not fed any power oirecily; 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 lor a horizontal 3-element Yagi array (solid line), and for a comparison dipoic (broken 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 transmitte' power, the effective radiated power of the Yagi in its favored direction is 5.2 times that of the dipole m 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 in 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 in Figs 6 and 7. Fig 6 shows the azimuth plane, the response m various J