07-25-1994 Council Packet - Laserfiche WebLink

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f II & t ■ K ■i:., |c i,:> |h:,,m k \ i‘ '-■ antenna from 35 to 70 fee* Evan grtater heights would provide stiN greater communications distances unoer the same conditions The radiation angle of the lowest lo&e for a hori zontal antenna above the ground may be determined mathematicaHy: $ > sin-I 0.2S where 9 « the wave angle or radiation angle h a the anter^na height above ground in w»v9i9ngths (see Table 2) In short, the higher the horizontal antenna, the lower is the lowes* looe of the pattern. Therefore, the 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 Charaderisttcs of HF Antennas As mentioned previously, a dipole antenna, when viewed from one end of the cortductor. radiates an equal amount of power in all directions. The plotted radiation pattern is a perfect circle, if the dipole is vertical, this circle represMts 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 comparirtg the light emanating from a bare electric lamp to that from an autorm^ile 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 trartsmitter power fed to the transmitting anMnna. beam antennas provide increased sigrtal 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 reiect signals from unwanted directions, and in effect boosts the strength of signals received from the desired direction. The increase in 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 in decibels. One type of beam antenna is a Vagi, 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 in 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 r-o Fig a Compmar cslculalaa azUnuth-plaiie ra diaW en panel Nne). Fig 7—Compyier-calculated eievation-plaiie or vertical- profile patiem of a horiaomal Vagi arr^, solid Mne. and for a campariaen dipole, broken line. These panama are those seen from the ends of the alemerMs with the antennas in free apace. 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 in the opposite direction. Values for a practical antenna of this type are quite close to theordticai. Computer-calculated radiation patterns for the horizontal Yagi array are presented m Figs 6 and 7. Fig 6 shows the azimuth plane, the response in various