08-22-1994 Council Packet - Laserfiche WebLink

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^ ' compass directions. a> a solid i;ne The broken line in Fig 6 shows the oattern of a half-wavelength dipole, for comparison. Fig 7 shows the elevation plane or vertical- profile pattern. This pattern is that seen from the ends of the elements, with the antenna in free space The solid line again represents the Yagi. and the broken line the dipole, that “perfect circle" which was mentioned earlier. Performance of most other Yagi-type arrays with three Of four elements will not be significantly different from the patterns shown here Another often-used type of beam antenna is the quad array. Typically, a quad is constructed with two or more wire elements in the form of a souare o. a diamond. The conducting wires are usually supported by X-shaped frames of wood or fiberglass material For the same number of elements and spacing, the performance of a quad beam is generally similar to that of a Yagi beam. The vertical patterns of Figs 4 and 5 were calculated by assuming the earth is a perfect reflector, an unrealistic situation. Actual earth does not reflect all of the radio- frequency energy striking it; some absorption takes place. Over real earth, therefore, the patterns will be slightly different, depending upon the electrical con ductivity and dielectric constant of the soil, and upon the radio frequency. Computer programs are available to take these factors into account, and raliable calculations of patterns over real earth may oe made. Yagi patterns over real earth are shown m Figs 8 through 14. These patterns were calculated with an Apple lie personal computer 3 "Average" soil conditions were used for these plots, that is. a conductivity of 5 milhsiemens per meter and a dielectric constant of 15 ■* These patterns compare the performance of Yagi arrays at heights of 35 and 70 feet. In Figs 6 through 12. the broken line is the plot for a 35-foot height, and the solid line for 70 feet. For 24.9 and 28 MHz. F-gs 13 and 14. it is helpful to present the 35- and 70-foot patterns m separate graphs, for clarity. Otherwise the multiple lobes become indis tinguishable ^A commercially available sofiA-are p'ligram ANNIE, was used to calculate all oattems presented in th's paper ••Such soil conditions may be considered typical for the central part of tne United Stales sucn as the Ohio ana Mississippi river val'eys. «? Fig 8—Vertical-profile pattern of a 7-MHz Yagi beam over average earth at a height of 70 feet (solid line) and at 35 (eel (broken line). Fig 9—Vertical-protile pattern of a 10.1*MHz Yagi beam at 70 feet (solid tine) and at 35 feet (broken line). »o Fig 10—Vertical-profile pattern of a 14-MHz Yagi beam at 70 feet (solid line) and at 35 feet (broken line). Fig 11 —Vertical-Mrofile pattern of an 18.1 -MHz Yagi beam at 70 feet (solid line) and at 35 feet (broken kre). Fig 12 —Vertical-profile pattern of a 21 -MHz Yagi beam at 70 feet (solid line) and at 35 feet (broken line). i| \ ■ -.g—