06-16-1997 Planning Packet - Laserfiche WebLink

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compass directions, as a solid line. The broken line in Fig 6 shows the pattern 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 or four elements will not be significantly different from the patterns shown here. Another often-used type of becm antenna is the quad array. Typically, a quad is constructed with two or more wire elements in the form of a square or 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 be made. Yagi patterns over real earth are shown in 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 millisiemens 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 8 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. Figs 13 and 14. it is helpful to present the 35- and 70-foot patterns in separate graphs, for clarity. Otherwise the multiple lobes become indis tinguishable. 3A commercially available software program. ANNIE, was used to calculate all patterns presented in this paper. ‘‘Such soil conditions may be considered typical for the central P3rt of the United States, such as the Ohio and Mississippi river valleys. 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 feet (broken line). Fig 9—Vertical-profile pattern of a 10.1 -MHz Yagi beam at 70 feet (solid line) and at 35 feel (broken line). 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-profile pattern of an 18.1-MHz Yagi beam at 70 feet (solid line) and at 35 feet (broken line). Fig 12—Vertical-profile pattern of a 21-MHz Yagi beam at 70 feet (solid line) and at 35 feet (broken line). 41