07-25-1994 Council Packet - Laserfiche WebLink

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r I fk fI' th r'- r compass dirsctions. as a solid tins. The broken line in Fig 6 showrs the pattern of a haif-waveiength dipole, for comparison. Fig 7 shows the elevation plane or vertical- profile pallem. This pattern is that seen from the ends line again repieaents the Vagi, and the broken line the dipole, that‘‘perfact cede** 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 oflan-used type of becm antenna is the quad array. TypicaNy. a quad is constructed with two or more wire alemama in the form of a square or a diamond. The conducting wires are usuaNy 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 Yagt beam. The vertical patterns of Figs 4 and 5 were calcuiaied by assuming the earth is a pertsct 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 wiN be slightly different, depending upon the electrical con ductivity and dielectnc constant of the soil, and upon the radio frequency. Computer programs are available to take these factors mto account, and reliable relculations 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 He personal computer.* “Average” soil conditions were used for these plots, that is. a conductivity of 5 miHisiemens per meter arKl a dielectric constant of 15.« These patterns compare the performance of Yagi arrays at heights of 3$ 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, tor clarity. Otherwise the multiple lobes become indis tinguishable. commercisHy avaiiabie software program. ANNIE, was used to calcuiaie ail patterns presented m this paper *$uen soil conditions may be considered typical (or the central part of tne United States, such as the Ohio and Mississippi river valleys. •C Fig S—Vertica l profile pattern of a 7-MHz Yagi beam over average earth at a height of 70 feet (solid line) and at 35 feel (broken line). ^0 at 70 feel (aoNd haa) and at 38 feat (brekan Nna) Fig 10—Vertical profile patiam of a i4-MHa Vagi beam at 70 feat (solid Hno) and at M foot (broken Kne). Fig 11—Vertical-prefilo pattom of an ig.l-MNz Yagi beam at 70 feet (solid line) and at 35 foot (broken lino). Fig 12—Vortlcai<^rofilo pattern of a 21-MHa Yagi beam at 70 feet (solid line) and at 35 feet (broken line). 1