01-18-1994 Planning Packet - Laserfiche WebLink

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Antenna Height and Communications Effectiveness B> GeralcJ L Hall. KlTD Associate Technical Editor The purpose of this paper is to p»'0\ije general information aoout communications effectiveness as related to the physical height of antennas The performance of honzontaf antennas at heights of 35 a^a 70 feet IS examined m oetail Venicai arrays are not considered here oecause at shcr-wa^e frequencies, over average terrain ana at lew raa.aticn angles they are less effective than are horizontal antennas. ionospheric Propagation Frequencies oerween 3 anc 30 megahertz (ab breviated MHz) are often called tne short-wave ’' bancs In engineering terms this range of frequencies is cefined as the high-freauency or HF poHion cf the radio spectrum. HF radio communicaticns between two points that are separated by distances of more than aocut 15 to 25 miles depend almost solely upon propagation of radio signals through the ionosphere. The ionosphere IS a region of the earth ’s upper atmosphere which is ionized by ultraviolet rays received from the sun. The ionosphere has the property that it wnl refract or ..end radio waves which pass through it. However, the ionosphere is not one single ‘'blanket ” of ionization. Instead, for reasons not fully understood, a few discrete layers are formed at different heights above the earth. From the standpoint of radio propagation, each ionized layer has distinctive characteristics, related primarily to different amounts of ionization in the various layers The ionized layer which is most useful for HF racio com- munications is callec the F layer. The F layer exists at heights varying from approxi mately 130 to 260 miles above the earth s surface Both the layer height and the amount of ionization depend upon the latitude from the ecuatcr the time of day the season of the year, ano upon the level of sunspot activity Sunspot activity varies generally m cycles that are approximately 11 years in duration although short-term bursts of activity may create changes in propagation conditions that last for less than an rour The ionosphere IS not hcmiogeneous. and is undergoing continual change The F layer disappears at night in periods of low and medium solar activity, as tne ultraviolet energy required to sustain ionization is no Icnge" received from the sun. The amount of bending tnat will be imparted to a passing radio wave is related directly to the intensity of ionization in this layer, and to the frequency of the radio wave. A triangle may thus be used to portray the cross- sectional path of ionospheric radio-wave travel, as shown in Fig 1. The base of the triangle is the surface of the » i N V • * .' 6 •C.'lutf I Fig 1 —A simplified cross-sectional representation of ionospheric propagation. Typically the F layer exists at a height of 150 miles above the earth at mid-latitudes. The distance between the transmitter and the receiver may range from a few miles to 2500 miles under normal conditions. earth between two distant points, and the apex of tne triangle is the point wihich represents refraction in the ionosphere. If all the necessary conditions are met. the radio wave will travel from the first point on the earth s surface to the ionosphere, wnere it will be bent suf ficiently to travel to the second point on the earth, many hundreds cf miles away Of course the earth's surface is not a flat plane, but instead is curved. High-frequency radio waves behave in essentially the same manner as light waves —they tend to travel in straight lines, but with a slignt amount of downward bending caused by refraction in the air For tnis reason it is not possible to communicate by a direct path over distances greater than aPout 15 to 25 miles in this freouency range The curvature of the eartn causes the surface to "fall away" from the path of tne radio wave witn greater distances Therefore, it is the ionosphere that permits HF radio communication to be made between points separated Py thousands of miles. The range of frequencies from 3 to 30 MHz is unique in this respect, as ionospheric propagation is not consistently supported for any frequencies outside this range. One of the necessary- conditions tor lonospneric communications is that the radio wave must encounter the ionosphere at the correct angle. This is illustrated in Fig 2. Radio waves which leave the earth at high angles above the horizon may receive only very slight bending, and are then lost to outer space. For the same fixed frequency of operation, as the radiation angle is lowered toward the horizon, a point is reached where the bending of the wave is sufficient to return the wave ■ --------------------