07-25-1994 Council Packet - Laserfiche WebLink

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Antenna Height and Communications Effectiveness By Gerald L Hail KITO Associate Technical Editor quite K the ot be lights tence The purpose of this paper is to provide general information about communications effectiveness as related to the physical height of antennas The performance of horizontal antennas at heights of 35 and 70 feet is examined in detail. Vertical arrays are not considered here because at shon-wave frequencies, over average terrain and at low radiation angles, they are less effective than are horizontal antennas. lofiospheric Propagation Frequerteies between 3 and 30 megahert': (ab* breviated MHz) are often called the short-wave" bands In engineering terms this range of frequencies is defirted as the high-frequency or HF portion of the radio spectrum. HF radio communications between two points that are separated by distances of more than about 15 to 25 miles depend almost solely upon propagation of radio signals through the icviosphere. 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 will refract or bend 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 radio com munications IS called 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 equator the time ot oay. the season of the year, and upon the level of sunspot activity Sunspot activity vanes generally m cycles that are approximately ll years m duration although short-term bursts of activity may create changes in propagation cont^dons that last for less than an hour. The ionosphere is not homogeneous, and is undergoing continual change. The F layer disappears at night in periods of low and medium solar activity, as the ultraviolet energy required to sustain ionization is no longer received from the sun. The amount of bending that 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- sectiortal path of ionospheric radio-wave travel, as shown in Fig 1. The base of the triangle is the surface of the a c ■« ••C. l ‘Via ts**- Fig 1—A simplHied eress-eectional repfesentation ot toAMphehe prepagatton. TypieaPy the F tayer exists at a hei^ of 190 mHes above the eaifh at mid-tatihidea. The distance between the trananiHler and the receiver may range from a few mHes to 2900 miles under normal conditions. earth between two distant points, and the apex of the triangle is the point which represents ref^’action in the ionosphere. If all the necessary conditions are met. the radio wave will travel from the first point on the eanh ’s surface to the ionosphere, where it will be bent suf ficiently to travel to the second point on the earth, many hundreds of 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 slight amount of downward bending caused by refraction m the air. For this reason it is not possible to communicate by a direct path over distances greater than about 15 to 25 miles in this frequency range. The curvature of the earth causes the surface to “fall away" from the path of the radio wave with greater distances Therefore, it is the ionosphere that permits HF radio communication to be made between points separated by 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 cortditirns for tonospnenc 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