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High-frequency Transmitting-station Sites
Author: Edmund A. Laport
The choice of a site for a high-frequency transmitting station for efficient radiation is determined almost wholly from considerations of the geometry of the wave-propagation circuits. Any site that has a horizon subtending vertically less than 2 degrees from level in any of the directions of transmission can be considered immediately a satisfactory site from the radiation standpoint.
In hilly or mountainous country the choice of a site for long-distance transmission (requiring very low beam angles) can be a difficult problem. In such cases, the best procedure is to set up a transit in the middle of the proposed site and plot the vertical horizon angle for all relevant azimuths in the manner shown in Fig. 3.12. Then on this same profile the beam centers, determined previously from geometrical studies of the desired propagation paths depending upon distance, layer heights, and number of hops, can be superimposed.
When the only possible site presents horizon obstructions in the preferred wave path, it may be necessary to design the antennas to use a higher order of hop and to direct the beam at a correspondingly higher angle to obtain the 2-to-l horizon clearance. If, for example, the computed vertical beam angle for a one-hop circuit was 6 degrees at an azimuth of 332 degrees and the horizon in this direction consisted of a range of mountains with a height of 8 degrees, the performance of the circuit would be greatly compromised by the obstruction of the mountains. In such a case it might be better to work this circuit with two hops. Then a vertical beam angle of 20 degrees can be used instead, with adequate horizon clearance for the wave path. Or if the circuit required 6 degrees for a two-hop circuit 5,400 kilometers long, with the same obstruction cited, one could change to a three-hop circuit, which for the same layer heights would permit the use of a beam at 14 degrees. This lacks the full 2-to-l horizon clearance desired, but it may be an acceptable compromise and perhaps preferable to using four hops. This latter example is one of the problems that frequently confronts the engineer where a decision cannot be made in advance.
Short-range high-frequency circuits using one-hop high-angle radiation give a great latitude of choice of sites. For F-layer transmission to distances of 500 miles and less, the beam angles are always greater than 30 degrees. Satisfactory sites for such transmission can often be in rather deep valleys without any compromise whatever on the circuit performance.
The presence of forests on or near the site requires some consideration. When it is remembered that the theoretical radiation pattern is calculated on the basis of perfect reflectivity from the ground, as from a mirror surface, it can be foreseen that there are some precautions necessary to obtain performance substantially in agreement with that anticipated. Therefore, attention is directed to choosing conditions as nearly perfect as possible, so far as the wave-reflecting surfaces around an antenna are concerned, by the complete absence or elimination of trees and buildings on the land out to the necessary distance from the antenna.
The point of wave reflection for the desired angle of radiation should be well clear, as shown by Fig. 3.13. Horizontal dipole antennas intended for radiation angles higher than 45 degrees should be located at the middle of a cleared area at least one wavelength square. If the site borders the sea or a lake and the surface of the water can be used as the wave-reflecting surface, this is an excellent choice. One therefore seeks to have the wave-reflecting surface as flat as possible and of the highest possible electrical conductivity and at the same time clear of trees, shrubbery, buildings, and other impediments.
Sites for high-frequency transmitting and receiving stations are not dependent upon soil characteristics to the extent that medium- and low-frequency stations using ground-wave propagation are. Nevertheless, the soil conductivity and inductivity have an important effect on the wave-reflection coefficient in the vicinity of an antenna, which influences the antenna efficiency and the pattern shape. Where a choice of soil exists among several possible sites, good engineering will give consideration to the soils having the highest reflectivity for the frequencies employed, other factors being equal.
It is not always possible to have a site that is on level ground, and here arise many problems of detail that cannot be formulated with precision. In undulating terrain many compromises are necessary. Rhombic antennas may occupy a considerable area encompassing variations of slope of the land. The best way to analyze such a situation is to construct an accurate profile of the terrain through a contemplated location out to a considerable distance in the desired direction of transmission, using the same vertical and horizontal scales. The antenna is shown on this profile to scale. Then from pure geometry one considers the locations of possible wave-reflection points and tries to visualize where spurious reflections may compromise the radiation pattern of the system or where the terrain can be used to advantage to produce reinforced radiation at the desired vertical angle. For instance, if a rhombic antenna can be located on a very long uniform slope of, say, 4 degrees in the direction in which it is desired to transmit (or receive), and it is desired to produce maximum radiation at an angle of 9 degrees from the horizon, then the rhombic-antenna dimensions can be chosen to produce a normal vertical angle of 13 degrees, provided that there is nothing ahead of the antenna to cause spurious reflections or impede this beam. The 4-degree forward slope of the land then brings the beam at the desired 9 degrees with respect to the horizontal. The variations of terrain within the area of a rhombic antenna determine whether different mast heights must be used to maintain the antenna in one plane or whether these variations are negligible.
Another type of problem related to sites is the layout of antennas to minimize interactions between them. A site of limited area may have to accommodate several antennas. Quantitative information of this sort is unavailable; yet situations of this kind are common enough in practice, and tolerable results have been obtained. One should distribute directive antennas in such a manner as to avoid the presence of one antenna in the beam of another by the largest practicable margins. Antennas should be located mutually so that each is in a position of least field strengths from the others. It must be recalled that the radiation patterns for antennas that are usually discussed are the patterns for great distances. Near extended systems, the field distributions are not the same, and one must then consider the effects of proximity to the nearest portions of other antennas. With dipoles and dipole arrays, minimum fields exist in line with the dipoles. In-line assemblies of dipole antennas may use common supports to good advantage. Where dipoles of various azimuthal orientations are to be used, it is well to locate them successively at the minimum angles from a common line, thus tending to form a polygon layout without inverted angles. When this is done, adjacent antennas have minimum coupling angles; and as the coupling angles get more unfavorable, there is a substantial distance between antennas. Such a layout of antennas makes good use of supporting masts and may eventually form a closed circle of antennas with the station house at the center.
Special problems of land utilization arise where several rhombic antennas are to be used and one wishes to know how close adjacent antennas can be without detrimental effects. The only practical advice that can be given with certainty is never to use more than one common mast for two adjacent rhombic antennas and then to have their orientations such that the nearest sides are as far from parallel as possible. The patterns for such antennas are derived from the assumption of an antenna that is completely isolated. The very large electrical dimensions of typical rhombic antennas imply that the radiation pattern is not formed for a very large distance from the system, and therefore the fields of the individual sides must be very strong for a considerable distance from each. Therefore, another antenna nearby will have some energy induced into it, which will cause its reradiation in some undesired manner. In spite of the temptation to place rhombic antennas near each other and to use common masts, good engineering design will provide the maximum available spacing. A rhombic antenna functions as a balanced system, and anything that disturbs its symmetry of fields from the four sides will disturb its performance. Furthermore, a rhombic antenna has almost no selectivity to discriminate against parasitic currents.
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