Antenna Patterns

Author: Edmund A. Laport

There are two main divisions of the signal variability to be considered - that due to the signal in space, and that contributed by the characteristics of the transmitting and receiving antennas. Customarily, both are considered as one. Actually, the antennas themselves may introduce a great deal of signal variation that may not be due to the space circuit at all. Consider, for example, a wave of constant field strength that is changing its azimuthal angle of arrival only by several degrees. If the receiving-antenna pattern is too sharp to have uniform response over the angle of azimuthal deviation, the receiving antenna introduces signal variation. If the receiving antenna has a deep null in its pattern 15 degrees off great-circle azimuth and the arriving wave swings as much as 15 degrees, the receiving antenna registers a deep fade which may not have been present in the arriving wave,

If azimuthal variation is a factor on a given circuit, it can be seen that the ideal receiving-antenna azimuthal pattern is one which is uniform over the range of angles of signal arrival and has zero response at all other angles. While it is theoretically possible to accomplish this, it is structurally and economically impractical to do so in present-day operations. In this case it may be the choice of the designing engineer to use a broader pattern, with lower gain and more wide-angle signal and noise pickup, to reduce signal variation caused by his receiving antenna. Another design engineer may find that the opposite choice best fits the circumstances, although usually the best solution is that which ensures the strongest and most constant signal.

The same effect occurs in the vertical plane. There may be several wave groups arriving simultaneously at different vertical angles, although one group will dominate the others in intensity from moment to moment. This gives, in effect, something like a single wave that is constantly changing its vertical angle of arrival. If now the field strength arriving is constant, but changing in angle, the vertical polar pattern of the receiving antenna, if nonuniform in response over the range of arrival angles, introduces signal variations as the signal comes in on different portions of its pattern. If the range passes a null in the pattern, it would look at the receiver as though the signal had faded to a very low value.

These effects due to the antenna response pattern occur in many cases, and the signal itself also varies typically over a considerable range of values. When the horizontal and vertical angles of arrival correspond to directions of minimum antenna response simultaneously with a minimum arriving signal strength, the signal at the receiver may go far under noise level and produce an interval of unintelligibility. Because of all these causes, the signal at the receiver goes through a wide range of values, sometimes exceeding 80 and 100 decibels, from second to second.

There is another very important aspect of the pattern of the receiving antenna. The fact that more than one wave group often arrives in the vertical plane and that the delay of the signal along each of the paths identified by its different arrival angle is different causes signal distortion even when signals may be strong. The delay is characteristically greater as the arrival angle becomes higher, since each wave traverses a longer path. Wave interference between the different wave groups at the receiving antenna introduces fading and elongation of signals due to the delays. If it were possible to select for reception only one of these wave groups and reject all the others, its field strength might be less variable and the delay differences would be eliminated. As a consequence, the signal intelligibility would be improved.

On some space circuits, there is sufficient angle between the multipath wave arrivals so that the vertical pattern of the receiving antenna can select one, by suitable angular response, and reduce or suppress the others. The shorter the circuit, the more likely is it that substantial angular difference in wave arrival will exist, including the effect of angular changes due to changes of layer height, to permit the selective reception of one dominant wave group by using the appropriate receiving-antenna pattern. However, on longer circuits, two or three orders of hops may arrive at so nearly the same angle that angular selection is impractical. In such a case there is no possibility of improving the intelligibility of the signal by this method, and lower signaling speeds and greater percentage of lost circuit time have to be accepted. If multipath delays cannot be eliminated, there is very little advantage to increasing transmitter power to improve signal intelligibility.

In the same respect as explained for receiving, the transmitting-antenna radiation pattern has a bearing on the situation. If one could radiate all the power at the one optimum vertical angle that gives the best transmission path, there would be relatively little radiated at the angles that give rise to multipath transmission. As stated previously, the angle of departure for a given wave path is roughly the same as that of its arrival a large portion of the time. Then if one is trying to eliminate a certain wave group at the receiver to improve signal intelligibility, it will usually help to transmit less power at the undesired angle. When the transmitting and receiving antennas are complementary, with maximum responses at the most favorable wave angles and relatively low responses at all the other angles, it is possible to improve operating margins by using greater transmitter power and increased operating speeds in telegraph services. In telephony, the improved intelligibility gives greater speed in completing calls satisfactorily and permits more calls to be placed.

It must be evident that antenna gain, of itself, is a secondary consideration in antenna design. The primary objective in design is to produce the most favorable radiation pattern in both vertical and horizontal planes from the standpoint of minimizing multipath propagation. Excessive directivity may be as detrimental as inadequate directivity. Using random patterns or patterns not expressly designed for the desired path can give results that are definitely bad. In these days of good ionospheric data, there is no longer an excuse to employ the hit-and-miss practices of the past. Years ago, one used high antenna gains to give high effective transmitting power and high-gain receiving antennas to receive as much power as possible - supposedly. Yet there were times when better communication resulted from the use of simple dipoles than from the superarrays. Modern propagation engineering makes this anomaly very clear and points the way to better performance. In years to come, greater and greater attention to these matters will be required as larger traffic volume has to be handled on fewer and fewer frequencies. The compromises employed today for economic expediency may well be intolerable in the future, when each circuit and frequency will be carefully engineered for peak performance, with tailored radiation patterns for that circuit only. A larger portion of the total investment will be in antennas.