Other Factors

Author: Edmund A. Laport

If the ground conductivity is high, some useful distances for communication can be obtained by high-frequency ground-wave propagation. From ground propagation curves it is evident that as the frequency becomes higher, the ground-wave attenuation increases very rapidly. Over sea water, the best conductivity that is available in nature, substantial distances can be covered with frequencies of 5 megacycles or sometimes more. This fact has often been utilized for interisland communication and for short-distance shore-to-ship communication, particularly in harbors and estuaries. In such applications, vertical polarization gives best results, and the station sites should be near the shore to avoid excessive attenuation over land.

In airways and aircraft communication using the high frequencies, it is essential to use horizontal polarization and transmission via the ionosphere even for short-distance working. Ground-wave coverage over land is so limited that an aircraft is quickly outside the communication distance. Also, regardless of polarization, the signal strength on the horizon at the high frequencies and typical ground conductivities is vanishingly small by the direct wave. Experience has long proved that best communication with aircraft is via the ionosphere as soon as the craft passes out of the range of the direct space wave. To obtain ionosphere reflections at nearly vertical incidence the frequency used must be below the vertical-incidence maximum usable frequency for the location of the transmitting station.

As a .consequence of the randomness of ionospheric waves in azimuth and vertical angles of arrival, as well as the large variations in intensity and the changes in polarization in transit, accurate direction finding at high frequencies has been difficult. Most successful high-frequency direction finding has been with Adcock receiving antennas responsive only to the vertically polarized component of the arriving signal. An extensive metallic ground system around the antennas eliminates wave tilt in the vicinity so that the electric-flux lines of the wavefronts are vertical. The vertical elements of the Adcock array respond to this electric field with a minimum of response to other components of the wave, as the polarization changes from moment to moment. A carefully calibrated system of this type can accurately indicate the direction of arrival of a wave (in azimuth), though this direction may not always be the true bearing of the transmitting station.

The antenna designer must recognize that high-frequency propagation is more complicated than can be predicted from the best available information. Fortunately, the accumulated information permits the prediction of single-hop circuits with sufficient accuracy for engineering use. It requires the most expert use of the available data to predetermine the variations of vertical propagation angles on multihop circuits through seasonal and sunspot cycles. There is very little information to guide the antenna engineer in the range of variation of azimuthal angles on long circuits, though it is known from observations with modern direction finders that the signals may arrive from angles considerably off the great-circle bearing from the transmitting station (). Antenna systems should be designed to utilize the optimum vertical angles and suppress others that contribute to multipath delay and distortion. On the other hand, excessive horizontal directivity is often detrimental to circuit performance when the horizontal angle of arrival swings as far as the first null in the receiving-antenna pattern. For this reason, the requisite vertical directivity may have to be associated with a rather broad horizontal pattern.

In using space diversity reception, the pattern for one receiving antenna can be turned slightly with respect to the other, giving the effect of broader horizontal pattern for both. Still more horizontal equalization of this kind can be employed in a third antenna when three-set diversity is employed.

It is essential in the design of communication circuits to direct the beams of the transmitting and receiving antennas at the angle that is most favorable for the particular circuit and operating frequency. In many, if not most, cases the best angle will be the same, within reasonable limits, at both ends of the circuit. On one-hop circuits this seems to be true always, and any departure from this principle seems to increase with the number of hops. To make the best use of this effect, it is desirable to employ complementary antennas for transmitting and receiving -antennas having the same vertical-plane patterns. Figure 3.10A, B, and C gives three examples of uncomplementary antennas; Fig. 3.10D) and E two which are complementary. Example B will be recognized as one typical of the high-frequency-broadcasting situation; C is an example of a case often found where a receiving antenna is too low to receive medium-angle signals and gives relatively poor results; and in A the receiving antenna is too high and presents low response to the angle of signal arrival. The same result would be obtained if the transmitting and receiving antenna patterns were interchanged. The best operational results can be expected from D and E, assuming that the angles of maximum transmission and reception are properly chosen propagationwise.

FIG. 3.10. Comparison of uncomplementary and complementary antennas.

In this discussion we have presented high-frequency wave trajectories on a straight geometric-ray basis, for any number of hops. Owing to ionosphere turbulence, wave trapping, refractions and reflections, and scattering in space and at reflection points on the earth, the actual trajectory of a wave is extremely complex and changes from moment to moment. However, there is a sufficient amount of successful engineering experience now to justify the ray theory for engineering applications.

From extensive measurements that have been made of multipath delays and angles of signal arrival it is indicated that the geometric-ray theory is perhaps a statistical average condition.

It is the utilization of this principle that has led to the use of the principle of complementary antennas for transmitting and receiving. When complementary antennas are used for a communication circuit, the composite radiation pattern for the circuit is the square of the pattern for one of the antennas and the angle of this composite pattern is chosen for the lowest practical order of hop for the distance, using the geometric-ray theory and the known ionosphere heights. When noncomplementary antennas are used, the angle of transmission is chosen to be that of maximum response for the composite pattern for the path obtained by multiplying the transmitting-antenna pattern by the receiving-antenna pattern. This is essential in services, such as broadcasting, where there is no control over the characteristics of the receiving antenna.