High-frequency Receiving-station Sites

Author: Edmund A. Laport

The remarks about the choice of a transmitting-station site apply in exactly the same way to a receiving-station site, with two notable exceptions:

1. The dominant angles of arrival of the incoming waves at the site are determined mainly by the characteristics of the transmitting antenna. Whenever possible, best results are obtained with complementary transmitting and receiving antennas. If a horizon obstruction exists at the optimum angle of wave arrival, a compromise noncomplementary antenna may be necessary. Whenever possible, the transmitting antenna had best be changed to be complementary with the receiving antenna when an obstruction is unavoidable at the receiving location on a fixed circuit. Figure 3.10 gives some examples of complementary and uncomplementary antenna patterns.

2. The receiving site must be as free as possible from electrical noise. The site should be an adequate distance from a city or other populated place that is a source of noise. Factories and other establishments are to be similarly avoided. Motor highways are also a source of noise (from ignition systems), and a substantial distance from highways that have passing motor vehicles should be allowed. The amount of man-made noise that can be tolerated at a particular receiving-station site depends upon the prevailing natural atmospheric noise levels. At a well-selected site, reception should always be limited only by natural atmospheric noise, which is the limitation imposed by nature. Any man-made noise at the site should always be substantially less than the atmospheric noise received during the low-noise periods.

There is one form of atmospheric noise that can be reduced by suitable antenna design, and that is the kind known as "precipitation static." This occurs when there is wind-blown dust, sand, snow, or fog; electrical noise from these sources is chiefly due to their charged particles hitting and imparting their charges to the metallic portions of the receiving antenna and feeder. It has been found that this kind of interference can be very substantially reduced by using thickly insulated wires for the antenna and feeder and by not having any exposed metallic surfaces. In low-noise regions, precipitation static may be the cause of limiting noise, so that the application of this technique may be very helpful in reducing the noise level.

The suitability of a receiving site will often depend upon the direction of arrival of noise. If, for instance, the dominant noise interference is always from a direction substantially different from that of desired signals, antenna directivity can be employed to favor the signal and discriminate against the noise. Another aspect of this is where man-made noise may be at low angles and the incoming signal at a high angle, in which case the proper antenna pattern will give best response to the signal and have relatively low response to the low-angle noise.

The suitability of a receiving-station site for optimum performance can be seen, from the foregoing, to depend to some extent on the characteristics of the antennas that will be employed for the particular services to be operated and on how antenna directivity can be advantageously employed to obtain the best operating signal-to-noise ratios. The most severe site-selection problem occurs when reception is required from all directions.

There are instances where a receiving station has to be located in a city or other region of severe man-made noise. It may be necessary to attempt to suppress noise from troublesome dominant sources in the neighborhood of the receiving station to obtain tolerable system performance. The measures that can be applied successfully depend upon the nature of the device and the kind of noise it emits, assuming that the source can be located.

FIG. 3.14. Vertical radiation angles for one-hop circuits.