Choice of Vertical Radiation Pattern

Author: Edmund A. Laport

An intelligent choice of a vertical radiation pattern for a particular application is made only after a computation of the ground-wave and sky-wave field strengths over the desired propagation paths. These two wave fields are separately computed, and special attention is directed to the distances at which their ratio is less than 2 to 1, because objectionable selective fading will occur at night at these distances. The location of this fading ring, or fading "wall" as it is often called, sometimes can be adjusted by the choice of vertical radiation pattern to fall where the least number of listeners is located. The variability of the sky-wave field strength from day to day will cause this fading ring to move about accordingly. Over ground where the direct wave is very rapidly attenuated, the fading ring may be quite narrow.

Consider a simple case of a broadcast station on 1,000 kilocycles in the center of a region having a uniform conductivity of 4·10^-14 electromagnetic unit. The station will operate with a power of 10,000 watts. It is desired to see what the coverage will be with an antenna of 60 degrees height compared with one of 190 degrees. It is assumed that there are no regulatory reasons why either cannot be used. It is a grade 3 noise area. The data of Fig. 2.7 are computed and plotted from the vertical patterns for these two antennas and the ground-wave and sky-wave propagation information and noise data provided, using the given power and frequency.

The direct ground-wave intensity for the 60-degree radiator reaches the 15-decibel signal-to-noise threshold of 560 microvolts per meter at a distance of 49 miles, and for the 190-degree radiator at 55 miles. The 60-degree-radiator sky wave, for 10 percent of the time at night, equals the ground-wave field strength at distance 49 miles (where it is also noise-limited) and the fading ring for 2 to 1 direct and indirect signal ratio extends from 39 miles to 62 miles 10 percent of the time. For 50 percent of the time, the center of the fading ring would be at about 73 miles and the near edge of the ring under these conditions at about 57 miles.

FIG. 2.7. Ground-wave and sky-wave curves for 10 kilowatts radiated from a 60-degree and a 190-degree radiator where σ = 4·10-14 electromagnetic unit, f = 1,000 kilocycles, in grade 3 noise area.

In this zone it appears that the signal is noise-limited before it is fading-limited, with this power.

For the 190-degree radiator, the sky-wave fields are not shown because it is known immediately from the chart thus far computed that the signal will be severely noise-limited before arriving at the distance where fading is objectionable.

This example was chosen to demonstrate the influence of natural atmospheric noise on the solution of a problem of this type. One can readily see that, under these circumstances, it would be wasteful to invest in a 190-degree radiator when a 60-degree radiator will provide essentially the same fading-free and noise-free coverage. However, this example should not be used for any conclusion for other cases without completely calculating the problem in the manner outlined. The use of lower powers and higher frequencies in regions of lower conductivities and equal or higher noise levels would always show poor justification for expenditures for high radiators. For higher powers on lower frequencies, in regions of lower noise levels and higher conductivities, there would almost always be a case to justify investments in higher radiators. Intermediate combinations of frequency, power, conductivity, and noise conditions will always require specific detailed study of the actual data before a decision can be made.

Some marginal intermittent service is given by daytime sky-wave propagation due to E-layer reflections. The computations for such propagation can be made by reference to the data published by the Central Radio Propagation Laboratory of the National Bureau of Standards, following the methods used for computing high-frequency propagation. The effects of E sporadic layers may also be computed from these data.