Intermittent Coverage from Sky Waves

Author: Edmund A. Laport

In circumstances where cochannel interference at night is negligible, it is frequently desired to know what service can be rendered intermittently at night by sky waves. The variability of the ionosphere makes this a statistical problem.

Considerable data on this type of propagation have been reduced to convenient curves. The FCC Standards of Good Engineering Practice Concerning Standard Broadcast Stations includes curves of sky-wave field strengths exceeding various percentages of the time from 5 to 95 percent, for varying distances. Several representative values taken from these curves are given in Table 2.3. These data are used for allocation purposes in the 550- to 1,600-kilocycle band. If one wished to know approximately what field strength could be delivered to a locality at some particular distance such that the only signal received there would be sky wave at night 95 percent of the time (corresponding to nearly 100 percent reliable night signals), the values read from the curve are adjusted to correspond to the actual field strength delivered at 1 mile in the right direction and at the proper vertical angle to arrive at the locality by reflection from the ionosphere. The vertical angle of radiation of the waves for a given distance may be determined by the curves of Fig. 2.3.

From the ambient-noise levels, the signal-to-noise ratios for a certain portion of time can be calculated as a statistical average.

To show how this information is used, consider the following case: A station contemplates using 50,000 watts in a region where grade 4 noise (see Appendix VI-A to VI-D) prevails for more than 6 months a year. It is desired to deliver a semiservice at night in certain cities varying in distance from 200 to 400 miles. What type antenna should be used, and what kind of service can be expected in these cities? (No interference from other stations is encountered on the frequency to be used.)

FIG. 2.3. Vertical radiation angles for sky-wave propagation.

From Fig. 2.3, the vertical angles of radiation for these distances vary from 30 to 15 degrees for a 60-mile-layer height. The antenna used must therefore have strong radiation at these angles. Guided by information from Fig. 2.6, we select an antenna approximately one-half wavelength high to obtain good ground-wave efficiency and yet have adequately large field strengths at vertical angles as high as 30 degrees. Referring to

A minimum acceptable service requires a signal-to-noise ratio of 15 decibels 90 percent of the time. During the dark hours the above field strengths are essentially those which will provide such a ratio according to Appendix VI-I. During the months when grade 3 noise exists, there is a somewhat better signal-to-noise ratio, provided that man-made noise does not dominate natural atmospheric noise on which this information is based.