Fading characteristics and bandwidth capability for within-the-horizon propagation at 1040 and 9300 MCs

Abstract

During the early months of 1961 the National Bureau of Standards established a 113 km tropospheric within-the-horizon path in Eastern Colorado in order to investigate propagation characteristics of signals in the 9300 Mc/s range over such a path. Special attention was given to the short-term fading characteristics of the received carrier enve¬ lopes, and to the bandwidth capability of the medium. The latter was studied by comparing the amplitude variations of two continuous-wave carriers separated by 100 Mc/s in the 9300 Mc/s frequency range.Separate antennas were used in transmission and reception of the two carriers; consequently, the results include the effects of space diversity.In addition to studying the correlation of carrier envelopes as a measure of the bandwidth capability, an analysis was also made of the distribu¬ tion of the carrier amplitude ratios, which is related to the correlation coefficient, and is believed to be a better indicator, at least for lineof-sight paths, of selective fading phenomena. Short-term fading in the 9300 Mc/s range can be characterized in terms of "prolonged space-wave fadeouts, " which have previously been observed and described for lower carrier frequencies over this and similar paths. Fadeouts appear to be well correlated in time of occurrence on the two frequencies 9350 and 9350 Mc/s. The fading durations range from a few tenths of a minute to 10 minutes, and the maximum observed depth was 25 db below the median of a five-day observation period. -2-The cross-correlation coefficient of instantaneous carrier envelopes separated by 100 Me/s had an overall value of 0. 91, as determined by sampling of the carrier amplitudes at the rate of one per second. The standard deviation of the amplitude ratios averaged 0. 76 db, with a maximum hourly value of 1. 81 db. These results support the feasibility of wide-band modulation techniques for withinthe-horizon paths if evaluated by amplitude variations of discrete frequencies within the bandwidth of interest. Sufficient margin in transmitter power has to be provided to allow for the short-term signal variations observed. Supporting measurements on 1040. 1 Mc/s over this path con¬ firmed previously established fadeout characteristics with regard to diurnal variations, depth, and duration of fadeouts.However, the distribution of hourly median signal levels on this frequency showed a much larger month-to-month variation than had previously been observed over this path. This is attributed to the small sample size available in the current measurement series.