Studies of within-the-horizon propagation at 9300 Mc

Abstract

Propagation characteristics of radio signals in the 9300-Mc frequency range were investigated over a 113-km tropospheric within-the-horizon path in Eastern Colorado. Special attention was given to the short-term fading characteristics of the received carrier envelopes and to the bandwidth capability of the medium in this frequency range, which was studied by the comparison of amplitude variations of two CW carriers separated by 100 Mc in frequency. For this purpose, correlation coefficients between carrier envelopes as well as the distributions of carrier amplitude ratios were analyzed. Although these parameters are related, the amplitude ratio has some advantages as an indicator of selective fading phenomena for within-the-horizon paths. By sampling the 9250 and 9350 Mc instantaneous carrier amplitudes at the rate of one per second, an over-all value of 0.91 was obtained for their cross-correlation coefficient. The standard deviation of the amplitude ratios expressed in db at 9250 and 9350 Mc averaged 0.76 db, with a maximum hourly value of 1.81 db. These results include the effect of space diversity, as separate antennas were used for transmission and reception of the two carriers, but they support the feasibility of wide-band modulation techniques for within-the-horizon paths if judged by the statistics of amplitude variations at discrete frequencies at the limits of the band considered. Short-term and long-term fading characteristics at 9300 Mc are similar to the ones previously observed on lower frequencies over this path. An analysis of prolonged space-wave fadeouts in this frequency range resulted in fadeout depths up to 25 db below weekly transmission loss medians, approximately log-normal distributions of fadeout durations, and the expected diurnal variations of fadeout occurrence typical of a continental climate.