VHF and UHF signal characteristics observed on a long knife-edge diffraction path

Abstract

During 1959 and 1960 long-term transmission loss measurements were performed over a 223 kilometer path in Eastern Colorado using frequencies of 100 and 751 Mc. This path intersects Pikes Peak which forms a knife-edge type obstacle visible from both terminals. The transmission loss measurements have been analyzed in terms of diurnal and seasonal variations in hourly medians and in instantaneous levels. As expected, results show that the long-term fading range is substantially less than expected for tropospheric scatter paths of comparable length. Transmission loss levels were in general agreement with predicted knife-edge diffraction propagation when allowance is made for rounding of the knife edge. A technique for estimating long-term fading ranges is presented and the results are in good agreement with observations. Short-term variations in some cases resemble the space-wave fadeouts commonly observed on within-the-horizon paths, although other phenomena may contribute to the fading. Since the foreground terrain was rough, there was no evidence of direct and ground-reflected lobe structure. In most cases comparatively high correlation exists between signals received simultaneously on two antennas with 8.3 and 14 meters vertical separation. These separations were chosen as being representative for practical space diversity systems designed for eliminating the effects of fading arising from direct and ground reflected phase interference phenomena. The comparatively high correlation observed suggests that space diversity will be relatively less successful in mountain obstacle paths with rough terrain near the terminals than on tropospheric scatter paths or on line-of-sight paths over smooth terrain. The enhancement of field strength associated with propagation over mountain ridges may cause concern in applications where mountains are being counted on to shield unwanted radio waves. Some radio astronomy installations have been located in mountain valleys for this reason, and it is possible that obstacle-gain effects may aggravate rather than alleviate interference.