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

Abstract

During 1959 a nd 1960 long-term tra nsmissio n loss measurem ents were p erform ed over a 223 kilom eter path in Eastern Colorado using fr equencies of 100 a nd 751 megacycles per second. This pat h i ntersects P ikes P eak which form s a kn ife-edge ty pe obstacle visible from both term inals. The transmissio n loss measurements have been analyzed in te rms of diurnal a nd seaso na l va riations in hourly m edians and in in stan taneous levels. As expected , results s h ow t hat the long-term fa di ng range is s ubstantially less t ha n expected for t ro pospheric seatter paths o f comparable length. T rans miss ion loss levels were in general agreem ent wi t h predicted k ni fe-edge d iffraction propagation whe n a ll owance is made for rounding of t he knife edge. A tec hnique for est imatin g long-term fading ra nges is p rese nted a nd t he res ults are in good ag reeme nt with obse rvat ions. Short-term variations in so me case resembl e t he s pace-wave fad eo uts commo nly obse r ved o n within-t he-horizon paths, a ltho ugh other phenomena may contribu te to t he fad in g. Since t he foreground te lTa in was rough there was no evidence of direct a nd grou nd-refl ected lobe structure. ' In most cases comparative ly 1 1igh co rrelation exists bct\\'ec ll signals received simultaneously on t wo a nte nn as with 8.3 an d 14 m eters ver t ical se paration. These se parations were chosen as being represe ntative fo r practical s pace d ive rsity systcms desig ned for eli minating t he eff ects of fading a ri sin g from direct and grou nd-rcf1ected phasc inter Fere ncc phc nom e na. The co mparatively hi gh co rrelatio n obse rved suggests t hat s pace diversity wi ll be re lativcly less successfu l in mountain obstac le paths with rou g h terrai n ncar t he ter min a ls than on t roposphe ri e seatter paths or on line-of-s ight paths over s mootll terrain. The enha ncement of fi eld strength associated with propagatioll over mountain ridges ITIay causc concern ill applicatio ns where Illountain are being co unted o n to hi elel uJlwantod radio waves. Somc rad io a stro nomy installations have bec n locateel in llloullLain vall eys fOf t his reaso n, and it is possible that obstacle-gai ll eii'ects ma.\" agg ravatc rather t ha n a ll e viaLe Interfe renc e.

Transmission loss variability and fading characteristics for long knife-edge diffraction paths

“…The discrepancy in the absolute levels may be ascribed to the already mentioned fact that for the purpose of this study possible reflections from the terrain in the vicinity of the terminals have not been taken into account in the calculation of the expected trans¬ mission loss distributions. Barsis and Kirby [1961] have already dis¬ cussed the effects of the assumption of a "rounded" knife-edge as the diffracting obstacle.…”

“…Peak forming the diffracting knife edge. Results of these measure¬ ments were described in a paper by Barsis and Kirby [1961] . More recently, additional measurements from this path and from two other long knife-edge diffraction paths have become available for analysis.…”

“…An extensive bibliography on knife-edge diffraction phenomena was contained in the paper by Barsis and Kirby [1961] . The present work deals essentially with three paths: the Pikes Peak path already -2discussed, a path along the coast of Alaska across the Fairweather range (data for which were supplied by Western Electric Co. ), and a knife-edge diffraction path across the Alps in Europe.…”

On Height‐Gain Studies Over a Long Mountain‐Obstacle Diffraction Path

“…The usefulness of mountain ridges in providing shielding of communication terminals (such as microwave relay sites) from unwanted signals has also become an important consideration in the siting of earth stations for satellite or space-communication systems. It is therefore timely to provide a revised assessment of comparisons between calculated and measured height-gain functions for a mountain-obstacle diffraction link discussed in earlier papers [Barsis and Kirby, 1961 Additional height-gain studies over the same path were performed at 4.5 and 9 GHz. The results of these measurements were discussed in the report by Hause and Peterson [ 1966].…”

“…was then concluded that the measured transmission-loss dependence on receiving antenna height did not at all agree with the calculated height-gain based on the four-ray geometric-optics models and the use of parameters derived from the terrain profiles involved. Although Figures 11 and 12 of an earlier paper[Barsis and Kirby, 1961] show Copyright ¸ 1970 by the American Geophysical Union. five lobes within the 2-33-meter height range for the calculated model, only a single lobe was derived from the measurements.…”