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During 1959 and 1960 long-term transmission loss measurements were perform ed over a 223 kilom eter path in Eastern Colorado using frequencies of 100 and 751 megacycles per second. This path intersects P ikes P eak which form s a kn ife-edge type obstacle visible from both terminals. The transmission loss measurements have been analyzed in te rms of diurnal a nd seaso nal variations in hourly medians and in instan taneous levels. As expected, results sh ow t hat the long-term fading range is substantially less t han expected for t ropospheric seatter paths of comparable length. T rans mission loss levels were in general agreem ent wi t h predicted k ni fe-edge d iffraction propagation when a llowance is made for rounding of t he knife edge. A technique for est imat in g long-term fading ra nges is presented a nd t he res ults are in good agreeme nt with obse rvat ions. Short-term variat ions in some case resembl e t he space-wave fad eo uts commo nly obse rved on withint 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-reflected lobe structure. ' In most cases comparative ly 1 igh co rrelat ion exists bct\\'ec ll signals received simultaneously on t wo a nte nn as with 8.3 an d 14 meters ver t ical se paration . These se parations were chosen as being representat ive fo r practical space d iversity systcms designed for eli minating t he effects of fading a risin g from direct and grou nd-rcf1ected phasc interFerencc phc nom ena. The co mparat ively hi gh correlatio n observed suggests t hat space diversity wi ll be re lativcly less successfu l in mountain obstac le paths with rough terrai n ncar t he termin a ls than on t roposphe ri e seatter paths or on line-of-s ight paths over smootll terrain. The enha ncement of fi eld strength associated with propagatioll over mountain ridges ITIay causc concern ill applicat ions where Illountain are being co unted on to hi elel uJlwantod radio waves. Somc rad io astro nomy installat ions 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 eviaLe Interfe rence.

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