SELECTIVE FADING OF SUPER-HIGH FREQUENCY SIGNALS 1189 



standard methods for studying multipath transmission (or selective 

 fading) and the results obtained were in good agreement. 



Some frequency-sweep transmission measurements were made during 

 the summer of 1950 over a typical radio relay path in the mid-continent 

 region of the United States for the purpose of obtaining additional data 

 cf a statistical nature which could be used in system engineering. 



The objective of the tests was to determine what per cent of the fading 

 was frequency selective and the degree of its frequency selectivity. Such 

 data were needed to evaluate the advantages of using frequency diversity 

 as a means of minimizing the effects of fading, and to provide quantitative 

 data for designing frequency-diversity measures. Recordings were made 

 throughout the months of July and August which serve as the basis for a 

 statistical picture of the fading occurring during those months in the 

 frequency band between 3,750 and 4,190 mc. 



Path Over Which Measurements Were Made 



The measurements were made over a 30.8-mile path between the 

 Lowden and Princeton (Iowa) towers of the Chicago-Omaha Radio 

 Relay System. This path was chosen as a typical radio relay path as to 

 length, clearance above terrain, and climatic conditions. Height-loss 

 runs made by the American Telephone and Telegraph Company (when 

 originally selecting the path as part of their radio relay route) indicated 

 that ground reflections on this path were unimportant. The reflection 

 coefficient was less than 0.1 



Method of Tests 



The frequency -sweep equipment used in these tests w^as similar to 

 that used in the previous studies.^ With this equipment about 50,000 

 record photographs were obtained of a cathode-ray tube presentation 

 of the path-loss vs frequency characteristic of the path between 3,750 

 and 4,150 mc. Fig. 1 shows three illustrations of these record photo- 

 graphs, which are more fully discussed below^ The taking of these records 

 was distributed throughout the months of July and August in such a 

 manner as to give complete coverage of all the fading during that pe- 

 riod. In addition the single-frequency path loss at 4,190 mc was con- 

 tinuously recorded throughout the two months. 



The data from these records were analyzed on a statistical basis; 

 and families of curves were obtained depicting several aspects of the 

 nature of the selective fading encountered during the tests. These are 

 described more fully below. 



