Selective Fading of Microwaves 



BY A. B. CRAWFORD AND W. C JAKES, JR. 



(Manuscript received October 25, 1951) 



The results of an extended survey of microwave ■propagation over two line- 

 of -sight paths in New Jersey are described. Angle-of-arrival 7neasurements 

 at 1.25-cm wavelength and selective Jading observations in a 450-mc fre- 

 quency band centered at 3950-mc show that the severe fading can be explained 

 in terms of multiple-path transmission. A cojnputer of the analogue type 

 was built to simulate the more complicated selective fading patterns. 



INTRODUCTION 



During the past few years, studies of microwave propagation have 

 been made by the Radio Research group at the Holmdel Laboratory 

 over two paths located in eastern New Jersey. Both of these are Une- 

 of-sight paths which might be considered to be typical links in a cross- 

 country microwave radio relay circuit. 



In conducting these studies, the usual continuous recordings of signal 

 levels were made but the greater interest was centered in special experi- 

 ments designed to reveal more of the processes which can cause fading. 

 The most relevant information has been obtained by exploring the inci- 

 dent wavefronts with a narrow-beam scanning antenna (angle-of-arrival 

 studies) and, more recently, by observing the transmission characteris- 

 tics of the paths by means of a frequency-sweep technique and also by 

 the use of very short pulses. 



Some results of angle-of-arrival observations have been reported pre- 

 viously^ and a companion paper describes the transmission tests con- 

 ducted with very short pulses.^ The present paper describes some of the 

 observed mechanisms associated with fading, presents typical data ob- 

 tained with the narrow-beam scanning antenna and gives examples of 

 the frequency-sw^eep observations, illustrating the frequency selective 



' W. M. Sharpless, "Measurement of the Angle of Arrival of Microwaves," 

 Proc. I.R.E., 34, Nov. 1946, pp. 837-845. A. B. Crawford and W. M. Sharpless, 

 "Further Observations of the Angle of Arrival of Microwaves," Proc. I.R.E., 

 34, Nov. 1946, pp. 845-848. H. T. Friis, "Microwave Repeater Research," Bell 

 System Tech. J., 27, Part I, "Propagation Studies" by A. B. Crawford, Apr. 

 1948, pp. 183-246. 



2 O. E. DeLange, "Propagation Studies at Microwave Frequencies by Means 

 of Very Short Pulses," Bell System Tech. J., 31, Jan. 1952, pp. 91-193. 



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