SELECTIVE FADING OF MICROWAVES 



73 



is usually superimposed a fine structure fading in which short duration 

 fades to levels as much as 45 decibels below free space have been ob- 

 served. For this reason it is desirable to avoid paths having small clear- 

 ance over intervening terrain and also paths which have a permanent 

 ground reflection of sufficient magnitude to depress the signal to critical 

 levels when, due to variable atmospheric refraction, the direct and re- 

 flected components are in phase opposition. 



The following sections describe the angle-of-arrival and frequency- 

 sweep experiments on which much of the preceding discussion was 

 based. 



ANGLE-OF-ARRIVAL OBSERVATIONS 



A photograph of the Cra^vford Hill receiving site is shown in Fig. 4. 

 The building housing the receiving equipment and the associated an- 

 tennas are mounted on a framework which can be rotated on a concrete 

 track, permitting investigation of the transmission characteristics of 

 either path. The parabolic antennas on the tower are used for continuous 

 recording of 4195 megacycle signals. The long object at the left of the 



Fig. 4 — The Crawford Hill receiving site. 



