146 A^ CLIMATOLOGY 



4.5. A Study of Fading Regions Within the Horizon 

 Caused by a Surface Duct Below a Transmitter 



4.5.1. Introduction 



Among the factors influencing the choice of an antenna site for a micro- 

 wave receiver operating on a line-of-sight path is the desirability of 

 locating in a region relatively free from space-wave fadeouts. Although 

 line-of-sight propagation is normally characterized by steady, high, de- 

 pendable signals, deep, prolonged space-wave fadeouts are observed from 

 time to time. Since serious disruptions occur during such fadeouts, a 

 systematic discussion of fading phenomena is of considerable interest to 

 the radio circuit engineer. 



There are [35] results in detail of a year's study of signal fadeouts 

 occurring at certain points within the horizon caused by ground-based 

 superrefractive layers. The path studied in this case [35] has its trans- 

 mitter at Cheyenne Mountain near Colorado Springs, Colo., at an eleva- 

 tion of 8800 ft. Receivers are at Kendrick, Colo., 49.3 mi from the 

 transmitter, and at an elevation of 5260 ft; Karval, Colo., 70.2 mi from 

 the transmitter, and at an elevation of 5260 ft; and Haswell, Colo., 96.6 

 mi from the transmitter, and at an elevation of 4315 ft. A study of the 

 profile of this path reveals that Kendrick is well within the radio horizon, 

 Karval is near, but still witnin the radio horizon, and Haswell is beyond 

 the radio horizon. 



The Cheyenne Mountain study found that throughout the period of 

 observation, fadeouts (of 5 dB or more) occurred regularly at Karval on a 

 frequency of 1046 Mc/s, in conjunction with superrefractive A'^-profile 

 conditions, and coincided with enhanced field strengths at Haswell: A 

 particular instance of this, the night of June 21-22, 1952, is shown in 

 figures 4.53 and 4.54. Figure 4.53 shows the field strengths recorded 

 simultaneously in a 12-hr period at Haswell, Karval, and Kendrick. Here 

 it is to be noted that with the sudden intense rise in field strength at 

 Haswell, a progressively deepening fadeout appears in the Karval data 

 relative to the monthly median, while only insignificant changes are noted 

 at Kendrick. Meanwhile, figure 4.54 indicates the shift in the refrac- 

 tivity profile at Haswell (presumably the same for Kendrick and Karval) 

 towards conditions of superrefraction throughout the evening of 21-22 

 June, 1952. 



The great number of fadeouts at Karval as compared to Kendrick 

 strongly suggested a dependence of fading on distance from the trans- 

 mitter within the horizon. The following sections describe the conditions 

 under which fading, variations in fading, and locations that favor the 

 occurrence of fading within the radio horizon (in the presence of a ground- 

 based duct) occur. 



