FADING REGIONS WITHIN THE HORIZON 147 



Fading, or fadeout, of a radio wave may be defined as a drop in power 

 or field strength below a specified level of intensity. For a given site, 

 criteria may be set up by the communications engineer to establish the 

 magnitude of drop that defines the onset of fading conditions. This 

 magnitude usually ranges from about 5 to 30 dB below the specified level 

 of intensity. The specified level is derived from some ideal propagation 

 condition. 



Fading is also a time-dependent occurrence. It may be classified as 

 (a) prolonged fading, which is fading of sufficient interval to cause con- 

 tinued communication disruptions, and (b) short-term Rayleigh fading, 

 which is only instantaneously observed by a receiver. 



One of the main causes of deep fading of field strength within the 

 horizon as compared with the free space field strength value is the A^ 

 structure of the atmosphere. Theoretically, the atmosphere can be con- 

 sidered horizontally homogeneous and in spherical stratifications con- 

 centric with the earth, and N can be considered to decrease exponentially 

 with increasing height above the earth's surface. However, in reality, 

 this picture of the atmosphere is rarely, if ever, realized because of the 

 synoptic meteorological conditions that are perpetually present. Strati- 

 fications caused by the synpotic meteorological pattern give rise to field 

 strength fading within the horizon by defocusing the lobe pattern of the 

 transmitter along a given path. Whenever the rate of change of refrac- 

 tivity from the surface value with height (called the gradient of A^ with 

 height) is less than —157 A^ units per kilometer, a "ducting" condition is 

 said to exist at the surface, and, as shown in figure 4.35, certain rays will 

 tend to be "trapped" or guided within the surface duct. It is this atmos- 

 pheric condition of surface ducting which will be further explored herein 

 with respect to fading within the radio horizon. 



It should be realized that not all within-the-horizon fading has been 

 attributed to refractivity gradient discontinuities in the lower atmos- 

 phere. Misme [36] shows the influence of frontal effects and frontal 

 passage on signal fading within the horizon, the frontal passage even 

 occurring at a time when one would most expect a fadeout caused by a 

 surface duct. 



4.5.2. Regions and Extent of Fading Within the Horizon 

 in the Presence of Superrefraction 



Serious disruptions in reception from a transmitter above a duct to 

 a receiver within a duct can occur at particular points within the horizon. 

 It is these disruptions, and these locations, which are of interest to the 

 communications engineer establishing a given transmitting-receiving 

 path. Whenever these disrui)tioiis occur between a transmitter and 

 receiver within the horizon, a corresponding increase in the field strength 

 characterized by steady, high, dei)endable signals, is usually expected 



