GUIDED PROPAGATION 



181. 



many, arc shown which leave the transmitter at 

 different angles with the horizontal. 



Ray I is bent so much that after some distance it 

 returns to the ground; there it is reflected and then 

 the same course is repeated again. In this way the 

 ray may be reflected a number of times in succession, 

 remaining always in the lowest layer. This super- 

 refraction "traps" the rays in a "duct" and results 

 in guided propagation of the radar waves. Trapping 

 does not occur under standard atmospheric condi- 

 tions. A ray, under standard conditions, may be re- 

 flected by the earth's surface only once before it 

 escapes into space. 



Ray 2 is also bent in the lowest layer but not 

 enough to keep it from escaping into the upper 

 atmosphere whence it does not return to earth. 



Ray 3 is similar to 2 except that it undergoes one 

 reflection by the ground before it escapes into the 

 upper atmosphere. 



Ray 4 separates the two types of rays illustrated 

 by rays 1 and 2. This ray becomes horizontal when 

 it reaches the top of the trapping layer or duct and 

 from there on travels along at the same height. All 

 rays are divided into two groups: those that leave 

 the transmitter at an angle with the horizontal less 

 than the critical angle and are trapped, and those 

 that leave the transmitter at a larger angle and 

 proceed into the upper atmosphere. 



Figure 5. Rays in an elevated duet. In this, another 

 common form of duct, the amount of bending may be 

 approximately normal both below and above the duct. 

 The rays oscillate between the upper and lower bound- 

 aries; maximum ranges in or near the duct may be even 

 greater than with a ground-based duct. 



The critical angle is aways small, practically never 

 larger than Yl degree. Its magnitude may be taken 

 as a measure of the intensity of guided propagation, 

 that is, of the amount of radiant energy trapped 

 within the duct. Rays that leave the transmitter at 

 a somewhat larger angle up to about twice the critical 



angle are sufficiently deflected while passing through 

 the lowest layers to distort that part of the radar 

 coverage pattern lying just above the duct. Rays 

 leaving the transmitter at a still larger angle are not 

 appreciably affected. 



The ground-based duct or trapping layer guides 

 the wave along the earth's surface in much the same 

 way that hollow metal tubes guide microwaves. 

 Within the duct there is less decrease of signal 

 strength with distance than there is above the duct. 

 Radar ranges on surface craft and low-flying aircraft 

 located within a duct, similar to the one illustrated 

 in Figure 5, are increased— sometimes to two, three, 

 or four times the normal ranges. Ground echoes 

 would be increased at the same time and might, in 

 some cases, obscure partly, or even entirely, the 

 echoes from incoming aircraft. 



When the radar is located within the duct, ranges 

 on aircraft flying above the duct will be decreased 

 only slightly, if at all. Often there may be a slight 

 increase in effective ranges. If the angle of elevation 

 of the aircraft is greater than 1 degree, the effects 

 become inappreciable and failure to detect the target 

 cannot be attributed to excessive refraction. 



If the duct does not include the radar within its 

 boundaries, as, for example, when a duct forms below 

 a high-sited radar, the effective ranges on surface 

 craft may be either increased or decreased. Similar 

 reasoning may be applied in the case of airborne 

 VHF radio communication. Usually there is no very 

 pronounced effect upon the signal strength when 

 VHF communication is carried on between two 

 aircraft, both flying above the duct. 



Interference between the direct rays and the rays 

 reflected from the ground — resulting in the well- 

 known lobe pattern of the coverage diagram — has 

 not been mentioned. Under standard conditions the 

 position of the lobes depends only on the wavelength 

 used and the height of the radar above the ground. 

 When a duct is present the lowest part of the coverage 

 diagram may be strongly distorted. 



Coverage depends upon a variety of factors of 

 which the most important are these: height of the 

 top and base of the duct, amount of refraction in 

 the duct, position of the transmitter relative to the 

 duct, frequency (or wavelength) of the radar equip- 

 ment, and height of the transmitter above ground. 



A coverage diagram for standard conditions is 

 shown in Figure 6, diagram 1, with height strongly 

 exaggerated. Only the lowest three lobes are shown, 

 and the higher lobes appear compressed as compared 



