190 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



atmosphere, in which the distributions of the temper- 

 ature, pressure, and humidity are the most important 

 elements. With refraction, rays are bent, and the 

 electromagnetic energy flows along the curved ray 

 paths. A situation frequently realized in practice is 

 that in which the curvature of the rays is independent 

 of height above ground. This is known as standard 

 rejraction. The term standard propagation is used to 

 designate propagation under conditions where the 

 refraction is of the standard type. 



During the war years the increased number of ob- 

 servations, which resulted from the world-wide use of 

 radar, showed that, under certain weather conditions, 

 radio field strengths may depart markedly from the 

 values expected with standard refraction. These 

 deviations are now known to be attributable to a 

 stratification of the atmosphere which is predomi- 

 nantly horizontal and is produced by vertical varia- 

 tions in water- vapor content and temperature. Since 

 these quantities control the index of refraction, and 

 therefore the curvature of the rays, it follows that 

 this curvature varies with the elevation above ground. 



Any stratification of the atmosphere tends to pro- 

 duce a distribution of the radiated energy different 

 from that which occurs in the standard atmosphere. 

 Of particular importance is a type of stratification 

 which results in a duct being formed in the atmos- 

 phere. In this event, a portion of the wave energy 

 may be guided horizontally along the duct and may 

 be effectively "trapped" within the duct's upper and 

 lower boundaries. This is known as "guided" propa- 

 gation. The radiation energy may then travel to dis- 

 tances far beyond the geometrical horizon, producing 

 unusually long ranges for short wave receivers or 

 radar targets. The phenomenon which tends to con- 

 strain the wave energy to follow the duct is called 

 "superrefraction." When this occurs, the rays in pass- - 

 ing through the inversion layer in the upper part of 

 the duct are bent downward with a curvature which 

 exceeds that of the curvature of the earth. The 

 regions covered by the inversion layer and the duct 

 are illustrated in Figures 15, 20, 22, and 23. The dis- 

 tribution of moisture and temperature in the atmos- 

 phere, responsible for the formation of ducts, is dis- 

 cussed in Section 17.3.1. 



As the stratification of the lower atmosphere that 

 produces superrefraction is part of the weather, the 

 prevailing meteorological conditions become of im- 

 portance for problems of propagation and coverage. 

 Meteorology as related to wave propagation is 

 treated in Section 17.3. 



17.1.3 



Reflection from the Ground 



A coverage diagram is a curve, or a set of curves, of 

 constant field strength in a vertical or horizontal 

 plane. The horizontal coverage diagram is deter- 

 mined chiefly by the antenna pattern itself. In the 

 vertical plane, however, the diagram depends pri- 

 marily upon the interference between the radiation 

 coming directly from the transmitter and that which 

 is reflected from the ground or sea surface. This effect 

 produces the lobe structure of the vertical coverage 

 diagram. At the lobe maxima the two rays reinforce 

 each other, while they cancel each other out, more or 

 less, at the lobe minima. 



The propagation problem in its full generality 

 leads to mathematical formulas of forbidding com- 

 plexity. In order to understand the processes at work 

 it is necessary to proceed in steps and gradually add 

 refinements to the basic features of the problem. 



Consider first the field radiated from an antenna 

 which is remote from the earth. This free space field 

 decreases in strength in inverse proportion to the dis- 

 tance, Ri, from the transmitter and varies with the 

 angular position in accordance with the shape of the 

 radiation pattern of the transmitting antenna. Let 

 this free space field strength at any point at distance 

 Ri be designated by E a . 



Figure 2. Interference of direct and reflected rays. 



If, instead, the transmitter is placed near the 

 ground, as at T in Figure 2, the field at any point in 

 space is produced partly by the direct wave (giving 

 the free space field E ) and partly by the wave which 

 is reflected from the ground. The resultant field is 

 given by the vector sum of the two component fields. 



The magnitude of the field strength of the reflected 

 beam depends upon: 



