198 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



shown that two points at elevations hi and /i 2 are 

 within sight of each other when their distance is less 



STANDARD ATMOSPHERE 



CURVED EARTH 

 RADIUS a 



NONSTANDARD ATMOSPHERE 



CURVED EARTH 



RADIUS ka 



(HOMOGENEOUS 



ATMOSPHERE) 



1 



/PLANE EARTH 



/(MODIFIED INDEX 



CURVES) 



INVERSION 

 LAYER v 



DUCT 



.1 



M M 



Figure 15. Types of index curves. 



than the horizon distance d n (Figure 16) given by 

 d h = V2kah 1 + \/2kah 2 , (13) 



where d h , a, and h are all expressed in the same units. 

 For the particular value of k=%, 



d h = Vlfhl + VTt/T, , (14) 



where d„ is measured in kilometers and h is in meters; 

 and 



d h = V2h[ + y/Wz , (15) 



where d n is given in statute miles and h in feet. 



The field strength at different elevations h (Figure 

 16) for a given range varies in the manner illustrated 

 in Figure 17. The field is given in decibels, relative to 

 the intensity at 1 m from the transmitter, for a range 

 of 50 miles over sea water for frequencies of 100, 200, 

 500, and 3,000 mc. The horizon elevation for this 

 point is 888 ft. Above point P in Figure 16, is the 

 interference region where, with increasing height, the 



INTERFERENCE 

 REGION 



field strength first increases rapidly and then oscil- 

 lates between maxima and minima determined by 

 the lobe patterns of the coverage diagrams. 



Below point P, the field strength declines rapidly 



-140 



DB 



-120 



Figure 16. Horizon distance. 



Figure 17. Diffraction and interference fields at height 

 h 2 . Field strength at 50 statute miles over sea water in 

 db relative to field at 1 m from transmitter. Horizontal 

 polarization. Transmitter height 30 feet. 



with decreasing height to a minimum at ground 

 level; the rate of decrease is larger for the higher fre- 

 quencies. Neither the direct nor the reflected rays 

 can penetrate into this region, which therefore, re- 

 ceives radiation entirely by diffraction of the energy 

 around the earth's curvature. 



Radar targets are rarely visible when they are in 

 the diffraction region. This is certainly true for air- 

 plane targets. Very large targets, such as warships or 

 islands, are occasionally visible in this region; but 

 more often the detection of targets is caused by 

 superrefraction. For communication work, on the 

 other hand, the diffraction region is of importance, 

 especially at the longer wavelengths. 



172 ATMOSPHERIC STRATIFICATION 

 AND REFRACTION 



17.2.1 Q r igj n f Refractive Index Variations 



The variation with height of the index of refraction 

 n controls the curvature of rays in the atmosphere. 

 The value of n exceeds unity by only a few hundred 



