206 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



ALTITUDE 

 FEET 

 7000 



-25 



25 50 75 



50 75 



•25 O 



25 50 75 

 D 



z. 



25 50 75 



E 



™ "100 

 BLIND ZONE.f^ 



NAUTICAL MILES 30~~ 



J I I ZONE OF DETECTION 40""- 



200 MC TRANSMITTER ELEVATION 100 FEET 



Figure 24. Calculated coverage diagram. 



mately 20. The height scale is exaggerated in the 

 ratio 40/1. Transmission over sea water is assumed. 

 The coverage range is adjusted to "define the 

 probable low-level zone of detection of a medium 

 bomber with fair aspect by an SC-1 or SC-2 radar 

 at 100-ft elevation. For SK radars and higher alti- 



tude installations, the diagrams are conservative. 

 For SC and SA radars or for lower altitude installa- 

 tions, they are optimistic." 



Figure 24A shows the lobe structure for the 

 standard atmosphere in which M increases 36 MU 

 per 1,000 ft. It also shows the value of M — Mi 0a ; 

 that is, the M curve is drawn so as to pass through 

 zero at the transmitter elevation of 100 ft. On 

 diagrams B through E the lower portion of the 

 standard lower lobe is indicated by a dash-dot line. 

 The blind zones are cross-hatched, and their boun- 

 daries represent the calculated limits of detection. 

 An interesting feature of these diagrams is the 

 appearance, in some cases, of blind zones of consider- 

 able range and altitude along the surface. These 

 cause "skip ranges" for ground targets that are 

 significant in operational problems. Ray diagrams 

 were used in calculating the field strengths in 

 Figure 24. 



The relative heights of the transmitter and the 

 duct have an important bearing on the mechanism 

 of transmission. The duct may develop entirely 

 below the transmitter site or entirely above, or the 

 duct may include the transmitter. With these alter- 

 natives a variety of propagation conditions is 

 possible. 



One of the important concepts of radiation theory 

 is contained in the principle of reciprocity. This 

 principle states that when a transmitter is at a point 

 in space A, and the receiver at a point B, the 

 received intensity is the same when they are inter- 

 changed, the transmitter being at B and the receiver 

 at A. (It is assumed in making this statement that 

 the transmitter and receiver may be regarded as 

 point sources.) Similarly, for radar the signal inten- 

 sity remains unaltered if the positions of radar and 

 target are interchanged. It is known that there are 

 serious limitations to the reciprocity principle where 

 ionospheric reflections are involved, but for shorter 

 waves and tropospheric propagation the principle 

 may be applied without restriction. By means of 

 the reciprocity principle any coverage diagram may 

 be used to obtain the field strength when the heights 

 of the target and the radar are interchanged. 



From a study of such evidence on coverage 

 diagrams as is available, it appears that (a) the 

 effects of superrefraction are most marked when the 

 transmitter lies in the duct; (b) they exist to a lesser 

 degree if the transmitter lies below the duct: in 

 particular no excessively long ranges for targets are 

 then found above the duct — sometimes the ranges 



