72 



TRANSMISSION EXPERIMENTS 



trapping, so that the field strength is greatest when 

 the transmitter or receiver or both are in the duct. 

 In terms of the height-gain functions of equation 

 (27), Chapter 5, it appears that these functions of the 

 lowest mode or modes have a pronounced maximum 

 in the duct and decrease rapidly above it. For S-band 

 transmission there is a transition between the com- 

 plete cutoff, indicated by a highly simplified wave- 

 guide theory, and complete trapping. This inter- 

 mediate effect is caused by some leakage of this wave 

 train from the duct and the retention by the duct of a 

 portion of its wave-guiding properties. The height- 

 gain functions, while still much larger in the duct 

 than in the case of standard propagation, no longer 

 have distinct maxima but show a gradual increase 



with height from the ground. This case is particu- 

 larly interesting because it clearly exemplifies the 

 possible variety of conditions intermediate between 

 trapping, as described by the ray tracing of geo- 

 metrical optics, and the diffraction around the earth's 

 surface characteristic of standard propagation. 



Figure 16 shows two regions with distinctly 

 different slopes in the curves of power versus dis- 

 tance. This probably indicates that two different 

 modes predominate in these two regions. The pattern 

 shown in Figure 16 can occur if for some distance 

 near the ground the height-gain function of the 

 second mode is greater than that of the first mode. 

 The second mode, however, is attenuated more 

 rapidly with distance than the first. At moderate 



20 40 60 80 100 120 140 160 



Figure 16. Signal strength as function of range. S band, Antigua experiments. 



