350 



BELL SYSTEM TECHNICAL JOURNAL 



For a fixed physical spacing, "a" varies inversely with the wave- 

 length, which results in an increasing steering range with increasing 

 wave-length. Since the critical angle of reflection from the ionosphere 

 increases with wave-length the upper limit of the range of useful 

 angles can be expected to increase also. By selecting the proper 

 spacing, the steering range and the critical angle can be made to agree 

 satisfactorily. Figure 7 shows a plot of 5 = cos~^ (1 — 1/a) against 

 wave-length for the unit antenna spacing of 200 meters which was 



50 



45 



40 



30 



25 



20 



15 



10 



20 25 30 35 40 



WAVELENGTH >v, IN METERS 



45 



50 



55 



60 



Fig. 7 — Measured upper limits of vertical angles as a function of wave-length, 

 compared with the upper limit of the array depicted in Fig. 6. The measured values 

 represent the highest angles observed; usually stronger waves of lower angles pre- 

 dominate. 



adopted for the experimental MUSA system to be described. The 

 points denote upper limits of earth angles obtained from measurements 

 made during the years 1933-1936 on signals from Rugby ^ and 

 Daventry, England. 



The foregoing analysis shows that 



(1) A MUSA system may be so proportioned that the upper limit 

 of its steering range follows, with fair accuracy, the upper limit of the 

 range of useful angles, as the wave-length is varied. 



