ment measuring the zero order and first two harmonics will show a = a 2 = a 2 = 1, 

 b x = b 2 — 0. Recombining these, 



R(d) = 1 + cos0 + cos 20. 



(7) 



This is plotted in Fig. 3, and is the exact equivalent of the directional sensitivity pattern 

 of a radio aerial or sound receiver. It shows a lobe covering approximately ±60°, 

 and the accuracy of detecting mean direction and beam width of the waves is deter- 

 mined by this lobe. A beam width which is narrow compared to this lobe will be 

 difficult to distinguish from a unidirectional beam. 



Figure 4. The directional sensitivity pattern of a device measuring the zero order and first six angular 

 harmonics of the wave spectrum. The equation of the curve is r = 1 + cos + cos Id + ... 

 + cos 60. 



52 



