DEVELOPMENTS IN SHORT-WAVE DIRECTIVE ANTENNAS 671 



end remote from the receiver. A simple example is illustrated in 

 Fig. 11. 



The end of the inverted V remote from the receiver R, m Fig. 11, is 

 so terminated as to absorb signals without reflections. In other words, 

 a termination equal to the antenna characteristic impedance is em- 

 ployed. Only the vectors for one leg of each of the inverted V's have 

 been drawn, as the second leg is simply a reproduction of the first, and 

 add directly thereto, after all phase relations have been determined. 



In Fig. 1 1-A, a wave from the right produces elementary load currents 

 which trace a semicircle, as previously discussed. Note that when the 

 wave arrives from the left as in Fig. U-B, the phase change is more 

 rapid and a closed circle is traced making the resultant zero, thus we 

 have achieved an infinite front-to-back ratio. It can be shown that 

 this advantageous condition exists for tilted wires where the wire length 

 of each element is an odd integral multiple, greater than one, of one- 

 quarter wave-length, provided that the previously mentioned optimum 

 tilt, in relation to the wave direction, is maintained. 



At first glance, it might appear that the frequency range is restricted, 

 since the above rule is limited to certain wire lengths expressed in 

 wave-lengths. The most disadvantageous case exists when the wire 

 length is an even integral multiple, greater than two, of one-quarter 

 wave-length. Fig. 12 illustrates one such case, the wire being one 



INDUCED 

 VOLTAGE 



CURRENT AT 



R-DIRECT 

 PROPAGATION 



CURRENT AT 



R VIA 

 TERMINATION 



r=Zc 



12 3 4 5 



RESULTANT CURRENT 



^ V f ^ ^l 



ZERO WHEN r = Zo 



r=Zo 



12 3 4 5 



RESULTANT CURRENT 



-. ♦ 





ZERO WHEN r = Zo 



Fig. 12 — Vector relations in an inverted V antenna— asymmetrical directivity. 



