40 



ANTENNAS 



The field pattern in^the equatorial plane is de- 

 pendent directly on the spacing and indirectly also, 

 since the spacing controls the mutual impedance and 

 thus the voltage induced in',the parasite. The current 



ohms; if longer, the inductive reactance is increased; 

 if shorter, it becomes at first less inductive, then 

 resonant with A'u = 0, and finally, capacitatively 

 reactive. Field patterns for s/X = 0.1 and s/\ = 0.25, 



Power 



cfi-7r 



Director Direction 



Reflector Diredion 



Elevation 



Power 



^ — ► Power 



t E 



9 



Figure 36. Half-wave dipole and parasite. 



in the parasite is further dependent on its self- 

 impedance, which can be changed by altering the 

 length of the parasite. Cut to a length of just X/2, 

 the self-impedance is inductive, Zu = 73.1 + J42.5 



"■2 



go 

 op 

 •^.o a. 

 -luj "■ 



a; o 





EQUATORIAL PLANE 8=90 



Figure 37. Relative field of half-wave antenna and 

 parasite. (Courtesy of I. R. E.) 



and for 5 = -h22.5°, 0°, and -22.5°, are plotted 

 in Figure 37. These illustrate that, by controlling 

 the spacing and length of parasite, it is possible to 

 direct the pattern maxinumi into either the R or D 

 direction, so that the parasite acts primarily either 

 as a reflector (Er > Ed) or as a director (Eo > Er). 

 1. Parasite as a reflector. For good reflector per- 

 formance, the spacing s/X should lie between 0.15 

 and 0.25 with the parasitic element made slightl}^ 

 longer (perhaps 5 per cent) than \/2 in order to 

 increase its inductive reactance. A few of the 

 equatorial field patterns are shown in the lower row 

 of Figure 37. To obtain the strongest field in the 

 R direction, it is necessary to lengthen the parasite 

 to a particular length (obtained liy trial). If this is 

 done, Figure 38 indicates that the field Er is a 

 maximum for s/X = 0.15 and that the ratio of 

 Er to E for the antenna alone is 1.83; for s/X = 0.25 

 it is 1.65. This does not, however, give the best 

 front-to-back ratio. 



