PARASITIC REFLECTORS AND DIRECTORS 



41 



2. Parasite as a director. Good director perform- 

 ance is obtained when s/X = 0.1 and the parasitic 

 element is cut slightly shorter (perhaps 4 per cent) 

 than V2 to produce a capacitative reactance. See 

 Figure 37, upper row, for the field patterns and 



1.8 

 1.6 

 1.4 



or 1.2 

 Ed/Eq 



1.0 



.8 



.6 



.4 



.2 

 



ER/Eanfenna alone 

 Eo/Eontenna alone 



. Reflector 

 Director 



.1 



.3 .4 



s/X 



Figure 38. Adjustment of parasite for strongest 

 fields Eg and E^. (Courtesy of I. R. E.) 



Figure 38 for the best ratio of E^ to E for the antenna 

 alone. The latter, again, does not give the best 

 front-to-back ratio. 



3.5.3 



Multiple Parasites. 

 Yagi Antennas 



By using several parasites, rather pronounced 

 directive effects can be achieved. Figure 39 shows a 

 typical example. This antenna uses three parasitic 



3 PARASITES 



FIELD 

 PATTERN 



a =.248 \ 

 b=.588X 

 C^335X 



•S-c — «• 



DRIVEN 

 ELEMENT 



Figure 39. Antenna with three parasite elements. 

 (From Radio Engineers' Handbook by Terman.) 



dipoles arranged in a triangle or parabolic curtain. 

 In order to obtain the most favorable pattern in 

 such cases, careful tuning of the parasites is required. 



The most commonlj' used of the multiparasitic 

 arrays of half-wave dipoles is the Yagi antenna 

 (Figiu-e -10) . It has one reflector and several (usually 

 2 to 5) directors. Since the voltage at the center of a 

 dipole is always zero, it is possible to weld all the 



DRIVEN 



REFLECTOR 



ELEMENT 



INPUT 



Figure 40. Yagi antenna with three directors. 



parasites to a central sustaining rod, as shown. By 

 increasing the number of directors, it is possible to 

 oljtain highly directive patterns. 



The spacings bet^^een the elements of a Yagi 

 array are not uniform. They are determined so that 

 the phase difference of the currents in adjacent ele- 

 ments is equal to their distance expressed in wave- 

 lengths. If this condition is fulfilled, the elements 

 are in phase with respect to radiation in the D 

 direction. In practice, the spacing is determined 

 experimentally rather than by calculations, which 

 become very cumbersome when several directors 

 are employed. 



3.5.4 



Reflecting Screens 



A plane-conducting screen placed behind a radiat- 

 ing dipole has a similar effect in the forward direction 

 as an image dipole whose distance s from the primary 

 dipole is twdce that of the screen and W'hich has a 

 phase shift of 180° from the primary dipole. Radia- 

 tion in the backward direction is confined to the 

 weak fields leaking around the edges of the screen. 

 The pattern in the forward direction is given by the 

 array formula of equation (22), and end-fire array 

 with i/- = 180° and s = X/2. Good results are 

 achieved when the distance from the screen to the 

 dipole is small (less than X/4) but larger spacings 

 are also used. The change in input impedance of the 

 primary element caused by the presence of the screen 

 is appreciable. 



Reflecting screens are used primarily in connection 

 with broadside arrays (curtains) to eliminate one 



