ANTENNAS 361 
and 0.25 with the parasitic element made slightly 
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 
ANTENNA 
s 
4 
PARASITE 
REFLECTOR 
DIRECTION 
R 
§ =-22.5° § =0° 8 =+22.5° 
CAPACITIVE RESONANT INDUCTIVE 
a& 
a5 
(2) 
Fac 
oa 
s S. 
=O) S=0,25 
$= 0.25 - 
EQUATORIAL PLANE 9 =90° 
Fiaure 37. Relative field of half-wave antenna and 
parasite. (Courtesy of I. R. E.) 
RF direction, it is necessary to lengthen the parasite 
to a particular length (obtained by trial). If this is 
done, Figure 38 indicates that the field Ep is a 
maximum for s/A = 0.15 and that the ratio of 
Ep to E for the antenna alone is 1.83; for s/\ = 0.25 
it is 1.65. This does not, however, give the best 
front-to-back ratio. 
2. Parasite as a director. Good director perform- 
ance is obtained when s/A = 0.1 and the parasitic 
element is cut slightly shorter (perhaps 4 per cent) 
than \/2 to produce a capacitative reactance. See 
Figure 37, upper row, for the field patterns and 
Figure 38 for the best ratio of Hp to E for the antenna 
alone. The latter, again, does not give the best 
front-to-back ratio. 
Multiple Parasites. 
Yagi Antennas 
By using several parasites, rather pronounced 
Er/Eantenna. alone 
Ep/Eantenna alone 
Reflector 
—o=-= Director 
Ficure 38. Adjustment of parasite for strongest 
fields Hp and Ep. (Courtesy of T. R. E.) 
3 PARASITES FIELD 
PATTERN 
—- 
o A) b 
Q=.248i ec ho ae 
b=.588\ t 
C=,535h DRIVEN 
ELEMENT 
Fiaure 39. Antenna with three parasite elements. 
(From Radio Engineers’ Handbook by Terman.) 
directive effects can be achieved. Figure 39 shows a 
typical example. This antenna uses three parasitic 
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 commonly used of the multiparasitic 
arrays of half-wave dipoles is the Yagi antenna 
(Figure 40). 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 
D DIRECTION 
? ZA 
oO 
ge 
eS 
> 
DRIVEN ——9/e—— ELEMENT 
i 
REFLECTOR |, | 
itt INPUT 
Ficure 40. Yagi antenna with three directors. 
parasites to a central sustaining rod, as shown. By 
increasing the number of directors, it is possible to 
obtain highly directive patterns. 
The spacings between 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. 
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 twice that of the screen and which 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 y = 180° and s=/2. Good results are 
achieved when the distance from the screen to the 
dipole is small (less than 4/4) but larger spacings 
-are also used. The change in input impedance of the 
