360 PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
The most common form of the ring antenna is the 
triple dipole shown in Figure 35. The elements are 
three half-wave dipoles spaced 120 degrees apart. 
PARASITIC REFLECTORS 
AND DIRECTORS 
Parasitic Antennas 
A parasitic antenna or “dummy” is an antenna 
which is not connected to the antenna input termi- 
nals; if placed in the vicinity of a driven antenna a 
current is induced in the former which modifies the 
radiation field. Parasitic antennas provide a simple 
means of producing a moderate increase of directiv- 
ity. Depending on the relative phase of the currents 
in the two antennas, the maximum of the radiation 
pattern either is found in the direction of the parasite 
and the latter is then called a director; or it is found 
in the direction of the primary element and the para- 
site is then called a reflector. In order to obtain good 
directive action the two dipoles must be close to- 
gether, that is, a fraction of a wavelength. 
Half-Wave Dipole and Parasite 
The geometrical arrangement corresponding to 
the following discussion is illustrated in Figure 36. 
The radiation field at any point in the equatorial 
plane (@ = 90°) is equal to 
Eg = oreo + un ei!B- (2=/A) 8 cos 4] ; (38) 
Plan View 
(2) = 77/2 
per I, 
= 
Director Direction | 
uous. 
=o 
Se 
fo 
aw 
where J is the center-fed input current to the an- 
tenna, J; is the center value of the current in the 
parasite, and # is the angle by which J, leads Ip. 
The relation between J; and Jp is given by 
| Zor | 38 
‘ 39 
(Zu | (39) 
1=1h 
where Zo, is the vector mutual impedance of antenna 
and parasite, and Zi, the vector self-impedance of 
the parasitic antenna, is equal to Ri +jXu = 
| Z1| e*. 65 is the phase angle of the parasite self- 
impedance. 
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 
in the parasite is further dependent on its self- 
impedance, which can be changed by altering the 
length of the parasite. Cut toa length of just 4/2, 
the self-impedance is inductive, Z1 = 73.1 + j42.5 
ohms; if longer, the inductive reactance is increased; 
if shorter, it becomes at first less inductive, then 
resonant with Xy, = 0, and finally, capacitatively 
reactive. Field patterns for s/A = 0.1 and s/A = 0.25, 
and for 6 = +22.5°, 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 maximum into either the R or D 
direction, so that the parasite acts primarily either 
as a reflector (Ez > Ep) or as a director (Ep > Ep). 
1. Parasite as a reflector. For good reflector per- 
formance. the spacing s/A should lie between 0.15 
Power 
p=0 
a 
Reflector Direction 
Antenna 
Transmitting i 
o 
Elevation 
Power 
FicurE 36. Half-wave dipole and parasite. 
