PARASITIC REFLECTORS AND DIRECTORS 



39 



(37) 



Again, for five similar dipoles, 



E = Ea {e'° + e-'")* 



= Ea (1 + ie-''" + Qe-'^" + ^e''"' + le"^'^"). 



Here the cophased current magnitudes are 1:4: 

 6:4:1, and there are no side lobes. 



The scheme is to follow the pattern of binomial 

 coefficients in adjusting the relative current values. 



m = 1 11 



2 12 1 



3 13 3 1 



4 14 6 4 1 



5 1 5 10 10 5 1 



Each number is the sum of the two immediatelj' 

 above. 



3.4.9 



Ring Arrays 



A set of radiating elements can be arranged on the 

 perimeter of a circle with equal angular distances and 

 equal phase shift between the elements ; the diameter 

 of the ring must be properly chosen; the resulting 

 radiation pattern can be made very nearly uniform, 

 i.e., circular, in the plane of the ring, while the 



Figure 35. Ring array. 



directivity of the pattern obtained in a plane per- 

 pendicular to that of the ring is increased compared 

 to that of the single element. 



If a number of such rings are stacked on top of each 

 other with a common vertical axis, a linear array is 

 formed whose elements are the rings. A radiation 

 pattern is thus produced that has pronounced direc- 



tivity in elevation while it is nearly uniform in 

 azimuth. This device is frequently used in micro- 

 wave beacons. 



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. 



3.5 



3.5.1 



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. 



3.5.2 



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 {6 = 90°) is equal to 



E„ = 



607o 



JO 



+ 



607i 



,j[?-{2jr/X}sco8 4 



(38) 



where h is the center-fed input current to the an- 

 tenna, Ji is the center value of the current in the 

 parasite, and /3 is the angle by which /i leads Iq. 

 The relation between /i and Jo is given by 



h = h 



J^ 



\Zn 



(39) 



where Zoi is the vector mutual impedance of antenna 

 and parasite, and Zn, the vector self-impedance of 

 the parasitic antenna, is equal to iSn -f jXn = 

 1 Zii I e^*. 5 is the phase angle of the parasite self- 

 impedance. 



