34 



ANTENNAS 



■spacings in different directions may be different fo)' 

 two- or three-dimensional arraj's. The use of arra5^s 

 permits groat increases in the amount of power 

 radiated, in directivity, and gain. 



Although the most common array element is a 

 half-wave dipole, the elements of an array may be 

 radiators of any type; in particular, the elements 

 may themselves Ije arrays. In this way it is possible 

 to interpret a two-dimensional array as an array of 

 arrays. A vertical curtain may be considered either 

 as a horizontal array of elements which, themselves, 

 are vertical, or it may be considered a vertical arra.y 

 of elements which, themselves, are hoiizontal arrays; 

 similarly for three-dimensional arrays. 



In most arrays the elements radiate \-ery nearly 

 equal power, but in the binomial array the elements, 

 although identical in structure, differ in the amount 

 of power radiated because of differing current dis- 

 tributions. In most arrays there is a constant phase 

 shift (which might be zero) between adjacent ele- 

 ments. B}' suitable phasing a great variety of an- 

 tenna patterns can l)e produced. 



3.4.2 



Basic Types of Dipole Arrays 



There are three basic types of dipole arrays. 



1. Broadside array. The centers of the elements 

 are arranged in a line, with the axes of the elements 

 parallel to each and perpendicular to the line. With 

 the currents adjusted all in phase, the maximum 

 radiation is broadside to the plane of the elements. 



2. End-fire array. The geometric arrangement is 

 the same as in the broadside array, but through 

 appropriate phasing of the currents in the elements 

 the maximum radiation can be directed primarily 

 along the line joining the centers. 



3. Colinear array. Here the axes of the antenna 

 elements are arranged along the line of centers with 

 the currents all in phase. The radiation is a max- 

 imum in the equatorial plane perpendicular to the 

 line of centers. 



To illustrate the principles most simply, two half- 

 wave dipole elements are considered first, and later 

 extension is made to arrays composed of a larger 

 number of elements. 



3.4.3 



Two-Dipole Side-by- Side Array 



Two half-wave dipoles are placed side by side with 

 spacing .s and the currents 7i and Ii are equal but 

 differ in phase by angle lA (see Figure 25). If 1% lags 



/i by time angle ^, the field of the second element 

 at /-" lags that of the first by angle a where a. is 



EQUATORIAL P\-f<H^,B = W'^ 

 Figure 25. Two dipole side-by-side array. 



compo.sed of ^ and the time delay caused by the 

 extra distance traveled, (2Tr/X)s cos ^ sin 5, 



a =^p+ (2Tr/X) s COS 4> sin d. (21) 



For equal currents, h = h — I, the field is equal 



to 



E, = 



60/ 



e'" + e- 



cos 



(fcos.) 



sin d 



cos 



(^cose) 



sin I 



(22) 



The first bracket gives the directional characteristic 

 of an array of two elements, while the second bracket 

 gives, the directional characteristic of the^ element 



itself. 



00 



UNI-DIRECTIONAL 



t'scdj LAGS r, ) 



BROADSIDE 

 S=X/2 

 ^= 0° 



END-FIRE 

 s=X/2 

 i/( = l80° 



EQUATORIAL PLANE 9-90° 



Figure 26. Radiation patterns (field strength) for two 

 dipole side-by-side array. 



Three special cases are particularly to be noted. 

 The field patterns for the equatorial plane {d = 90°) 

 and 1 /, I = 1 /i I are plotted in Figure 26. 



