ANTENNA ARRAYS 



33 



metry, as in Figure 18. In this case the radiation is 

 horizontally polarized. It can also be used as an 

 inverted \ in a vertical plane with the point of the 

 V directed upwards; the radiation is then vertically 

 polarized. This antenna, also called a semi-rhomlaic, 

 is represented bj'' the upper half of Figure 20. 



in 







The field in the axial direction is equal to 



Q.^ 



2 4 6 



L/X 



8 10 12 



Figure 22. Angles for major lobes for traveling-wave 

 antenna. 



3.3.3 



Rhombic Antenna 



This type of antenna is based on the same prin- 

 ciple as the traveling-wave V antenna. The rhombic 

 antenna consists of four wires arranged in the form 

 of a rhomboid or diamond (Figure 23). The reflec- 

 tionless termination of the wires is achieved by 

 connecting the two \\ires at the end opposite the 

 input to a resistance equal to their characteristic 

 impedance. 



CHARACTERISTIC 

 IMPEDANCE 



-AXIS 



Figure 23. Rhombic antenna. 



As in the case of the V antenna, the I'hombic 

 antenna can be used both horizontally and vertically ; 

 at the longer waves the horizontal arrangement is 

 usually more practical. The optimum tilt angle of 

 the rhombic (angle (j) oi Figure 23) is not very 

 critical provided the legs are not less than two wave- 

 lengths long. The radiation pattern is not veiy 

 sensitive to frequency and the rhombic antenna can 

 therefore be used over a fairly mde frequency range 

 (of the order of 2 to 1). Rhombic antennas have 

 appreciablj^ higher gains than V antennas. 



-^ axial 



2407i cos (j) 



1 — sin </) 



■sm- 



X 



(1 — sin (p) 



(20) 



Effect of Perfectly Conducting Ground. If the 

 rhombic is placed in a horizontal plane, height H 

 above ground, the effect of the image rhombic 

 must also be considered. The net residt is that the 

 direction of the resultant lobe maximum is tilted 

 up by an angle e. It can be shown that, for a given 

 angle e and wa^'elength X, to point the major lobe 

 at vertical angle e the following relations for H, 

 L, and (p must hold : 



X 



H = 



L = 

 4> = 



4 sine 



0.371X 



■ , ' 

 sm- e 



90° - 



Figure 24 iUustrates the radiation pattern (relative 

 field strength) for a particular case. 



ANGLE IN DEGREES 

 90 70 50 40 30 20 



ANGLE IN DEGREES 

 90 70 60 50 40 



PLAN VIEW 



L = 4.1 X 

 H = 0.e3X 

 ■^ = 7 2.5° 

 < =17.5° 



Figure 24. Rhombic antenna above ground (relative 

 field strength). (Courtesy of Bell System Technical 

 Journal.) 



24 ANTENNA ARRAYS 



3.4.1 Principle of Arrays 



An antenna array is a combination of several 

 antennas, usually of equal strength and equally 

 spaced in any one given direction. One-, two-, and 

 three-dimensional arrays may be distinguished. The 



