Rhombic Antenna Arrays

Author: Edmund A. Laport

Some of the basic deficiencies of the rhombic antenna are correctable by more complicated arrays of rhombic elements. For example, a two-layer system on the same supports, one above the other, can do a great deal to suppress the higher angle minor lobes, as shown in Fig. 3.88. Such a pattern has relatively desirable vertical-plane characteristics for the angular discrimination against multi-path propagation on some circuits. The horizontal pattern may be undesirable at the higher frequencies, not so much because of the spread of its several forward radiation lobes, but because a signal swinging in azimuth may swing into and past the quasi nulls between the lobes.

When azimuthal swing of signals is a disturbing factor, two such arrays can be used in diversity, by turning the axis of the second array off the great-circle bearing by an angle equal to that between the first null and the peak of the main beam. When two such antennas are spaced as in typical space-diversity operation and oriented in this manner, the maxi-mums of one rhombic pattern will fill the minimums of the other. To obtain this benefit, each rhombic must be associated with its own receiver and their outputs combined in diversity. This form of utilization is obviously a special application to avoid the effects of arriving signals that deviate from the great-circle bearing by an amount which exceeds half the beam width of the main lobe.

Figure 3.89 shows the construction for a two-layer rhombic system having two wires per side in each layer. Figures 3.90 and 3.91 show what is possible by using arrays of these two-layer rhombic systems.

FIG. 3.88. Two-layer rhombic antenna patterns of following geometry (After Christiansen):

FIG. 3.89. Construction for two-layer rhombic antenna.

FIG. 3.90. Directive patterns for two two-layer rhombic antennas of the type of Fig. 3.88, with apex spacing as follows: (a) apex spacing 1.33; (6) apex spacing 2.0; (c) apex spacing 3.0. (After Ckristiansen.)

FIG. 3.91. Radiation patterns for two arrays of the type of Fig. 3.90 in broadside but with major axis spacings as follows: (a) 2.33λ; (6) 3.50λ; (c) 5.25λ. (After Christiansen.)

These figures, together with Figs. 3.78 and 3.88, represent optimum-design parameters for a frequency range of 2.25 to 1, based on idealized conditions. Comparison measurements on rhombic antennas of the types having the patterns of Figs. 3.78 and 3.88 are listed in Table 3.3, as published by Christiansen.

1	Power lost in terminal resistance for unity power input to antenna system.