Horizontal Rhombic Antenna

Author: Edmund A. Laport

The commonest practical form of high-frequency antenna using the traveling-wave principle is the horizontal rhombic antenna, constructed as shown in Fig. 3.77. It is widely used in high-frequency applications for both transmitting and receiving. It has some marked advantages and disadvantages.

The advantages include simple construction, low-cost supporting structures, low cost of material, relatively high gain for the cost, broad frequency response from the impedance standpoint, minimum antenna potentials and currents for the power transmitted, inconspicuousness, easy maintenance and repair, almost no field adjusting required after installation, and the ease with which the height can be changed to obtain the optimum vertical angle as layer heights change through the sunspot cycle.

The disadvantages include large amount of land required; loss of power in the terminating load; a multiplicity of lobes of radiation in almost all directions, in addition to some rather large secondary lobes under the best conditions of design; compromises necessary from a propagation standpoint as the radiation pattern changes with frequency; limitation of gain and signal-to-noise ratio due to the multiplicity of radiation lobes; difficulty of predetermining its complete performance due to the complications of computation and to the effects of attenuation and partial standing waves always present in practice; and inability to control the horizontal and vertical patterns separately.

For optimum performance, a rhombic antenna should be designed for use at one frequency or a very small band of frequencies, the pattern for which is best suited to the propagation conditions of the space circuit.

FIG. 3.77. Horizontal rhombic antenna (common three-wire form).

Usually about all that a designer attempts to compute about this system is the characteristic of the main lobe. The enormous labor of computation has obscured its complete radiation characteristics.

There has been widespread confusion between the radiation performance of the horizontal rhombic and its circuitry. The input impedance may be uniform over a frequency range of 8 to 1 but its radiation characteristics are seldom satisfactory over more than 2 to 1 range.

The patterns shown in Figs. 3.78 and 3.79 give an idea of the complete radiation patterns for typical rhombic antennas having parameters near the optimum for a frequency within a band of 2.25 to 1. The conditions are for a fixed structure as the frequency is changed over this range.

FIG. 3.78. Directive patterns for a rhombic antenna with an apex angle A = 22 degrees: (a) l = 3.33λ, h = 0.8λ; (6) I = 5.0λ, h = 1.2λ; (c) / = 7.5λ, h = 1.8λ. (After Christiansen.}

FIG. 3.79. Directive patterns for rhombic antenna same as Fig. 3.78 except that apex angle A = 18 degrees. (After Christiansen.)

The outer periphery of each chart is the horizon of a flat earth, and the center is the zenith. The contours are in 3-decibel steps, the smallest shown being only 18 decibels below the amplitude of the main beam. There are many others below this value in the forward half of the hemisphere and also in the rear half, which is not shown. Imperfect construction undoubtedly accentuates these spurious lobes beyond the relatively ideal theoretical conditions represented in these figures. Christiansen has described methods by which these patterns can be improved, using tiered, broadsided and overlaid rhombic elements.

FIG. 3.80. Horizontal rhombic antenna with feeders arranged for reversing pattern.