The influence of gain and current attenuation on the design of the Rhombic antenna

With the increased use of the ionospheric scatter mode of propagation in the VHF range, the horizontal rhombic antenna is employed in many instances because of its simplicity, high performance, and low maintenance costs. Designers of these long rhombics have no doubt realized that the "maximum output" and "maximized" designs described by Harper and others do not take into account current attenuation due to radiation and cannot generally be employed when l/\lambda>8 because of the requirement that the first ground factor maximum should agree closely with the free space vertical pattern maximum. This leads to the conclusion that power gain is the logical basis for design. In order to formulate an expression for power gain, a relation must be established between the radiation resistance with uniform current distribution and radiation resistance with exponential current distribution. The expression derived by Lewin is compared to that derived by Zuhrt. Gain curves are drawn using the formulas, and the "maximum output" and lobe alignment design are compared on the basis of gain with the design based on maximizing the vertical pattern function at the desired angle of radiation. It is found that only under certain conditions does the "maximum output" design have greater gain for the same leg length. The maximum gain condition is discussed together with optimum termination loss, attenuation rates, and surge impedance formulas for multiple wire rhombics. In general, the analysis does not invalidate the design conditions previously derived, but rather increases the emphasis on the general alignment condition and gain and decreases the importance of the "maximum output" and "maximum alignment condition" which were derived on the basis of a constant input current and uniform current distribution.