Directive Antennas Using Unequal-Height Radiators

Author: Edmund A. Laport

A directive-antenna system using two or more radiators of identical physical dimensions, and therefore having intrinsically equal, or near-equal, self-impedances, is computed on the basis that the field strength contributed from each radiator is proportional to the radiator current. Therefore we can talk about radiation patterns directly in terms of radiator currents. When a directive array includes radiators of unequal heights and other dissimilar physical dimensions, it is impossible to use a direct proportionality between radiator currents and their contributing fields. The effective field strength of each radiator must be taken into account in computing a pattern. The use of unequal radiators greatly increases the engineering complications of a design.

In an array of dissimilar radiators, the current ratios for equal horizontal field strength must be computed first. When the desired pattern has been determined, the field contributions from each radiator may be ascertained. The current ratios for the different radiators in the system are then derived. The different self-impedances are then entered into the network equations with their associated currents and mutual impedances to determine the input impedance and the power input to each radiator. After this the synthesis of the phasing, impedance-matching, and power-dividing networks can be carried out, taking into account the arrangement of the feeders.

The computation of the vertical radiation patterns for the array in various directions is also complicated by the fact that the vertical pattern for each radiator of different physical dimensions will be different.

Mutual impedances between vertical radiators of unequal heights, over the range of values ordinarily encountered in broadcast arrays, have been published.

Precautions must be taken to avoid excessive ground-terminal losses in any array when the input resistance to any radiator is driven down to a very low value by virtue of mutual impedances - especially if a substantial amount of power is to be put into such a radiator. In a system of unequal-height radiators this effect has to be more carefully watched because of a lower self-resistance of a short radiator and the probable need for a higher current in it to produce the desired effective field strength.

In general, very short radiators in a system of higher radiators are justified only where a radiator is required to make only a small contribution to the pattern and where input power and base current will be relatively small. Where unequal radiators are used, there are usually no more than two different radiator sizes. The purpose of such a practice is to avoid the expense of a full-size radiator when only a small portion of the total power is to be contributed by it. Occasionally, the reason is that some existing towers are employed together with one or more new radiators. Dissimilarity is also introduced when a television or other very-high-frequency antenna is mounted on top of one radiator in an otherwise uniform system of identical radiators.