Feeding Power to Dipole Arrays Using Half-wave Spacings

Author: Edmund A. Laport

The attainment of a desired radiation pattern depends upon the realization of certain specified spatial current distributions. The radiators are located in a prescribed geometrical arrangement and have current amplitudes and phases that are derived from the design of the radiation pattern. The circuital design of a radiating system starts with a study of ways and means to obtain the prescribed currents in the various radiators of the entire system and to bring the correct impedance at the main feed point to terminate the power transmission line. The other aspects of the circuital design for a system include a determination of the potentials and currents which will exist when the system is energized at specified power input, and the specifications for the conductors and the insulators are based on these.

Feed-system techniques are as varied as the types of systems that may be used. Considerable inventive ingenuity may be required to design a feed system for one of the more complicated arrays. The simplest feed method occurs in systems where several identical half-wave radiators with equal cophased currents are used in a symmetrical array with half-wave spacing. The properties of a half-wave section of feeder have special utility in such applications, tending to equalize small irregularities in impedances and currents among the various elements. Large beam arrays of the type shown in Fig. 3.100 are constructed for operation on a specified frequency by cutting the radiators to a length slightly less than one-half the free-space wavelength to account for end effects and end capacitance due to insulators and attachments, the feeders being cut to one-half wavelength. If the dipoles are to be end-fed from standing waves on the feeder, the dipoles are located at potential-maximum points on the feeder. Balance of the feeder is maintained by attaching dipoles on opposite sides, which makes a pair of dipoles thus fed have equal cophased currents. The next pair of dipoles is located one-half wavelength along the feeder, where equal potentials, of opposite phase, are located. To cophase this second pair of dipoles with the first, the feeder is transposed 180 degrees. In this way a vertical stack of dipole pairs can be fed. If the number of pairs of such dipoles is very large, the attenuation in the feeder, due to its distributed loading, begins to be a factor to consider, as it will cause the outermost dipoles to have lower current amplitudes than those nearest the source. This effect is usually negligible until the number of pairs of radiators exceeds four. Beyond this number, it is desirable to bring the main power feeder up to the middle of the dipole feeder and branch symmetrically from this point.

In order to transpose a balanced feeder of the two-wire type, it is necessary to use insulators to maintain constant spacing through the transposition. The presence of these insulators can reduce the velocity of propagation in the feeder and change its electrical length. If the velocity reduction is more than 2 or 3 percent. it may be necessary to reduce the spacing of the dipole pairs along the feeder by an equivalent amount or there will be a cumulative phase discrepancy in the currents in the successive dipole pairs.

The feeding of collinear dipoles from one end, as in Fig. 3.38, requires the employment of means to connect them in series through quarter-wavelength balanced feeder stubs to cophase and equalize currents in the dipoles not directly attached to the main feeder. Figure 3.49B is a schematic example of a 6 by 4 array with one central feeder, using series excitation in the three dipoles each side of the center.

Nearly uniform currents can be obtained throughout an array of the larger sizes by the method of Fig. 3.49A, where each vertical feeder has but one pair of dipoles attached at each level. The main power feeder is branched into the desired number of vertical feeders, taking the necessary precautions to excite the resulting sections of the array uniformly in current and phase. However, if there is a phase difference between the two sections of an array of the type of Fig. 3.49A, the main beam is deflected away from the broadside position, one way or the other, depending upon the polarity of the phase difference. Use is made of this fact to slew a beam a few degrees.

The feed-point impedance of a pair of end-fed radiators will depend upon the characteristic impedance of the conductors comprising the dipoles, upon their electrical exact length, and upon the radiation couplings to all other dipoles, ground images, and reflector images. The impedance of a pair will change with any change in its position in the system with respect to other dipoles. The dominant element of mutual impedance from other dipoles will of course be from the nearest parallel pair(s) or from the ground images for very low antennas. The nearest collinear pair has a mutual impedance that is relatively small compared with that from the parallel pair. Other echelon pairs will have intermediate effects.

Mutual impedances decrease as distance between dipoles increases. In arrays requiring systematic current distributions it is necessary to compute as accurately as possible, in advance, the impedance at every feed point and to provide the proper excitation at each such point to realize the desired performance.

A broadside array of the horizontal half-wave-dipole type with half-wave spacings is often built to use a second identical curtain of dipoles in a close-spaced parallel plane to obtain reinforcement of fields on one side and partial suppression on the other. One curtain is energized by feeder and the other energized parasitically from the field of the first. A short circuit is placed at the proper point in the main feeder for the reflector curtain to obtain optimum unidirectivity. The radiator and reflector curtains can be interchanged by switching to reverse the direction of the main beam. Unidirectivity can also be obtained, and much more effectively, by using an untuned, passive reflector.