Beam Slewing for Broadside Arrays

Author: Edmund A. Laport

Beam slewing is a practice often used where one array can serve for two azimuths having a small angular difference. The main beam can be set off of the normal a few degrees by giving a small phase difference to the currents in the two halves of the array. The method for doing this is indicated in Fig. 3.49A, where a phase lag is introduced into the right-hand half by having an extra length of matched feeder on that side.

The horizontal pattern for a slewed unidirectional beam from a horizontal array of four half-wave spaced elements follows the relation

The appearance of the constant phase-difference angle φ in the last cosine factor of this equation accounts for the tilting, or slewing.

FIG. 3.45. Radiation pattern for six cophased dipoles with reflecting screen.

It can be seen from this factor that its maximum will always be to one side or the other of the normal to the array when φ is other than zero. It is also evident that a null is going to appear on the off side of a slewed beam, which, when φ becomes sufficiently large, causes a split in the pattern. It is this split that sets a practical limit to effective slewing, both from loss of gain on the main beam and the growth of the secondary beam to objectionaable size.

The most effective form of beam slewing is to introduce an equal phase difference in the current of each dipole in succession. The more uniformly the phase difference is distributed across the array, the greater is the slewing angle before the beam splits.

Fig. 3.47 exemplifies two values of beam slewing for an array of four dipoles in line.

FIG. 3.46. Radiation patterns for horizontal row of vertical dipoles with unit current, cophased. (From RAF Signal Manual.)