Directed Radiation

Author: J.B. Hoag

A transmitting antenna should be able not only to radiate energy, but also to direct that energy into the areas where it is to be received.

Fig. 8 E. Radiation patterns from vertical Marconi antennas

Figure 8 E shows typical field patterns from a Marconi antenna. Arrows drawn from the origin to the curves will have lengths proportionate to the field intensity emitted in their directions. The half-wave antenna gives low-angle radiation while the full-wave antenna gives high-angle radiation.

Fig. 8 F. Top view of radiations from horizontal Hertz antennas

Fig. 8 G. High- and low-angle radiations from vertical half-wave Hertz antennas one-half and one wave-length above the ground

Figure 8 F shows the field intensities established in different directions around an isolated Hertz antenna of different lengths, while Fig. 8 G shows the effect of the ground on a half-wave antenna of this type.

Many combinations of the simple antennas just described have been used to increase the directive effect. An example of a directive array is shown in Fig. 8 H.

Fig. 8 H. A half-wave broadside array with strong uni-directional radiation

The half-wave radiators are properly spaced and are so connected that the current flows through the different wires in proper order to give the uni-directional characteristic shown.

For the very high frequencies (u.h.f. and microwaves), parabolas may be used to focus the waves along a beam, like light rays from an automobile headlamp.