Horizontal Dipole with Parasitic Elements

Author: Edmund A. Laport

This type of array is composed of two parallel horizontal dipoles, one of which is fed and the other is self-resonant and excited by the field of the radiator.

FIG. 3.32. Relative field strength from a half-wavelength dipole parallel to and one-quarter wavelength in front of a sheet reflector 0.85X square. (Data by Carter.)

FIG. 3.33. Horizontal dipole with passive screen reflector for unidirectional radiation.

Maximum radiation takes place in the direction of the director. When the spacing d is 36 electrical degrees and the height above ground h is about 0.65 wavelength (height at which the mutual impedance with the images is resistive only), the currents in both elements are very nearly equal and the vertical pattern is

With changes in height it is possible to make small adjustments to maintain a maximum field strength on the beam. A gain of 4 to 6 decibels is available in the arrangement shown in Fig. 3.34.

Using rigid members and operating at a sufficiently high frequency, this array can be mounted on one support, which can be rotated to any azimuth.

The pattern can be inverted and the director changed to a reflector by tuning the parasitic element off self-resonance on the side that will give it a positive reactance. This can be done by inserting inductance at the center or by adjusting the length of the parasitic element. Figure 3.35 shows the relations between two radiation-coupled dipoles in free space when their lengths are such as to have zero self-reactance.

FIG. 3.35. Effect of parasitic dipole. (After Brown.)