Composed of two identical radiators, the distance between the two is typically a quarter of a wavelength. This is a weak directional antenna, low gain, wide half-power angle, the general use of passive reflectors, because the antenna input impedance changes more stable than with the director.
Usually feed only to the radiator, and in the reflector connected to a variable reactor, by changing the reactance value to make the forward (by the direction of the radiator) radiation highest or backward (by the direction of the reflector) radiation the weakest. But there are also two antenna elements feed at the same time, using the network to adjust the two current and phase to meet the needs of the antenna array radiation pattern. A four-element directional antenna array is composed of four identical radiating elements. The plane layout of the antenna array and the relation of current among them are shown in fig. The distance between two radiators or two reflectors, D. Typically half a wavelength, the distance between the emitter and the reflector is d. It's usually 1/4 wavelength.