Directional antennas are antennas characterized by an anisotropic, a directional radiation pattern with a small beam angle and by a high front-to-back ratio, the return loss.
Depending on the antenna type, the beam bundling can be 30 degrees and more, but it can also have an extremely strong beam bundling with an opening angle of 5 degrees to 10 degrees. The strong beam bundling results in a high antenna gain, which means that the transmit power can be kept low while the field strength remains the same.
The return loss is the parameter for the ratio of the radiation hitting the antenna from the front and the back. It is specified as the front-to-back ratio and reaches decibel values of 40 dB and more, depending on the design of the antenna and its reflectors.
Directional antennas are used in all frequency ranges; in radio and VHF broadcasting as well as in television and especially in directional radio for point-to-point connections of two converters. Depending on the frequency range, these are then Yagi antennas, which are used up to about 1 GHz, and parabolic antennas, which are used at higher frequencies. For microwaves above 10 GHz, there is also the horn radiator, which forms the termination for waveguides.
A special form of directional antenna is the pencil beam antenna, which has an extremely strong beam bundling.