An antenna that is a more effective transmitter or receiver of energy in some directions than in others. A common method of obtaining such directivity is by using a passive antenna and an active antenna in conjunction. The active antenna is one connected directly to the transmitter or receiver. The passive antenna is not so connected but influences the directivity by reacting with the active antenna. In transmission there is an induced e.m.f. produced in the passive antenna; in reception the reaction between the antenna elements results from their mutual inductance. A passive antenna placed behind an active antenna is called a reflector; one placed in front is a director. See also antenna array; omnidirectional antenna.