This article is about antennas. For the speaker enclosure, see Passive radiator (speaker).

A passive radiator or parasitic element is a radio antenna element which does not have any wired input. Instead, it absorbs radio waves radiated from another active antenna element in proximity, and re-radiates it in phase with the active element so that it adds to the total transmitted signal. This will change the antenna pattern and beam width.

Parasitic elements can also be used to alter the radiation parameters of nearby active elements. An example of this is to place a parasitic microstrip patch antenna above another driven patch antenna. This antenna combination resonates at a slightly lower frequency than the original element. However, the main effect is to greatly increase the impedance bandwidth of the antenna. In some cases the bandwidth can be increased by a factor of 10.

An example of an antenna that uses passive radiators is the Yagi, which typically has a reflector behind the driven element, and one or more directors in front of the driven element, which act respectively like the reflector and lenses in a flashlight to create a "beam".

Compare this to a log periodic antenna, which is similar in appearance to a Yagi, but all of its elements are driven directly by the transmitter.

See also

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• driven element.

Radio frequency antennas