In a
radio antenna, a
passive radiator or
parasitic element is a conductive element, typically a metal rod, which is not electrically connected to anything else. Multielement antennas such as the
Yagi-Uda antenna typically consist of a "
driven element" which is connected to the
radio receiver or
transmitter through a
feed line, and parasitic elements, which are not. The purpose of the parasitic elements is to modify the
radiation pattern of the radio waves emitted by the driven element, directing them in a beam in one direction, increasing the antenna's
directivity (
gain). A parasitic element does this by acting as a passive
resonator, something like a guitar's sound box, absorbing the radio waves from the nearby driven element and re-radiating them again with a different
phase. The waves from the different antenna elements
interfere, strengthening the antenna's radiation in the desired direction, and cancelling out the waves in undesired directions.