The Yagi–Uda antenna consists of a number of parallel thin rod elements in a line, usually half-wave long, typically supported on a perpendicular crossbar or "boom" along their centers. There is a single driven element in the center (consisting of two rods each connected to one side of the transmission line), and a variable number of parasitic elements, a single "reflector" on one side and optionally one or more "directors" on the other side. The parasitic elements are not electrically connected to the transceiver, and serve as passive radiators, reradiating the radio waves to modify the radiation pattern. Typical spacings between elements vary from about to of a wavelength, depending on the specific design. The directors are slightly shorter than the driven element, while the reflector(s) are slightly longer. The radiation pattern is unidirectional, with the main lobe along the axis perpendicular to the elements in the plane of the elements, off the end with the directors.