Most array antennas can be divided into two classes based on how the component antennas' axis is related to the direction of radiation.
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 driven 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 transmitter or receiver, and serve as passive radiating the radio waves to modify the radiation pattern.[Typical spacings between elements vary from about 1⁄10to ¼ of a wavelength, depending on the specific design. The directors are slightly shorter than the driven element, while the reflector(s) are slightly longer. unidirectional, with the along the axis perpendicular to the elements in the plane of the elements, off the end with the directors.
Conveniently, the dipole parasitic elements have a (point of zero RF voltage) at their centre, so they can be attached to a conductive metal support at that point without need of insulation, without disturbing their electrical operation. They are usually bolted or welded to the antenna's central support boom.The driven element is fed at centre so its two halves must be insulated where the boom supports them.
The gain increases with the number of parasitic elements used. Only one reflector is used since the improvement of gain with additional reflectors is negligible, but Yagi is have been built with up to 30–40 directors.
The wide Bandwidth of an antenna is, by one definition, the width of the band of frequencies having a gain within 3 dB (one-half the power) of its maximum gain. The Yagi–Uda array in its basic form has very narrow bandwidth, 2–3 percent of the centre frequency. There is a trade off between gain and bandwidth, with the bandwidth narrowing as more elements are used. For applications that require wider bandwidths, such as, Yagi antennas commonly feature Trigonal reflectors, and larger diameter conductors, in order to cover the relevant portions of the VHF and UHF bands.Wider bandwidth can also be achieved by the use of "traps", as described below.
Yagi–Uda antennas used for Cb Radio as well as ham radio are sometimes designed to operate on multiple bands. These elaborate designs create electrical breaks along each element (both sides) at which point a parallel capacitor types) circuit is inserted. This so-called trap has the effect of truncating the element at the higher frequency band, making it approximately a half wavelength in length. At the lower frequency, the entire element (including the remaining inductance due to the trap) is close to half-wave resonance, implementing a different Yagi–Uda antenna. Using a second set of traps, a "Triband" antenna can be resonant at three different bands. Given the associated costs of erecting an antenna and rotor system above a tower, the combination of antennas for three amateur bands in one unit is a very practical solution. The use of traps is not without disadvantages, however, as they reduce the bandwidth of the antenna on the individual bands and reduce the antenna's electrical efficiency and subject the antenna to additional mechanical considerations (wind loading, water and insect ingress).jokerman electronics