Yagi-Uda antenna

reference ： wikipedia

A Yagi-Uda antenna, commonly known as a Yagi antenna, is a directional antennaconsisting of multiple parallel elements in a line, usually made of metal rods.Yagi-Uda antennas consist of a single driven element connected to the transmitter orreceiver with a transmission line, and additional parasitic elements: a so-called reflectorand one or more directors.

The reflector element is slightly longer than the driven dipole, whereas the directors are a little shorter.This design achieves a very substantial increase in the antenna's directionality and gain compared to a simple dipole.

It has moderate gain which depends on the number of elements used, typically limited to about 17 dBi, linear polarization, unidirectional (end-fire) beam pattern with high front-to-back ratio of up to 20 db. and is lightweight, inexpensive and simple to construct.The bandwidth of a Yagi antenna, the frequency range over which it has high gain, is narrow, a few percent of the center frequency, and decreases with increasing gain, so it is often used in fixed-frequency applications. The largest and most well-known use is as rooftop terrestrial television antennas, but it is also used for point-to-point fixed communication links,in radar antennas,and for long distance shortwave communication by shortwave broadcasting stations and radio amateurs.

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 moredirectors on the other side.The parasitic elements are not electrically connected to the transmitter or receiver, and serve aspassive radiators, reradiating the radio waves to modify the radiation pattern.Typical spacings between elements vary from about 1/10 to 1/4 of a wavelength, depending on the specific design. The lengths of the directors are slightly shorter than that of 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。

Conveniently, the parasitic elements have a node (point of zero RFvoltage) 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.