Yagi antennas, created by Shintaro Uda and Hidetsugu Yagi in 1926, are directional, high-gain dipoles with an added reflector and at least one director. There are innumerable variations on the original Yagi antenna. A UHF Yagi antenna is designed specifically for the ultra-high frequency band, from 300 megahertz (MHz) to 2 gigahertz (GHz). The different types of UHF Yagi antenna include those used to receive television, amateur radio, citizen band (CB) radio and satellite signals.
The original Yagi design was intended for short-wave radio use in aircraft. Later designs expanded the Yagi’s use to high-frequency bands, including 28, 21 and 14 MHz. Multiband Yagis use pairs of coils and capacitors called traps, which isolate radio signals from multiple bands. In exchange for the increased functionality, however, some efficiency is lost. Using traps also reduces the antenna’s bandwidth.
The Yagi also was adapted for use in receiving television signals in the UHF band, because of its directionality and gain. The popularity of satellite and cable transmission has reduced the need for a UHF Yagi antenna for television signals, but the Yagi is still in common use for amateur radio, CB radio and satellite communications. The UHF Yagi antenna has remained one of the most efficient antenna designs for UHF transmission and receiving.
There are almost as many versions of the UHF Yagi antenna as there are users. In addition to commercially manufactured Yagis, individual amateur radio users constantly design and build their own versions. What defines an antenna as a Yagi is the use of a half-wavelength dipole as the driven element, a reflector that is greater than one-half wavelength and one or more directors that are less than one-half wavelength. This combination creates a phased array that increases gain. The size and spacing of each of these elements controls the bandwidth in which the antenna will be resonant.
The U.S. National Bureau of Standards, now known as the National Institute of Standards and Technology, examined six basic Yagi antenna designs and measured their performance in a study conducted in 1976. The report serves as a reference for adapting the Yagi to frequencies outside the UHF bands.