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A Yagi antenna, also known as a Yagi-Udo antenna or a yagi, is a type of antenna array often used by amateur radio operators. It can also be used for other purposes, for example as a TV antenna, to collect radio signals from space, and as a cellular antenna. Depending on its construction, it can be used for communications on frequencies from short wave to microwave, covering the spectrum from very high frequency (VHF) through ultra high frequency (UHF). The Yagi antenna is a directional antenna, meaning its transmission and receiving power is focused in one direction and that it needs to be pointed in the specific direction one wants to receive or transmit. It is also high-gain, meaning it allows for increased signal strength both when receiving, by capturing more of the radio signal, and when transmitting, by allowing more of the transmitted power to be sent.
The Yagi antenna was invented in 1926 by two Japanese scientists. At its most basic, it consists of an array of three long, parallel pieces of metal attached to a central boom. The three metal pieces are the antenna elements: a reflector, a driven element, and a director.
The reflector, which absorbs and re-reflects radio waves, is positioned at the back of the array. The director element is at the front and enables transmission and reception. The driven element, also called the active element, is the only part connected to the cable, or source feed, that powers the antenna. The other elements are called parasitic or passive.
A Yagi antenna usually consists of one reflector, one driven element, and one or more directors. Depending on the length and spacing of these parts, the power of the antenna is changed and it will receive and transmit at different frequencies. Mathematical formulas are used to calculate how long the various elements should be, and how they should be spaced for various frequencies. For use on UHF frequencies, the antenna is usually 1 foot (30 cm) wide, several feet long and features 10-20 directors. A Yagi antenna used on VHF frequencies usually has fewer antenna elements, spaced wider apart.
A Yagi antenna is sometimes combined with a rotator so it can be easily turned in different directions. It needs to be installed as high up as possible, preferably in a location without trees or structures that impede its transmission and reception. A Yagi antenna is relatively easy to build and install, and can be constructed from welding pipe, aluminum tubing or copper house wire.