Yagi–Uda antenna

Lore & Background

The antenna was invented by Shintaro Uda of Tohoku Imperial University, Japan, in 1926, with a lesser role played by Hidetsugu Yagi. However, the name Yagi has become more familiar, while the name of Uda, who through experiment developed and put into practice Yagi's initially vague conception, is often omitted. Yagi filed a patent application in Japan on the new idea without Uda's name and later transferred the patent to the Marconi Company in the UK; in the US, the patent was transferred to RCA Corporation. Yagi antennas were first widely used during World War II in radar systems by Japan, Germany, the United Kingdom, and the United States. After the war, they saw extensive development as home television antennas.

The Yagi–Uda antenna typically consists of a number of parallel thin rod elements, each approximately a half wave in length, supported on a perpendicular crossbar or boom. There is a single dipole driven element and a variable number of parasitic elements: reflectors on one side and optionally one or more directors on the other. The parasitic elements are not electrically connected to the transmission line. 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 off the end with the directors. Gain increases with the number of parasitic elements; only one reflector is normally used, but Yagis have been built with 40 directors and more.

The bandwidth of a Yagi–Uda array in its basic form is narrow, 2–3 percent of the centre frequency, with a tradeoff between gain and bandwidth. For wider bandwidths, such as for terrestrial television, designs commonly feature trigonal reflectors and larger diameter conductors. Yagi–Uda antennas used for amateur radio are sometimes designed to operate on multiple bands using traps—parallel LC circuits inserted along the elements—which truncate the element at higher frequencies and allow resonance on multiple bands, though traps reduce bandwidth and efficiency.

Reader's Guide

The Yagi–Uda antenna is significant for its directional properties, providing moderate to high gain of up to 20 dBi and a front-to-back ratio of up to 20 dB, making it ideal for fixed-frequency applications such as rooftop terrestrial television, point-to-point fixed communication links, radar, and long-distance shortwave communication by broadcasting stations and radio amateurs. Its legacy includes widespread use on the HF, VHF, and UHF bands, and it is relatively lightweight, inexpensive, and simple to construct. The antenna's narrow bandwidth, a few percent of the center frequency, suits fixed-frequency uses but requires design compromises for wider coverage. The historical naming controversy—where Yagi's name became familiar while Uda's is often omitted—reflects the patent and transfer process. The antenna's development during World War II for radar systems and its post-war adoption for television solidified its place as a common directional antenna.

Did You Know?

• Yagi filed a patent in Japan without Uda's name and later transferred it to the Marconi Company in the UK.
• The antenna can have up to 40 directors or more, and gain increases with the number of parasitic elements.