Yagi-Uda antenna

Yagi-Uda antenna
Name	Yagi-Uda antenna
Caption	A typical Yagi-Uda antenna showing the driven element, reflector, and directors.
Invented year	1926
Invented by	Shintaro Uda and Hidetsugu Yagi
Frequency range	HF to UHF
Polarization	Typically linear
Radiation pattern	Directional

Yagi-Uda antenna. A Yagi-Uda antenna is a highly directional radio antenna consisting of a linear array of parallel dipole elements. It was invented in the 1920s by Shintaro Uda at Tohoku University with the assistance of his colleague Hidetsugu Yagi, who later popularized the design internationally. This antenna's simple construction and high gain make it one of the most common designs for point-to-point communication, television reception, and amateur radio applications worldwide.

History and development

The fundamental principles of the antenna were first developed by Shintaro Uda, publishing his work in Japanese in the Journal of the Institute of Electrical Engineers of Japan. His colleague, Hidetsugu Yagi, recognized the design's potential and presented it to the Western world in a seminal 1928 paper in the Proceedings of the Institute of Radio Engineers. Early adoption was slow, but the design gained significant traction during World War II for radar and VHF communication systems used by the Allies. Post-war, the antenna became ubiquitous for television broadcasting and was instrumental in the expansion of networks like the BBC and CBS.

Design and operation

The antenna operates as an end-fire array, where a single driven element is coupled with parasitic elements to achieve directionality. The driven element is typically a resonant half-wave dipole connected directly to the transmission line. A single longer element, the reflector, is placed behind the driven element to reject signals from the rear, while multiple shorter elements, called directors, are arranged in front to focus energy forward. This configuration creates a unidirectional radiation pattern by manipulating the phase velocity of currents induced in the parasitic elements through mutual coupling.

Construction and components

Traditional construction involves mounting all elements on a central supporting beam called a boom, which is often made from aluminum or fiberglass for durability and low weight. The elements themselves are typically metal rods, with the driven element being center-fed. A balun is frequently employed at the feed point to match the balanced dipole to an unbalanced coaxial cable. Critical adjustments include precisely spacing the elements and tuning their lengths, which are optimized for a specific design frequency, such as those in the FM broadcast band or amateur radio bands like the 2-meter band.

Performance characteristics

Key performance metrics include high directivity and front-to-back ratio, which minimize reception of unwanted signals from the sides and rear. Gain is directly related to the number of director elements, with practical designs achieving gains of up to 20 dBi for long arrays. The bandwidth is relatively narrow, often only a few percent of the center frequency, which makes it sensitive to precise tuning. The input impedance is typically low and can be matched to standard 50 ohm or 75 ohm cables using impedance matching techniques like the gamma match.

Applications

Its most famous historical application was for analog television reception, making it a common sight on rooftops during the era of NTSC and PAL broadcasts. It remains a staple in amateur radio for contests and communication via satellites like OSCAR and modes such as EME. Other critical uses include point-to-point radio communication for services like microwave radio relay, wireless internet provision by WISPs, and as a sensor element in radio direction finding systems used by organizations like the Federal Communications Commission.

Variations and enhancements

Numerous variations exist to optimize for specific needs. The Moxon antenna uses a two-element design with a rectangular shape for a compact footprint. The Quad antenna replaces linear dipoles with loop elements arranged in a similar configuration. For UHF and SHF bands, designs like the log-periodic antenna offer much wider bandwidth. Modern enhancements include using computer-aided design software for optimization and incorporating phased array techniques to create electrically steered versions for applications in satellite television and 5G NR networks.

Category:Radio frequency antenna types Category:Japanese inventions