What Are Yagi-Uda Antennas?

Yagi-Uda antennas are a class of directional wire antennas that use a single driven element and one or more parasitic elements to produce a unidirectional radiation pattern with moderate to high gain. The design was developed in Japan in the 1920s by Hidetsugu Yagi and Shintaro Uda at Tohoku Imperial University and was first published in English by Yagi in 1928. The antenna consists of a driven dipole, one or more reflector elements placed behind it, and one or more director elements placed in front, all arranged collinearly along a boom. Mutual coupling between the driven and parasitic elements produces constructive interference in the forward direction and destructive interference to the rear, shaping the radiation pattern into a well-defined main lobe. The Yagi-Uda antenna draws on classical antenna theory rooted in Maxwell's equations and reciprocity, and it remains one of the most widely deployed directional antenna types across frequencies from tens of megahertz to millimeter waves.

Operating Principle and Element Configuration

The reflector element is cut slightly longer than the driven dipole, approximately 5 percent longer, causing it to present an inductive impedance that reflects energy forward. Each director element is cut slightly shorter than the driven dipole, presenting a capacitive impedance that guides energy in the forward direction. The spacing between elements, typically 0.1 to 0.25 wavelengths, and the number of directors are the primary variables that determine gain and front-to-back ratio. Adding directors increases gain at the cost of a longer boom and a narrower bandwidth. A procedure for designing Yagi-Uda arrays to maximize gain is analyzed in IEEE Transactions research on gain optimization for Yagi-Uda arrays, which provides design curves relating element count and spacing to achievable directivity.

Gain and Directivity

The gain of a practical Yagi-Uda antenna ranges from about 6 dBi for a three-element design to more than 17 dBi for a long-boom array with many directors. Gain increases roughly logarithmically with the number of directors beyond the first few, so diminishing returns set an economic limit on array length for most applications. Impedance matching is a design consideration because the input impedance of the driven element shifts when parasitic elements are nearby; a folded dipole is often used as the driven element because its intrinsically higher impedance better matches common feed line impedances after the coupling effect is accounted for. IEEE research on high-performance Yagi-Uda arrays at 28 GHz demonstrated that the design principles scale to millimeter-wave frequencies relevant to 5G wireless infrastructure, achieving gain suitable for base station applications.

Design Variations and Frequency Scaling

The Yagi-Uda configuration scales directly with wavelength, making it straightforwardly adaptable from VHF television bands through microwave frequencies. Printed and planar versions implemented on dielectric substrates replace the wire elements with etched conductors, reducing fabrication cost for high-volume applications. Log-periodic Yagi hybrids and stacked arrays of multiple Yagi-Uda elements extend bandwidth or increase aperture gain beyond what a single planar design can achieve. In millimeter-wave systems, the antenna is often integrated on-chip or into a package alongside the transceiver circuitry. IEEE conference publications document circular Yagi-Uda array configurations for 60 GHz wearable communications, illustrating the range of geometries that the fundamental parasitic-element principle supports.

Applications

Yagi-Uda antennas have applications in a wide range of communication and sensing systems, including:

• Terrestrial television reception and over-the-air broadcast signal reception
• Amateur radio and point-to-point HF and VHF links
• Weather radar and VHF surveillance radar arrays
• Wireless access point and client antennas for directional 802.11 links
• Radio astronomy receiving arrays for specific frequency bands
• Millimeter-wave 5G base station and device antennas