Yagi-Uda antenna

The Yagi-Uda antenna, usually shortened to Yagi, is a directional antenna made from a row of parallel rod elements mounted along a supporting boom. Only one element, the driven element, is connected to the feed line; the others are parasitic, meaning they are not fed directly but pick up energy from the driven element and re-radiate it with a phase that reinforces the beam in one direction. The design was developed in Japan in the 1920s by Shintaro Uda and Hidetsugu Yagi, whose name became attached to it in the wider world.

The elements have specific roles set by their length and spacing. The driven element is typically a half-wave dipole at the operating frequency. Behind it sits a slightly longer reflector that pushes radiation forward, and in front of it one or more slightly shorter directors that draw the beam in their direction. Adding more directors narrows the beam and raises the gain, which is why long Yagis carry many closely related director elements along the boom.

The result is an antenna with substantial forward gain, a fairly narrow main beam and good rejection of signals from the rear, all in a structure that is light and inexpensive. These qualities made the Yagi the standard rooftop television and FM aerial for decades, and it remains widely used in amateur radio, point-to-point links and other applications where moderate directional gain over a modest bandwidth is wanted. Because it is directional, a Yagi must be pointed toward the wanted station and matched in polarization with it.

Frequently asked questions

What are parasitic elements?

They are the reflector and director rods that are not connected to the feed line; they couple to the driven element and re-radiate energy to shape the beam.

How do you increase a Yagi's gain?

Adding more director elements in front of the driven element narrows the beam and raises forward gain, at the cost of a longer boom and narrower bandwidth.

Where did the Yagi antenna come from?

It was developed in Japan in the 1920s by Shintaro Uda and Hidetsugu Yagi, which is why it is properly called the Yagi-Uda antenna.