In August 1926, a young assistant professor at Tohoku Imperial University in Sendai, Japan — Shintaro Uda — noticed that a row of parallel metal rods, spaced at specific fractions of a wavelength, could squeeze radio waves into a tight, forward-pointing beam. His supervisor, Hidetsugu Yagi, translated the work into English, promoted it internationally, and filed patents in Japan (JP 69115), the UK, and the United States. The U.S. patent, US 1,860,123, was filed September 3, 1926 and granted May 24, 1932, titled "Variable Directional Electric Wave Generating Device."

The invention is beautifully simple. A single "driven" dipole is connected to the transmitter. In front of it sit one or more slightly shorter "director" rods; behind it, a slightly longer "reflector." Nothing is wired to these parasitic elements — they just float in space. The passing wave induces currents in them, and those currents re-radiate with phase shifts that add constructively in one direction and cancel in the other. The result is a cheap, passive antenna with astonishing forward gain, made from what looks like a fish skeleton.

Japan's telecom establishment showed almost no interest. Yagi presented the work at conferences in the U.S. and Europe, where the British and Americans understood immediately what they were looking at. By the mid-1930s, Yagi arrays were being built into experimental radar sets in the UK and Germany.

The most famous moment in the antenna's history came in February 1942, when Japanese forces overran Singapore and captured British radar equipment along with a technical notebook. The notes kept referring to a "Yagi array." A captured British technician was interrogated, and — according to the story that became legend inside Japanese engineering — he expressed astonishment that Japanese officers didn't recognize the name of their own countryman. Japan had been fighting a radar war using an antenna it had invented and forgotten.

The Yagi-Uda design turned out to be one of the most durable inventions of the 20th century. It scales cleanly with frequency, works from HF to millimeter-wave, needs no active components, and can be built by anyone with a tape measure and a hacksaw. That's why you still see it:

• Every rooftop TV antenna in the world (UHF Yagis with 10–20 directors are standard for DVB-T and ATSC reception).
• Amateur radio and long-range Wi-Fi — the "cantenna" and every 2.4 GHz directional link is a Yagi variant.
• UHF RFID readers in warehouses, toll booths, and inventory scanners use short Yagi arrays for the reader-to-tag link.
• Radio astronomy feeds and weather radar profilers often use Yagi arrays for VHF/UHF work — the MU radar in Shigaraki, Japan is a phased array of 475 Yagis.
• LoRa, Meshtastic, and rural ISM-band links — long-range IoT gear routinely ships with detachable Yagi antennas.

Could it be built better now? In some sense, it already has been: modern electromagnetic solvers (HFSS, NEC-4) let engineers tune element spacing to fractional-millimeter precision, and printed Quasi-Yagi variants are etched directly onto PCBs for 5G mmWave beamforming modules. But the topology — driven dipole, one reflector, N directors — is exactly what Uda drew on his notebook page a hundred years ago this fall.