One of Japan’s best-known radio researchers in the 1920s-1930s era was Professor Hidetsugu Yagi, who was initially at Tohoku University. He had become very interested in the magnetron, built and named by Albert W. Hull at General Electric in 1921. While Hull’s magnetron was a HF device, Yagi was convinced that it could also be a generator of VHF or even UHF signals. Kinjiro Okabe was one of Yagi’s first doctoral students and was encouraged by his mentor in this pursuit.
In 1926, Okabe developed a magnetron device that significantly decreased the operating wavelength of oscillations. He filed for a US patent in 1926, which was granted in 1929 (No. 1,735,294). His work continued, and based on developing the split-anode device.
Okabe developed an experimental apparatus using a VHF transmitter and receiver with Yagi-Uda antennas separated some distance. In 1936, he successfully detected a passing aircraft by the Doppler-interference method; this was the first recorded demonstration in Japan of aircraft detection by radio. With this success, Okabe’s research interest switched from magnetrons to VHF equipment for target detection. During and following the war years, Okabe remained very active at the Osaka Laboratory. While Okabe’s initial proposal for radio detection was not accepted by the military, time would see this position greatly change.