English: Experimental UHF radio transmitter using two type 932 tubes in a push-pull Barkhausen-Kurz oscillator, constructed by James Millen in 1933. During the 1930s the frontier in radio research was the UHF wavelengths. Ordinary vacuum tube feedback oscillators could not reach these frequencies due to electrode capacitance and inductance and transit time effects. The Barkhausen-Kurz oscillator, the first transit time oscillator, was the first circuit which could produce power at UHF frequencies. It used specially designed triode tubes, the 932 (left) in which the electrons, instead of moving from cathode to plate, oscillated back and forth between cathode and plate through the grid, which was biased at a high voltage. The transmitter uses two coupled Lecher lines (parallel wire transmission line stubs) (right) as the tank resonator. It generated 5 W at 300-400 MHz. The device at extreme left was a motor-generator set to generate the DC plate voltage