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 would not reach these frequencies due to electrode capacitance and inductance. The Barkhausen-Kurz oscillator, the first transit time oscillator, was the first circuit which could produce power at UHF frequencies. It used a specially designed triode tube 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 lines) as the tank resonator. The dipole antenna (right) in front of the parabolic reflector, is fed by another parallel line. Millen is using another Lecher line to measure the output frequency. This transmitter radiated 5 W at 300-400 MHz.