English: An experimental 3 GHz microwave AM transmitter and receiver using a triode vacuum tube in Barkhausen-Kurz mode for both transmitting and receiving. In Barkhausen-Kurz mode the plate is biased near zero voltage while the grid is biased at a high positive voltage. The electrons oscillate back and forth through the grid, attracted by the grid and repelled by the low voltages on both the cathode and plate. The Barkhausen-Kurz was the first tube that could oscillate in the UHF range, and by the mid 30s could generate microwave frequencies. It was the only source of microwaves until the klystron was invented. The output frequency is determined by the geometry of the tube and the 4 cm half-wave antenna soldered directly to the grid terminal. The tubes and antennas are mounted at the focus of parabolic reflectors which are pointed at each other. In the receiver the Barkhausen-Kurz tube is operated below oscillation as an amplifier