Barkhausen–Kurz tube

The triode vacuum tube developed by Lee de Forest in 1906 was the first device that could amplify, and was used in most radio transmitters and receivers from 1920 on.

In 1920, Heinrich Barkhausen and Karl Kurz at the Technische Hochschule in Dresden, Germany used the velocity modulation theory in developing a "retarded-field" triode.

Although severely limited in output power, the Barkhausen–Kurz tube was quickly adopted world-wide for UHF research.

The best known result of this research was the klystron tube[4][5] invented 1937 by Russell and Sigurd Varian, which is widely used as a high power source of microwaves to the present.

The Barkhausen–Kurz tube was a triode operated with the grid (a thin mesh of wires) at a positive potential relative to both the cathode (or filament) and the anode (or plate).

Compared to a conventional triode oscillator, the number of electrons actually hitting the anode plate and grid is small, so the plate and grid alternating currents are small, and the output power of the B-K oscillator is low.

Experimental low power 3 GHz AM communication link from 1938 uses Barkhausen-Kurz tubes for both transmitting and receiving
The first experimental microwave relay system, a 1.7 GHz link 40 miles across the English channel in 1931, used a Barkhausen-Kurz tube mounted at the focus of the 10 foot dish shown. It had a radiated power of about 1/2 watt.