Barkhausen-Kurz tube

The Barkhausen–Kurz tube, also called the retarding-field tube, reflex triode, B–K oscillator, and Barkhausen oscillator was a high frequency vacuum tube electronic oscillator invented in 1920 by German physicists Heinrich Georg Barkhausen and Karl Kurz. It was the first oscillator that could produce radio power in the ultra-high frequency (UHF) portion of the radio spectrum, above 300 MHz. It was also the first oscillator to exploit electron transit time effects. It was used as a source of high frequency radio waves in research laboratories, and in a few UHF radio transmitters through World War 2. Its output power was low which limited its applications. However it inspired research that led to other more successful transit time tubes such as the klystron.

After the development by Lee de Forest of the triode vacuum tube in 1906, it was realized that the upper frequency at which the device could be used was limited by the spacing between internal components. Even with the smallest of spacing, the frequency limit of early triodes was in the low megahertz range. A technique called velocity modulation was theorized to overcome this limitation.

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. They found it could operate at frequencies into the UHF region, the first vacuum tube to do so. Although severely limited in output power, the Barkhausen–Kurz tube was quickly adopted world-wide for UHF research. This device is also called the retarded-field and positive-grid oscillator. Versions of the Barkhausen oscillator were used in some of the first applications of microwaves, such as the first microwave relay system, a 1.7 GHz link across the English Channel in 1931, and in early radar systems used in World War 2.

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