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The Barkhausen–Kurz tube, also called the retarding-field tube, reflex triode, or B–K 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,. and inspired research that led to other transit time tubes such as the klystron. 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high frequency radio waves in research laboratories, and in a few UHF radio transmitters through World War 2, but was superseded by other tubes like the klystron.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 the retarded-field triode that could provide UHF operation. 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.
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). The negative electrons emitted from the cathode are accelerated toward the positive grid, but easily pass through the grid’s wide spacing. Past the grid, the negative anode repulses the electrons, stopping their velocity just before reaching the anode surface. They are then accelerated back toward the grid, again pass through the wide spacing, and are then repelled by the negative cathode, stopping just before reaching the surface. Bunching occurs when the velocity becomes zero, resulting in a moving cloud of electrons, oscillating back and forth.
This oscillatory motion of the electron cloud continues, with some electrons lost to the grid at each passage and constituting the output signal. The lost electrons are replenished by new emissions by the cathode. The time for one complete cycle is the period of the generated signal. This time, and thus the output frequency, could be somewhat varied by changing the relative potentials.
Subsequent to the development of the Barkhausen–Kurz device, a number of other similar devices for UHF applications were invented. All of these are classified as "reflex oscillators," the best known being the klystron.,
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