Barkhausen–Kurz tube

The Barkhausen–Kurz tube—also known as the retarding-field tube, reflex triode, B–K oscillator, or Barkhausen oscillator—was a high-frequency vacuum tube oscillator invented in 1920 by German physicists Heinrich Georg Barkhausen and Karl Kurz. It was the first oscillator capable of generating radio power in the ultra-high frequency (UHF) range, above 300 MHz, and the first to make use of electron transit time effects.

**History** The triode, invented by Lee de Forest in 1906, was the first device to amplify signals and became the backbone of radio transmitters and receivers after 1920. However, its maximum operating frequency was limited by the spacing between its internal components; even with the smallest gaps, early triodes could only reach the low megahertz range. To overcome this, researchers theorized a technique called velocity modulation. In 1920, Heinrich Barkhausen and Karl Kurz at the Technische Hochschule in Dresden, Germany, applied this theory to create a "retarded-field" triode. They found it could operate at UHF frequencies—the first vacuum tube to do so. Despite its low output power, the Barkhausen–Kurz tube was quickly adopted worldwide for UHF research. It was also called the retarded-field or positive-grid oscillator. Variants of this tube appeared in some of the earliest microwave applications, including the first experimental microwave relay system—a 1.7 GHz link across the English Channel in 1931—and in early World War II radar systems. The tube’s ability to generate microwaves spurred research into similar devices without its power limitations, leading to other "reflex oscillators." The most notable result was the klystron, invented in 1937 by Russell and Sigurd Varian, which remains a widely used high-power microwave source. By World War II, the klystron and magnetron had replaced the Barkhausen–Kurz tube, rendering it obsolete.

**How It Works** The Barkhausen–Kurz tube was a triode operated with its grid (a fine wire mesh) at a positive voltage relative to both the cathode (or filament) and the anode (or plate). Electrons emitted from the cathode were accelerated toward the positive grid. Most passed through the grid wires and continued toward the anode, but the anode’s negative potential repelled them, causing them to reverse direction before hitting the anode surface.