The Barkhausen–Kurz tube, also called the retarding-field tube, reflex triode, B–K oscillator, and Barkhausen oscillator was a high frequency
vacuum tube
A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
electronic oscillator
An electronic oscillator is an electronic circuit that produces a periodic, oscillating or alternating current (AC) signal, usually a sine wave, square wave or a triangle wave, powered by a direct current (DC) source. Oscillators are found ...
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
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter ...
(UHF) portion of the
radio spectrum
The radio spectrum is the part of the electromagnetic spectrum with frequencies from 3 Hz to 3,000 GHz (3 THz). Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particula ...
, 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 transmitter
In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna with the purpose of signal transmissio ...
s 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
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequenci ...
, which made the low power Barkhausen-Kurz tube obsolete.
History
The
triode
A triode is an electronic amplifier, amplifying vacuum tube (or ''thermionic valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated Electrical filament, filament or cathode, a control grid, grid ...
vacuum tube
A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
developed by
Lee de Forest #REDIRECT Lee de Forest
{{redirect category shell, {{R from move{{R from other capitalisation ...
in 1906 was the first device that could amplify, and was used in most radio transmitters and receivers from 1920 on. It was found that the highest
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
at which the triode 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
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base ...
range. A technique called velocity modulation was theorized to overcome this limitation.
In 1920,
Heinrich Barkhausen
Heinrich Georg Barkhausen (2 December 1881 – 20 February 1956) was a German physicist who established an influential research laboratory in Dresden. The phenomenon by which ferromagnetic domains align during magnetization and produce discr ...
and Karl Kurz at the
Technische Hochschule
A ''Technische Hochschule'' (, plural: ''Technische Hochschulen'', abbreviated ''TH'') is a type of university focusing on engineering sciences in Germany. Previously, it also existed in Austria, Switzerland, the Netherlands (), and Finland (, ) ...
in
Dresden
Dresden (; ; Upper Saxon German, Upper Saxon: ''Dräsdn''; , ) is the capital city of the States of Germany, German state of Saxony and its second most populous city after Leipzig. It is the List of cities in Germany by population, 12th most p ...
, Germany used the velocity modulation theory in developing a "retarded-field" triode. They found it could operate at frequencies into the
UHF
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter ...
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 experimental
microwave relay
Microwave transmission is the transmission of information by electromagnetic waves with wavelengths in the microwave frequency range of 300 MHz to 300 GHz (1 m - 1 mm wavelength) of the electromagnetic spectrum. Microwave signal ...
system, a 1.7 GHz link across the
English Channel
The English Channel, also known as the Channel, is an arm of the Atlantic Ocean that separates Southern England from northern France. It links to the southern part of the North Sea by the Strait of Dover at its northeastern end. It is the busi ...
in 1931,
and in early
radar
Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
systems used in World War 2.
The success of the Barkhausen-Kurz tube in generating radio waves at microwave frequencies inspired research to develop similar tubes which did not have its power limitations, resulting in the invention of other tubes which were known as "reflex oscillators". The best known result of this research was the
klystron
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequenci ...
tube invented 1937 by Russell and Sigurd Varian, which is widely used as a high power source of microwaves to the present. Sources like the klystron and
magnetron tube replaced the B-K tube around World War 2 and it became obsolete.
How it works

The Barkhausen–Kurz tube was a
triode
A triode is an electronic amplifier, amplifying vacuum tube (or ''thermionic valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated Electrical filament, filament or cathode, a control grid, grid ...
operated with the
grid
Grid, The Grid, or GRID may refer to:
Space partitioning
* Regular grid, a tessellation of space with translational symmetry, typically formed from parallelograms or higher-dimensional analogs
** Grid graph, a graph structure with nodes connec ...
(a thin mesh of wires) at a positive potential relative to both the
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...
(or
filament
The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including:
Astronomy
* Galaxy filament, the largest known cosmic structures in the universe
* Solar filament ...
) and the
anode
An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...
(or
plate). The negative electrons emitted from the cathode are accelerated toward the positive grid. Most pass between the grid wires and continue toward the anode plate, but the negative potential on the anode repels them and they reverse direction before they hit the surface of the anode plate and are accelerated back toward the relatively higher potential grid through which they have just passed. Again, most pass through the grid wires, but they are then repelled by the negative potential of the cathode and reverse direction just before reaching the surface of the cathode. The electrons continue oscillating back and forth through the grid until one by one they strike the grid wires.
The oscillating grid potential induced by the passage of the electrons through the grid excites oscillations in a
tank circuit
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can act ...
attached to the grid, usually consisting of a quarter
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
of parallel
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
shorted at the end, called a
resonant stub
In microwave and radio-frequency engineering, a stub or resonant stub is a length of transmission line or waveguide that is connected at one end only. The free end of the stub is either left open-circuit, or short-circuited (as is always the c ...
. In turn the oscillating voltage on the tank circuit varies the potential of the grid, causing the electrons to ''bunch'' into a cloud of electrons moving back and forth through the grid
in phase
In physics and mathematics, the phase (symbol φ or ϕ) of a wave or other periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is expressed in such a s ...
at the resonant frequency.
The oscillatory motion of the electron cloud continues; this cloud constitutes the alternating output current. Some electrons are lost to the grid on each pass, but the electron supply is continually replenished by new electrons emitted by the cathode. 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. Higher power devices like the klystron were later developed to overcome this limitation.
The frequency of oscillation depends on the spacing and potentials of the electrodes, and can be tuned within a limited bandwidth by altering the electrode voltages.
References
External links
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{{DEFAULTSORT:Barkhausen-Kurz tube
Vacuum tubes
German inventions
1920 in science