Sutton tube

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The 5836, a typical reflex klystron used as a low-power microwave source. Note the terminal on the top of the tube, used to power the keep-alive. RK5836.JPG
The 5836, a typical reflex klystron used as a low-power microwave source. Note the terminal on the top of the tube, used to power the keep-alive.

A Sutton tube, or reflex klystron, is a type of vacuum tube used to generate microwaves. It is a low-power device used primarily for two purposes; one is to provide a tuneable low-power frequency source for the local oscillators in receiver circuits, and the other, with minor modifications, as a switch that could turn on and off another microwave source. The second use, sometimes known as a soft Sutton tube or rhumbatron switch, was a key component in the development of microwave radar by Britain during World War II. Microwave switches of all designs, including these, are more generally known as T/R tubes or T/R cells.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Radar Object detection system using radio waves

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.

United Kingdom Country in Europe

The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom or Britain, is a sovereign country located off the north­western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north­eastern part of the island of Ireland, and many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state, the Republic of Ireland. Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world. The Irish Sea separates Great Britain and Ireland. The United Kingdom's 242,500 square kilometres (93,600 sq mi) were home to an estimated 66.0 million inhabitants in 2017.


The Sutton tube is named for one of its inventors, Robert Sutton, an expert in vacuum tube design. The original klystron designs had been developed in the late 1930s in the US, and Sutton was asked to develop a tuneable version. He developed the first models in late 1940 while working at the Admiralty Signals and Radar Establishment. Sutton tubes were widely used in a variety of forms during World War II and through the 1960s. Their role has since been taken over by solid state devices like the Gunn diode, which started to become available in the 1970s. "Rhumbatron" refers to the resonant cavity design that was part of many klystrons, referring to the rhumba because of the dance-like motion of the electrons.


A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian, which is used as an amplifier for high radio frequencies, from UHF up into the microwave range. Low-power klystrons are used as oscillators in terrestrial microwave relay communications links, while high-power klystrons are used as output tubes in UHF television transmitters, satellite communication, radar transmitters, and to generate the drive power for modern particle accelerators.

Admiralty Signals and Radar Establishment

The Admiralty Signal and Radar Establishment (ASRE) originally known as the Experimental Department and later known as the Admiralty Signal Establishment (ASE) and Admiralty Surface Weapons Establishment (ASWE) was a research organisation of the British Royal Navy established in 1917 it existed until 1959 when it was merged with the Admiralty Gunnery Establishment to form the Admiralty Surface Weapons Establishment its headquarters were located in Haslemere, Surrey, England.

World War II 1939–1945, between Axis and Allies

World War II, also known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from more than 30 countries. The major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 70 to 85 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, and the only use of nuclear weapons in war.

Basic klystron concept

In a two-cavity klystron, the electrons "bunch up" as they move between the cavities, re-creating the original signal. Klystron.enp.gif
In a two-cavity klystron, the electrons "bunch up" as they move between the cavities, re-creating the original signal.

Klystrons share the basic concept that the microwave output is generated by progressively accelerating then slowing electrons in an open space surrounded by a resonant cavity. The easiest klystron designs to understand have two cavities.

Electron subatomic particle with negative electric charge

The electron is a subatomic particle, symbol
, whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure. The electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum (spin) of a half-integer value, expressed in units of the reduced Planck constant, ħ. Being fermions, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of both particles and waves: they can collide with other particles and can be diffracted like light. The wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a longer de Broglie wavelength for a given energy.

The first cavity is connected to a source signal, and is designed to resonate at the desired frequency, filling its interior with an oscillating electric field. The cavity's dimensions are a function of the wavelength, most are flat cylinders the shape of a hockey puck of varying sizes. A hole is drilled through the middle, at the center of the "puck". [1]

Hockey puck sports equipment

A hockey puck is a disk made of vulcanized rubber that serves the same functions in various games as a ball does in ball games. The best-known use of pucks is in ice hockey, a major international sport. A hockey puck has also been referred to as a "Flat Ball."

A stream of electrons fired from an electron gun passes through the hole, and the varying field causes them to either accelerate or decelerate as they pass. Beyond the cavity the accelerated electrons catch up to the decelerated ones, causing the electrons to bunch up in the stream. This causes the stream to re-create the original signal's pattern in the density of the electrons. This area of the tube has to be fairly long to allow time for this process to complete. [2]

Electron gun electrical component producing a narrow collimated electron beam

An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode ray tubes (CRTs), used in nearly all television sets, computer displays and oscilloscopes that are not flat-panel displays. They are also used in field emission displays (FEDs), which are essentially flat-panel displays made out of rows of extremely small cathode ray tubes. They are also used in microwave linear beam vacuum tubes such as klystrons, inductive output tubes, travelling wave tubes, and gyrotrons, as well as in scientific instruments such as electron microscopes and particle accelerators. Electron guns may be classified by the type of electric field generation, by emission mechanism, by focusing, or by the number of electrodes.

The electrons then pass through a second cavity, similar to the first. As they pass, the bunches cause a varying electric field to be induced in the cavity, re-creating the original signal but at much higher current. A tap point on this cavity provides the amplified microwave output. [2]

Local oscillators

The reflex klystron essentially folds the two-cavity design in half, using two opposing accelerating fields. Reflex.sch.enp.svg
The reflex klystron essentially folds the two-cavity design in half, using two opposing accelerating fields.

The introduction of the cavity magnetron caused a revolution in radar design, generating large amounts of power from a compact and easy-to-build device. However, it also required several additional developments before it could be used.

Among these was a suitable local oscillator about 45 MHz different than the transmitter signal, which fed the intermediate frequency section of the receiver circuits. [3] The problem was that the magnetron's frequency drifted as it warmed and cooled, enough that some sort of tuneable microwave source was needed whose frequency could be adjusted to match. A second magnetron wouldn't work, they would not drift in sync. [4]

As the receiver circuit requires only very little output power, the klystron, first introduced only two years earlier, was a natural choice. Sutton, a well-known expert in tube design, was asked if he could provide a version that could be tuned across the same range as the magnetron's drift. [5] An initial model available in 1940 allowed tuning with some effort. While it worked, it was not suitable for an operational system. Sutton and Thompson continued working on the problem, and delivered a solution in October 1940. [3] Thompson named it for Sutton, while Sutton referred to it as the Thompson Tube. [6] The former stuck.

Their advance was to use a single resonator and clever physical arrangement to provide the same effect as two cavities. He did this by placing a second electrode at the far end of the tube, the "reflector" or "repeller", which caused the electrons to turn around and start flowing back toward the gun, similar to the Barkhausen–Kurz tube. By changing the voltage of the reflector relative to the gun, the speed of the electrons when they reached the cavity the second time could be adjusted, within limits. The frequency was a function of the velocity of the electrons, providing the tuning function. [5]

This modification effectively folded the klystron in half, with most of the "action" at the center of the tube where the input and output from the single cavity were located. Furthermore, only the interior of the cavity was inside the tube, the outer surface was in the form of a metal shell wrapped around the tube. Larger changes to the frequency could be made by replacing the outer shell, and this also provided a convenient location for mounting. [5]

Unfortunately, the system needed two high-voltage power supplies, one for the initial acceleration in the gun, and a second between the gun and the reflector. And, due to the way it worked, the system was generally limited to milliwatts of power.[ citation needed ]

Soft Sutton tube

One of the advantages of using microwaves for radar is that the size of an antenna is based on the wavelength of the signal, and shorter wavelengths thus require much smaller antennas. This was vitally important for airborne radar systems. German aircraft, using longer wavelengths, required enormous antennas that slowed the aircraft between 25 and 50 km/h due to drag. [7] Microwaves required antennas only a few centimetres long, and could easily fit within the aircraft nose.

This advantage was offset by the lack of a switching system to allow a single antenna to act as both a transmitter and receiver. This is not always a major problem; the Chain Home system made do with two sets of antennas, as did early airborne radars like the Mk. IV. In 1940 Bernard Lovell developed a solution for microwave radar by placing two sets of dipoles in front of a common parabolic dish and placing a disk of metal foil between them. However, this was not terribly successful, and the crystal diodes used as detectors frequently burned out as the signal bled through or around the disk. [8] A solution using two spark gap tubes was also used, but was less than ideal. [9]

A better solution was suggested by Arthur H. Cooke of the Clarendon Laboratory, and production development was taken up by H.W.B. Skinner along with A.G. Ward and A.T. Starr at the Telecommunications Research Establishment. [9] They took a Sutton tube and disconnected the electron gun and reflector, leaving just the cavity. This was filled with a dilute gas, initially helium or hydrogen, [10] but eventually settling on a tiny amount of water vapour and argon. [11]

When the transmission signal was seen on the input, the gas would rapidly ionize (helped by a heater coil or radium). [12] The free electrons in the plasma presented an almost perfect impedance source, blocking the signal from flowing to the output. As soon as the transmission stopped, the gas de-ionized and the impedance disappeared very rapidly. [10] The tiny echoes caused by reflections from the target, arriving microseconds later, were far too small to cause the ionization, and allowed the signal to reach the output. [3]

The usable soft Sutton tube arrived in March 1941, and was put into production as the CV43. [3] It was first used as part of the AI Mk. VII radar, the first production microwave radar for aircraft. [10] The system was widely used from then on, appearing in almost all airborne microwave radars, including the H2S radar and ASV Mark III radar. [10]

Post-war intelligence revealed that the Germans were baffled by the purpose of the soft Sutton tube. Several examples fell into their hands, notably in the Rotterdam Gerät, an H2S that was captured in fairly complete form in February 1943. Interviews with German radar engineers after the war demonstrated that they couldn't understand the purpose of the unpowered tube. [9]

The soft Sutton tube was used in a circuit known as a "T/R switch" (or many variations on that theme). Other spark tubes had been used for this purpose, in a design known as the "Branch-Duplexer". This consisted of two short lengths of waveguide about 1/4 of a wavelength, both of which turned on when the signal arrived. Because of the geometry of the layout, the two paths resulted in a reflection of the signal. [13] Sutton tubes were used in a simpler design known as the "shunt branching circuit", which was T shaped with the transmitter and antenna located at either end of the horizontal portion of the T, and the receiver at the end of the vertical portion. By locating the Sutton tube at the right location along the waveguide to the receiver, the same effect as the branch-duplexer could be arranged. [14] [15]

Related Research Articles

Microwave Form of electromagnetic radiation

Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter; with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF bands. A more common definition in radio-frequency engineering is the range between 1 and 100 GHz. In all cases, microwaves include the entire SHF band at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

Cavity magnetron device for generating microwaves

The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavities. Electrons pass by the openings to these cavities and cause microwaves to oscillate within, similar to the way a whistle produces a tone when excited by an air stream blown past its opening. The frequency of the microwaves produced, the resonant frequency, is determined by the cavities' physical dimensions. Unlike other vacuum tubes such as a klystron or a traveling-wave tube (TWT), the magnetron cannot function as an amplifier in order to increase the intensity of an applied microwave signal; the magnetron serves solely as an oscillator, generating a microwave signal from direct current electricity supplied to the vacuum tube.

Traveling-wave tube device used to amplify radio frequency signals in the microwave spectrum

A traveling-wave tube or traveling-wave tube amplifier is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range. The TWT belongs to a category of "linear beam" tubes, such as the klystron, in which the radio wave is amplified by absorbing power from a beam of electrons as it passes down the tube. Although there are various types of TWT, two major categories are:

Resonator Device or system that exhibits resonance

A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator can be either electromagnetic or mechanical. Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal. Musical instruments use acoustic resonators that produce sound waves of specific tones. Another example is quartz crystals used in electronic devices such as radio transmitters and quartz watches to produce oscillations of very precise frequency.

H2S (radar)

H2S was the first airborne, ground scanning radar system. It was developed for the Royal Air Force's Bomber Command during World War II to identify targets on the ground for night and all-weather bombing. This allowed attacks outside the range of the various radio navigation aids like Gee or Oboe, which were limited to about 350 kilometres (220 mi). It was also widely used as a general navigation system, allowing landmarks to be identified at long range.

History of radar aspect of history

The history of radar started with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects. This possibility was suggested in James Clerk Maxwell's seminal work on electromagnetism. However, it was not until the early 20th century that systems able to use these principles were becoming widely available, and it was German inventor Christian Hülsmeyer who first used them to build a simple ship detection device intended to help avoid collisions in fog. Numerous similar systems, which provided directional information to objects over short ranges, were developed over the next two decades.


A gyrotron is a class of high-power linear-beam vacuum tubes which generates millimeter-wave electromagnetic waves by the cyclotron resonance of electrons in a strong magnetic field. Output frequencies range from about 20 to 527 GHz, covering wavelengths from microwave to the edge of the terahertz gap. Typical output powers range from tens of kilowatts to 1–2 megawatts. Gyrotrons can be designed for pulsed or continuous operation. The gyrotron was invented by soviet scientists at NIRFI, based in Nizhny Novgorod, Russia.

Backward-wave oscillator

A backward wave oscillator (BWO), also called carcinotron or backward wave tube, is a vacuum tube that is used to generate microwaves up to the terahertz range. Belonging to the traveling-wave tube family, it is an oscillator with a wide electronic tuning range.

SCR-584 radar

The SCR-584 was an automatic-tracking microwave radar developed by the MIT Radiation Laboratory during World War II. It was one of the most advanced ground-based radars of its era, and became one of the primary gun laying radars used worldwide well into the 1950s. A trailer-mounted mobile version was the SCR-784.

Barkhausen–Kurz tube high frequency vacuum tube electronic oscillator

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, which made the low power Barkhausen-Kurz tube obsolete.

COHO, short for Coherent Oscillator, is a technique used with radar systems based on the cavity magnetron to allow them to implement a moving target indicator display. Because the signals are only coherent when received, not transmitted, the concept is also sometimes known as coherent on receive. Due to the way the signal is processed, radars using this technique are known as pseudo-coherent radar.

Sir John Turton Randall, was an English physicist and biophysicist, credited with radical improvement of the cavity magnetron, an essential component of centimetric wavelength radar, which was one of the keys to the Allied victory in the Second World War. It is also the key component of microwave ovens.

Type 271 radar British WWII naval surface search radar

The Type 271 was a surface search radar used by the Royal Navy and allies during World War II. The first widely used naval microwave-frequency system, it was equipped with an antenna small enough to allow it to be mounted on small ships like corvettes and frigates, while its improved resolution over earlier radars allowed it to pick up a surfaced U-boat at around 3 miles (4.8 km) and its periscope alone at 900 yards (820 m).

AI Mk. VIII radar

Airborne Interception radar, Mark VIII, or AI Mk. VIII for short, was the first operational microwave-frequency air-to-air radar. It was used by Royal Air Force night fighters from late 1941 until the end of World War II. The basic concept, using a moving parabolic antenna to search for targets and track them accurately, remained in use by most airborne radars well into the 1980s.

GL Mk. I radar Early British Army anti-aircraft radar

Gun Laying radar, Mark I, or GL Mk. I for short, was an early radar system developed by the British Army to provide range information to associated anti-aircraft artillery. There were two upgrades to the same basic system, GL/EF and GL Mk. II, both of which added the ability to accurately determine bearing and elevation.

GL Mk. III radar

Gun Laying radar, Mark III, or GL Mk. III for short, was a radar system used by the British Army to directly guide, or lay, anti-aircraft artillery (AA). The GL Mk. III was not a single radar, but a family of related designs that saw constant improvement during and after World War II. These were renamed shortly after their introduction in late 1942, becoming the Radar, AA, No. 3, and often paired with an early warning radar, the AA No. 4, which was also produced in several models.

Type 277 radar

The Type 277 was a surface search and secondary aircraft early warning radar used by the Royal Navy and allies during World War II and continuing use into the post-war era. It was a major update of the earlier Type 271 radar, offering much more power, better signal processing, new displays, and new antennas with greatly improved performance and had much simpler mounting requirements. It allowed a radar with performance formerly found only on cruisers and battleships to be fit even to the smallest corvettes. It began to replace the 271 in 1943 and was widespread by the end of the year.

ASV Mark III radar British WWII anti-submarine search radar

Radar, Air-to-Surface Vessel, Mark III, or ASV Mk. III for short, was a surface search radar system used by RAF Coastal Command during World War II. It was a slightly modified version of the H2S radar used by RAF Bomber Command, with minor changes to the antenna to make it more useful for the anti-submarine role. It was Coastal Command's primary radar from the spring of 1943 until the end of the war. Several improved versions were introduced, notably the ASV Mark VI, which replaced most Mk. IIIs from 1944 onward, and ASV Mark VII radar, which saw only limited use until the post-war era.



  1. Caryotakis 1998, p. 3.
  2. 1 2 Caryotakis 1998, pp. 1-2.
  3. 1 2 3 4 Watson 2009, p. 146.
  4. "Magnetron Theory of Operation", p. 3.
  5. 1 2 3 Lovell 1991, p. 61.
  6. Reg Batt, "The Radar Army: Winning the War of the Airwaves", Hale, 1991, p. 61.
  7. Jean-Denis G.G. Lepage, "Aircraft of the Luftwaffe, 1935-1945", McFarland, 2009, p. 61.
  8. Lovell 1991, p. 62.
  9. 1 2 3 Hodgkin 1994, p. 192.
  10. 1 2 3 4 Lovell 1991, p. 63.
  11. Watson 2009, p. 165.
  12. Robert Buderi, "The Invention That Changed the World", Touchstone, 1998, p.118.
  13. Christian Wolff, "Branch- Duplexer"
  14. C.G. Montgomery, "Microwave Duplexers", MIT
  15. A.L. Samuel, J.W. Clark and W.W. Mumford, "The Gas-Discharge Transmit-Receieve Switch", Bell System Technical Journal, 1946, p. 54.


  • Hodgkin, Alan (1994). Chance and Design: Reminiscences of Science in Peace and War. Cambridge University Press. ISBN   9780521456036.
  • Watson, Jr, Raymond (2009). Radar Origins Worldwide. Trafford Publishing. ISBN   9781426991561.
  • Lovell, Bernard (1991). Echoes of War: The Story of H2S Radar. CRC Press. ISBN   9780852743171.
  • Caryotakis, George (April 1998). "The Klystron: A Microwave Source of Surprising Range and Endurance" (PDF). Stanford Linear Accelerator Center.

Further reading