Traveling-wave tube

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A traveling-wave tube (TWT) is an electronic device used to amplify radio frequency signals to high power, usually in an electronic assembly known as a traveling-wave tube amplifier (TWTA).

The TWT was invented by Rudolf Kompfner in a British radar lab during World War II, and refined by Kompfner and John Pierce at Bell Labs. Both of them have written books on the device.[1][2] In 1994, A.S. Gilmour wrote a modern TWT book[3] which is widely used by U.S. TWT engineers today, and research publications about TWTs are frequently published by the IEEE.

Cutaway view of a TWT. (1) Electron gun; (2) RF input; (3) Magnets; (4) Attenuator; (5) Helix coil; (6) RF output; (7) Vacuum tube; (8) Collector.
Cutaway view of a TWT. (1) Electron gun; (2) RF input; (3) Magnets; (4) Attenuator; (5) Helix coil; (6) RF output; (7) Vacuum tube; (8) Collector.

The device is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A magnetic containment field around the tube focuses the electrons into a beam, which then passes down the middle of a wire helix that stretches from the RF input to the RF output, the electron beam finally striking a collector at the other end. A directional coupler, which can be either a waveguide or an electromagnetic coil, fed with the low-powered radio signal that is to be amplified, is positioned near the emitter, and induces a current into the helix.

The helix acts as a delay line, in which the RF signal travels at near the same speed along the tube as the electron beam. The electromagnetic field due to the current in the helix interacts with the electron beam, causing bunching of the electrons (an effect called velocity modulation), and the electromagnetic field due to the beam current then induces more current back into the helix (i.e. the current builds up and thus is amplified as it passes down).

A second directional coupler, positioned near the collector, receives an amplified version of the input signal from the far end of the helix. An attenuator placed on the helix, usually between the input and output helicies, prevents reflected wave from travelling back to the cathode.

The bandwidth of a broadband TWT can be as high as three octaves, although tuned (narrowband) versions exist, and operating frequencies range from 300 MHz to 50 GHz. The voltage gain of the tube can be of the order of 70 decibels.

A TWT has sometimes been referred to as a traveling-wave amplifier tube (TWAT),[4][5] although this term was never really adopted. "TWT" is sometimes pronounced by engineers as "TWIT".[6]

Contents

[edit] Coupled-cavity TWT

Helix TWTs are limited in peak RF power by the current handling (and therefore thickness) of the helix wire. As power level increases, the wire can overheat and cause the helix geometry to warp. Wire thickness can be increased to improve matters, but if the wire is too thick it becomes impossible to obtain the required helix pitch for proper operation. Typically helix TWTs achieve less than 2.5 kW output power.

The coupled-cavity TWT overcomes this limit by replacing the helix with a series of coupled cavities arranged axially along the beam. Conceptually, this structure provides a helical waveguide and hence amplification can occur via velocity modulation. Helical waveguides have very nonlinear dispersion and thus are only narrowband (but wider than klystron). A coupled-cavity TWT can achieve 15 kW output power.

Operation is similar to that of a klystron, except that coupled-cavity TWTs are designed with attenuation between the slow-wave structure instead of a drift tube. The slow-wave structure gives the TWT its wide bandwidth. A free electron laser allows higher frequencies.

[edit] Traveling-wave tube amplifier

A TWT integrated with a regulated power supply and protection circuits is referred to as a traveling-wave tube amplifier[7] (abbreviated TWTA and often pronounced "TWEET-uh"). It is used to produce high-power radio frequency signals. The bandwidth of a broadband TWTA can be as high as one octave, although tuned (narrowband) versions exist; operating frequencies range from 300 MHz to 50 GHz.

A TWTA consists of a traveling-wave tube coupled with its protection circuits (as in klystron) and regulated power supply (EPC, electronic power conditioner), which may be supplied and integrated by a different manufacturer. The main difference between most power supplies and those for vacuum tubes is that efficient vacuum tubes have depressed collectors to recycle kinetic energy of the electrons and therefore the secondary winding of the power supply needs up to 6 taps of which the helix voltage needs precise regulation. The subsequent addition of a linearizer (as for inductive output tube) can, by complementary compensation, improve the gain compression and other characteristics of the TWTA; this combination is called a linearized TWTA (LTWTA, "EL-tweet-uh").

Broadband TWTAs generally use a helix TWT, and achieve less than 2.5 kW output power. TWTAs using a coupled cavity TWT can achieve 15 kW output power, but at the expense of bandwidth.

[edit] Uses

TWTAs are commonly used as amplifiers in satellite transponders, where the input signal is very weak and the output needs to be high power.[8]

A TWTA whose output drives an antenna is a type of transmitter. TWTA transmitters are used extensively in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection systems.[9] In these types of applications, a control grid is typically introduced between the TWT's electron gun and slow-wave structure to allow pulsed operation. The circuit that drives the control grid is usually referred to as a grid modulator.

Another major use of TWTAs is for the electromagnetic compatibility (EMC) testing industry for immunity testing of electronic devices.[citation needed]

[edit] See also

Other types of microwave power tubes include:

[edit] References

  1. ^ Pierce, John R. (1950). Traveling-Wave Tubes. D. van Nostrand Co.. 
  2. ^ Kompfner, Rudolf (1964). The Invention of the Traveling-Wave Tube. San Francisco Press. 
  3. ^ Gilmour, A.S. (1994). Principles of Traveling Wave Tubes. Artech House. 
  4. ^ Military Acronyms, Initialisms, and Abbreviations on Federation of American Scientists web site
  5. ^ Patent #20050157817, "Method for receiving two decorrellated signals transmitted on a single channel and receiver for implementing said method"
  6. ^ Henry W. Cole (1985). Understanding Radar. Collins. 
  7. ^ John Everett (1992). Vsats: Very Small Aperture Terminals. IET. ISBN 0863412009. 
  8. ^ Dennis Roddy (2006). Satellite Communications. McGraw-Hill Professional. ISBN 0071462988. 
  9. ^ L. Sivan (1994). Microwave Tube Transmitters. Springer. ISBN 0412579502. 

[edit] External links