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A traveling wave tube (TWT) is an electronic device used to produce high-power radio frequency signals.
The device is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A solenoid coil wrapped around the tube creates a magnetic field which focuses the electrons into a beam, which then passes down the middle of a wire helix that stretches the length of the tube, finally striking a collector at the other end.
TWTs are commonly used as amplifiers in satellite transponders.
They are also used extensively in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection systems. Typically a control grid is introduced between the electron gun and the slow-wave structure to allow pulsed operation.
There is also another way to use twt beside as TX tube.
That is receiver first stage amplifier (instead of transistor based LNA) in radars. This was widely used, especially by Russian constructors.
In the following picture you may see comparision for different type of high power rf tubes.
Just one thing, attenuators (maybe is better to say absorbers) in tube are used to absorb wave that goes in the backward direction. Because it causes self-oscillation , which is useful when you want to use twt like oscillator (so called backward tube) - today this type of twt is practically disappeared.
One more thing helix is used to 'slowdown' rf signal, so that phase speed of the EM wave equals to speed of electrons. This is broadband structure, which is important for broadband amplification (for lesser powers).
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