Traveling-wave tube

A traveling-wave tube (TWT, pronounced "twit"^[1]) or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range.^[2] It was invented by Andrei Haeff around 1933 as a graduate student at Caltech, and its present form was invented by Rudolf Kompfner in 1942–43. 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.^[2] Although there are various types of TWT, two major categories are:^[2]

• Helix TWT - in which the radio waves interact with the electron beam while traveling down a wire helix which surrounds the beam. These have wide bandwidth, but output power is limited to a few hundred watts.^[3]
• Coupled cavity TWT - in which the radio wave interacts with the beam in a series of cavity resonators through which the beam passes. These function as narrowband power amplifiers.

A major advantage of the TWT over some other microwave tubes is its ability to amplify a wide range of frequencies i.e. a large bandwidth. The bandwidth of the helix TWT can be as high as two octaves, while the cavity versions have bandwidths of 10–20%.^[2][3] Operating frequencies range from 300 MHz to 50 GHz.^[2][3] The power gain of the tube is on the order of 40 to 70 decibels,^[3] and output power ranges from a few watts to megawatts.^[2][3]

TWTs account for over 50% of the sales volume of all microwave vacuum tubes.^[2] They are widely used as the power amplifiers and oscillators in radar systems, communication satellite and spacecraft transmitters, and electronic warfare systems.^[2]