What is a Traveling Wave Tube?
A traveling-wave tube, or TWT, is a specialized vacuum tube used to amplify high-frequency signals. The signals are amplified by absorbing power from a beam of electrons as it goes through the tube. TWT are linear beam tubes and are used extensively for high-power, high-frequency amplification. They usually operate from 300 MHz to frequencies over 100 GHz (there are cases where these go as high as 650 GHz) and provide power levels from a few watts to megawatts. Traveling wave tubes are widely used as power amplifiers in radar systems, communication satellite and spacecraft transmitters, and electronic warfare systems.
There are two main types of traveling wave tubes: Helix and Coupled Cavity.
Helix TWT: In Helix TWT, the waves interact with the electron beam while traveling down a wire helix that surrounds the beam. Though these TWTs provide wide bandwidth, their output power is limited to only a few hundred watts.
Coupled Cavity TWT: In coupled cavity TWT, the waves interact with the electron beam in a series of cavity resonators through which the beam passes. These function as narrowband power amplifiers.
Important Parameters to consider in a TWT:
- Operating Frequency: This is the frequency range at which a TWT is fully functional or provides the best performance.
- Output Power (W/dBm): It is the magnitude of power that the TWT will provide at the output after amplifying a signal.
- Gain (dB): Gain is the ratio of output to input power or amplitude and is measured in Decibels (dB).
- Cathode Voltage (V): Cathode voltage is the voltage applied across the cathode to project the electrons.
- Cathode Current (mA/A): Cathode current is the total emission of electrons from the cathode of the tube after voltage is applied.
- Cooling: A TWT requires sufficient cooling at the collector, electron gun, as well as body to provide the best performance. TWTs are usually designed for water, forced air, or convection cooling to maintain the temperature at a safe operating level.
