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A backward-wave tube (BWT) is a type of microwave traveling-wave tube (TWT) that operates with the unique characteristic of energy flow in the slow-wave circuit moving in the opposite direction to the electron beam. This technology is utilized in various applications due to its capability as both a stable, low-noise amplifier and an oscillator.

Key Concepts:

  • Traveling-Wave Tube (TWT): A TWT is an electronic device used to amplify radio frequency (RF) signals by interaction between an electron beam and a slow-wave structure.
  • Slow-Wave Structure: This structure slows down the RF wave so that it can interact more effectively with the electron beam. In a BWT, the slow-wave circuit’s energy flow is opposite to the direction of the electron beam.

How it Works:

  1. Electron Beam: Electrons are emitted from a cathode and accelerated towards the anode, forming an electron beam that travels through the tube.
  2. Slow-Wave Circuit: A circuit or structure, such as a helix or a series of cavities, slows down the RF wave. In a backward-wave tube, this circuit is designed such that the wave propagates in the opposite direction to the electron beam.
  3. Energy Interaction: As the RF wave and the electron beam interact within the slow-wave circuit, energy is transferred from the electrons to the RF wave. This energy transfer amplifies the RF signal.
  4. Backward Energy Flow: In BWTs, the amplified wave moves in the opposite direction to the electron beam, which distinguishes them from other types of TWTs where the energy flows in the same direction as the electron beam.


  • Amplifiers: BWTs can serve as stable, low-noise amplifiers, which are crucial in communication systems for boosting weak signals without introducing significant noise.
  • Oscillators: As oscillators, BWTs can generate microwave frequencies. They can be easily tuned over a wide frequency range by adjusting the beam voltage, making them versatile for various frequency applications.


  • Wide Frequency Range: The ability to tune over a wide frequency range by changing the beam voltage allows for flexibility in different applications.
  • Low Noise: The design of BWTs helps maintain low noise levels, making them suitable for sensitive communication and radar systems.
  • Stability: BWTs provide stable performance, which is essential for reliable amplification and oscillation in microwave systems.


The backward-wave tube is a specialized microwave traveling-wave tube where the energy in the slow-wave circuit flows in the opposite direction to the electron beam. This design allows BWTs to function effectively as both amplifiers and oscillators, offering wide frequency tuning capabilities, low noise, and stable performance, making them valuable in various communication and radar applications.

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