Figure 19. Traveling-wave tube with focusing coil.
1-24.
OPERATION
a. The TWT can be compared with a klystron in that both tubes velocity-modulate a high-velocity
electron beam. The electron beam takes on energy from the dc power through the acceleration of the electrons.
An RF signal along the path of the electron beam causes the electron velocities to change so that some are
speeded up and others are slowed down. Changing the individual velocities of the electrons in the beam forces
them into groups, or bunches. At the output end of these tubes, the electron bunches are decelerated, causing
them to give up their energy to an output circuit.
b. In the klystron the RF signal that interrupts the electron velocities is from a resonant cavity, and in the
TWT the RF signal is from the helix. Here the similarity between the TWT and the klystron ends.
c. The TWT differs from the klystron in that the electrons interact with a traveling wave rather than a
standing wave. This interaction is distributed along the helix, not localized as in the klystron. There are no high-
Q resonant circuits in the TWT, so it can amplify over a broad band of frequencies.
d. A simple concept of a traveling-wave tube is shown in figure 20. An RF signal is applied to a
straight-wire transmission line at the input side, and an electron beam is passed along, parallel with the straight-
wire line. If the output of the straight-wire line is terminated in its characteristic impedance, the line is
nonresonant and will pass a broad band of frequencies. When an RF signal is applied to the straight-wire line,
part of the RF signal's electric field is parallel with the direction of travel of the electron beam. This will cause
interaction between the RF signal and the electron beam.
e. If the electrons in the beam could be accelerated so as to travel faster than the RF signal
(electromagnetic wave) on the straight wire, bunching would occur because of the effect of the RF signal on the
electron beam. Some of the bunches that are decelerated give up energy to the RF signal on the straight-wire line
and increase the amplitude of the original RF signal. This action is possible over a wide range of frequencies,
allowing the TWT to act as a broadband amplifier.
344 L1
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