broader band of frequencies than the waveguide but they still have a high Q compared with the traveling-wave
tube.
Figure 22. Traveling-wave-tube couplers.
c. The direct coax-helix coupling shown in C of figure 22 is sometimes called direct-pin coupling. It is
a very simple method of coupling because the center conductor of a coaxial line is connected directly to the helix
of the traveling-wave tube. This type of coupling usually is used in high-power TWT's. The power-handling
capability is limited only by the amount of heat generated by the standing waves that are on the pin. The pin goes
through a glass seal on the tube, and heating sometimes causes the glass seal to break and damage the tube.
d. The coupled helix in D of figure 22, is a commonly used coupling. The short helix encircles and
magnetically couples to the ends of the main helix. This type of coupling has a broader bandwidth
than the waveguide and cavity coupling and it also has better standing-wave characteristics than
344 L1
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