direct coax-helix coupling. Even though this type of coupling appears to be most desirable, it is limited to
handling low power because it is structurally unable to handle high power.
1-26.
ADVANTAGES AND DISADVANTAGES
a. The broadband characteristics of the traveling-wave tube have satisfied the demand for a broadband
device more than any other tube developed today. This unique advantage of the TWT is almost offset by several
disadvantages. The external magnetic focusing coil may require as much as 11 amperes of solenoid current with a
voltage drop across the coil of 15 to 85 volts. None of this power can be recovered in the output.
b. This high power required by the solenoid generates a tremendous amount of heat which must be
dissipated, because the tube must be kept cool. This usually requires large air conditioning units. Even slight
heating can distort the helix and cause nonuniformity in it. A nonuniform helix causes "holes" or areas of no
transmission in the output of the TWT. Areas of no transmission relate to the inability of the tube to operate at
some frequencies. Holes may exist in a normal TWT because it is almost impossible to manufacture one that is
perfect.
c. Because the holes are known to exist, their positions (frequencies) must be accurately determined. To
locate these holes, one of the most elaborate tube testers has been developed to evaluate a traveling-wave tube.
So much difference exists between each tube that they must be calibrated individually.
d. Despite their many disadvantages, the TWT's are allowing the development of more new electronic
equipment. For example, a microwave repeater in a communication link can be made with a single TWT.
Section VII. BACKWARD-WAVE OSCILLATORS
1-27.
INTRODUCTION
A backward-wave oscillator is similar to a traveling-wave tube, except for the following differences:
a. Unlike the TWT, the backward-wave oscillator has no attenuator. As a result, the RF signals that
travel backward toward the cathode are not suppressed.
b. The helix is terminated with a matching impedance.
c. The output is taken from the end of the helix nearest the electron gun.
d. The insertion of a terminating impedance causes dissipation of the RF signal traveling in a forward
direction toward the collector; this does not occur in a traveling-wave tube.
e. The electron beam interacts with an RF signal traveling in the opposite direction on the helix
(backward) toward the cathode.
344 L1
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