A large signal has a greater influence on electron velocity than a small signal. If an electron reaches the cavity at
gap A when the RF voltage is zero, the speed of the electron is unaffected. However, any electron reaching gap A
when the voltage is positive will be accelerated, while those electrons reaching the gap when the voltage is
negative will have their speed reduced. Now, if the electrons that were accelerated travel long enough, they will
eventually catch up with those that were slowed down shortly before on a previous negative half-cycle. Thus,
velocity modulation becomes a density modulation.
b. The RF section is made up of the drift tube and four resonant cavities which surround it at intervals
along its length. The drift tube is a round interrupted tube with a length almost 20 times its diameter. There are
four interruptions, or gaps, along the length of the drift tube. They are arranged so that the sides of the drift tube
protrude into the cavity wall. These opposing high-voltage points are surrounded by ceramic windows. Thus,
these drift tube tips become capacitance-loading elements when the cavity is excited. The external demountable
tuning boxes (resonant cavities) are assembled around the ceramic sections.
c. As the electrons pass through the remaining cavities, the bunching becomes more pronounced. As the
bunches pass through the output cavity, oscillations are set up in the cavity in much the same way that pulses of
current excite the plate-tank circuit of a class C amplifier.
d. Since power delivered to the output cavity is greater than power delivered to the input cavity,
amplification results. Power output is transferred to the antenna through a directional coupler by the output
coupling loop in the output cavity.
The collector section of the klystron consists of one electrode, the collector. Approximately 30 percent of
the beam energy is absorbed by the collector. The collector electrode gathers the unused electrons and passes
them out of the klystron into an external circuit leading to the positive terminal of the beam power supply.
MAGNETIC CONTROL OF ELECTRONS
a. A magnetic field is used to control the electrons in the drift tube. This magnetic field is created by
controlling amounts of direct current flowing in electromagnetic coils surrounding the klystron, as shown in
figure 16. The number of coils required varies with the tube type.
b. The prefocusing coil is inclosed in a special magnetic shell containing an annular airgap. The flux
outside the airgap forms a magnetic lens on the axis of the klystron at the point where the convergent paths of the
electrons are focused. The magnetic lens keeps the electrons from striking the drift tube wall before the beam
enters the main magnetic field created by the body coils.
c. The magnetic field in the body coils is adjusted to control the diameter and direction of the electron
beam as it passes through the klystron, so that as little beam current as possible will strike the drift tube wall