2-31
The output of a magnetron is reduced by the bombardment of the filament by electrons which travel
in loops, shown in figure 2-22, views (B) and (C). This action causes an increase of filament temperature
under conditions of a strong magnetic field and high plate voltage and sometimes results in unstable
operation of the tube. The effects of filament bombardment can be reduced by operating the filament at a
reduced voltage. In some cases, the plate voltage and field strength are also reduced to prevent destructive
filament bombardment.
ELECTRON-RESONANCE MAGNETRON.—In the electron-resonance magnetron, the plate is
constructed to resonate and function as a tank circuit. Thus, the magnetron has no external tuned circuits.
Power is delivered directly from the tube through transmission lines, as shown in figure 2-25. The
constants and operating conditions of the tube are such that the electron paths are somewhat different
from those in figure 2-24. Instead of closed spirals or loops, the path is a curve having a series of sharp
points, as illustrated in figure 2-26. Ordinarily, this type of magnetron has more than two segments in the
plate. For example, figure 2-26 illustrates an eight-segment plate.
Figure 2-25.—Plate tank circuit of a magnetron.
Figure 2-26.—Electron path in an electron-resonance magnetron.
The electron-resonance magnetron is the most widely used for microwave frequencies because it has
reasonably high efficiency and relatively high output. The average power of the electron-resonance
magnetron is limited by the amount of cathode emission, and the peak power is limited by the maximum
voltage rating of the tube components. Three common types of anode blocks used in electron-resonance
magnetrons are shown in figure 2-27.
