2-5
Velocity Modulation
The microwave tube was developed when the use of the frequency spectrum went beyond 1,000
megahertz and into the microwave range. The microwave tube uses transit time in the conversion of dc
power to radio-frequency (rf) power. The interchange of power is accomplished by using the principle of
electron VELOCITY MODULATION and low-loss resonant cavities in the microwave tube.
A clear understanding of microwave tubes must start with an understanding of how electrons and
electric fields interact. An electron has mass and thus exhibits kinetic energy when in motion. The amount
of kinetic energy in an electron is directly proportional to its velocity; that is, the higher the velocity, the
higher the energy level. The basic concept of the electron energy level being directly related to electron
velocity is the key principle of energy transfer and amplification in microwave tubes.
An electron can be accelerated or decelerated by an electrostatic field. Figure 2-2 shows an electron
moving in an electrostatic field. The direction of travel (shown by the heavy arrow) is against the
electrostatic lines of force which are from positive to negative. The negatively charged electron will be
attracted to the positively charged body and will increase in velocity. As its velocity increases, the energy
level of the electron will also increase. Where does the electron acquire its additional energy? The only
logical source is from the electrostatic field. Thus, the conclusion is clear. An electron traveling in a
direction opposite to electrostatic lines of force will absorb energy and increase in velocity (accelerate).
Figure 2-2.—Moving electron gaining velocity and energy.
As figure 2-3 illustrates, the opposite condition is also true. An electron traveling in the same
direction as the electrostatic lines of force will decelerate by giving up energy to the field. The negatively
charged body will repel the electron and cause it to decrease in velocity. When the velocity is reduced, the
energy level is also reduced. The energy lost by the electron is gained by the electrostatic field.
