TABLE OF CONTENTS Vacuum tubes are used for nearly all high-power transmitter systems including power grid tubes, klystrons, helix-type TWTs, CCTWTs, crossed-field amplifiers (CFAs), magnetrons, gyrotrons oscillators, and gyrotron amplifiers.
Much of the information in this section is adapted from [1].
Grid-type vacuum tubes find extensive use as high-power amplifiers and modulators for frequencies below about 4 GHz. Their main application is at VHF and lower frequencies. Types of power grid tubes used include triodes, tetrodes, and pentodes. The triode is shown in Figure 19.1(a). Elements of the triode include the thermionic cathode, which emits electrons; the control grid, which controls the flow of electrons; and the anode, which collects most of the electrons. If the grid of a triode is biased to a sufficiently negative voltage, no current flows. As the grid is made less negative, more current flows to the anode. When the grid becomes positive with respect to the cathode, both the grid and the anode draw current.
In operation as an RF power amplifier, the triode must be either neutralized or operated in the common-grid mode. If that is not done, internal capacitance between the anode and the grid produces positive feedback that may cause oscillation.
Very large triodes may be used at the lower frequencies in high-power applications. As frequency is increased, however, the triodes must become smaller. This limitation is the result of reduced...
