7 - Thermionics, Power Grid and Linear Beam Tubes

Abstract

The earliest radio frequency tubes were triodes and multi-grid tubes. At low frequencies, the principle of operation of these tubes is quite simple. The rf signal is applied to the grid facing the cathode, which is a source of electrons sensitive to the applied electric field, connected to the negative pole of a power supply. Parasitic impedances, which are negligible at lower frequencies, become important. These are the reactances because of the inter-electrode capacitances as well as the stray capacitances between connections. Electron transit times between electrodes, especially between cathode and modulating grid, become of the same order of magnitude as the rf period. These transit times are simply because of the fact that electrons are massive particles and obey the laws of classical dynamics. Any empty volume surrounded by a good conducting material can be considered as a resonant cavity. If the conductivity is large enough for the losses to be considered negligible, the cavity, when excited by a microwave source, will exhibit electromagnetic fields only at discrete frequencies that form an infinite spectrum.