Microwave tube

(electronics) A high-vacuum tube designed for operation in the frequency region from approximately 3000 to 300,000 megahertz.

A high-vacuum tube designed for operation in the frequency region from approximately 3000 to 300,000 MHz. Two considerations distinguish a microwave tube from vacuum tubes used at lower frequencies: the dimensions of the tube structure in relation to the wavelength of the signal that it generates or amplifies, and the time during which the electrons interact with the microwave field. See also Vacuum tube.

In the microwave region wavelengths are in the order of centimeters; resonant circuits are in the forms of transmission lines that extend a quarter of a wavelength from the active region of the microwave tube. With such short circuit dimensions the internal tube structure constitutes an appreciable portion of the circuit. For these reasons a microwave tube is made to form part of the resonant circuit. Leads from electrodes to external connections are short, and electrodes are parts of surfaces extending through the envelope directly to the external circuit that is often a coaxial transmission line or cavity. See also Cavity resonator; Transmission lines.

At microwaves the period of signal is in the range of 0.001-1 nanosecond. Only if transit time is less than a quarter of the signal period do significant numbers of electrons exchange appreciable energy with the signal field. Transit time is reduced in several ways. Electrodes are closely spaced and made planar in configuration, and high interelectrode voltages are used.

Tubes designed by the foregoing principles are effective for wavelengths from a few meters to a few centimeters. At shorter wavelengths different principles are necessary. To obtain greater exchange of energy between the electron beam and the electromagnetic field several alternative designs have proved practical.

Instead of collecting the electron beam at a plate formed by the opposite side of the resonant circuit, the beam is allowed to pass into a field-free region before reacting further with an external circuit. The electron cloud can be deflected by a strong static magnetic field so as to revolve and thereby react several times with the signal field before reaching the plate. See also Klystron; Magnetron.

Instead of producing the field in one or several resonant circuits, the field can be supported by a distributed structure along which it moves at a velocity comparable to the velocity of electrons in the beam. The electron beam is then directed close to this structure so that beam and field interact over an extended interval of time. See also Microwave; Traveling-wave tube.