Magnetron, Tunable

(voltage-tunable magnetron), a generating device of the magnetron type whose operating frequency varies over a broad range in proportion to the anode voltage. It is sometimes called a mitron.

The effect of voltage on the frequency tuning of a magnetron was first revealed by the American engineers D. Wilbur and P. Peters in 1949. In 1950 they proposed a tunable magnetron with a central cathode, and in 1955, a tunable magnetron with an electron gun. Tunable magnetrons with a power output of up to 1 watt (W) are used extensively in radio measuring equipment, in heterodyne broad-band radio receivers with rapid frequency tuning, and in master generators for radar sets; those with a power output of 1-10 W are used in radio altimeters, telemetry equipment, and other apparatus where frequency modulation is required over a broad band of generated frequencies; and those with a power output of more than 10 W are used in broad-band radio transmitters and television and telemetry apparatus for airborne systems.

Many types of tunable magnetrons that operated at frequencies from 0.2 to 10 gigahertz (GHz) were produced in the 1950’s and 1960’s. Tunable magnetrons with a power output of up to 1 W (inclusive) have a tuning range of about 1.0-1.5 octaves; those with a power of 1-10 W have a tuning range of up to 50 percent of the average frequency; and those with powers of 10-500 W have a tuning range of 10-20 percent. The effciency of a low-power tunable magnetron is usually not more than 10 percent, but it reaches 70 percent in the most powerful types.

A tunable magnetron differs from the ordinary multiresonator magnetron in that the g-factor of its oscillatory system is lower and the electron current in the interaction space is less. The oscillatory system is a cylindrical anode in the form of opposing pins built into a resonant cavity or a section of a line such as a section of a radio wave guide, a strip line, and so on. The current in the interaction space may be reduced by underheating the cathode, thereby limiting the electron emission, or by using an electron gun at one end and replacing the central emitting cathode with a nonemitting electrode. The second method has prevailed, since it makes possible changes in the current, and thus in the power of the magnetron, by means of a control electrode. As in a multiresonator magnetron, during the generation of oscillations the electron bunches move with a tangential velocity such that in each half-period of the oscillations they are shifted by a distance equal to the pitch of the anode pin system. This condition for synchronization is expressed by the following linear relationship between the anode voltage U_a (V) and the operating frequency/(GHz): U_a = 2π/N ×10^-5 ×B(r_a² - r_c²) ×f, where B is the induction of the magnetic field (in gauss), N is the number of pins, and r_a and r_c are the radii (in cm) of the anode and the central nonemitting electrode, respectively.