Radar System Engineering

CHAPTER 10
THE MAGNETRON AND THE PULSER
BY G. B. COLLINS, J. V. LEBACQZ,AND M. G. WHITE
THE
MAGNETRON
BY G. B. COLLINS
Strenuous efforts were made by the British, beginning about 1938,
to develop high-power pulsed sources of radiation at very high frequencies,
because of the operational need for microwave radar. Two lines of
attack were followed. One consisted of attempts to improve and modify
conventional types of transmitting tubes, and the other of efforts to
devise entirely new forms of transmitting tubes.
Modifying the conventional type of transmitting tube to satisfy the
requirements of radar met with difficulties. To a first approximation
the electronic characteristics of a low-frequency oscillator or amplifier are
preserved at high frequency only if all dimensions of the tube are scaled
in proportion to the wavelength A; as a result, it is necessary to reduce the
size of tubes as the frequency is increased. The practical consequences
of this become serious as frequencies of 1000 Me/see are approached.
The reduction of cathode and plate areas, which under these conditions
vary as A2, rapidly reduces the available peak emission and plate dissipation.
Electrode clearances become so small that they are difficult to
maintain accurately. Although many improvements in the design of
triodes or tetrodes for use at microwave frequencies have been made, the
general limitations just outlined have resulted in greatly reduced efficiency
and power output in the microwave region.
Fortunately, a new type of pulsed microwave generator was invented
whose performance was indeed spectacular. These generators are now
known as ‘‘ microwave” or ‘i cavity” magnetrons, and they constitute
the most important single contribution to microwave radar. The
description, method of operation, and characteristics of this type of
magnetron are the subject of the first part of this chapter.
As a source of high-power microwaves the multicavity magnetron
represents a very great advance over both the conventional space-charge
and the velocity-modulated or klystron-tube types. A few numerical
comparisons will emphasize this superiority. Above frequencies of
3000 Me/see, space-charge tubes such as triodes cease entirely to be
practical sources of r-f power, while magnetrons produce pulse powers
320