Radar System Engineering

SEC. 10.5] MAGNETRON CHARACTERISTICS 341
nal to the tube and are accessible to change. In microwave magnetrons,
the circuit elements are an integral part of the tube and must be incorporated
with great care into every new design.
Thus any radar system designed to meet a new set of conditions or to
operate on a new frequency will require the development of a new magnetron
type, or at least a critical evaluation of the characteristics of
existing types. Since the over-all characteristics of the radar system
are so closely related to, and restricted by, the performance of the magnetron,
a general knowledge of the important characteristics of magnetrons
is essential.
Listed below with a discussion of each are the characteristics of
magnetrons of particular importance to system design. These characteristics
are not usually independent of one another and their relationships
are also considered.
Wavelen@h S’caling.-Since the wavelength of the radiation from a
magnetron is fixed or at best variable over a limited range, operation on
different wavelengths requires different tubes. To a first approximation,
magnetrons of different wavelength are derived from one another by a
simple over-all scaling process. All essential dimensions of the tube are
altered by the scaling factor a = A/XO, where A is the new wavelength
desired and XOis the wavelength associated with the original dimensions.
If thk is done, the new tube at wavelength ~ will operate at the original
voltage and current, and at a magnetic field H = ~ Ho, where Ho is the
operating magnetic field of the original magnetron. The power input,
and thus the power output, increases with increasing wavelength. A
rough rule is: The pulse power output (or input) of sculed magnetrons
varies as the square of their wavelength. The change in the size of the tube
with wavelength is the basis of this rule. The pulse power input is often
limited by cathode emission and, since the cathode area is proportional
to hz, pulse power input is also proportional to ~? Similar reasoning
shows that if the pulse power output is limited by r-f voltage breakdown
tvithin the tube, the same variation of power with wavelength is to be
expect ed.
This rule is an important one from the standpoint of system design.
At any given time it may not be exact, because special emphasis may
have been given to obtaining high peak powers at a particular wavelength
and a better design evolved as a result. In the long run, however,
the validity of the rule is reestablished, because any new design can,
within limits, be used to advantage at other wavelengths.
Pulse Power.-The most outstanding characteristic of pulsed microwave
magnetrons is their extremely high pulse power output, made
pchsible by the very large emission yielded by oxide cathodes when pulsed,