Radar System Engineering

352 THE MAGNETRON AND THE P ULSER [SEC. 106
magnetron-magnet combination is now a small fraction of the weight of
the pulser required to drive it. The weight of magnetrons and their
magnets has thus ceased to be a critical design consideration.
10.6. Magnetron Characteristics Affecting Pulser Design—The
magnetron, standing as it does be Lween the pulser and the r-f system,
imposes restrictions on the design of both these components. Pulser
design in particular has been complicated by some very strict requirements
which the magnetron places on pulse shape and voltage regulation.
These requirements arise from undesirable magnetron characteristics
which it has so far been impossible to remove in the design of the tubes.
They may perhaps be eliminated from magnetrons in the future.
The stringency of the conditions that the magnetron places on the
pulser or on the r-f components increases rapidly as the maximum operating
conditions are approached. Thus the surest way to obtain reliable
performance is to operate the tubes at conservative ratings.
Little need be said here about the interaction between the magnetron
and the r-f components. The relationship between frequency stability,
VSWR of the r-f system, and the magnetron pulling figure has been
covered in Sec. 10.4. The related subject of long-line effect is discussed
in Sec. 11”1.1
Because the interaction between the pulser and the magnetron has
only recently been understood, many serious difficulties have been
alleviated in the past by the unsatisfactory process of cut-and-try. Four
characteristics of magnetrons are largely responsible for these troubles.
Change of Frequency with Current.—The input impedance of a magnetron
varies with voltage as shown in Fig. 10.29. The average input
impedance is usually in the range of 400 to 1000 ohms. Very small
changes in anode potentials produce large changes in anode current.
For example, the performance chart of a 4J31 magnetron (Fig. 10.16)
shows that, at a magnetic field of 2300 gauss, a change of voltage from
20 kv to 22.5 kv causes a change in current from 20 to 50 amp. Variations
in voltage during a pulse must be kept within small limits to prevent
large current variations and consequent distortion of the r-f pulse shape.
The rate of change of frequency with anode current, dv/dI, determines
the amount of frequency modulation and undesirable distortion
in the energy spectrum of the pulse which will be caused by current
variation during the pulse. In the usual operating range for most magnetrons,
dv/dZ is about 0.1 Me/see per amp, but it can be as large as
+ 1 Me/see per amp. The exact value depends to a considerable degree
on the operating point selected. To illustrate this problem, consider a
magnetron with du/dI = 0.4 Me/see per amp driven by a l-psec pulse
which “droops” from 55 to 50 amp. This 5-amp change in current will
1 See also Microwave Magnetrons, Vol. 6, Sec. 7.2, Radiation Laboratory Series.