Radar System Engineering

340 THE MAGNETRON AND THE PULSER [SEC. 10.5
diagram, which corresponds to a matched load, represents a reasonable
compromise between efficiency and frequency stability.
It is possible to adjust the loading on the magnetron to any reasonable
value by the suitable use of r-f transformers (Sec. 11.1) in the output line.
As an example, suppose it is desirable to operate the magnetron represented
by Fig. 10.17 at a point of high efficiency and low frequency
stability corresponding to point A. This can be accomplished by introducing
a transformer which sets up a 2-to-1 VSWR and making its distance
from the magnetron such that the phase of this VSWR corresponds
to point A. By moving this transformer along the line in either direction
one-quarter wavelength, operation corresponding to point B can be
obtained. In comparing the eflect of different loads corresponding to
various points on the Rieke diagram, it should be realized that these
points represent transformations that reduce the size of a circle of constant
VSWR as its center is moved away from the center of the diagram.
In Fig. 10.17 the circles about points A and B represent the VSWR = 1.5
circle when displaced different distances from the center of the diagram.
The load points A and B, with their associated variations in load in
the above example, are especially simple cases. In general, the load
variations correspond to a very irregular path on the Rieke diagram
whose behavior is unpredictable. A safe policy in design is to estimate
the maximum variation in VSWR to be expected from the r-f circuits, and
to employ a loading of the magnetron which does not produce a frequency
change too large to be accommodated by the radar receiver even when
this variation in VSWR is of such a character as to produce the maximum
possible frequency shift.
Appreciation of the effects of the r-f load on the performance of magnetrons
has contributed more than any other single factor to magnetron
reliability. As a corollary, it is also true that many troubles attributed
to magnetrons result from a failure to use properly the information provided
by a Rieke diagram.
10.5. Magnetron Characteristics Affecting Over-all Systems Design.
One of the shortcomings of microwave magnetrons is their limited
adaptability to different requirements. This circumstance has forced
the design and production of an extremely large number of tube types.
Although the development of microwave magnetrons in this country
began only in late 1940, there now exist over 100 distinct types of magnetrons,
despite early and continuing attempts at standardization.
During the past war it was true, almost without exception, that each new
radar system made new demands on the magnetron and required the
development and production of a new type. This has not been necessary
in the case of conventional types of tubes, since the associated circuit
elements which are largely responsible for over-all performance lie exter-