Radar System Engineering

10 INTRODUCTION [SEC.14
step in output power was due to an improvement in the magnetron itself.
The increase at 10-cm wavelength in the early part of 1941 was brought
about by the development of modulators of higher power.
It is important to realize that the curves of Fig. 1.5 lie above one
another in the order of increasing wavelength not because development
was begun earlier at 10 cm than at 3 cm, and earlier at 3 cm than at 1 cm,
but because magnetrons of the type used in radar are subject to inherent
limitations on maximum power which are more severe the shorter the
wavelength. The same is true of the r-f transmission lines used at
microwave frequencies. The horizontal dashed lines shown in Fig. 15a
show the maximum power that can be handled in the standard sizes of
‘‘ waveguide” used for r-f transmission at the three bands.
A similarly spectacular decrease in the minimum detectable signal,
due to the improvement of microwave radar receivers, has marked the
war years. In the wavelength bands above about 10 m, natural “static”
and man-made interference set a rather high noise level above which
signals must be detected, so that there is little necessity for pursuing the
best possible receiver performance. This is not true at microwave frequencies.
Natural and man-made interference can be neglected at these
frequeficies in comparison with the unavoidable inherent noise of the
receiver. This has put a premium on the development of the most
sensitive receivers possible; at the end of 1945 microwave receivers were
within a factor of 10 of theoretically perfect performance. Improvement
by this factor of 10 would increase the range of a radar set only by the
factor 1.8; and further receiver improvement can today be won only by
the most painstaking and difficult attention to details of design.
Why Microwave s?-The reader will have observed that when radar is
discussed in what has gone before, microwave radar is assumed. This is
true of the balance of this book as well. So far as the authors of this
book are concerned, the word m.dar implies not only pulse radar, as has
already been remarked, but microwave pulse radar. Though it is true
that the efforts of the Radiation Laboratory were devoted exclusively to
microwave pulse radar, this attitude is not entirely parochialism. The
fact is that for nearly every purpose served by radar, microwave radar
is preferable. There are a few applications in which longer-wave radar
is equally good, and a very few where long waves are definitely preferable,
but for the overwhelming majority of radar applications microwave radar
is demonstrably far more desirable than radar operating at longer
wavelengths.
The superiority of microwave radar arises largely because of the
desirability of focusing radar energy into sharp beams, so that the direction
as well as the range of targets can be determined. In conformity
with the well-known laws of physical optics, by which the sharpness of