Radar System Engineering

314 ANTENNAS, SCANNERS, AND STABILIZATION [SEC. 9.21
the other is transmitting a pulse. On account of the curvature of the
earth and for reasons of microwave propagation it is usually advantageous
that the scanner be installed as high as possible. Radomes are not widely
used to house shipboard scanners, the tendency being to make the
antennas of such low wind resistance and such sturdy construction as to
render radomes unnecessary.
Ground-based radars may be grouped, according to their mobility,
as transportable, mobile, and fixed. Because the transportable scanners
are disassembled for moving, their installation requirements include
easy assembly. The AN/TPS-10 scanner shown in Fig. 9.15 disassembles
into small parts for easy mobility in mountainous terrain. A firm
and adjust able foot ing must be incorporated for transportable scanners.
A truck or trailer on which a scanner is mounted must also be firmly
braced while the radar is in use; the scanner itself must be adjustably
mounted to the vehicle, if accurate position-finding is desired.
The siting of ground-based scanners is to be regarded as an aspect of
the installation problem. A suitable site must be one where no mountain
seriously blocks off the region beyond. A more subtle siting requirement
is the avoidance of ground “clutter”; permanent echoes are bothersome
at certain sites, particularly if the radar targets frequently pass at nearly
the same range and direction as such echoes. Moving-target indication,
dkcussed in Chap. 16, is now practicable to alleviate ground clutter.
9.21. Radomes.1-It is necessary to protect all airborne and some
surface-based microwave antennas from wind and weather. The protective
housings have come to be known as ‘‘ radomes.” A fuller discussion
than the following will be found in Vol. 26 of this series.
The circular cylinder shape was early used for the radome. For such
a shape the radiation falls on the radome essentially at normal incidence,
the angle of incidence being defined as the angle between the incident
radiation and the normal to the surface at the point of incidence.
The development of microwave airborne radar systems forced the
development of the science and art of radome design. It resulted in
amplification of the electrical theory for radomes through which the radar
beam passes at high angles of incidence. Plywood, first used for radome
construction, gave way to more satisfactory synthetic materials; and
aerodynamics, at first ignored, was given consideration in the design.
Excellent installations are possible if the radar systems are considered in
the initial design studies of aircraft.
Since the airborne radome involves all the problems of the surfacebased
installation in addition to many others peculiar to itself, this
discussion will be limited to the airborne case. Some of the factors controlling
the design of an airborne radome are the radar antenna and its
1Ssca 9,21-9.25 by F. J. Mehringer.