Radar System Engineering

SEC. 15.12] DESIGN OBJECTIVES AND LIMITATIONS 613
Limitations Imposed by Aircraft Installation.—The performance
requirements just mentioned can be simply stated, and it is easy to
check whether the completed radar meets them successfully. Neither
of these remarks applies to the extremely important set of design limitations
that arise from the fact that the radar is to be used in aircraft.
The more important of these limitations are mentioned below.
The outstanding requirement is for low total weight. Every part
of the design is influenced by the necessity for making the final weight
of the radar as small as possible.
Low power consumption is important. Many aircraft types have
relatively low electrical-generating capacity, and it is most desirable
to be able to install a radar meant primarily as a navigational aid without
the necessity of revising the electrical installation of the aircraft. The
set must be able to operate either from a fixed-frequency 400-cps motoralternator
set, or from a variable-frequency engine-driven alternator,
which may supply power at a frequency as high as 2400 cps.
Aerodynamic drag produced by the antenna housing must be as low as
possible. This requirement puts a great premium on as small an antenna
as possible. Taken in conjunction with the requirement for a narrow
beam, this means that the wavelength on which the radar operates
should be as short as is practicable.
J3aseof installation in all t ypes of aircraft is important. This suggests
that the radar be built in the form of several small components instead
of one or two large units, since small units afford greater flexibility in
installation. Maintenance is also simplified by dividing the equipment
into units that can be replaced for checking and removed for repair.
The necessity for connecting such units with cables adds considerably
to the total weight of the set, however, and the physical fayout must be
carefully planned to reduce the number of cables and to keep their
lengths as small as possible.
Equipment mounted in aircraft is subject to extreme variations of
pressure and temperature. External air pressure can vary from that at
sea level to the pressure of less than one-quarter atmosphere found at
30,000 ft. At the reduced pressures of high altitude, clearances required
to prevent arc-over from high-voltage points are likely to become excessive.
Further, the low temperatures of high altitude are likely to cause
condensation, in the r-f line, of the water vapor that is present in warm
sea-level air. The simplest solution for these difficulties is to seal the
transmitter and the r-f line pressure-tight so that they can be maintained
at sea-level pressure at any altitude.
Such pressurization of the modulator and the r-f system increases
the problems of cooling. To meet operating conditions on the ground
in the tropics, the radar must operate indefinitely at an ambient tem-