Radar System Engineering

SEC. 9,5] CONSTRUCTION OF RADAR ANTENNAS 279
Beacon antennas are often made up of a vertical array of radiating
elements excited in phase. By correct design of such an antenna the
pattern can be made reasonably uniform in azimuth and at the same time
well confined to the region of the horizon. Like other beacon antennas,
the antennas used for IFF equipment are not strictly radar equipment
and will not be discussed. The same statement applies to the antennas
used in the search and jamming functions of radar countermeasures.
9.6. Construction of Radar Antemas.-The main requirements in the
mechanical construction of airborne antennas are accuracy of form, the
ability to withstand field service conditions, and light weight. Surfacebased
antennas must be no less accurate, and considerations of weight,
inertia, and wind forces are paramount. Considerable advances have
been made in the mechanical design of airborne, and particularly of surface-based,
antennas for microwave radar during the recent war. In
this section only antennas in which the feed is distinct from the reflector
are considered; array antennas are not discussed.
Sheet aluminum or magnesium alloy is always used for the airborne
reflectors. Small reflectors up to 30 in. in diameter are simply spun or
otherwise formed, a bead around the rim being added for stiffness, as in
Fig. 9.17. The tolerance allowable in the surface of the reflector is about
~+ of a wavelength.
The larger sizes of airborne reflectors, whether paraboloid or shaped
cylindrical surfaces, thus far installed in a streamlined airplane are considerably
wider than high and are used for circular scanning. On these
it is good practice to use aircraft construction methods. The selection of
materials used depends upon the size of the reflector and the stresses
involved. Magnesium or aluminum alloy will serve for airborne equipment.
Since aluminum alloy has better forming characteristics than
magnesium, it is considered the best material for the reflector face. The
most accurate method of forming aluminum alloy sheet for the reflector
face is by stretching the material over a metal die which has been cast
and ground to the desired contour; these dies are usually made from
Kirksite, a lead and zinc alloy. After forming, the sheet metal conforms
to the exact contour of the die providing the draw is shallow enough, as is
the case with most radar reflectors. The next problem is to support this
reflector face rigidly and to maintain the contour already obtained by
forming. This can be done, as in the 5-ft reflector of AN/APQ-13 shown
in Fig. 9.8, by riveting to the rear surface of the reflector several stamped
or machined ribs contoured thereto and boxing in by riveting another
sheet of aluminum or magnesium alloy across the back; flush rivets are
preferable on the front face. A 42- by 10-in. reflector with single curvature
can be made within a tolerance of t 0.005 in.
In the case of a large reflector, over 5 ft in width, weight may be saved