January-February - Air Defense Artillery

Figure 4. PPI presentation. Compare aerial radar scope photo with actual chart of island (Nantucket). Land, ships,
aircraft appear white. Bright spot in center is position of plane which took picture.
is in contrast to most other types of range finders which have
a constant percentage error in determining the range.
Azimuth and Elevation Determination
The use of timed pulses, as we have seen, gives a simple
means of measuring range. How then is the direction in
which a target lies determined? This is done by providing
the radar with a directional antenna, which sends out the
pulses in a narrow beam, like a searchlight. Since azimuth
and elevation determination are similar, only azimuth determination
will be discussed here. There ar~ in use two systems,
the single lobe and double lobe.
Single Lobe System
Here all energy is directed in a single lobe. The antenna
may be rotated as the pulses are sent out, and we get back
a "pip" when the antenna is pointed toward its target. \Ve
get the strongest pip when the beam of energy sent out by
the radar is pointed directly at the target. The azimuth of
the center of the antenna gives us the azimuth of the target.
Double Lobe System
The method used in the SCR-268 and several other sets
for azimuth determination is a double lobe system. Two
identical lobes whose axes are displaced by som~ fixed angular
distance are used. The two lobes intersect at onl" one
point, known as the crossover point. Each lobe receives a
pip from the target. \ Vhen the t\\'O pips are of equal size
the antenna is pointed directly at the target. The use of two
lobes instead of a single lobe greatly increases the precision
of azimuth measurement because the operator has a more
sensiti"e indicator to show whether or not he is off target.
(Figure III.) In addition, it provides the operator with a
sense of direction. If the antenna is off target the inequality
of the pips shows him the direction to turn. These advan-
Hence the single lobe system is most often used in s I
sets while the double lobe system is usually employ
fire control systems.
The problem of antenna design is one of the major
lems in radar, incomprehensible as this may seem t
operator of a home radio receiver who finds a few yar
wire strung up on his roof adequate for his purpose. A
antenna must possess the following characteristics:
a. It must be directional; that is, it must concentrat
radio energy into a definite beam. All of the power sl
go into this beam and none should leak off into "side 1
in other directions, for such side lobes may be fatall"
fusing .. ,
b. It must be capable of being directed from a re
place and must be stabilized on board ships and airpl
to take out the motion of the craft itself.
Antennas are made directional by building them up
array of small antennas suitably spaced and timed to
centrate the energy in one direction, or they may be
on a searchlight principle of spraying the energy into a 1
parabolic reflector which focuses the energy into a bea
Illdicators
Two principal classes of indica tors are used in radar
meters and cathode-ray tubes.
,\ Ieters have been ~sed principally for azimuth and
vation indication: the needle deflecting to one side \
the antenna was off target. However, meters present
principal disadvantages (I) their slowness of reaction
(2) their inability to distinguish between multiple tar
The cathode-ra" tube indicators mav be subdivided
two general classes. In one, termed "d~f1ection modula
the echo shows as a deflection in a bri~ht line. The
type of presentation was illustrated in Figure II. Ther