Radar System Engineering

182 THE GATHERING AND PRESENTATION OF RADAR DATA [SEC.6.10
The set was built both for mobile use (SCR-270) and for fixed-station
installation (SCR-271). During the general advance in radar technique,
the set went through many detailed changes and improvements and was
still in active use at the close of the Japanese war.
The SCR-2701 and the CXAM are quite similar in their operating features.
The SCR-270 operates at a frequency of 106 Me/see, has a peak
power of 100 kw, a pulsewidth from 10 to 25 psec, and a reliable range
against single bombing aircraft of more than 100 miles. The repetition
rate is 621 pps. The same antenna is used for both transmission and
reception. This antenna (Fig. 6.18) consists of a dipole array originally
four dipoles +de and nine high, with a reflector; the antenna gain is 140
and the beamwidths 28° in azimuth and 10° in elevation. Provision is
made either for continuous rotation of the antenna at 1 rpm or for manual
training to maximize target signals.
The SCR-270 has an A-scope display, with a control for so varying
the start of the sweep that a selected target signal can be brought under a
marker on the face of the tube; a scale on the phasing control then indlcates
target range directly. Target bearing is determined by reading off
the antenna azimuth when the echo signal from the target in question
appears to be strongest. The azimuth accuracy depends on proper
siting, as in the case of all radar making use of ground reflections, and
attainable accuracy in practice is about + 4°. Height-finding can be
performed by the method of nulls (Sec. 6.11).
Later models of this set include, among other improvements, an antenna
giving a beam narrower in azimuth, and a PPI indicator. The simplicity,
reliability, and ruggedness of the SCR-270 made it a very useful and very
widely used equipment despite its great bulk and weight, its poor lowangle
coverage, its vulnerability to ground clutter in anything less than a
very carefully chosen site, its lack of range resolution and azimuth discrimination,
and its low traffic-handling capacity.
6.10. PPI Radar for Search, Control, and Pilotage.’-In the last
section, an important historical development has been traced. The
first radar sets used fixed antennas and floodlighting technique, and
required that a manipulation be performed to find the azimuth of each
target. Next were designed relatively narrow-beam, continuously scanning
radars, in which each target in the field of view is displayed periodically,
so that its azimuth and range can be read off at regular intervals.
A further step was made with the development of the plan-position indicator,
or PPI, which, although the radar beam was still narrow and still
1Sincethe radardesignof the SCR-270 does not differfrom that of the SCR-271,
the designationSCR-270 will be used,as a matterof convenience,to standfor either
equipment.
~FfyL. N. Ridenour.