by the radar is transmitted automatically to
control circuits which keep the searchlight or anti-aircraft gun trained
in the direction (azimuth and elevation) indicated by the radar.
Thus when the radar is on the target the searchlight or gun is also on
the target and can illuminate or fire at the will of the artillery officer.
The reflection of radar signals is a highly inefficient
process. The power radiated in the pulse disperses with the square
of the distance as it travels to the target, and the power in the echo
disperses at the same rate as it returns to the radar. It follows
that the power available to actuate the receivers falls off as the fourth
power of the distance to the target. This means |
that an increase of 16 times in power (12 db)
is required to double the maximum range of a radar viewing a given target
in empty space. Thus an aircraft which is easily visible at 5 miles, using
moderate power and receiver sensitiv- |
 |
ity, may be completely invisible at 20 miles.
This reasoning points to the necessity
of employing the highest possible power in the transmitter and the greatest
possible sensitivity in the receiver. In the SCR-268, the peak power
of the pulses is 75 kilowatts, and the receivers are sensitive to a signal
power of 0.1 of a micromicrowatt (10-" watt). Judged by pre-war standards,
these are remarkable achievements at the frequency (205 me) at which the
radar operates. But by present standards the transmitter power is
not outstanding, and the receiver performance is poor. Nevertheless,
the power and sensitivity are adequate to detect aircraft at distances
up to 22 miles, provided the aircraft is not too close to the horizon.
General Description
The SCR-268 is pictured in Fig. 2. The
radar components are mounted on a trailer. In addition, four large
trucks are required, one supplying primary power, another supplying high
voltage for the transmitter, and two for the transportation of the radar
components.
The trailer consists of a rotatable pedestal which
carries three antenna arrays. From left to right as seen from the
back of the equipment, these are the azimuth receiving array, the transmitter
array, and the elevation receiving array. Behind each receiving array
is the corresponding receiver. Atop the pedestal is the transmitter.
Directly below the transmitter are three oscilloscopes, with bucket seats
in front of them. Here the three operators observe the azimuth, elevation
and range, turning handwheels which keep the radar pointed at the target.
The handwheels are con-
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