September 1945, Electronics - The SCR-268 RADAR
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     This article is a technical overview of the SCR-268.  The SCR-268 was recognized as the "backbone" of America's radar defense at the end of the war by the National Defense Resource Council. 
     Developed by the Signal Corps with little funds in the 1930s this radar and the SCR-270 and SCR-271 were ready when America entered WWII.  It represents an amazing achievement by the engineers of Fort Monmouth.
    Initial work on the radar was done at Fort Monmouth, then moved to Fort Hancock on Sandy Hook for security.  Once the war had begun in Europe the Signal Corps realized how exposed Sandy Hook was to sea attack.  The radar work was relocated to Camp Evans in early 1942.  There the systems were upgraded during the balance of the war to meet the challenges presented by the enemy and to take advantage of new components. 

See:THE SIGNAL CORPS DEVELOPMENT of U.S. ARMY RADAR EQUIPMENT, PART 1
Early Research and Development - 1918 - 1937 by H. M. DAVIS, 1st Lt., Signal Corps March 1943
for an account of the early development of radar at Fort Monmouth.

  Contents of this article for quick reference: 

Page 100:
   Fig. 1 - Artist's sketch illustrating a typical setup
   Basic Elements
Page 101:
   Fig. 2 - The SCR-268 radar photo
   Fig. 3 - Simplified block diagram showing basic components of the equipment
Page 102:
   Fig. 4 - Fundtional block diagram of the SCR-268 radar keying unit
   Fig. 5 - Simplified schematic of the modulator
Page 103:
   Fig. 6 - Modulator wave forms
Page 104:
   The Keying Unit
   Fig. 7 - Simplified schematic of the SCR-268 radar transmitter
   The Modulator Unit
Page 105:
   Fig. 8 - Basic directive array scheme
   The Transmitter Proper
   Fig. 9(a) - Directive pattern of receiving array, (b) Double receiving antenna pattern
Page 106:
   Transmission Line and Radiator
   Receiving Arrays and Lobe Switching
   Fig. 10 - SCR-268 radar receiving antenna lobe-switching
Page 107:
   Fig. 11 - Functional block diagram of one of the two receivers
   The Receiver
Page 108:
   Fig. 12 - Diagram showing how the SCR-268 radar range unit functions
   The Range Unit
   The Indicator Units
Page 109:
   Fig. 13 - Functional block diagram of one of the three oscilloscope indicating units
Range Indicator
   References


ELECTRONICS
September 1945

By McGraw-Hill Staff
Page 100 - 109
evans logo
The SCR-268 RADAR


Developed before the war by the Signal Corps, the SCR-268 has seen service on all fronts, -
detecting enemy aircraft, directing searchlights and guns toward them. Surpassed in per-
formance, and captured by the Germans and Japanese, it may now be described
ON THE EVENING of May 26, 1937, Mr. Harry Woodring, the Secretary of War, stood with a group of officers and civilian scientists on a field of the Signal Corps Laboratories at Fort Monmouth, New Jersey.  Before them, spread over the field, were the transmitter and receivers of a radio detector, the SCR-268 prototype.  Connected with the radar were the controls of a standard anti-aircraft searchlight.
     As prearranged, an Army aircraft flew over the field in the darkness.  Three enlisted men, viewing oscilloscopes, put the radar "on target" and tracked it across the sky. When tracking was established, the command was given to light the searchlight.  As the light pierced the darkness, the aircraft, caught in the beam, became visible to those on the ground.
     The next day, orders were given
to move the equipment to a remote corner of Fort Hancock, where secrecy could be better preserved.  In the eight years that have intervened, secrecy has been the watch-word of radar development.  But in November 1944, the SCR-268 was reduced in classification from "confidential" to "restricted".  By that time, the equipment had been captured, with crews and instruction books, by the Germans and the Japanese. Moreover it had been far surpassed by later radar equipments.  While still in use, the 268 was obsolescent, almost antique.
    When the editors of ELECTRONICS first approached Signal Corps authorities for permission to describe the SCR-268, they were asked "Why do you want to describe that set?"  The answer, not obvious to those working in the radar field, is that the greater part of the electronics industry has never seen a radar
set, or a technical description of one, in any form. The SCR-268, by virtue of its early development and wide employment in the war, is not only a significant milepost in radar development, but the first radar to reach such a venerable position that describing it would offer no aid or comfort to the enemy.

Basic Elements

    A radar detects the presence and position of objects by means of reflected radio waves.  In the case of the SCR-268, the "object" is ordinarily an aircraft, and the radio waves are projected in sharp bursts or "pulses" at a rate of 4098 per second.  A pulse is transmitted every 244 microseconds.  Each pulse is of extremely short duration, approximately six microseconds.  Between pulses, there is a period of about 240 microseconds during which the radar receivers may detect the echoes reflected from the target aircraft.
    When the radar detects a target, the position of the aircraft is indicated in three coordinates, known as the slant range, the elevation (or angular height), and the azimuth.  The slant range is the distance from the radar to the target.  The elevation is the vertical angle subtended at the radar by the target and the ground plane.  The azimuth is the horizontal angle subtended at the radar by the target and true north.  The target coordinates are illustrated in Fig. 1.
     The radar measures the range of the target by timing the interval between transmission of a pulse and reception of the echo.  Since radio waves travel at a velocity of 0.186 miles per microsecond, for each microsecond in the interval the wave travels 0.186 miles round trip from radar to target and back. Accordingly, the distance to the target is 0.093 miles for each microsecond

                                     100                                                                                September 1945 - ELECTRONICS


Page updated January 15, 2004  page created September 1, 2003


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