page_title: SCR-268 radar
section_title: Description
hierarchical_section_title: SCR-268 radar / Description
caption_attribution_description: Lot-2292-4: WWII-Communications: Radar in Operation. Oscilloscope operators on an SCR 268 radar maintain watch near Menella, Italy. Nearest the camera is the range scope, next is the azimuth scope, and, at the right, the elevation scope. U.S. Army Signal Corps Photograph: SC-200984-S. Courtesy of the Library of Congress. (2018/02/23).
context_page_description: The SCR-268 was the United States Army's first radar system. Introduced in 1940, it was developed to provide accurate aiming information for antiaircraft artillery and was also used for gun laying systems and directing searchlights against aircraft. The radar was widely utilized by both Army and Marine Corps air defense and early warning units during World War II. By the end of World War II the system was already considered out of date, having been replaced by the much smaller and more accurate SCR-584 microwave-based system.
context_section_description: The SCR-268 antenna system consisted of a number of dipole elements arranged in three groups, each in front of a passive reflector, mounted on a large aimable cross. The cross consisted of a short upright pedestal sitting on top of a large base platform, mounting long cross-arms extending from the midpoint of the upright pedestal. The antenna system was about forty feet wide and ten feet high overall. Both the pedestal and the cross-arm could be rotated around their axis for aiming, in azimuth and altitude respectively.
  The left side of the cross arm, as seen from the back, contained a set of dipoles that were set to be sensitive in angle, while almost insensitive to elevation. It was arranged six dipoles wide and four dipoles high, each with its own reflector. On the far right side was a similar, but smaller, arrangement rotated 90 degrees in order to be sensitive in elevation and not angle. This portion was two dipoles wide and six dipoles high with corresponding reflectors. Finally in the "middle" of the cross, between the upright pedestal and the elevation antenna, was the broadcasting array which created a circular beam about 10 degrees wide.
  The three radar operators sat at consoles mounted on the pedestal just below the antenna cross-arm, each with their own oscilloscope display. One controlled the azimuth, another the elevation, and the third measured the range. Pointing the antenna was controlled by rotating large handwheels, the range being reported by a similar wheel.
  The accuracy of the antennas themselves was not very high, about 9-12 degrees, so simply rotating the antenna and looking for a maximum would not point it very accurately. To help with this the antennas were deliberately designed to have two directions of high sensitivity, or "lobes". Signals from both lobes were displayed, slightly separated, on the layer's displays. By adjusting the antenna until the returns from both were equally strong, accuracies of about one degree were possible.
  Range information was taken from the elevation array, and worked, as it did for most radars of the era, by triggering the trace on an "A-line" oscilloscope and read against a scale at the bottom. A second blip was also generated by equipment attached to the rangefinder's handwheel. By rotating the handwheel until the reference blip overlapped the one returned from the antenna, the timing could be read from the wheel. Range accuracy was about plus or minus 200 yards.
  The system also included two sets of "repeaters" that sent the directional information to a searchlight, and both the directional and range (as dialed in on the rangefinder's handwheel) to a gun. The accuracy was not enough for direct gunlaying, but in combination with a searchlight the gun's existing optical equipment could "fine tune" the radar's guidance.
  The radar operated at 205 MHz with a PRF of 4098 pulses per second of 6 μS (microsecond) duration, with an inter-pulse time of 240 μS. Radio waves (light) travel at about 0.093 miles/μS round-trip, so the system had a 22 miles (35 km) maximum range (240 × 0.093). It broadcast about 75 kW of power, which was, in theory, more than enough to offer longer range.
  The radar was mobile, requiring four prime movers for support. Two towed the radar base and the antennas themselves, another pulled a K-34 trailer van providing power, and the fourth a van that converted the power to high voltage for the radio equipment. In total, including the trucks, the SCR-268 weighed 82,315 pounds. That the system was mobile at all was more a testament to the US's overwhelming industrial might than any quality of the radar itself.
  The SCR-268 was combined with the Sperry M-4 gun director to create automatic radar-controlled gun laying; however, the relatively long wavelength (1.5 meters) resulted in poor accuracy. This system was eclipsed by the SCR-584, which utilized a 3 GHz magnetron oscillator from Britain, completely automatic tracking, and the Bell Telephone Laboratories electronic M-9 gun director.
  The SCR-268 was one of the first radar sets to use lobe switching of 
caption: Oscilloscope operators with range scope at left, azimuth scope in middle, and the elevation scope at right
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