SCR-268 Radar - Description

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 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 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 mile 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 not a particularly advanced design by the standards of the era. Early units were technically behind the times in terms of tracking in particular; while the SCR-268 was being introduced, the Germans were upgrading the Würzburg radar, first to use lobe switching, and finally to conical scan the next year. These sorts of changes were simply not possible on the SCR-268 due to its long wavelength, which demanded a non-circular antenna.

The only way to "fix" this problem was to move to a microwave radar of their own, an effort that started after the Tizard Mission. This led to the introduction of the SCR-584, which fit the entire radar system on a single trailer, added conical scanning, and was generally much more accurate and far easier to operate. SCR-268s were removed from service almost immediately with the ending of the war.

The SCR-268 was one of the first radar sets to use lobe switching of its receiving antennas as a means to locate AA (anti-aircraft) searchlight beams on aircraft. Since it did not lobe-switch its transmitted signal it would be classes as one of the first LORO radars.