ELECTRONICS
September 1945

By McGraw-Hill Staff
Page 100 - 109

nected also to the converter which transmits the target coordinates to the associated searchlight control or gun director. On the ground beside the trailer are the keyer and modulator units which drive the transmitter.
    The SCR-268 weighs 82,315 pounds, complete with all auxiliaries and trucks. It consumes 15 kva of power. Despite these large figures it can be (and has been) set up and ready for operation within a few hours of being put ashore at a beachhead.
    The block diagram, Fig. 3, shows the flow of the signal through the equipment. At the left are the transmitter components, which comprise a separate group. The keyer establishes the basic timing of the transmitted pulses at 4098 per second and produces the short (3 to 9 microsecond) pulses which drive the modulator. The modulator raises the power level of the keyer output and applies modulating pulses to the transmitter. The transmitter is a self-excited tuned-plate tuned-grid oscillator consisting of 16 tubes in a ring circuit. The transmitter output is applied to the transmitting array, which radiates a beam of circular cross section approximately 10 degrees wide.
    When the transmitted beam encounters a target, the echo is returned to the two receiving arrays. The directivity of these arrays is about 12 degrees in the azimuth array and 9 degrees in the elevation array. By means of receiving antenna "lobe switching", to be described later, the directivity in azi-muth and elevation is refined to about ±1 degree in each coordinate.
    The signal received by the elevation array is passed to the elevation receiver and thence to two cathode-ray indicators. One indicator displays the range of the target, the other the elevation. Both are "type A" indicators, that is, they are es-sentially similar to a conventional test oscilloscope. The cathode-ray beam is driven by a sweep circuit at constant speed from left to right. The echo signal from the associated receiver is applied to the vertical deflection plates so that the beam is deflected upward whenever an echo is received. At the left end of the oscilloscope trace the transmitted pulse appears. This estab-

lishes the "zero" of the time scale. The distance from the transmitted pulse to the echo, measured on the range indicator, gives the distance to the target, or range.
The elevation indicator is similar, except that two echoes are displayed, slightly displaced from one another. The two echoes, known as the "split image", are produced by shifting the directivity pattern of the elevation array slightly, at a rapid rate, by the receiving antenna lobe-switching method. The operator adjusts the elevation array until the two echoes in the split image have the same height. The array is then pointed directly at the target within the one-degree error previously mentioned.

The azimuth receiver is connected to a similar oscilloscope which displays a split image produced by shifting the pattern of the azimuth array. The operator at this indicator adjusts the orientation of the azimuth array until the two pulses in the split image are of equal height. The azimuth array is then pointed at the target within 1 degree accuracy.
The remaining items shown in Fig. 3 are the repeaters and converters which transmit the target information in suitable form to the searchlight or gun director. The principal electronic component here is the range unit, which introduces a time shift in the deflection circuits of the range indicator. When

September 1945 - ELECTRONICS 103

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ELECTRONICS September 1945

ELECTRONICS
September 1945