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ELECTRONICS |
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| a signal wave front arriving parallel
to the array. The wavefront induces voltage in the two halves of
the array at the same instant. Consider the array from the back and
assume that the left-hand termination is in use. The signal induced
in the left half of the array arrives at once at the termination, but the
signal from the right half of the array arrives at the left-hand termination
only after a delay caused by its passage through the phasing stub.
Hence the net voltage arriving at the left-hand termination is a vector
sum, and this vector sum is the same as though the wave had arrived from
an angle to the left and as if no phasing stubs were present. The
phasing stubs thus cause the antenna pattern to shift to the left when
the left-hand termination is in use.
By the same process, the stubs cause the directivity of the array to shift to the right when the right hand termination is in use. By shifting alternately from left-hand to right-hand termination, the effect |
of two lobes is produced. Accurate direction
finding is obtained when the signals from the two terminations have the
same amplitude. In the elevation array, the array is divided into
top and bottom halves and the same effect is produced in the elevation
coordinate.
In practice, the phasing stubs are adjusted when receiving a wave-front of known orientation until the peaks of the lobes are spaced approximately 10 degrees and a symmetrical variation in signal is obtained on either side of the center direction. Thereafter this adjustment is preserved until the radar moves to a new location. A signal generator ("tweeter") produces the required wavefront and a technique has been developed to avoid the effects of external reflections. As in the case of the transmitting array, matching stubs are employed on the transmission lines to match the receiving array impedance to the line. One possible source of trouble in the operation of the radar is the existence of side lobes, that is, |
minor directivity patterns offset from the main pattern
by some 25 degrees. Proper adjustment of the array will keep the
sensitivity of the array in these side directions about 30 db below that
of the main lobe, but the effect cannot be elim-inated entirely.
Thus a nearby target centered in a side lobe may produce a stronger signal
than a distant target centered in the main lobe. Moreover, at the
intersection between the left-hand main lobe and a right-hand minor lobe
signals of equal amplitude will appear. The array will then be pointed,
not at the target, but some 25 degrees away from the target. The
true lobe intersection can, however, always be determined by noting the
symmetry of the signal variation as the array is moved either side of the
equal-amplitude direction. The false balance is not symmetrical.
Two identical receivers are employed in the SCR-268, connected to the elevation and azimuth arrays. A block diagram of the receiver is |
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