The effective subarray antenna gains as a function of the tilt angle, or mechanical misalignment, are shown in Figure IV.A.2-12. Three subarray sizes, 4, 10 and 18 meters, were studied. A 2% loss in antenna efficiency was allocated for mechanical misalignments which corresponds to 8.0, 3.2, and 1.9 arc minutes for the 4, 10 and 18 meter square subarrays. The total cost of the transmit antenna as a function of subarray size is shown in Figure IV.A.2-13. The transmit antenna is a 1 km, 5GW system -- the only change is the $64,000 cost for the phasing electronics associated with each subarray. The curve which is calculated using the relationship indicates that small subarrays are impractical due to the large increase in cost. A 4m X 4m subarray configuration cost $2.7 X 10$ more than a 10m X 10m system. Therefore, the model system uses a 100 m2 subarray (approximately 10m X 10m) which is a compromise between the more stringent alignment requirements for larger subarrays and the increased cost of smaller subarrays. The total number of these subarrays within the 1 km total array is 7,850. The mechanical alignment requirement is + 3 arc minutes, giving a 2% loss in antenna efficiency. The corresponding mTsalignment tilt in length is .44 cm for the subarray and .44m for the total array. The stringent mechanical alignment requirement for the subarrays is the tilt angle from boresight to the ground; the vertical displacement of the subarrays with respect to each other is not that critical since the electronic phasing can compensate for different path lengths to the ground. IV.A. 2(h) BASIC SYSTEM PERFORMANCE REQUIREMENTS The preceding analysis has considered only a perfect antenna; the degradations associated with phase and amplitude errors, together with failure rates, will now be determined.
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