Fig. 10. Planar scanner concept. vironment (maintained within 1.1°C), and an r.f. anechoic environment. Recent work at Georgia Tech has demonstrated that accurate antenna patterns can be obtained via near-field techniques (5, 6). The National Bureau of Standards has shown that for planar near-field scanning, the near-field derived patterns are as accurate as far-field measured patterns when considering all error sources involved (7). A planar scanner concept applicable to SPS subarray antenna measurements is shown in Fig. 10. An investigation of error contributors to near-field measurements (8) has indicated that the required probe X-, Y-, and Z-positioning accuracy is X/200 for pattern accuracy compatible with SPS measurement objectives. Trade-off studies at Georgia Tech have suggested that the planar near-field concept has potential for array measurements of an SPS mechanical module (30 m2). Problem areas to be resolved include computer requirements and the complexity of scanning over a much larger surface with acceptable precision. A previous study performed by Georgia Tech for NASA indicated that the cylindrical near-field technique is attractive for the measurement of electrically and physically large ground station antennas (9). Near-Field Facility Concept Based on the linear planar scanner mechanism shown prior in Fig. 10, the near- field measurement facility depicted in Fig. 11 was conceived. This system presumes that the ceiling of the measurement chamber is totally covered with microwave absorber material, side walls are only partially covered. The purpose of the antenna handling mechanism is for array setup and handling. The system further presumes full high power testing will be employed. Utilizing key elements of the microwave electronics depicted prior in Fig. 4, and an appropriate microcomputer, a cost estimate of the near-field concept is shown in Table 6. These data indicate projected costs for a near-field facility to be approximately 5% greater than the far-field measurement facility concept.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==