Near-Field Spherical Scanning: Quiet-Zone Field Evaluation! R. H. Direen, M. H. Francis, and R. C. Wittmann National Institute of Standards and Technology, Boulder, CO 80305, Email: francis@boulder.nist.gov 1. Introduction Determination of the illuminating electromagnetic fields within a test zone has long been of interest, for example, in the evaluation of compact-range performance. Re- cently, the National Institute of Standards and Technology (NIST) has explored the use of an interior, near-field spherical scanning method for measuring this illumina- tion [1], [2]. To test effectiveness, we have compared test-zone fields found by applying both the interior [1] and the (standard) exterior [3], near-field, spherical scanning techniques. In both techniques, the response is measured while moving a probe over the surface of a measurement sphere. In the interior method (Figure 1), the probe points radially outward to determine the fields interior to the sphere; all sources are assumed to be outside of the sphere. In the external spherical near-field scanning method (Figure 2), the probe points radially inward to determine the fields exterior to the sphere; sources in this case are interior to the sphere. The interior approach has an advantage in that measurements are required only in the region of interest. The exterior approach can be impractical when the source antenna is very large as is frequently the case for compact ranges. On the other hand, the interior algorithm may suffer from poor conditioning due to probe insensitivity issues. We compare the fields of a 46 cm Ku-band dish determined in a region where both the interior and exterior methods apply. To qualify the comparison, we provide preliminary estimates of measurement uncertainty. 2. Measurement and Data Reduction For practical reasons, we use specially constructed J1 == ±1 probes [4]. At a given sample location r == rr, measurements are made in two orientations separated by a 90° rotation of the probe about its axis. These measurements may be combined to form a vector probe response w' (r) lao, where ao is the excitation. Although we suppress the details, proper normalization is necessary when absolute field compar- isons are required. The initial step is to construct the expansion (1) IU.S. Government work not subject to U.S. Copyright u.s. Government work not protected by U.S. copyright