Methods for Locating Stray-Signal Sources in Anechoic Chambers

Two complementary numerically efficient frequency-domain methods for locating stray-signal sources in anechoic chambers are investigated and applied in combination to actual measurement data. Both methods use single-frequency near-field data collected on a planar surface and process them to reconstruct field values (images) elsewhere. The first method, which is based on the fact that the probe output satisfies the Helmholtz equation, uses plane waves to backpropagate the scan-plane data and is well suited for fast-Fourier-transform (FFT)-based rapid reconstruction of images on planar surfaces parallel to the scan plane. The second method uses the simple spherical-wave focusing technique and is flexible, in that, it can be used to generate images on either planar or nonplanar surfaces from the data collected on either planar or nonplanar surfaces. When data and image points are both located on a regular grid, the method can be implemented using the FFT-based fast convolution technique. Both methods include a spatial filter for isolating selected plane-wave spectrum components. The two methods are used in combination to successfully locate the strong multiple-bounce stray signals that degrade the quiet zone of a near-field bistatic radar cross-section facility. Subsequent scan data confirm that the suppression of these stray signals indeed substantially improves the quality of the quiet zone. The spherical-focusing method is also used to evaluate the effectiveness of the various absorber configurations applied to selected edges of the reflector to control edge-diffracted fields. It is shown that the reduction of the edge-diffracted fields further improves the quiet zone.