Fast Spherical Near-Field to Far-Field Transformation for Offset-Mounted Antenna Measurements

In spherical near-field measurements, the number of measured samples grows with the radius of the minimum sphere enclosing the antenna under test (AUT). This leads to unnecessary long acquisition times for antennas mounted in offset positions with respect to the measurement sphere. In this letter, a general technique is proposed to reduce the sampling rate requirements of offset mounted antennas. The approach is based on centering the spherical wave expansion (SWE) over the AUT by a shift in the coordinate system to compensate for the offset. The purpose of this shift is to reduce the number of significant spherical waves required to represent the AUT field. In the subsequent steps of the algorithm, the coordinate system is brought to its original location to preserve the efficiency and probe-correction capabilities of the traditional spherical near-field to far-field transformation technique. This is performed by a proper translation of the SWE coefficients between both coordinate systems. Higher order probe correction is also supported intrinsically. The proposed algorithm is numerically tested to assess its performance. Electromagnetic simulation and anechoic chamber tests are used to validate it, showing reductions in measurement times with low transformation errors.