On the Influence of Probe Positioning Errors due to Mechanical Uncertainties in Spherical Near-field Measurements at Terahertz Frequencies in Modern Positioner Systems

Spherical near-field measurements are regarded as the most accurate technique for the characterization of an Antenna Under Test's (AUT) radiation. The AUT's far-field radiation characteristics can be calculated from the Spherical Mode Coefficients (SMC), or spherical wave coefficients, determined from near-field data. At Terahertz frequencies, the far-field approximation distance classically defined by the Fraunhofer distance is easily met. Still, the spherical near-field framework can also be applied to measurements beyond the AUT's near field, providing a reliable way of determining its radiation pattern with any resolution from a relatively coarse measurement. Moreover, the AUT's radiated power can be calculated from the SMC. However, the mechanical uncertainty at these frequencies is comparable to the wavelength, thus introducing basis mismatch between the measured grid and the grid assumed for the transformation into the SMC domain, which results in reconstruction error. In this work, a theoretical stochastic analysis at 0.3 THz is performed. Random superpositions of spherical modes are generated and, for each one, a number of reconstruction experiments affected by a random grid error in line with standard measurement systems is executed. The results for the ideal and the distorted grids are compared and the mean and maximum error are statistically evaluated.