Inverse Source-Based Three-Antenna Methods in the Near Field

The two- and three-antenna methods are well established measurement techniques for determining the gains of unknown antennas under test (AUTs) without the need for a known probe antenna. In its original versions, both approaches require far-field (FF) conditions and can only be approximately applied to near-field (NF) antenna measurements. We present a three-antenna method suitable for any measurement distance, including NF measurements. Based on an inverse-source formulation, nonlinear systems of equations corresponding to an NF FF transformation (NFFFT) with unknown probe behavior are formulated and solved via two iterative approaches. Simulations indicate that accurate results can be obtained despite the inherent nonconvexity of the problem even at close distances where the classical three-antenna method is highly imprecise.