Design and Validation of Quasi Ideal Ultra-Wideband 3dB/180° Couplers for High Precision Spherical Near-Field Probes

Spherical Near Field (SNF) measurement systems are primarily limited in usable bandwidth by the probe frequency coverage. This limitation mainly arises from the presence of higher-order azimuthal modes in the probe pattern [1]. In case of electrically large or offset AUTs, such a limitation may be overcome by a full probe correction algorithm for the NF/FF transformation [2]. However, probes approximating first order performance over the full bandwidth are generally preferred. Traditionally, first-order probes based on geometrically symmetric Ortho-Mode Junctions (OMJ) with externally balanced feeding have been widely accepted. These probe designs rely on couplers that provide equal amplitude and opposite phase distribution at their output ports [3]. In this paper, the design and validation of a novel 3dB/180° coupler is presented. The concept is based on the natural anti-symmetric properties of the electric field within the component, providing a quasi-perfect amplitude and opposite phase distribution. To achieve these properties, an architecture based on slot coupling is selected. The design has been implemented in several frequency bands, from UHF to Ku-band, as stand-alone cased components. Experimental data at L/S-band is presented in this paper, showing excellent performance in terms of matching, balance, and isolation between output ports, well in-line with full-wave electromagnetic predictions. In addition, the impact of the coupler accuracy is also assessed on a relevant SNF test case [1].