A desirable feature of probes for Spherical Near Field (SNF) measurements is the absence of radiated higher order spherical modes in the pattern [1]. This implies that the probe pattern can be fully described by a linear interpolation of its principal plane patterns leading to a significant simplification in the NF/FF transformation when probe pattern compensation is included. Probes with such properties are inherently narrow band. Although probe correction algorithm for NF/FF transformation with any probe pattern are available [2], there is a need for probes approximating first-order performance across a large bandwidth. Traditional first-order probes rely on geometrically symmetric Ortho-Mode Junctions (OMJ) and externally balanced feeding. Such probes utilize couplers that ensure equal amplitude and opposite phase of the balanced feeding scheme [4]. This paper introduces the design and validation of a novel 3dB/180° coupler, based on the natural antisymmetric properties of the electric field within the coupler to achieve quasi-perfect amplitude and opposite phase distribution. To realize these properties, an architecture based on slot coupling is selected. The coupler concept is suitable for various frequencies and has been realized at UHF to Ku-band, as a stand-alone cased component. The paper includes experimental data specifically at L/S-band, showcasing outstanding coupler performance concerning matching, balance, and isolation, aligning well with full-wave electromagnetic predictions. Furthermore, the study assesses measured probe design improvements due to enhanced coupler accuracy on a 1.4-4.2GHz SNF probe design
