Wideband field probes for advanced measurement applications

New developments in ortho-mode junctions (OMJ) and probe technologies has enabled dual polarized Near-Field and Far-Field measurement probes with as much as 1:4 bandwidth. The large bandwidth can be achieved while maintaining the high performance standards of traditional narrow band probes [1]-[3]. The new design approach is based on an inverted ridge structure providing four symmetrical access points for external balanced feeding, thus stabilizing the frequency dependence of the OMJ. The wideband and stable performance makes the probes highly suitable as field probes for Far Field and Near Field measurement scenarios in Spherical/Planar/Cylindrical configurations [4]-[14]. This paper briefly review the theory of balanced feeding and the inverted ridge structure as presented in [6]-[7]. Performance guide lines are derived on the external beam forming network for achieving high port-to-port isolation and matching on a wide bandwidth. The relationship between excitation errors in the balanced feeding scheme and the mode index μ≠1 spherical modal content of the probe is also investigated and upper bounds on acceptable excitation errors are established.