Small aperture crossed waveguide broad-wall directional couplers

The crossed-waveguide broad-wall-to-broad-wall directional coupler is a compact device capable of good performance over a wide frequency band. However, no complete design procedure is available. The small aperture coupling theory of this broad-wall (Moreno) coupler is reviewed and extended. It is shown that a lower limit on the directivity is given by the ratio of magnetic polarisability to electric polarisability for the coupling apertures. Hence, directivity may be improved by using very narrow crossed slots for coupling. It is also shown that the coupling and lower limit on directivity may be calculated for any slot width by using published values of crossed slot polarisabilities. The results of coupling measurements on Moreno couplers are presented in the paper to demonstrate the accuracy of the small-aperture design procedure. Application of these results should permit the broad-wall cross coupler to be used in a wider range of applications