Composite waveguide coupling aperture having a thickness dimension

The present invention relates to antennas in general, and to planar waveguide fed slot-antenna arrays suitable for terrain mapping side looking radar airborne antennas (SLAR) in particular. It provides a novel coupling aperture or slot which has a variable cross sectional area along the coupling path; thus, it has a significant dimension perpendicular to the slot plane. A new composite coupling aperture or slot having at least three significant dimensions is achieved. These dimensions are length, width and thickness. In its broadest aspect, the composite coupling aperture is characterized by having non-uniform cross sectional areas along its thickness dimension. In its narrowest aspect the cross sectional area changes abruptly between the opposite waveguide coupled ends of the aperture. This coupling aperture is designed for use with a side looking radar antenna which requires coupling between two waveguides separated by a thin wall. It was devised to adjust the degree of coupling while accommodating the necessary mechanical restraints.