The admittance of a rectangular waveguide radiating into a dielectric slab

The variational analysis of Compton is modified to account for the surface-wave pole contribution to the admittance of a waveguide-fed slot on a ground plane covered with a low-loss dielectric slab. The results of an extensive experimental study to determine the effects of ground-plane and dielectric slab size upon waveguide admittance are compared with calculations using the dominant waveguide mode field as a trial solution. It is found that a waveguide terminated in a flat flange is of sufficient size to approximate the infinite ground plane assumed in the theory. A disagreement of 20 percent was noted for some thin sheet cases. Based upon a thorough theoretical and experimental investigation, it was concluded that tolerances do not explain the error; however, a measurement of the aperture field for cases where maximum error occurs reveals significant distortions. Hence, it is concluded that induced higher-order modes are the probable cause of disagreement.