Radiation from elliptical waveguides terminating in a ground plane is analysed using a Green's function approach. In the waveguides the field is expressed as a sum of the TEcmn, TMcmn,TEsmnandTMsmnelliptical waveguide modes, and expressions for the mutual admittances of modes excited at the apertures are obtained. From these expressions the mode reflection and conversion coefficients are determined. Of the techniques available for calculating the mutual admittances, the transform representation and direct integration approaches are used, with the latter preferred for detailed calculations. These show that for a single aperture the TEcmnmode couples only to the TEcm'n'andTMsm'n'. modes whenmandm' have the same parity. Similarly, the TEsm'n'mode couples only to TEsm'n'and TMcm'n'modes. Computed and measured results are given for reflection coefficient in a single waveguide due to the open end, and calculations of mutual coupling between two elliptical waveguides are presented for E- and H-plane dispositions. Results obtained for elliptical waveguides are compared with corresponding results for rectangular waveguides in a ground plane. Within limits, the less computationally intense rectangular waveguide solution can be a useful first approximation to the elliptical waveguide solution.