A recent innovation in the development of slotted-waveguide array antennas has shown that for a rectangular waveguide radiating into a 'half-space' through slots located in the broadwall, polarisation selectivity can be achieved by even or odd mode excitation of a suitable combination of longitudinal and transverse slots in an axially bifurcated form of the waveguide. In practice, however, the external region of the guide is seldom a true half-space, particularly in the case of linear arrays, which are often modified by a short section of parallel plate waveguide to provide some degree of polarisation purification and/or spatial filtering. An analysis of this geometry is described, in which the presence of the parallel plate region has been accommodated within a moment method model of a polarisation agile slot arrangement by employing an 'imaged' Green function to represent the parallel plate enclosure. It is also shown that particular care in identifying suitable basis functions is required in the case of the T-shaped element of the polarisation agile slot pattern which straddles the waveguide bifurcation.
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