Comparison between different rectangular horn elements for array antenna applications

The coupling and directivity characteristics of an array with fundamental smooth-walled, soft and hard open-ended waveguide elements with rectangular cross-section are considered. Exact and approximate methods to analyse coupling in arrays are discussed. It is known that the coupling, which can be expressed by a quadruple integral across the rectangular apertures, can be reduced to double integrals by introducing a co-ordinate transformation. Calculations show that the soft waveguide elements are suited for applications where low return loss and mutual coupling are required. Arrays with hard waveguide elements yield high directivity for limited scan applications. Both elements can, in contrast to conventional smooth-walled horns, operate in dual polarisation with high isolation between the orthogonal modes