We report on the design and realization of horn antennas required to achieve high performances with respect both to the return loss and the cross polarization level over a wide band.

In recent years the increasing interest in feed systems with high p:rfomance has supported the development of design tools for the analysis and modelling of cylindrical or conical hom antennas (corrugated, stepped or smooth-walled, square or circular) 11.21. A iiew electromagnetic modelling activity has been carried out with as result a significant improvement in design and optimization capability of the feed system consisting of !he hom connected lo the ortho-mode-transducer (OMT). In this paper the tools for the analysis of the hom, the OMT and their combination. resulting in an integral analysis of the feed system. are described. The adopted approach is suitable for the extension of the analysis to waveguide structures, like marching networks, typical of a feed system, as shown in Fig. I . Part of the analytical results is supported by experimental results. The recent development of two methods for analysis of the OMT is emphasized. n HORN ANALYSEIn the last fifteen yean the design and analysis of smooth-wall and corrugated flared horns has received particular anention [ 1.21. In general these structures are considered as a combination of proper waveguide discontinuities connecting a waveguide region to a proper radiating aperture. Mode-matching technique combined with the scattering matrix representation is usually adopted. A limitation of this approach is the heavy computational time required when the horn geometry involves a large number of discontinuities (>loo) as usually present in wide band or dual band corrugated homs. A new computational approach [B) has been developed to overcome this limitation. The main features of the method m the following: each waveguide discontinuity is described by means of the most suitable representation (2 matrix or Ymatrix) in order to minimize the number of matrix operations; the cascade composition of the discontinuities is performed by a n:cursive algorithm which starts from the radiating aperture a n d goes towards the input waveguide; the input retum loss, the total aperture field and consequently the radiation performance are simultaneously available at each iteration thus providing a complete control of the hom performance.The electroniagnetic niodel has been developed for the analysis of circular and coaxial homs. Figure 2 shows two typical configurations of smooth-wall and corrugated horns. The implemented computer code is a quite efficient tool both in terms of analysis capability of complex horn configurations and required computational time. The accuracy of the theoretical model has been verified by considering a conical corrugated horn with the following geometrical parameters: input waveguide diameter 2d=14.2". aperture diameter 2D=130", half flare angle &7" and 155 constant width slots. The computed and measured input retum loss and radiation patterns, for both co-polar and cross-polar con~ponents. are shown in the Figs.3 and 4 which confirm a quite satisfactory agreement. 'This work was supported in part by ESTEC Contract N.9551/9I/Nl,/LC.428 0-7803-1246-!N...