Variable permittivity dielectric material loaded stepped-horn antenna

17Figure 2.5 Scattering matrix representation of cascaded discontinuities.Al, and the number of modes in the expansion, NT. The number of sections along thehorn antenna must be chosen so that the pyramidal tapered section is accurately modeledand modes excited by the artificial steps do not influence the radiating aperture fields. It isimportant to notice that the elements of the generalized scattering matrix [8] are in the formof infinite series summations. In reality, the series must be truncated with finite number ofterms. Selecting the number of terms is very critical from convergence point of view. Asimple formula derived for determining M and N for convergence [11], is the nearesthigher integer ofwhere A and B are the dimensions of the horn aperture.2.3 Method of MomentsThe generalized reflection matrix SA of the aperture as shown in Figure 2.6, is formulatedby incorporating the mismatch between the aperture section of the horn antenna and thefree space.The approximation of the horn aperture terminated by an infinite ground plane isbased on negligible effect of the induced current on the horn metallic walls. For practical