Time-domain electromagnetic simulation of hard surfaces

The concept of hard surfaces in electromagnetics is valuable tool for antenna design. In principle, hard horns and feeds can support a TEM mode with uniform field distribution and straight field lines over the aperture of an arbitrary cross-sectional shape. This makes them compact with high gain, and thus convenient to apply in cluster feeds and arrays (Kildal and Lier, 1990). Low cross-polarization is also expected. Hard surfaces are realised by dielectric-filled longitudinal corrugations or by longitudinal strips on a dielectric coating. Their potential has been previously been demonstrated experimentally. However, the theoretical relations given so far apply only to a limited class of hard structures. They are typically derived assuming that the period of corrugations is small compared to the wavelength and that the corrugations are rectangular and much wider than the edges between them (Kildal, 1990). The aim of the present paper is to show that the finite-difference time-domain method applied to hard surfaces provides deeper insight into their behaviour, and also permits fast evaluation of their characteristics.