  Spatial coordinate transformation is a suitable tool for the design of
complex electromagnetic structures. In this paper, we define three spatial
coordinate transformations which show the possibility of designing a taper
between two different waveguides. A parametric study is presented for the three
transformations and we propose achievable values of permittivity and
permeability that can be obtained with existing metamaterials. The performances
of such defined structures are demonstrated by finite element numerical
simulations.
