  We propose discretisation of a regularised combined field integral equation
(regularised CFIE) only with the Rao-Wilton-Glisson (RWG) basis function. The
CFIE is a formulation of integral equations, which avoids the so-called
ficticious frequencies of integral equations. The most typical CFIE, which is a
linear combination of the electric field integral equation (EFIE) and magnetic
field integral equation (MFIE), is known to be ill-conditioned and requires
many iterations when solved with iteration methods such as the generalised
minimum residual (GMRES) method. The regularised CFIE is another formulation of
the CFIE to solve this problem by applying a regularising operator to the part
of the EFIE. In several previous studies the regularising operator is
determined based on the Calderon preconditioning. This regularising operator
however takes much more computatonal time than the standard CFIE since
discretising the EFIE with the Calderon preconditioner requires the dual basis
function. In this article we propose a formulation of the regularised CFIE,
which can be discretised with the Galerkin method without the dual basis
function.
