Abstract: Evaluation of Hankel functions of zeroth and first order arises in the finite element solution of two dimensional electromagnetic scattering problems as the iterative Robin boundary condition (IRBC) is used as the openregion boundary condition. In practice, approximation of these functions is required when the target is electrically large and the computation is very timeconsuming. In this paper, four approximation algorithms are first investigated, including series expansion, small and large argument approximation, and two different kinds of numerical methods. Then the accuracy and speed are compared in the computation of IRBC. The small and large argument approximation, combined with a certain numerical method is shown to be very effective. Some examples of electrically large size are also presented.

Key words: iterative Robin boundary condition (IRBC), finite element method (FEM), electrically large targets, Hankel function