Here considered is the problem of transient electromagnetic scattering from
overfilled cavities embedded in an impedance ground plane. An artificial boundary
condition is introduced on a semicircle enclosing the cavity that couples the
fields from the infinite exterior domain to those fields inside. A Green's
function solution is obtained for the exterior domain, while the interior problem
is solved using finite element method. Well-posedness of the associated variational
formulation is achieved and convergence and stability of the numerical scheme
confirmed. Numerical experiments show the accuracy and robustness of the method.