Abstract-The Poincaré-Steklov operator provides a direct relation between the tangential electric and magnetic field at the boundary of a simply connected domain, and a discrete equivalent of the operator can be constructed from the sparse finite element (FE) matrix of that domain by forming the Schur complement to eliminate the interior unknowns. Identifying the FE system matrix as a discretized version of the Poincaré-Steklov operator allows us to describe and analyze FE and hybrid finite element-boundary integral equation (FE-BIE) formulations from an operator point of view. We show how this operator notation provides substantial theoretical insight into the analysis of spurious solutions in hybrid FE-BIE methods, and we apply the theory on a TM scattering example to predict the breakdown frequencies of different hybrid formulations.
Index Terms-Hybrid methods, Electromagnetic scattering
I. INTRODUCTIONThe hybrid FE-BIE method is a widely used approach to numerically solve electromagnetic scattering or radiation problems. It combines the versatility of the FE method to model complex inhomogeneous and anisotropic structures with the accuracy and efficiency of the BIE method to model large homogeneous and potentially unbounded domains. Traditionally, the FE formulation is set in a variational framework, which is extended with the BIE formalism to form a hybrid variational formulation. However, this approach relies on the internal field densities in the FE domain, and since the exact method used to couple both formulations seems to be very important to avoid spurious solutions, we expect that a hybrid formalism specifically focusing on the behavior of the formulations at the boundary will provide more theoretical insights.In this paper, we use the concept of a Poincaré-Steklov (PS) operator to describe the FE formulation in a domain by the relation it provides between the tangential electric and magnetic fields at the boundary of the domain. Hybrid formulations are easily formed by properly combining the PS and BIE operators. Unlike the classical variational framework, this operator notation retains the individual identities of the FE and BIE operators in FE-BIE formulations, and different properties regarding spurious solutions are easily derived. After outlining the general equations in Section II, we use the new operator notation in Section III to analyze the problem of spurious solutions in commonly used hybrid FE-BIE formulations. The theoretical results are then applied in Section IV to compare the analytical and the simulated breakdown frequencies of the different FE-BIE formulations for a TM scattering problem involving a single dielectric cylinder.