Using a waveguide T junction and an orthogonal mode transducer as illustrative examples, the admittance matrix elements for a passive microwave device are computed by three-dimensional finite element analysis from the magnetic field formulation having natural Neumann boundary conditions and from the electric field formulation having enforced Dirichlet boundary conditions. The results from these two formulations are compared using both linear and bilinear postprocessing functionals. ¿ American Geophysical Union 1993