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Numerical electromagnetic models, such as the finite difference time domain (FDTD) method, have many applications. The authors focus on the non-orthogonal FDTD method, which offers an improved geometric flexibility compared to other standard techniques. Results from numerical electromagnetic analysis methods, such as the FDTD method, are often degraded by an error known as numerical dispersion. For metallic structures this dispersion error is often higher than expected from theoretical considerations. The source of this additional error is due to the reciprocal field interpolation scheme used in the non-orthogonal FDTD algorithm. The error is illustrated by means of a microstrip waveguide and a microstrip antenna. Techniques for reducing this error are evaluated; careful construction of the mesh at the metallic boundary being the most reliable solution.