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Wave propagation in microstrip lines not parallel to the FDTD grid is modelled accurately using enhanced FDTD equations for sharp diagonal edges. It is assumed that the metal strip is infinitely thin, and its edges are diagonal to FDTD cell faces. A microstrip line, at 60/spl deg/ from the grid, is analysed using enhanced equations, and the phase constant and the effective dielectric constant are computed over a wide range of frequencies. The same diagonal microstrip line is analysed using standard staircase and split-cell models. It is found that only the enhanced-equations technique generates accurate results when the grid is very coarse. For example, the error in the computed effective dielectric constant of a microstrip line at 40 GHz is 0.8%, 6.1% and 12.8%, using enhanced, split-cell and staircase techniques, respectively. The enhanced equations are found to be stable even when the time step is set to the maximum limit.