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The lumped element (LE)-FDTD method allows the incorporation of lumped element models including passive, active and non-linear devices directly into the FDTD grid. Furthermore the implemented code allows plane wave excitation using the total-field/scattered-field transform and radiation pattern determination of the scattered field. These powerful features make the LE-FDTD method ideal for the analysis and design of quasi-optical structures. The paper begins with a passive validation of the method using a single dipole without a ground plane, which is fed by a double strip line. Input impedance, return loss and radiation patterns are shown. The authors then discuss the analysis of a dual frequency crossed dipole structure under plane wave excitation including lumped nonlinear devices. The main idea of this approach is to achieve orthogonal polarized radiation at the two frequencies and the possibility to tune the antenna at the different frequencies almost independently.