Analysis and experiments on microstrip antennas with varactor tuning are presented. Results are given for the frequency tuning of rectangular half and quarter wavelength microstrip antennas. Double tuning range was observed for quarter wavelength antennas. Emphasis is also given to the study of harmonic radiation generated because of the varactor nonlinearities. A description of the varactor model as a nonlinear capacitor in the finite difference time domain method and its validation is reported. Harmonic radiation levels and patterns were measured and investigated by full wave analysis.