Active antennas, such as active metamaterial antennas, reconfigurable antennas and non-foster antennas, have received increasing attention over the last several years. One of the most important factors influencing the performance of active antennas is the nonlinear behaviors generated from the incorporated active devices including diodes, transistors and switches. Therefore, it would be invaluable to develop an effective and practical method to accurately model and analyze the nonlinear behaviors of these active radiators. In this work, a hybrid EM-circuit simulation method with Ansoft designer and HFSS are used in the nonlinear analysis. The discussion will be focused on the power capacity, harmonic radiations, and efficiency of active antennas.