A monopole antenna with frequency diversity has been designed. Depending on the state of one PIN diode, this antenna has the ability to alter between two frequencies. The first one is 2.45 GHz which corresponds to the WiFi applications. The second is 3.45 GHz which corresponds to the WiMAX applications. This monopole antenna has been excited by a 50Ω microstrip line. The radiating element and a partial ground plane, that was used to achieve better gain and better bandwidth, have been printed respectively at each side of an FR4 substrate having total dimensions of 30mm × 40mm × 1.6mm. The parameters related to this antenna are analyzed using the HFSS software. For the two cases of operation of the antenna, the reflection coefficient and the gain have been simulated and showed good results. Also, for the two resonance frequencies, the VSWR is strictly lower than the value 1.2.