Solid-state devices can be directly integrated with planar antenna to form active antenna elements. A voltage controlled oscillator (VCO) is designed and fabricated at 2.4 to 2.5 GHz using microstrip patch antenna. A varactor diode is used as a variable reactance. The predicted frequency tuning range of the VCO is 2.448 to 2.498 GHz in the design procedure and the fabricated VCO has 2.446 to 2.498 GHz frequency tuning range when the varactor tuning voltage is varied from 0 to 11 V. Transmitted power output of the patch antenna which serves both as a resonator and a radiating element for VCO is almost 18 mW over this tuning range. The phase noise of VCO is -90 dBc/Hz at center frequency.