In this paper, a novel configuration of postvaractor loaded circular microstrip antenna is presented. A comprehensive cavity model analysis of such electrically thin antenna leads to very interesting results like compactness and high tunability. In earlier published works, varactor diode had been directly connected between the antenna and the ground plane. In this case, however, a metallic post is used to help connect the varactor diode in SOT-23 package to the ground. The metallic post acts as an inductance, and therefore the circuit changes from capacitive loading to series L-C loading resulting in vastly different results. The numerical results agree well with experimental results.