In this article, the design of a frequencyswitchable, square-shaped microstrip patch antenna with simultaneous circular-polarization (CP) reconfigurability is presented and experimentally verified. From the measurement of a fully functional prototype, it is confirmed that the resonant frequency of the proposed antenna can be continuously altered between 1.97 and 2.42 GHz by electronically varying its effective length. A wideband 90° phase-shifting and switching network is also designed to excite the antenna for generating CP radiation at all the tunable frequencies and altering the sense of rotation [both left-hand CP (LHCP) and right-hand CP (RHCP)]. More importantly, the antenna shows an axial ratio (AR) value that is less than 3 dB within a wide beamwidth (130-150°) at all the tuning states, and it has a maximum realized gain of 5.8 dB isotropic circular (dBic) at 2.42 GHz. The frequency with CP-reconfiguring characteristics makes the proposed antenna suitable for advanced wireless communication systems operating in the S-band, which can withstand both frequency-based challenges and signal-quality issues arising due to climatic changes. © 1990-2011 IEEE.