In this paper a design method for a circular polarized small-size microstrip antenna on a short dielectric layer is presented. It's known that exciting two detuned modes with a single coaxial probe makes possible circular polarization (CP) but as yet little efforts were made in order to control CP frequency and exact 50 ohm matching. A straight forward circuital model for the device can solve both this issues and gives hint for the best radiator design; our prototype is a circular disc slotted by a concentric elliptical cut in order to slightly perturb the central symmetry and obtain the modal degeneration. It includes a SPDT to allow the switching between the two CP. The prototype is designed for 2.44 GHz and exhibits a return loss of 19 dB, an impedance BW of 120 MHz a directivity of 6 dB and an estimated gain of 1.8 dB.