In this paper, the Minkowski fractal is used for the design of a microstrip slot antenna by the characteristic mode theory, whereby the radiation mode generating the desired radiation patterns is excited. The right-hand circular polarization (RHCP) antenna is first designed and then the method of polarization reversal from RHCP to left-hand circular polarization is presented by the design of the feed system. The FR-4 substrate is used for the design of the slot antenna at 2.45 GHz. Also, the frequency spectrum of ISM includes that of WiFi, which has a single feed port. The use of Minkowski fractal has resulted in 0.28λ0 × 0.28λ0 miniaturization of the slot antenna. For the proof of concept, a prototype model of the proposed slot antenna is simulated, designed, fabricated and measured, which shows good agreement between the simulation results and measurement data.