A quad-mode dual-band dual-sense circularly polarized (CP) single patch antenna is proposed. The four boresight radiating modes are obtained by simply loading a stub and U-slot on the patch. The characteristic mode analysis (CMA) is utilized to reveal the operational principles of the patch. The control over the antenna's frequency ratio and the sense of circular polarization is achieved by independently manipulating the different modes. This is accomplished by altering the dimensions of the patch, U-slot, and the stub. To further validate the effectiveness of the proposed design method, a patch antenna prototype has been designed and fabricated for experimental validation. In both bands, left-hand circular polarization and right-hand circular polarization were obtained separately. The measured impedance bandwidths in the low and high bands are 12.7% and 13%, respectively, corresponding to a 3 dB axial ratio (AR) bandwidths of 2.47% (3.2-3.28 GHz) and 1.17% (4.26-4.31 GHz).