A simple design for circularly-polarized (CP) patch antennas with broadband operation is presented. The design uses a cross slot antenna with an ellipse-shaped ground plane, and it is found that a broadband CP operation can be realized by selecting a proper aspect ratio of the ground plane. A design guideline is given. Three examples are designed by the guideline, and their antenna prototypes are also accomplished and tested. Measured results indicate that the 3dB-axial-ratio bandwidths of the three prototypes, centered at 0.9, 1.4, 2 GHz, are 26, 37, and 32 %, respectively. HFSS Simulated results validate the experiments. In addition, the possible methods of further improving the 3dB-axial-ratio bandwidth are also addressed.