By mounting a regular circularly polarized microstrip antenna on a three-dimensional (3-D) square ground structure, beamwidth enhancement of the circular polarization (CP) radiation can be obtained. Experimental results show that, owing to the 3-D square ground structure, the 3 dB beam width of the CP radiation can reach more than 110 degrees, which is about 30 degrees greater than that of a corresponding regular microstrip antenna. Details of the proposed 3-D square ground structure are described, and the effects of various dimensions of the proposed square ground structure on the beamwidth enhancement of CP radiation are experimentally studied. (C) 2002 John Wiley & Sons, Inc.