A broadband rhombic loop antenna is introduced to radiate a circularly polarized wave. This antenna has a single feed and is located above a ground plane. The perimeter of the loop is typically about 1.3 wavelength. One gap is made on the loop to produce a traveling wave distribution of current. Antenna characteristics are calculated by the method of moments and compared with the measured data. By adjusting a perimeter and a gap position of the loop, circular polarization is obtained. In addition, with the appropriate vertex angle of the rhombus, the bandwidth of about 20% for the axial ratio (2dB) is attained and the possibility of controlling the input impedance is found. Finally, it is observed that sense of circular polarization can be changed easily from left-hand to right-hand, and vice versa by switching one gap position to the other on the rhombic loop.