K.-F Tong, T.-P. Wong, “Circularly Polarized U-Slot Antenna ", IEEE Transactions on Antennas and Propagation, Volume: 55  Issue: 8, Aug. 2007, Page(s): 2382-2385

The publication can be downloaded from http://ieeexplore.ieee.org/xpl/tocresult.jsp?isnumber=4285964&isYear=2007