W. Lui, C. Cheng, H. Zhu, “Improved Frequency Notched Ultrawideband Slot Antenna Using Square Ring Resonator", IEEE Transactions on Antennas and Propagation, Volume: 55  Issue: 9, Sept. 2007, Page(s): 2445-2450

The publication is available to download from http://ieeexplore.ieee.org/xpl/tocresult.jsp?isnumber=4298161&isYear=2007