Y. Duroc, Y.-P. Vuong, S. Tedjini, “A Time/Frequency Model of Ultrawideband Antennas ", IEEE Transactions on Antennas and Propagation, Volume: 55  Issue: 8, Aug. 2007, Page(s): 2342-2350

The publication is available to download from http://ieeexplore.ieee.org/xpl/tocresult.jsp?isnumber=4285964&isYear=2007