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Novel ultrawideband (UWB) antennas with multiresonant split-ring loops and with coplanar waveguide (CPW) feed are presented in this communication. Dual concentric microstrip split-ring loops with different geometrical ground planes have been analyzed and compared. UWB antennas are designed, of which the dimension of each split-ring loop varies proportionally to the arithmetic progression sequence. The dimensions of the rectangular ground planes and the split-ring loops are tuned for achieving a very wide bandwidth. A tapered transmission line is adopted to improve the return loss performance in the required band. Measurement results show that the proposed UWB antennas have a wide bandwidth from 2 to 20 GHz, and the measured return losses are less than--10 dB in this band combined with relative stable radiation characteristics. Index Terms--Coplanar waveguide (CPW), split-ring loops (SRLs), ultrawideband (UWB) antenna.