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Wireless broadband communications such as UWB, WiMAX et. al. have been put into services in Japan, in order to realize high-speed transmission systems. Such a system enables to transmit movies from a person to the other person in the winking of an eye. To realize such wireless broadband communications, wideband antennas have been required. This paper describes the bandwidth limitations of ultra-wideband antennas and its downsizing techniques for wireless broadband communications. First, the bandwidth limitation of wideband antennas is shown by data of wideband characteristics of many kinds of antennas reported in the recent papers. Next, the bandwidth limitations of ultra-wideband antennas are described. Finally, in order to downsize ultra-wideband antennas, rounded disc monopole antennas are proposed, and the limitations of the relative bandwidth and the lowest frequency of small- sized card antennas are clarified.