I. Introduction
Ultrawideband (UWB) technology has experienced a significant rise in popularity in the past few years. Federal Communication Commission (FCC) allocated the spectrum from 3.1 to 10.6 GHz for UWB technology with EIRP less than −41.3 dBm/MHz. So far different kinds of ultra-wideband antennas have been introduced in the literature [1]–[4]. These antennas exhibit good impedance stability over the very large frequency band. In addition to high bandwidth, the low dispersion characteristic of the antenna should be considered as one of the most important factors to achieve the best performance in UWB applications. In short-range UWB communications, the trade off between bandwidth, size, omni-directional pattern, radiation efficiency, low cost and dispersion characteristics should be optimized to obtain an acceptable design. Although some trade offs such as size-bandwidth and size-gain are more challenging, we should take into account the dispersive behavior of the antenna in design procedure.