Wideband Antenna Characteristics for UWB Radio Systems

Ultra Wideband (UWB) Radio Systems require small and efficient antennas with a suitable transient radiation behavior like small bi-conical and mono-cone antennas and their planar relatives. For some applications like ceiling mounted access points conformal antenna solutions like spiral antennas are of special interest. INTRODUCTION Spectrum is presently one of the most valuable goods worldwide as the demand is permanently increasing and it can be traded only locally. Since the United States Federal Commission on Communication (FCC) has opened the spectrum from 3.1 GHz to 10.6 GHz, i.e. a bandwidth of 7.5 GHz, for unlicensed use with up to 41.25 dBm/MHz EIRP numerous applications in communications and sensor areas are showing up: the ultra wideband radio technology promises high resolution radar applications, sensor networks with a large number of sensors for industrial or home surveillance as well as high data rate communication over short range for personal area networks (PAN). The new applications gain from the experience in pulse propagation theory and ultra wide band radar systems, which has been obtained in the past decades. The new FCC regulation does not specify a special modulation or multiple access scheme. Therefore there is plenty of room for different new approaches like pulse position modulation with time hopping as well as multi band solutions like spectral keying [1]. The pulse position modulation occupies the whole bandwidth within one ultra short pulse whereas the spectral keying hops through the bandwidth occupying only a sub band of e.g. 500 MHz within the duration of one pulse. The transient radiation and receiving properties of the employed antennas as well as the ultra wideband channel characteristics in total are of great importance for the design of such systems. According to the reciprocity theorem the transient antenna transmit characteristic is the derivative of its receive characteristic [2]. For ultra wide bandwidth this means a significant impact on the received power: an antenna that has a constant gain G over the regarded bandwidth will have an effective area Ae which decreases proportional to the square of frequency f .