tailieunhanh - Mobile and wireless communications network layer and circuit level design Part 5

Tham khảo tài liệu 'mobile and wireless communications network layer and circuit level design part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | UWB Ultra wideband wireless communications UWB Printed Antenna Design 111 2. Ultra Wideband Antenna Requirements There are further challenges in designing a UWB antenna as compared to a narrowband one. A UWB antenna is different from other antennas in terms of its ultra wide frequency bandwidth. According to the FCC s definition a suitable UWB antenna should provide an absolute bandwidth no less than 500 MHz or a fractional bandwidth of at least . This is the minimum bandwidth but generally the UWB antenna should operate over the entire GHz frequency range resulting in spanning GHz Liang 2006 Yang Giannakis 2004 . The UWB antenna performance is required to be consistent over the whole equipped band. Ideally antenna radiation patterns gains and impedance matching should be stable across the entire band Wong et al. 2005 . The radiation efficiency is another significant property of the UWB antenna. Since the transmit power spectral density is extremely low in UWB systems high radiation efficiency is required because any unwarranted losses incurred by the antenna could affect the functionality of the system Liang 2006 . A suitable antenna should be physically compact and preferably planar to be compatible to the UWB unit especially in mobile and portable devices. It is also greatly desired that the antenna attributes low profile and compatibility for integration with a printed circuit board PCB Liang 2006 . Finally a UWB antenna should achieve good time domain characteristics. In narrowband systems an antenna has mostly the same performance over the entire bandwidth and fundamental parameters such as gain and return loss that have slight discrepancy across the operational band. Quite the opposite UWB systems occupy huge operational bandwidth and often utilize very short pulses for data transmission. Consequently the antenna has a more critical impact on the input signal. Indeed minimum pulse distortion in the received waveform is a main concern of a .