The thesis will introduce some popular ultra wideband (UWB) antennas in the world and design a small size microstrip-fed monopole antenna for RFID application. The UWB antenna cooperates with PWM receiver and IR-UWB transmitter to constitute the circuit of the power converter. They are integrated in a Liquid-Crystal Polymer (LCP) substrate. Designing an UBW antenna is very difficult, because of extremely large bandwidth of UWB signals. For the expected application of the RFID systems, such as a supply chain integration, track and trace, and so on, the small and efficient printed antenna is required.