Bandwidth Enhancement of Small Square Monopole Antennas by Using Defected Structures Based on Time Domain Reflectometry Analysis for UWB Applications

摘要

In this paper a simple printed monopole antenna with enhanced bandwidth by using defected structures for UWB applications is presented, which is designed due to time domain designing method. The proposed antenna consists of a square radiating patch with a defected mircostrip feed-line by a pair of T-shaped slits and a defected ground plane by a pair of rectangular slots with a T-shaped strip protruded inside the slots, which provides a wide usable fractional bandwidth of more than 130 % (3.06 GHz - 14.27 GHz). By cutting two modified T-shaped slits with variable dimensions on the feed-line and also by inserting a pair of rectangular slots with T-shaped protruded strip inside the slots on the ground plane, additional resonances are excited and hence much wider impedance bandwidth can be produced. This large operating bandwidth is obtained by choosing suitable dimension for the proposed defected structures. To verify the validation of the proposed antenna, the equivalent circuit based on time domain reflectometry analysis is presented. Detailed simulation and numerical investigations are conducted to understand their behaviors and optimize for broadband operation. The designed antenna has a small size of 12 × 18 mm~2, about 0.15λ × 0.25λ at 4.2 GHz. The proposed antenna exhibits almost omnidirectional radiation patterns with low cross polarization level.