On Wideband And Dual-Frequency Dual Circularly Polarized Microstrip Antennas

Recently, more and more attention has been paid to the wideband and multi-polarized antennas combined with the development of the wireless communication. The microstrip antenna is widely studied for its advantages of low profile, easy fabrication and integration. However, the narrow bandwidth is the main disadvantage of the microstrip antenna, which strongly limits its application. Therefore, based on the scientific projects, this thesis analyses the wideband, dual-frequency and dual polarization techniques of the microstrip antenna. The method of using the L-shaped probe to broaden the microstrip antenna's bandwidth is studied and a design of dual-frequency dual circularly polarized microstrip antenna is proposed.First, some principles and analysis methods of the microstrip antenna, such as Transmission Line Model Theory, Cavity Model Theory and Finite Element Method (FEM), are introduced. The techniques of the circular polarization are also presented. Based on the discussion above, the wideband and dual-frequency dual circularly polarized microstrip antennas are studied, and several antenna configurations with good performance are proposed. With regard to the wideband microstrip antennas, the antenna fed by L-shaped probe is studied. The influence of the L-shaped probe to the impedance bandwidth is discussed. An improved antenna structure with two L-shaped probes is presented. Calculated and measured results show that the antenna with two L-shaped probes improves the impedance bandwidth to 27%. Concerning dual-frequency dual circularly polarized microstrip antenna, the L-shaped microstrip is used to feed the annular-ring slot on the ground plane. The energy coupled from the L-shaped microstrip line to the annular-ring, and the two orthogonal modes of the antenna are excited with the same amplitude and 90 phase difference at 2420MHz and 4200MHz, which results in a good circular polarization radiation. We design and simulate the antenna by Ansoft HFSS software. The simulated and measured results indicate that the antenna realize the RHCP at 2380MHz~2460MHz with the axial ratio CP bandwidth of 3.3% and the LHCP at 4180MHz~4220MHz with the axial ratio CP bandwidth of 0.95%.