Coupler Terminated By Complex Impedance And Its Application On RFID Reader Microstrip Antenna

The branch-line directional coupler has both power dividing and 90° phase shifting function.It is widely used in the feed network of wideband circularly polarized antenna and beam switchable antenna array.It is an important microwave device in communication system.The main study directions of the branch-line directional coupler are broadband coupler,multi-frequency coupler and miniaturized coupler.The coupler terminated by complex impedance and its application on RFID reader microstrip antenna are studied in this paper supported by the National Natural Science Foundation of China(Foundation number:61401056).The mainly completed works are as follows:(1)An branch-line directional coupler terminated by complex impedance is proposed and verified by actual design.In this paper,the design formula of the branch-line directional coupler terminated by complex impedance is deduced by using the even-and odd-mode analysis method,and the final design formula is given.The coupler can achieve a complex port impedance,while the coupler's straight port and coupling port can achieve different power outputs.(2)A single-layer capacitive coupled feeding broadband circularly polarized microstrip antenna is designed based on the branch-line directional coupler terminated by complex impedance.The antenna using the advantages of capacitance coupled feed which can be equivalent to a resonant circuit,the widening of the antenna impedance bandwidth is achieved by selecting the input impedance of the complex number.In order to eliminate the matching circuit between the antenna feed point and the 3dB coupler,branch-line directional coupler terminated by complex impedance is designed by using the design formula,and the conjugate matching is realized by cascade.(3)To improve the antenna gain flatness,a double-layer wideband circularly polarized microstrip antenna with capacitive coupled feeding is designed to improve the single-layer capacitive coupled feed broadband circularly polarized microstrip antenna.The antenna also uses the capacitive coupling feed to achieve the complex input impedance,and then use the conjugate matching method to cascade branch-line directional coupler terminated by complex impedance to achieve circular polarization wave radiation.And compare with single-layer capacitive coupled feeding broadband circularly polarized microstrip antenna.