Differentially-fed Broadband Circularly Polarized Antennas

Miniaturized highly-integrated broadband communication systems are required due to the increasing demands of communications.Differential circuits are widely used in communication systems.Differentially-fed antenna can be directly connected to differential circuits,avoiding the use of converting components,such as baluns,which improves the integration of communication systems.Circularly polarized antenna can effectively resolve the problems caused by polarization mismatching.Besides,circularly polarized antennas have the advantages of rotational orthogonality and reversible rotation direction,which helps to improve the capacity of wireless communication systems.Therefore,differentially-fed broadband circularly polarized antennas have high research value.The main focus of this paper is on the research of differentially-fed broadband circularly polarized antennas.This thesis mainly includes the following three aspects of research:1.A differentially-fed broadband circularly polarized antenna is proposed.The antenna is made up of two folded off-center-fed dipoles,which are connected in parallel through a pair of coplanar strip lines and are driven by a short microstrip line.The off-center-fed dipoles can generate circularly polarized radiation.By introducing two parasitic elements below the folded off-center-fed dipoles,the bandwidth of the antenna is enhanced.Both the off-center-fed dipoles and the microstrip line are printed on the same substrate,making the antenna simple and planar.Differential signals are fed to the short microstrip line through two coaxial cables for differentially driving the antenna.Experimental results show that the impedance bandwidth for differential reflection coefficient <-15 d B is 1.62-2.94 GHz and the bandwidth for axial ratio(AR)for AR< 3 d B covers the frequency band 1.67-3 GHz.The average gain of the antenna is about 8 d Bi with stable radiation patterns.2.A differentially-fed broadband circularly polarized antenna above an artificial magnetic conductor(AMC)surface is presented.The AMC surface is introduced to replace the metallic reflector in order to reduce the antenna height.It is shown that the antenna height is significantly lowered to 45% of the original height while the broadband performances of the antenna are still maintained.The antenna has a bandwidth of 54.9% covering 1.47-2.56 GHz for differential reflection coefficient <-15 d B and a bandwidth of 47.3% covering 1.68-2.72 GHz for axial ratio <3 d B.The average antenna gain is 6.5 d Bi and the radiation patterns are stable.The differentially-fed broadband circularly polarized antenna above an AMC structure features low profile and planar structure characteristics,which are beneficial for system integration.3.An improved differentially-fed broadband circularly polarized antenna is proposed.The antenna also consists of a pair of off-center-fed dipoles,which are connected in parallel through coplanar strip line and are differentially fed by a short strip line.The longer arms of the dipoles are folded twice for circular polarization and antenna miniaturization.Stable high antenna gain is achieved by reducing the antenna height.Experimental results show that the antenna has a bandwidth of 59.6% covering 1.53-2.83 GHz for differential reflection coefficient <-15 d B and a bandwidth of 47.3% covering 1.68-2.72 GHz for axial ratio <3 d B.The antenna gain is higher than 9 d Bi and the radiation patterns are stable.The antenna has the advantages of planar structure,high gain and easy integration.These differentially-fed broadband circularly polarized antennas may find wide applications in radar and communication systems.