Low-profile Broadband Antennas For Base Stations

With the development of modern mobile communications,the integration of mobile communication systems is getting higher and higher,and at the same time,the limited space resources for base-station systems have become more expensive.Thus,the miniaturization/lowprofile for broadband base-station antennas have become an important development trend.Due to the development of antenna design methods,the technology of electromagnetic metasurfaces has been found valuable for antenna designers.By using a deliberately designed metasurface,the antenna may become miniaturized,which releases more strained space resources for mobile communication systems and improves the integration of systems.Therefore,this dissertation investigates low-profile broadband antennas for base-station applications by using artificial magnetic conductors(AMC)and metasurfaces.The main contributions of this work are summarized as follows.1.A low-profile broadband antenna based on AMC is developed.The antenna is composed of a radiator,a metal disc,an AMC reflector and a metal strip wall.The radiator of the antenna consists of a pair of crossed dipoles with a "horseshoe-shaped" slot for bandwidth enhancement.The metal disc is located between the radiator and the AMC reflector to improve the impedance matching of the antenna.The AMC structure uses a square metal patch,with a certain height of air layer in the middle,and a PEC reflector at the bottom.The bandwidth for the in-phase reflection phase of the AMC is 1.4-2.85 GHz.The in-phase reflection phase leads to a low antenna height which is about 0.105λL(λL is the wavelength at 1.4 GHz in free space.)Use of metal strips can improve the antenna radiation pattern and impedance matching.Simulated and measured results show that the low-profile broadband antenna based on AMC covers 1.4-2.7 GHz(|S11|<-15 d B,63.4%)with isolation > 30 d B and gain > 7 d Bi.2.A low-profile broadband antenna based on a metasurface is proposed.The antenna is composed of a radiator,a metasurface array,a metal disc and a metallic wall.The radiator of the antenna is a pair of crossed dipoles.By loading the metasurface above the radiator,the profile of the antenna is lowered to 0.105λL.The metal disc improves the impedance matching of the antenna and the metallic wall stabilizes the radiation pattern and the gain of the antenna.Simulated and measured results show that the low-profile broadband antenna based on a metasurface covers 1.4-2.7 GHz(|S11|<-15 d B,63.4%)with isolation > 30 d B and gain(31)8.7 d Bi.3.A miniaturized low-profile broadband antenna based on a metasurface is investigated.The antenna is composed of a radiator,a metasurface,a metal square ring and a disc.A pair of crossed dipoles serves as the radiator of the antenna.By loading the metasurface above the radiator,the height of the antenna is reduced.The metal square ring and the metal disc are located below the radiator,which mainly contributes the improvement of impedance matching at lower frequencies,thus helpful for antenna miniaturization,leading to an overall antenna size of 0.39λL×0.39λL×0.105λL.Simulated and measured results show that the antenna covers 1.4-2.7 GHz(|S11|<-10 d B,63.4%)with isolation >30 d B and gain > 9 d Bi.A four-element antenna array is also studied based on metasurface-based miniaturized low-profile broadband antenna.Simulated and measured results show that the antenna array achieves good radiation and impedance performances,suitable for high-gain base-station applications.The low-profile broadband antennas developed in this dissertation may found wide applications in base stations for modern mobile communications.