Novel AMC Structures And Their Applications

In modern mobile communication systems, base station antennas are usually required to have a wide bandwidth, a high gain and low profile. In order to realize directed radiation and high gain, these planar base station antennas need to be placed ~l/4 above a perfect electric conductor(PEC) ground plane. It will lead to a large profile, especially at the frequencies for LTE700/GSM900 base stations. As the reflection phase of an artificial magnetic conductor(AMC) surface varies with frequency continuously from 180° to -180°, this paper focuses on research and design broadband artificial magnetic conductor structures for height reduction. The main contributions of this work include: 1. The surface wave band gap and the in-phase reflection phase characteristics of the AMC structures have been investigated. Then, a novel broadband AMC cell is proposed for lowering the height of planar antennas. The bandwidth of in-phase reflection phase is from 0.8GHz to 1.2GHz. 2. The AMC structure in reducing the height of planar antennas is researched. Three planarantennas for LTE700/GSM900 base stations, including a linearly polarized antenna, a dual-polarized antenna, and a circularly polarized antenna, are investigated. The antenna heights are lowered from 0.263l0.83 GHz to 0.083l0.83 GHz,(where l0.83 GHz is the wavelength in free space at the frequency 0.83GHz). The linearly polarized antenna has a bandwidth of 38%(0.69-1GHz) for return loss >15d B, the dual-polarized antenna has a bandwidth of 35%(0.678–0.965GHz) for return loss >15d B with an isolation of higher than 35 d B, the bandwidth of the circularly polarized antenna for S11 ￡ -15d B is 46% (0.68-1.09GHz) and the bandwidth for axial ratio is 37.8%. All these planar antennas above the AMC surface have a gain of higher than 7d Bi. Which demonstrate that the designed broadband AMC structure can effectively reduce planar antennas of various polarizations. 3. An improved AMC structure is achieved by modify the novel broadband AMC structure. Similarly, this improved AMC structure is applied to three planar antennas stated above. The antenna heights of all these planar antennas are significantly reduced while they still maintain a wide bandwidth. This proved that the achievements of this paper have practical value and the design method has instructional value.