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Abstract-A compact polarization reconfigurable wideband omnidirectional circularly polarized (CP) antenna is presented in this paper. This design is based on a low profile omnidirectional CP antenna which consists of a microstrip antenna working on TM01/TM02 modes with slots etched on the ground plane. Vias are introduced between the patch and the ground plane, in order to excite the TM01 and TM02 modes which generate vertically polarized electric field within a wide frequency band. Sequential bended slots are introduced on the ground plane to radiate horizontally polarized electric field. As the result of the combined radiations from both the patch antenna and the sequential slots, left hand circularly polarized (LHCP) omnidirectional radiation pattern is obtained. The polarization reconfigurable is realized by introducing PIN diodes on the slots and through controlling the states of the PIN diodes, the effective orientation of the slots on ground plane can be adjusted and the polarization of this antenna can be altered between LHCP and RHCP (right hand circularly polarization). To verify this design concept, one LHCP and one polarization reconfigurable prototypes at 2.4GHz band were designed and fabricated. Experimental results show good agreement with the simulation results. The LHCP antenna exhibits a wide impedance bandwidth of 18.4% from 2.12 to 2.57 GHz and 3dB-axial ratio (AR) bandwidth of 450MHz. For the polarization reconfigurable antenna, 19.8% (2.09 GHz -2.55 GHz) overlapped 3 dB AR and 10 dB impedance bandwidths is obtained. The present design has thickness of only 0.024 and exhibit stable CP omnidirectional conical-beam radiation patterns within the operating frequency band with good circularly polarization.

In this paper, a method to reduce the inevitable mutual coupling between antennas in an extremely closely spaced two-element MIMO antenna array is proposed. A suspended meta-surface composed periodic square split ring resonators (SRRs) is placed above the antenna array for decoupling. The meta-surface is equivalent to a negative permeability medium, along which wave propagation is rejected. By properly designing the rejection frequency band of the SRR unit, the mutual coupling between the antenna elements in the MIMO antenna system can be significantly reduced. Two prototypes of microstrip antenna arrays at 5.8 GHz band with and without the metasurface have been fabricated and measured. The matching bandwidths of antennas with reflection coefficient smaller than −15 dB for the arrays without and with the metasurface are 360 MHz and 900 MHz respectively. Using the meta-surface, the isolation between elements is increased from around 8 dB to more than 27 dB within the band of interest. Meanwhile, the total efficiency and peak gain of each element, the envelope correlation coefficient (ECC) between the two elements are also improved by considerable amounts. All the results demonstrate that the proposed method is very efficient for enhancing the performance of MIMO antenna arrays.

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