This letter presents a closely coupled dual-band multiple-input-multiple-output (MIMO) patch antenna that resonates at 3.7 and 4.1 GHz. The MIMO antenna is composed of two mirror-symmetrical single-feed patch antennas that are closely placed with approximately 0.034 λ0 (where λ0 is the wavelength at 3.7 GHz). The decoupling structure consists of the modified array antenna decoupling surface (MADS) and H-shaped defect ground structures for the lower band and upper band, respectively. Through simulation and measurement, the isolation is determined to be greater than 30 dB in both frequency bands, showing a noticeable improvement compared to the original antenna array. Under the effect of the MADS, the measured gain increases by 2.2 and 0.8 dB at the resonance frequencies of 3.7 and 4.1 GHz, respectively. The measured results indicate that the proposed decoupling structure is quite suitable for closely spaced dual-band MIMO antennas.

This paper introduces an effective solution for improving isolation in linear antenna array. The array is composed of three-element E-plane single feed patch antennas (the overall size is 0.59 λ0 × 1.02 㮰), which are closely placed with approximately 0.037 λ0, where λ0 is the free-space wavelength at 4 GHz. The decoupling structure consists of T-shaped and rectangular ring shaped defect ground structure (DGS) with six metal-vias. The decoupling mechanism is illustrated by investigating the current vector on the ground plane. The current distribution indicates that the decoupling structure along the outer edge of the radiation patch sever as coupling current converter to produce reverse current opposite to the direction of the ground coupling current, which transmits along the outer edge of the radiation edge. The measured results show that the enhancement in the isolation at 4 GHz is 10.8 dB for |S21|, 17.5 dB for |S23| and 16.6 dB for |S31|. After applying the decoupling structure, the 10-dB impedance bandwidth (3.95 -4.04 GHz) of antenna 1 remains unchanged. Meanwhile, the 10-dB impedance bandwidth of the antenna, which is located in the middle of the three-element antenna array, increases by 86 MHz (from 114 MHz to 200 MHz). When the decoupling structure is loaded, the maximum gain only decreases by 0.12 dB (from 3.94 to 3.82 dB). The decoupling structure has the advantages of simple structure, easy processing, and independent design of the array, so it can be applied to multielement patch array, such as massive multiple-input multipleoutput (M-MIMO) system.

A metamaterial-based wideband and high-gain circularly polarized (CP) mushroom antenna is proposed and analyzed in this letter. The proposed antenna consists of a 4 4 mushroom-cells array and an L-shaped slot coupling-fed layer. The metamaterial-based mushroom cells are adopted to adjust the axial ratio (AR) to realize wideband CP and high gain. With the optimization, the proposed dielectric-filled mushroom antenna is with a low profile of 0.08 (is the operating wavelength in free space) while covering an area of 55mm 50mm only. The measured impedance bandwidth of the proposed antenna is 48.2% (5.2$times$8.5 GHz) and the measured 3-dB axial ratio bandwidths are 35.5% (5.8-8.3 GHz). Moreover, the antenna performed a stable high gain which is better than 10.5 dBic from 6 to 7.5 GHz across the CP region.

In this letter, a novel polarization conversion (PC) structure is introduced, and its use with a wideband, wide 3-dB axial-ratio (AR) beam, low-radar-cross-section (RCS) circular polarization (CP) slot antenna is suggested. The unit cell of the proposed PC metasurface (PCM) consists of square and L-shaped patches separated by an L-shaped slot printed on an FR4 substrate. The mechanisms for linear to circular polarization and bandwidth improvement of a conventional antenna are analyzed theoretically. Through numerical optimizations and parametric studies, a simulated 10-dB impedance bandwidth of 42.7% (4.15-6.4 GHz) and a 3-dB AR bandwidth of 26.2% (4.8-6.25 GHz) are achieved by the proposed antenna. As the simulation demonstrates, the proposed antenna achieves an average 20-dB RCS reduction in a wide band from 5.25 to 6.8 GHz compared to a conventional slot antenna covered by the PC band. In addition, compared to previously reported work, the PCM-based CP antenna exhibits a larger 3-dB AR beam width of 200° in the xoz plane and 120° in the yoz plane, and a wider CP bandwidth within the operational bandwidth. Finally, an overall volume of 0.55λ0×0.55λ0×0.05λ0 centered at 5.275 GHz is fabricated and measured, demonstrating good agreement with the simulation results.