In this paper, a novel decoupling technique for closely packed patch antennas using near-field resonator (NFR) above each antenna element is proposed. The decoupling mechanism is illustrated by investigating the electric-field (E-field) and magnetic-field (H-field) distributions. The E-field distributions indicate that the NFRs above the patches serve as coupling-mode transducers to produce an orthogonal coupling mode at the desired resonance, leading to high port isolation between the patches. The H-field distributions demonstrate that the H-fields in the substrate are confined within the excited element, leading to the effective suppression of antenna mutual coupling. The NFR can be easily applied to MIMO antennas having multiple patch elements. Three practical decoupling examples are demonstrated and the simulation and measurement results show that impedance matching for each antenna port and isolation of better than 20 dB are achieved for all these examples using the NFRs. Moreover, for the H- and E-plane decoupling of wideband 2-port MIMO antennas, wide decoupled impedance bandwidths of 6.1% and 5.8% are obtained, respectively. More results of radiation patterns reveal that good radiation performance is reserved with no reduction in realized gain or front-to-back ratio (FBR).

A low-profile dual-polarized dipole antenna using wideband artificial magnetic conductor (AMC) reflector with stable radiation pattern for use in 2G/3G/4G base stations operating from 1.7 to 2.7 GHz is presented. The antenna consists of a pair of crossed-dipoles and a wideband AMC reflector which consists of an AMC surface, a metallic ground plane, and the air substrate between them. The AMC is designed to operate with 90° reflection-phase bandwidth of 1.64-2.88 GHz (54.8%). Compared with using perfect electric conductor reflector, the profile height of the dual-polarized dipole antenna using AMC reflector is reduced by half from 36 to 18 mm, resulting in the lowest-profile 2G/3G/4G base station antenna. Six metallic ground walls are used to stabilize the radiation pattern, and the resulted half-power beamwidths are around 68±3° and 81±3° in the H- and V-planes, respectively, across the operating band. More measured results show that wide impedance bandwidths of 1.67-2.98 and 1.64-3.05 GHz for the two input ports, isolation of more than 25 dB, and cross-polarization of less than -20 dB are achieved.

A general and systematic approach is the first time proposed in this paper to design the neutralization line (NL) for isolation enhancement in multiple-input multiple-output (MIMO) antenna arrays. Each NL consists of a metal strip and a reactive component attached at its middle position. It is connected with every two antenna radiators to generate an additional coupling path against the original antenna coupling for isolation enhancement. A general network model that leads to a systematic design procedure is developed to meet the required isolation criteria. Three practical examples are presented and used to verify the proposed NL-based decoupling technique (DT). The measurement results align well with simulations. For all these examples, good impedance matching, isolation over 25 dB, total radiation efficiencies over 70%, and very low envelop correlation coefficients (ECCs) can be achieved simultaneously after adopting the proposed NL DT. Compared with previous decoupling works using parasitic elements, decoupling networks or NLs, the proposed method can achieve either a much wider decoupling bandwidth or a much smaller antenna separation distance while keeping a low design complexity along with systematic design procedure.

Mutual coupling always seriously degrades the antenna performance in multiple-input multiple-output (MIMO) systems but this issue has been rarely investigated for antennas with circular polarization. In this paper, a planar and compact circularly polarized MIMO patch antenna with the polarization diversity is presented. Three grounded stubs and a mirrored F-shaped defected ground structure are used to achieve simultaneous matching and isolation between the two patches with offset feeding for circular polarization. The antenna elements are closely packed with the edge-to-edge distance of 0.06λ0 at the desired frequency of 2.5 GHz. The measured results indicate that the proposed antenna achieves the impedance matching (S11<-10 dB), high isolation (S12<-20 dB), and circular polarization (axial ratio<3 dB) within the frequency band of 2.5-2.55 GHz. The radiation patterns and realized gains are measured, showing good agreement between the simulation and measurement.

This communication proposes a novel decoupling approach for multiple-input multiple-output (MIMO) dielectric resonator antennas (DRAs) using a dielectric superstrate above the array. Utilizing the interaction with electromagnetic fields, the superstrate affects the coupled field distributions inside the adjacent DRA and so reduces coupling between two closely located DRAs. Five decoupling cases are investigated to show the advantages of this method. Results show over 25-dB isolation enhancement for all cases. Good radiation characteristics are obtained with the improvements in total efficiency and realized gain and reductions in envelop correlation coefficient and pattern slant angle. Moreover, the superstrate is independently designed and implemented above the coupled array without requiring modification on the original array. Therefore, the proposed one provides a unique approach for isolation enhancement of MIMO DRAs.