Wideband MIMO Antenna Isolation Enhancement Using 4th-Order Cross-Coupled Decoupling Circuit

This article presents a new fourth-order stub-based decoupling circuit for isolation improvement in a wideband tightly coupled multiple-input–multiple-output (MIMO) antenna array. The proposed decoupling circuit uses a fourth-order coupling-resonator filter effect and is designed using the structure’s admittance parameter. In the decoupling circuit, the pair of dual-band second-order stub-based filters is used for the desired band’s lower and upper frequencies. Finite transmission zeros provide cross-coupling, which helps maintain the admittance slope stopband between these bands and reduces coupling among MIMO elements. The proposed fourth-order decoupling circuit concept can be utilized for different wideband MIMO antenna applications to redesign the targeted band and cross-coupling scenario properly. In addition, to demonstrate the proposed decoupling circuit concept’s influence in achieving good wideband isolation in a compact dimension of $45\times 40\times1.6$ mm ³ , octagonal- and circular-shaped patch MIMO antennas are analyzed. To analyze the impact of the proposed decoupling circuit of the dimension $11.3\times11$ mm ² in the design, edge-to-edge separation between MIMO elements is kept at a minimum of 0.2 mm ( $0.0035\lambda _{0 \thinspace }$ at 5.4 GHz, WLAN band). This demonstrates that the decoupling circuits are antenna-independent and can be applied to other MIMO antennas. The measured S-parameters of the proposed wideband MIMO antenna present high isolation (>20 dB) between the antenna elements. Moreover, the proposed solutions’ radiation behavior and diversity performance are evaluated. The MIMO antenna element measured radiation efficiency is increased from 50% to 75%. It demonstrates good properties that make it a suitable contender for advanced scientific applications.