In this article, a general decoupling method based on a new perspective of common mode (CM) and differential mode (DM) cancellation is proposed. For two closely spaced antennas, the mutual coupling effect can be analyzed and solved by exciting them simultaneously with in-phase (CM) and out-of-phase (DM) signals. It is theoretically proved that, if CM and DM impedances are the same, the mutual coupling effect between two separated antennas can be totally eliminated. Therefore, we can solve the coupling problem by CM and DM impedance analysis and exploit the unique field properties of characteristic modes to assist in antenna decoupling in a physical intuitive way. To validate the feasibility of this method, two practical design examples, including the decoupling between closely spaced dipole antennas and planar inverted-F antennas, are proposed. Both design examples have demonstrated that the proposed method can provide a systemic design guideline for antenna decoupling and achieve better decoupling performance compared to the conventional decoupling techniques. We forecast the proposed decoupling scheme, with a simplified decoupling procedure, has great potential for the applications of antenna arrays and multi-input multi-output (MIMO) systems.

Antenna Decoupling by Common and Differential Modes Cancellation

A ceramic superstrate-based decoupling method (CSDM) is proposed to reduce the mutual coupling between two closely packed dipole antennas while maintaining cross-polarization suppression. Compared with other superstrate-based methods, this proposed method can effectively reduce the mutual coupling between the antennas without using any periodic structures on the superstrate. The ceramic superstrate is a 2 mm thin slab with a relative dielectric constant of 20.5 and is suspended over the antennas coupled in H-plane with the spacing of only 0.28 wavelength at 3.5 GHz. It is demonstrated by both simulation and measurement that the isolation between two dipole antennas can be improved from 10 to more than 25 dB within the operation band while their reflection coefficients remain to be below −10 dB after the ceramic superstrate is introduced. The radiation patterns of the antenna maintain stable at different operation frequencies within the band of interest and the peak gain has increased by around 1.5 dB. Meanwhile, the total efficiency is enhanced by about 15% and the envelope correlation coefficient (ECC) between the two antennas is reduced from 0.7 to 0.4.

Ceramic Superstrate-Based Decoupling Method for Two Closely Packed Antennas With Cross-Polarization Suppression

This brief presents a novel design of high-isolation dual-band antenna with a close distance for 5G applications. The proposed antenna consists of two symmetrical monopole antenna elements. Two resonance frequencies of the monopole antenna are independently tunable. The defected ground structure (DGS) and grounded branches are loaded to reduce mutual coupling of the lower band by changing the current distribution on the ground plane. And a T-shaped parasitic element is placed between the two antenna elements. The parasitic element introduces an inverse path for mutual coupling cancellation, which greatly increases the isolation of the higher band. As a result, the proposed antenna achieves 5G dual-band of 3.409–3.601 and 4.76–5.04 GHz ( ${S} _{11}$ & ${S} _{22}{ dB, ${S} _{12}$ & ${S} _{21}{ dB). Compared with the single monopole antenna, the bands of the proposed antenna don’t shift and the impedance matching is improved. With the decoupling structure, the isolations are −29 and −41.4 dB at 3.5 and 4.85 GHz, respectively. In addition, the proposed antenna also has the advantages of small size, more flexibility, and simple structure.

Isolation Enhancement in Dual-Band Monopole Antenna for 5G Applications

A pair of tri-band multiple-input-multiple-output (MIMO) antennas with high isolation is investigated in this paper. The proposed antenna consists of two monopole antenna elements with an edge to edge spacing of 4 mm ( $0.03~\lambda _{0}$ at 2.3 GHz). The monopole antenna element chose the bending line structure that can operate in tri-band and realize miniaturization. To achieve compact dimension and high isolation, symmetrical distribution layout is adopted to decouple the low frequency band, the U-shaped neutralization line (NL) contacting with two microstrip lines, and it can to improve the isolation in high frequency band, and the inverted U-shaped NL contacting with two radiation patch can decoupling in the middle frequency band. And then study the envelope correlation coefficient is lower, and the radiation pattern in operation bands is quasi-omnidirectional. It indicates that the antenna has obtained a satisfactory diversity performance within the whole operation bands which can be a good candidate for some portable MIMO applications.

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Neutralization Line Decoupling Tri-Band Multiple-Input Multiple-Output Antenna Design

In this paper, a penta-polarization reconfigurable antenna with mushroom-type metamaterial loading is proposed, which can operate in $x$ -direction linear polarization ( $x$ -LP), $y$ -LP, 45°-LP, left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP). In order to realize polarization reconfigurability, a dual-port dual LP mushroom antenna excited by crossed H-shape slot is designed by characteristic mode analysis (CMA). Through shifting the states of PIN diodes on the reconfigurable feeding network, the amplitude and phase distributions of two ports of the dual LP antenna can be dynamically controlled and the five polarization modes can be achieved. A prototype with a profile of $0.06\lambda _{0}$ at 5.2 GHz ( $\lambda _{0}$ is the operated wavelength in free space) is simulated and measured. The measurement results exhibit a wide impedance bandwidth over 33.2% for LPs and over 38.7% for CPs. In addition, the measured axial ratio bandwidths are 28.0% for LHCP and 30.3% for RHCP, and the maximum measured boresight gains of all modes are higher than 8.2 dBi. The simulated and measured results verify that the proposed antenna can achieve a balance between low-profile and broadband on the basis of penta-polarization reconfigurability. The proposed antenna can be applied to polarization diversity and C band satellite communication.

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Broadband and Low-Profile Penta-Polarization Reconfigurable Metamaterial Antenna

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.

Isolation Enhancement in Closely Coupled Dual-Band MIMO Patch Antennas

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 $4 \times 4$ 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 $0.08{\lambda _0}$ ( ${\lambda _0}$ is the operating wavelength in free space) while covering an area of $60\,{\text{mm}} \times 60\,{\text{mm}}$ only. The measured impedance bandwidth of the proposed antenna is 48.2% (5.2–8.5 GHz), and the measured 3 dB AR 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.

Bandwidth and Gain Improvement of an L-Shaped Slot Antenna With Metamaterial Loading

A metasurface-based low-profile crossed dipole antenna with wide circularly polarized bandwidth for 2.45 GHz ISM band wireless communications is proposed and fabricated in this article. Consisting of four slit-loaded rectangular patches, the double-sided printing crossed dipoles are fed by a pair of vacant-quarter printed rings which circularly polarized (CP) radiation could be generated. With slits loaded, by properly combining the fundamental mode of the two inverted L-shaped dipole, the slot mode and extra resonance generated by the AMC surface, a wideband circularly polarized operation can be obtained. After optimization, the final design with an overall size of 0.44λ0 × 0.44λ0 × 0.1448λ0 at 2.4 GHz had measured a 31.6% (2–2.75 GHz) impedance bandwidth and 3 dB axial ratio bandwidths of measured were 23.2% (2.1–2.65 GHz), respectively. In addition, the antenna performed a small gain variation (7.0–7.5 dBic) and a front-to-back ratio (FBR) of over 25 dB across the whole CP region.

A metasurface‐based slit‐loaded wideband circularly polarized crossed dipole antenna

A novel dual-band, dual-circularly polarized antenna is proposed and fabricated. The proposed antenna consists of an asymmetric U-shaped slot and an inverted L-shaped slot which are designed to excite two orthogonal E vectors with equal amplitude and 90° phase difference (PD), in addition, fed by a coplanar waveguide (CPW) Furthermore, a left-hand circular polarization in the direction of z > 0 and a right-hand circular polarization instead of the opposite direction both at the lower and upper bands are exhibited by the radiations of the antenna. Good agreement is achieved between the measurement and simulation, which indicates that a 10-dB bandwidth of 38.75% from 2.56 to 3.8 GHz and 21.8% from 10.01 to 12.53 GHz, while a 3-dB axial-ratio bandwidth (ARBW) of 13.4% from 2.77 to 3.2 GHz and 9.23% from 10.25 to 11.25 GHz at two operation bands, respectively, are covered in the designed antenna. To explain the mechanism of dual-band dual-circular polarization, the analysis of magnetic fields distributions and a parametric study of the design are given. Meanwhile, compared to other recent works, a single layer structure, wider axial ratio and impedance bandwidths and a more compact size are the key features of the proposed antenna.

Novel dual-band asymmetric U-shaped slot antenna for dual-circular polarization

Based on convoluted interwoven element, a miniaturized frequency selective surface (FSS) with stable band-stop response is proposed in the paper. By extending the four dipoles into the adjacent elements, the equivalent inductance and capacitance are increased, and therefore the proposed FSS realizes promising miniaturization characteristics. The simulation results indicate that the resonant frequency is 1.19 GHz, and the dimension is only 0.027λ0. Compared to traditional crossed elements, the size is reduced by 94.6%. Besides, the FSS has excellent angle-stability under both TE and TM waves. Finally, the proposed FSS is fabricated and measured, and the experiment results prove the satisfactory consistency with the simulation results.

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Qiang Chen

Papers

Isolation Enhancement in Closely Coupled Dual-Band MIMO Patch Antennas

Bandwidth and Gain Improvement of an L-Shaped Slot Antenna With Metamaterial Loading

A metasurface‐based slit‐loaded wideband circularly polarized crossed dipole antenna

Novel dual-band asymmetric U-shaped slot antenna for dual-circular polarization

A Outstanding Miniaturized Frequency Selective Surface Based on Convoluted Interwoven Element