An ultra-wideband (UWB) multiple-input, multiple-output (MIMO) antenna with a reasonably compact size of 30 × 18 × 1.6 mm³ is presented in this paper. The proposed antenna contains two radiating components, each of which is made up of three elliptically shaped patches situated 60 degrees apart, and resembles the shape of a flower. Moreover, the proposed antenna design incorporates a T-like ground branch that functions as a decoupling structure, and is composed of two modified inverted-L branches and an I-shaped stub, offering an isolation of more than 20 dB over the whole operation band (4.3–15.63 GHz). Furthermore, the proposed antenna system was fabricated and tested, and the envelope correlation coefficient (ECC), diversity gain (DG), and total active reflection coefficient (TARC), as well as the radiation characteristics and MIMO performance, were analyzed. The proposed UWB-MIMO antenna may be a suitable candidate for diverse UWB applications, based on the simulated and measured results of this study.

To satisfy the demand for 5G communication to smartphone terminal antennas in element quantity and isolation, an eight-element wideband MIMO antenna set of high isolation level is proposed. Each antenna element in the array is a double-L antenna consisting of an L-shaped slot and an L-shaped 50 Ω microstrip line. The L slot is formed by adding an I-shaped open circuit directly to the side of the rectangular slot. In addition, the antenna arrays are located on the long bezels on both sides of the mobile phone motherboard, and the coupling feed is made in the frequency range of 3.6–4.7 GHz through the L-shaped microstrip line, so as to cover the 5G NR band (N77/N78/N79). Finally, the four element pairs (8 components) achieve isolation of greater than 11 dB, the performance frequency range is 3.6–4.7GHz, the return loss is −6 dB, the total efficiency is greater than 85%, and the envelope correlation coefficient is less than 0.08. Other MIMO performances are also calculated, the design process is discussed in detail, and one-handed grip mode and two-handed grip mode are discussed to demonstrate their stabilities in real-world applications.

A practical method, which based on eighth-mode substrate integrated waveguide (EMSIW), is investigated in this paper to design low-profile antenna with dual-sense circular polarization (CP) and enhanced bandwidth. First, an eighth-mode radiating cavities is presented and excited by single coaxial probe for size reduction and directional radiation. Then, two EMSIW antennas are placed symmetrically and a triangular parasitic patch is introduced between two EMSIW cavities. Attribute to the weakly coupled effect between the radiation aperture and triangular parasitic patch, dual-sense circular polarization characteristic and an additional resonance are acquired. When the triangular patch is transformed into a square parasitic patch, the current path on the parasitic patch is shortened, and a new resonance point is generated at 3.55 GHz. While the resonant mode of the two eighth-mode radiating cavities are almost unaffected and the CP bandwidth varies with the changed current path. Finally, a dual-sense CP EMSIW antenna with extended bandwidth and simple structure is designed and fabricated. The overall size of the fabricated prototype is only $0.57\lambda _{0} \times 0.54\lambda _{0} \times 0.02\lambda _{0}$ ( $\lambda _{0}$ is the free-space wavelength at 3.4 GHz). Measured impedance bandwidth is 10.5% from 3.24 GHz to 3.60 GHz with an isolation up to 11.6 dB and a maximal gain better than 5.74 dBic in two ports is also achieved. Moreover, the characteristics of dual-sense CP and the cross-polarization suppression capability of up to 20 dB in the main beam are obtained in 3.48-3.60 GHz.

A compact, multiband two-port MIMO antenna is proposed in this paper for various wireless transmission networks, where the overall size of the antenna is only 30 × 20 × 1.6 mm3. The proposed MIMO antenna consists of two radiating patches, each of which comprises a semicircle and a semi-regular hexagon, as well as the surface-etched C-slot and U-slot to tailor the antenna’s return loss characteristics. In proposed antenna, a parasitic branch forms when the ground plane’s meandering branches are symmetrically distributed. On one hand, it can increase the ground plane’s effective area and enhance the antenna’s return loss characteristics. A neutralization line, on the other hand, is generated, thereby limiting the current transmission on the ground plane. A cross-shaped slit in the ground’s center is also employed to further promote isolation between the radiation elements. According to obtained results, the antenna can cover the frequency bands 0.67-7.29 GHz, 8.07-12.11 GHz, 14.07-15.41 GHz, and 16.04-22 GHz (S11<−10 dB). Moreover, an RF isolation larger than 18 dB exists between the two ports. Lastly, in terms of ECC, DG, TARC, CCL, and MEG, the diversity performances are all satisfactory.

Based on shared U-shaped patch, a six port antenna with frequency and radiation pattern diversity is investigated in this paper. Two evolutionary U-shaped patches are cascaded together to form a three port antenna with a common ground. Three probes located at the two ends and the middle of the U-shaped patch are used to provide excitation with different phases. In order to improve the impedance bandwidth of each port, both T-shaped and rectangular stubs are loaded on the ground plane. When different ports are excited, the vector currents on the U-shaped patch and the T-shaped stubs show diametrically opposite flow directions, resulting in the diversity of the antenna in frequency and radiation pattern. Two three port antennas form a six port antenna with a relatively compact distance of 0.23λ. The measured results show that the frequency diversity is specifically manifested as when port 1, 3, 4, 6 are excited respectively, the six port antenna operates in 4.73-5.02 GHz with a peak gain of 5.38 dBi. And, when port 2, 5 are separately excited, the antenna works at 3.80-4.40 GHz with a peak gain of 5.01 dBi. Meanwhile, the six port antenna exhibits diverse radiation patterns with better total efficiency up to 84.8%. Furthermore, the port isolation in the two operating bandwidth is better than 21.7 dB and 18.5 dB, which also indicates that the designed six port antenna can be applied for 5G.