This article focuses on reducing mutual coupling between the ports of dielectric resonator antenna (DRA) using defected ground structures (DGSs). The antenna has the dimension of 50 mm × 50 mm × 8.5 mm. The resonating element in the proposed two-port radiator consists of a cylindrical structure of alumina ceramic (ɛr = 9.8). The rectangular-shaped aperture is utilized to excite both of the resonating elements. The resonating ceramic elements acting as radiators are offset-fed to enhance the antenna’s coupling. Combining interdigital-shaped and semicircular arc-shaped DGSs improves isolation between two resonating elements, embodying the structural novelty. The measured operating frequency range of Port-1 and Port-2 is 5.19–6.7 and 5.15–6.68 GHz, resonating at 5.58 and 5.56 GHz, respectively. The measured mutual coupling between the two ports is −35.5 dB. The measured gain for Port-1 is depicted to be 5.5 dB. The presented multiple-input–multiple-output (MIMO) radiator in this article is an appropriate candidate for WLAN (5.25–5.35, 5.47–5.725, 5.725–5.85, 5.850–5.925 GHz) and WiMAX(5.5 GHz) applications. All the simulated and experimentally observed MIMO parameters of the radiator are discovered to be within optimal bounds.

This manuscript presents a 50 Ω microstrip-fed quad-element high isolated ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna with band-notched characteristics. The overall area of the proposed structure is 0.33λo × 0.33λo mm2 (where λo depicts the free space wavelength corresponding to the lower cutoff frequency, i.e. 2.54 GHz), etched on an FR-4 substrate of thickness 0.8 mm. The top layer has four semicircular disc-shaped radiating elements that are identical and orthogonal to obtain better inter-element isolation and compactness. A reverse two-shaped slot is etched onto the radiating patches to attain a band-rejection capability. Moreover, a decoupling structure is also placed at the top layer to suppress the unwanted surface waves. The bottom layer consists of a ground plane, which is further modified with quasi-self complementary and meandered line slots. A rectangular slot is also etched below the feed line to better match the impedance near the lower cutoff frequencies. The simulated reflection coefficients (S11) of the proposed antenna are less than −10 dB over 2.54 to 10.74 GHz frequencies except at 3.37 to 4.15 GHz (WiMAX/C band), and the simulated inter-port isolation (S21) is greater than −15 dB over the entire UWB range of frequencies (3.1 to 10.6 GHz). Also, the measured S-parameter results well agreed with the simulated ones. Furthermore, the simulation study of the 20-element UWB-MIMO antenna is also investigated using the proposed quad-element structure.

As the demand for high data rates is increasing day by day, fifth-generation (5G) becomes the leading-edge technology in wireless communications. The main objectives of the 5G communication system are to enhance the data rates (up to 20 Gbps) and capacity, ultra-low latency (1 ms), high reliability, great flexibility, and enhance device to device communication. The mentioned objectives lead to the hunting of the millimeter-wave frequency range which lies from 30 to 300 GHz for 5G wireless communications. To design such high capacity, low latency, and flexible systems, antenna design is one of the crucial parts. In this paper, a survey is presented on various antenna designs with their fabrication on different types of substrates such as Rogers RT/duroid 5880, Rogers RO4003C, Taconic TLY-5, etc., at different 5G frequency bands. The different configurations of antennas that covered antenna arrays, multiple-input multiple-output (MIMO) antennas, phased antennas, and beamforming antennas are discussed in detail with their applications. The design of MIMO antennas in the 5G frequency band occupied less space so mutual coupling reduction techniques are required for maintaining the required gain, efficiency, and isolation. This paper is also focused on the mutual coupling reduction techniques and diversity in MIMO antennas.

This paper presents an efficient approach for the design of a non-uniform single ring circular antenna array (CAA) for the synthesis of the optimal far-field radiation pattern. A recently proposed meta-heuristic-based optimization technique known as ant lion optimization (ALO) is applied in this paper to determine the optimum set of current amplitude excitation weights and the inter-element distance among the array elements to reduce the side lobe level (SLL) and 3-dB beam width considering the mutual coupling effect. The results achieved by employing the ALO algorithm are compared with the uniform radiation pattern and with those of the recently reported literature containing equal sets of elements to prove the superiority of ALO algorithm. Three different design examples of 8, 10, and 12 elements CAA are presented, and their performances are compared to illustrate the capability of the ALO algorithm-based approach over those of the recently reported literature.

For the requirements of low mutual-coupling MIMO antennas for WLAN, a new complementary split-ring resonator (CSRR) unit cell is introduced in this paper. A microstrip-fed Vivaldi antenna array is designed for WLAN applications, where compact triple-band gap-complementary split-ring resonator unit cells are loaded between two antennas to examine the effect of unit cells on the rate of mutual-coupling reduction. By inserting the CSRR, the final design offered an improvement in decoupling by 8.5, 10.5, and 18 dB at 3.65, 4.9, and 5.8 GHz, respectively, compared with the reference antenna. By suppressing surface waves, antenna gain and front-to-back ratio are improved.

A novel multiple-input and multiple-output (MIMO) fractal antenna excited by a coplanar waveguide was investigated in this study. A novel technique was used to improve the isolation of 20 dB between the dual radiating elements by inserting a strip line into the outer edges of the ground plane. A sunflower structure was used to configure the antenna in three steps. At each step, an additional sunflower structure was added with half the size of that used in the previous step to enhance the impedance bandwidth. The measured values of envelop correlation coefficient and total active reflection coefficient indicated that the proposed MIMO antenna has high-diversity performance between radiating elements. Wide dual operating bands of 2–2.9 and 5–10 GHz were obtained, which can support different wireless communications, such as 3G, LTE (2.6 GHz), WLAN (2.4 GHz/5 GHz), WiMAX (2.4 GHz/5GHz), ISM (2.4 GHz/5 GHz), 5G (5–6 GHz), and satellite communications (6–8 GHz). The MIMO fractal antenna with a small size achieved a maximum efficiency of 85% and a peak value gain of 6 dBi, low-channel capacity loss of 0.15–0.4 b/s/Hz, and high isolation between radiating elements is suitable for portable communication devices.

In this paper, a novel decoupling technique for closely spaced E-plane patch antennas using defect ground structure (DGS) is proposed. The electric field coupling between the antennas is suppressed by etching DGS which consists of a pair of rectangular slots and four stubs on the ground plane. Moreover, unlike the other methods, the DGS is not etched in the middle of the antennas but loaded along the outer edge of the radiated patch. Thus, through the adopted technology the distance between the antenna elements is reduced and the isolation is increased. To validate the improvements by adopting the proposed technology, the array with DGS loading has been fabricated and then measured. The measurement results show that designed antennas have 95 MHz 10-dB impedance bandwidth, which is 25 MHz higher than that of the antenna without DGS. More importantly, isolation improvements have been increased from 8.5 to 31.3 dB by using the decoupling technique when the antennas are placed with a 0.032 λ0 edge-to-edge distance, where λ0 is the free-space wavelength. Therefore, this technique can be widely applied to improve isolation in a compact and low profile antenna system.

In this paper, a multiple input multiple output antenna which operates at 5.8 GHz for wireless local area network applications is proposed. The proposed antenna is composed of two sets of four elements antenna array (2 × 2) on the top and a novel metamaterial structure on the ground plane. Here, the ground plane, which includes a lattice of 2 × 5 unit cells of metamaterial structure, is utilized in order to improve parameters of the antenna. Thanks to the proposed metamaterial structure, not only gain and bandwidth of antenna are enhanced, but also mutual coupling is reduced. The final design, with an overall size of 137 × 77 × 3.048 mm3, resulted in a |S11| <−10 dB bandwidth of 1.78 GHz and a peak gain of 9.2 dBi. In addition, the isolation is higher than 18 dB although the close separation from edge to edge of the two antennas is only 2 mm and radiation efficiency of 73% at the operating frequency band. All is simulated based on CST Studio software and the simulated S-parameter results of the antenna are in good agreement with measurement results.

Multiple input multiple output (MIMO) antenna is at core of the presently available wireless technologies. The design of MIMO antennas over a limited space requires various approaches of mutual coupling reduction, otherwise gain, efficiency, diversity gain, and radiation patterns will be severely affected. Various techniques have been reported in literature to control this degrading factor and to improve the performance of the MIMO antennas. In this review paper, we have carried out an extensive thorough investigation of diversity and mutual coupling (correlation) reduction techniques in compact MIMO antennas.

Compact modified T-slot circular patch antenna for multiple input multiple output (MIMO) applications, which has good response for four operating frequency bands is developed. Here the design consists of a circular patch with a new slot loaded T- shape on one side of the substrate and a feeder line on the other side of the substrate. A shorting pin is located between the patch and the feeder line. This proposed antenna resonates at the frequencies of 2, 4.8, 5.4, and 6.2 GHz covering the S-band, C-band, and LTE applications. The independent tuning of frequency bands is achieved by varying the length of the slots. By independent tuning of frequency bands, the selection of interested frequency band is achieved. The single element with miniaturized size of 1.945λ × 1.945 λ (30 × 30 mm2) is achieved including the feed line. The antenna has been tested for MIMO applications. The measured envelope correlation coefficient between the two elements, which are spaced 0.139 λ is <0.1(low correlation) and mutual coupling is found to be in the range of −20 dB to −80 dB. The simulation and measurement results of reflection coefficient, mutual coupling, and radiation pattern are presented.

A compact quad band slot antenna with high isolation suitable for multiple-input–multiple-output (MIMO) applications is developed. The quad bands are achieved by introducing slots in a modified, size reduced circular patch antenna. The single-antenna element consists of a substrate sandwiched between a modified circular patch with F shape slot, a feeder line and a via running between the radiating patch and the feeder line. The proposed design resonates at the frequencies of 1.8 GHz (1.7–1.88 GHz), 3.6 GHz (3.50–3.76 GHz), 5.4 GHz (5.25–5.38 GHz), and 7.2 GHz (7.15–7.35 GHz) covering the GSM II, WIMAX, wireless local area network (WLAN), and C-band applications, respectively. The independent tuning of frequency bands is achieved by varying the length of the slots. Orthogonally placed two- and four-element MIMO antenna system are fabricated, tested, and the measurement results are presented. The separation between each element is reduced to 0.085λ while introducing slotted and pulsed stubs to improve isolation between elements. A detailed analysis, including mutual coupling, low correlation, diversity gain, and total array reflection coefficient had been reported. The two- and four-element MIMO antennas achieved correlation as low as 0.005, mutual coupling ≤−15 dB, and diversity gain nearly 10 dB.

A circularly polarized (CP) and high gain Microstrip antenna is designed in this paper using metamaterial concepts. The antenna, built on a metamaterial substrate, showed significant size reduction and less mutual coupling in an array compared with similar arrays on conventional substrates. Demonstrated to have left-handed magnetic characteristics, the methodology uses complementary split-ring resonators (SRRs) placed horizontally between the patch and the ground plane. In order to reduce mutual coupling in the array structure, hexagonal-SRRs are embedded between antenna elements. The procedure is shown to have great impact on the antenna performance specifically its bandwidth which is broadened from 400 MHz to 1.2 GHz for X-band and as well as its efficiency. The structure has also low loss and improved standing wave ratio and less mutual coupling. The results show that a reduction of 26.6 dB in mutual coupling is obtained between elements at the operation frequency of the array. Experimental data show a reasonably good agreement between simulation and measured results.

A novel compact multiple-input–multiple-output (MIMO) antenna system operating from 5 to 7.3 GHz is proposed for wireless applications. It comprises of two similar antennas with microstrip feeding and radiating patches developed on a reduced ground plane. The developed antenna system resonates at a dual-band of 5.4 and 6.8 GHz frequencies, giving an impedance bandwidth of 38% (based on S11 < −10 dB). The unique structure of the proposed MIMO system gives a reduced mutual coupling of −27 dB at 5.4 GHz resonant frequency and −19 dB at 6.8 GHz resonant frequency and in the entire operating band the coupling is maintained well below −16 dB. The envelope correlation coefficient of the proposed MIMO system is calculated and is found to be less than 0.05 in the operating band. The measured and simulation results are found in good agreement.

This paper presents a new planar particle that shows negative effective permittivity under irradiation by an electromagnetic wave. The mutual coupling between the couples of these particles is studied in particular. The response of this particle sensitive to an electric field is strongly anisotropic. The particle is aimed to be used to compose an isotropic epsilon-negative metamaterial in two forms. First, a unit cell of the metamaterial consists of a cube bearing six particles on its faces, located with specific orientations. The experiments showed that this unit cell is suitable for manufacturing an isotropic epsilon-negative metamaterial obtained by arranging these cells in a 3D cubic periodic system. The second form of an epsilon-negative metamaterial with an isotropic response consists of the planar particles themselves, distributed quasi-randomly, composing a 2D system and/or of particles placed in spherical shells and distributed fully randomly in a hosting material forming a 3D system. The isotropy of these systems was verified by measurements in a rectangular waveguide.