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A compact 4 element MIMO antenna operating in three Sub-6 GHz 5G bands is proposed. The single element antenna consists of a square-shaped patch where circular and rectangular-shaped slot cuts are incorporated. The single element is symmetrically arranged along the horizontal direction and in mirrored fashion along the vertical and diagonal direction. The 4 element MIMO antenna achieves a symmetric structure on top with the connected ground at back. With no extra isolation element, the proposed FR-4 based design achieves compact size of 0.39λ02 (f0 = 3.72 GHz) operating in three frequency bands ranging from (2.65%) 3.72–3.82 GHz, (2.33%) 4.65–4.76 GHz and (4.75%) 6.16–6.46 GHz. The satisfactory isolation amongst inter-elements (≥ 16 dB) with directional radiation pattern and an average gain of 2.5 dBi is attained for proposed bands. Furthermore, the antenna meets the MIMO diversity criteria by achieving DG > 9.7, ECC

In this paper, a semi-flexible 2 × 1 array antenna is proposed with epoxy glass fiber and graphene as patch and ground, respectively. Microstrip patch antenna with a center parasitic patch of half-wavelength and slot in the radiating patch have been incorporated for the bandwidth enhancement in order of 79.56% (2.21–5.13 GHz). The antenna has an overall size of 0.30λ × 0.24λ at a lower frequency of operation (2.45 GHz). The incorporation of slotted Graphene in radiating element leads to a wideband regime with satisfactory gain values of 2.73 and 3.744 dBi at 2.40 and 4.0 GHz, respectively. Antenna radiation efficiency in the range of 78% with linear polarization makes the antenna appropriate for WLAN band and smart wireless devices application.

A compact dual parallel slotted-patch antenna having a single offset feed on the corner side is presented. The antenna geometry comprises parallel and horizontal coupled S dual slotted structure at the top, with a complete ground plane at the bottom, which is oriented to enhance the multiband frequency bands. The patch is patterned on an FR4 substrate having a dielectric constant of 4.4 and loss tangent of 0.02. The antenna has overall dimensions of 36 × 28 × 1.6 mm3, with an offset feed which is optimized to achieve 50 Ω impedance matching. The proposed antenna is simulated using a FEM-based high-frequency structure simulator, after which it is optimized, fabricated, and measured, with the simulated and measured results showing good agreement. The proposed compact antenna resonates in the pentaband frequency range which includes 2.00–2.16 GHz local TV transmission, 3.14–3.23 GHz for radio-frequency identification and detection, and 4.34–4.61 GHz for long-distance radio telecommunications. The antenna shows good gain ranging from 0.5 to 3.5 dBi with omnidirectional and bidirectional radiation patterns, making it suitable for its use in wireless applications with space constraints.

The compact slotted antenna having offset feed radiating in the multi-frequency bands is proposed. The antenna geometry comprises of a vertical and horizontally coupled slotted structure at the top with a complete ground plane at the bottom to achieve the proposed application bands. The patch is patterned on the FR4 substrate having a dielectric constant of 4.4 and loss tangent of 0.008. The antenna has an overall dimension of 0.18λ × 0.15λ mm2 (f = 1.52 GHz) with offset fed which is optimized to achieve 50 Ω impedance matching. The proposed compact antenna resonates in pentaband frequency bands which includes 1.52–1.60 GHz for GPS (ISM band in India), 2.97–3.02 GHz for Radio frequency identification and detection, mobile communication, logistics, manufacturing, transportation and healthcare, 3.73–3.84 GHz for Amateur Fixed Mobile except aeronautical mobile (R), 4.42–4.52 GHz for radio communications, TransferJet USB Adapter (Toshiba Corporation), and 4.83–4.96 GHz for Aviation Private Land Mobile applications. The proposed antenna has demonstrated a decent gain ranging from 1.07 to 3.92 dBi having omnidirectional and bidirectional radiation patterns, hence, making it suitable for various wireless applications.

In this paper, a novel multiband coplanar waveguide-fed slotted transparent antenna is presented. The structural design of the radiating element is based on the semicircular curves in the form of slots on the patch to achieve multi-band characteristics with improved bandwidth. The overall structure having a size of λ0/2.67 × λ0/2.11 achieves transparency and a lightweight due to the presence of Plexiglas as a substrate material and AgHT-8 as a conductive material including the circular slotted patch and related CPW feeding line at the center frequency of the lowest operation band. The proposed antenna has the multiband operation characteristics with the center frequencies of 2.58 GHz, 3.67 GHz, 6.2 GHz having the impedance bandwidth of 19.84% (2.27—2.77 GHz), 1.76% (3.61—3.74 GHz) and 53.58% (4.50–7.79 GHz) respectively. The agreement between the numerical computation and measurement results of RF performance characteristics point out the technical potential of the proposed optically transparent antenna for short range multiband versatile wireless applications.

A four-port dielectric resonator-based connected ground multiple-input multiple-output (MIMO) antenna is designed. The presented antenna was excited through the aperture feeding technique. The dual bands are achieved by optimally feeding the rectangular dielectric resonator through engineered triangular slots. The antenna has operating modes of T E 111 X and T E 111 Y at 4.5 GHz and 5 GHz, respectively. It presents a 2 : 1 VSWR bandwidth of 2.64% (4.48 GHz–4.60 GHz) and 1.2% (4.96 GHz–5.04 GHz) in the lower and upper bands, respectively. The edge-to-edge distance between array elements is around 7.5 mm. The single antenna dimension is 30 mm × 30 mm, whereas the four-port antenna dimension is 60 mm × 60 mm. The optimum isolation was achieved by carefully placing the antenna elements on the substrate through multiple iterations. The antenna provides port isolation better than 20 dB at both resonances with full ground profile. The advantage of the antenna is that it provides fair antenna and MIMO parameters without additional isolation techniques. The antenna has efficiency in order of 88.02% and 86.31%. The peak gain is 7.67 dBi and 8.32 dBi at 4.5 GHz and 5 GHz, respectively. The optimum envelope correlation coefficient (ECC) is 0.037, channel capacity coss (CCL) is 0.2 bits/sec/Hz, diversity gain (DG) is 9.99 dB, and total active reflection coefficient (TARC) is −18.87. The antenna elements are orthogonally placed with adequate separation to achieve polarization diversity and spatial diversity. The antenna provides the utilization in Sub-6 GHz 5G and WLAN communication applications.

A multi-band planar antenna inspired from a meander line structure is proposed. The antenna is printed on a low-cost FR4 substrate having loss tangent, dielectric constant and height of 0.02, 4.4, and 1.6 mm respectively. The conductive patch consists of design inspired by Meander lines which is optimized to achieve targeted frequency bands. Partial ground plane acting as a defected ground structure is incorporated on the bottom side which is engineered for achieving acceptable bandwidths for multi-frequency bands. The proposed antenna has an overall size of 40 × 40 × 1.6 mm2 which resonates at 2.47–2.54 GHz, 4.14–4.23 GHz, 5.43–5.78 GHz and 6.71–7.42 GHz, respectively having the reflection coefficient (S11) better than 10 dB and a peak gain of 0.5 dBi to 4.5 dBi for all the resonant frequencies. The simulated and measured results show great correlation making the antenna suitable for its use in WiMAX, LTE, WLAN, and INSAT C-Band applications.

Multiple-input multiple-output (MIMO) antennas with four and eight elements having connected grounds are designed for ultra-wideband applications. Careful optimization of the lines connecting the grounds leads to reduced mutual coupling amongst the radiating patches. The proposed antenna has a modified substrate geometry and comprises a circular arc-shaped conductive element on the top with the modified ground plane geometry. Polarization diversity and isolation are achieved by replicating the elements orthogonally forming a plus shape antenna structure. The modified ground plane consists of an inverted L strip and semi ellipse slot over the partial ground that helps the antenna in achieving effective wide bandwidth spanning from (117.91%) 2.84–11 GHz. Both 4/8-port antenna achieves a size of 0.61 λ × 0.61 λ mm2 (lowest frequency) where 4-port antenna is printed on FR4 substrate. The 4-port UWB MIMO antenna attains wide impedance bandwidth, Omni-directional pattern, isolation &gt, 15 dB, ECC &lt, 0.015, and average gain &gt, 4.5 dB making the MIMO antenna suitable for portable UWB applications. Four element antenna structure is further extended to 8-element configuration with the connected ground where the decent value of IBW, isolation, and ECC is achieved.

A dual-band high gain monopole antenna interfaced with an artificial magnetic conductor (AMC) reflector is introduced in this paper. The antenna is composed of an open-loop shape radiator fed with a coplanar waveguide (CPW) to achieve the desired frequency bands with a total size of 28 mm × 36 mm × 1.6 mm (0.224λ0 × 0.288 λ0 × 0.012 λ0 at 2.4 GHz). A dual-band AMC array structure is integrated on the back of the antenna to enhance the achieved gain. The total size of the integrated model is 79.9 mm × 79.9 mm × 8 mm (0.63λ0 × 0.63 λ0 × 0.064 λ0 at 2.4 GHz). The suggested models are fabricated and tested in terms of the S11, radiation pattern, and peak gain to validate the simulation results. The tested results of the antenna with the AMC array illustrate that the antenna operates at two bands where S11 ≤ −10 dB from 2.37 GHz to 2.5 GHz and from 4.45 GHz to 4.9 GHz. Furthermore, peak gain values of 5 dBi and 7.5 dBi are achieved at both bands, respectively. The suggested model can be used in sub-6 GHz 5G indoor and outdoor applications.

Interconnected three-element and four-element wideband MIMO antennas have been proposed for millimeter-wave 5G applications by performing numerical computations and carrying out experimental measurements. The antenna structure is realized using Rogers 5880 substrate (εr = 2.2, tan δ = 0.0009), where the radiating element has the shape of an inverted L with a partial ground. The unit element is carefully designed and positioned (by orthogonally rotating the elements) to form three-element (case 1) and four-element (case 2) MIMO antennas. The interconnected ground for both cases is ascertained to increase the practical utilization of the resonator. The proposed MIMO antenna size is (0.95λ × 3λ) for case 1 and (2.01λ × 1.95λ) for case 2 (at the lowest functional frequency). Both the designs give an impedance bandwidth of approximately 26–40 GHz (43%). Moreover, they achieve greater than 15 dB isolation and more than 6 dBi gain with an ECC value lower than 0.02, which meets the MIMO diversity performance thus making the three-element and four-element MIMO antennas the best choice for millimeter-wave 5G applications.

A tapered symmetrical coplanar waveguide (S-CPW) fed monopole antenna is initially studied. To achieve multiband characteristics, the radiating element of this monopole antenna is loaded with multiple narrow slots and multiple slotted stubs (MSS). The designed slot-loading monopole is further transformed into a two-antenna MIMO type with a gap distance of only 0.12λ (at 5 GHz), and thus it has a small overall size of 32 × 20 × 0.8 mm3. By deploying five concentric ring elements between the two adjacent antenna elements, the desirable isolation of better than 20 dB is yielded. As the low band and high band operation of the proposed two-antenna MIMO is 81.08% (3.3–7.8 GHz) and 40% (8.0–12.0 GHz), respectively, it can therefore satisfy the Sub-6 GHz 5G New Radio (NR) n77/78/79, IEEE 802.11ac/ax, X-band/C-band wireless and satellite applications. Furthermore, it has shown a desirable gain of above 3 dBi and a radiation efficiency greater than 69% throughout the two bands of interest.

A dual-port transparent multiple-input multiple-output (MIMO) antenna resonating at sub-6 GHz 5G band is proposed by using patch/ground material as transparent conductive oxide (AgHT-8) and a transparent Plexiglas substrate. Two identical circular-shaped radiating elements fed by using a microstrip feedline are designed using the finite element method (FEM) based high-frequency structure simulator (HFSS) software. The effect of the isolation mechanism is discussed using two cases. In case 1, the two horizontally positioned elements are oriented in a similar direction with a separate ground plane, whereas in case 2, the elements are vertically placed facing opposite to each other with an allied ground. In both cases, the transparent antennas span over a −10 dB band of 4.65 to 4.97 GHz (300 MHz) with isolation greater than 15 dB among two elements. The diversity parameters are also analyzed for both the cases covering the correlation coefficient (ECC), mean effective gain (MEG), diversity gain (DG), and channel capacity loss (CCL). The average gain and efficiency above 1 dBi and 45%, respectively with satisfactory MIMO diversity performance, makes the transparent MIMO antenna an appropriate choice for smart IoT devices working in the sub-6 GHz 5G band by mitigating the co-site location and visual clutter issues.

A single radiator that is a part of four-port diversity Multiple-Input Multiple-Output (MIMO) antenna design is composed of four octagonal rings embedded between the two opposite sides of a T-shaped conductive layer surrounded by inverted angular edge cut L-shaped and E-shaped structures. The radiators are placed at the four corners with common ground at the center of a smartphone to form a four-element mobile MIMO antenna. The printing of the antenna is carried out on the flexible polyamide substrate (dielectric constant = 3.5 and loss tangent = 0.0027) with dimensions of 70 × 145 × 0.2 mm3. A wide impedance bandwidth of (84.12%) 2.39 to 5.86 GHz is achieved for all four radiators. The compact size of the radiators along with their placement enables the proposed MIMO antenna to occupy much less area while preserving the space for 2G/3G/4G antennas. The placement of the antennas results in self-isolation between antenna elements by achieving isolation greater than 17.5 dB in the desired operating bands. Furthermore, besides showing a high efficiency of 85% and adequate gain above 4 dBi, good diversity performances such as Envelope Correlation Coefficient (ECC) of less than 0.05, Diversity Gain (DG) of above 9.8 dB, Mean Effective Gain (MEG) of −3.1 dB, Channel Capacity of 21.50 bps/Hz, and Total Active Reflection Coefficient (TARC) of below −10 dB are achieved by the flexible MIMO smartphone antenna. The effect of bending along the X and Y-axis on the performance of the proposed MIMO antenna is also analyzed where decent performance is observed. This makes the proposed flexible four-element MIMO antenna a potential candidate to be deployed in future smartphones.

A four-port MIMO antenna array with wideband and high isolation characteristics for imminent wireless systems functioning in 5G New Radio (NR) sub-6 GHz n77/n78/n79 and 5 GHz WLAN bands is proposed. Each array antenna element is a microstrip-line fed monopole type. The novelty of the antenna lies in loading an “EL” slot into the radiating element along with two identical stubs coupled to the partial ground in order to improve the impedance matching and radiation characteristics across the bands of interest. To further attain high port isolation without affecting the compactness and radiation performance of each antenna element, the technique of introducing an innovative un-protruded multi-slot (UPMS) isolating element (of low-profile 2 × 19 mm2) between two closely spaced antenna elements (with an edge-to-edge distance of approx. 0.03λ at 4.6 GHz) is also presented. Besides demonstrating a small footprint of 30 × 40 × 1.6 mm3, the proposed four-port MIMO antenna array has also shown wide 10-dB impedance bandwidth of 58.56% (3.20–5.85 GHz), high isolation of more than 17.5 dB, and good gain and efficiency of around 3.5 dBi and 85%, respectively, across the bands of interest. Finally, the MIMO performance metrics of the proposed antenna are also analyzed.

A novel electrically compact and high isolation microstrip diplexer is engineered for Radio Occultation (RO) System for future planetary systems. The radio occultation technique is being utilized extensively in examining properties of a planetary atmosphere. A detection of shift in microwave signal is primary method to measure the occultation. The On-board RO transmitter utilizes the frequency synthesizers at two distinct frequency bands. The proposed microstrip diplexer has self-symmetrical structure that permits reduction in the tradeoff between miniaturization and port isolation. The balanced geometry of two intrinsic filters permits several transmission zeros creating sharp edge, wider stopband and significant port isolation. The further tuning of the diplexer is possible without junction matching however; the insertion loss and isolation may vary based on the frequency shift. The electrical size of the proposed diplexer is 0.52λ × 0.625λ at lower frequency. The optimized structure provides fraction bandwidth of around 4.6% at 6.7 GHz and 4.34% at 8.4 GHz frequencies. The isolation is better than 27 dB for both frequencies. The validation of simulated results is carried out through measurements on fabricated prototype.

A novel dual branch multiband compact slotted antenna for digital broadcasting, wireless and sub 6 GHz 5G is presented. Antenna achieves multiband characteristics at (750–790 MHz) Digital broadcasting, (1.41–1.45 GHz) wireless medical telemetry, (2.10–2.14 GHz) Universal Mobile Telecommunication Service (UMTS), (3.44–3.51 GHz) WiMAX/sub 6 GHz 5G, (3.80–3.87 GHz) Sub 6 GHz 5G, (5.17–5.20 GHz) wireless local area network (WLAN) and (5.97–6.38 GHz) fixed satellite communication by introducing slots in the form of inverted U and E shape having varied branch lengths and full ground plane. The antenna has an overall size of 30 × 30 × 1.6 mm3. The proposed antenna achieves compact operation along with an omnidirectional radiation pattern having gain and efficiency in the range over 1.1 dBi and 78% for the proposed frequency bands. The antenna is fabricated after optimizing the geometrical dimensions where measured results match well with simulated ones.

The demand of compact antenna is ever increasing as the inception of wireless communication devices. In the age of wireless com- munication, requirement of miniaturized antennas is quite high. It is quite often that antenna dimensions are decided based on application- based requirement compared with practical antenna constraints. The trade-off in efficiency and other antenna parameters against to antenna size is always a debatable issue. The article presents detailed review of fundamentals of electrically small antennas (ESAs) and its potential applications. In addition, constraints and challenges of ESAs are also presented in the article. V C 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2386-2388, 2015; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.29335