This paper presents a single-fed, single-layer, dual-band antenna with a large frequency ratio of 4.74:1 for vehicle-to-vehicle communication. The antenna consists of a 28 GHz inset-fed rectangular patch embedded into a 5.9 GHz patch antenna for dual-band operation. The designed dual-band antenna operates from 5.81 to 5.99 GHz  and 27.9 to 30.1 GHz (5G millimeter-wave (mm-wave) band). Furthermore, the upper band patch was modified by inserting slots near the inset feed line to achieve an instantaneous bandwidth of 4.5 GHz. The antenna was fabricated and measured. The manufactured prototype operates simultaneously from 5.8 to 6.05 GHz and from 26.8 to 31.3 GHz. Notably, the designed dual-band antenna offers a high peak gain of 7.7 dBi in the DSRC band and 6.38 dBi in the 5G mm-wave band. Vehicle-to-Vehicle communication V2V) is the future of the Intelligent Transportation System . The FedV is the Multiband systems use separate antennas, each operating at a specific band, or a single antenna structure resonating at various bands. Examples of dual-band single-fed planar antennas include patch antennas ,4, co-plA simple way to realize dual-band antennas with a large frequency ratio is to integrate two different frequency antennas on the same aperture ,8,9,10. Indeed, designing a single-layer single-port dual-band antenna with a large frequency ratio is daunting due to the large difference in antenna dimensions. In ref. , a multiIn ref. , a microIn , a stackIn this paper, a low-profile, dual-band antenna with an integrated inset-fed upper band patch and a coupled-fed lower band patch antenna is presented, as depicted in The paper is organized as follows: The dual-band antenna is designed for the Rogers RT/Duroid 5800LZ with thickness t = 1.27 mm, dielectric constant The dual-band antenna was designed and simulated using a full-wave simulator. The surface current distribution of the designed dual-band antenna at the operating frequencies is first analyzed for a better understanding. First, the current distribution at 28 GHz (phase = 0) is examined, as shown in The simulated S11 (dB) of the dual-band antenna see a shows aA parametric study was carried out to further analyze the patch lengths’ effect on the operating frequencies. The bandwidth of the upper band patch can be further improved by adding parallel slits near the inset feed line, as shown in Antenna II operates from 5.8 GHz to 5.95 GHz and 26.8 GHz to 31.3 GHz, as shown in As shown in The designed dual-band antenna with enhanced bandwidth (Design II) was fabricated and measured. The fabricated antenna prototype is shown in The measured radiation pattern of the dual-band antenna with enhanced bandwidth at 5.9 GHz is presented in The measured radiation pattern of the fabricated antenna at 28 GHz is displayed in This section compares the implemented dual-band antenna with other dual-band antennas with a large frequency ratio, as shown in Frequency tunability: This antenna design proposes a frequency-tunable dual-band patch antenna with a high-frequency ratio that can be tuned between 5.9 GHz and 28 GHz.Reduced harmonics between the two bands: The design was optimized for a frequency that included the width and length parameters for the lower band patch. By optimizing the upper patch (lower band patch) design for 5.9 GHz, it was found that the higher-order harmonics at frequencies above 5.9 GHz were reduced because of the lack of proper coupling. Therefore, optimizing the design of the upper patch is a tool that can be used to reduce the effects of harmonics and higher-order modes that could occur at other frequencies.Bandwidth improvement at the upper band: slits introduced additional resonance for the small patch. As a result, the additional resonances were well-coupled to the patch resonance, enhancing antenna bandwidth. In our design, the bandwidth at the upper-frequency band is improved from 8% to 16% by adding parallel slots near the inset-feed line.High gain at both bands: The designed dual-band antenna offers a high peak gain of 7.7 dBi in the DSRC band and 6.38 dBi in the mm-wave band. Both antennas radiate in the boresight direction.; andSingle-layer structure, simple design, and low-cost PCB fabrication.This paper presents a single-layer, single-feed, dual-band antenna with enhanced bandwidth. The designed antenna consists of an inset-fed 28 GHz patch embedded into the 5.9 GHz patch to provide dual-band operation. Furthermore, the bandwidth at the upper-frequency band is improved from 8% to 16% by adding parallel slots near the inset-feed line. The fabricated antenna operates from 5.8 GHz to 6.05 GHz and 26.8 GHz to 31.3 GHz, covering the DSRC and 5G mm-wave bands. Overall, the designed dual-band antenna offers the following:The FCC is modernizing the 5.9 GHz frequency to improve Wi-Fi and automotive safety. The 28 GHz band, within the Ka-band, is utilized by the satellite industry. Both frequencies are also relevant to implementing 5G networks, covering the FR1 and FR2 bands. These frequencies play a crucial role in advancing wireless communication, satellite services, and the development of high-speed connectivity.


This inventive design features a truncated corner monopole accompanied by branched stubs fed by a coplanar waveguide. The stubs, varying in length, serve as quarter-wavelength monopoles, facilitating multi-band functionality at 2.45, 3.5, and 5.8 GHz. Given the antenna’s intended applications in flexible devices and body-centric networks, the conformability of the proposed design is investigated. Furthermore, an in-depth analysis of the Specific Absorption Rate (SAR) is conducted using a four-layered human tissue model. Notably, the SAR values for the proposed geometry at 2.45, 3.5, and 5.8 GHz stand at 1.48, 1.26, and 1.1 W/kg for 1 g of tissue, and 1.52, 1.41, and 0.62 W/kg for 10 g of tissue, respectively. Remarkably, these values comfortably adhere to both FCC and European Union standards, as they remain substantially beneath the threshold values of 1.6 W/kg and 2 W/kg for 1 g and 10 g tissues, respectively. The radiation characteristics and performance of the antenna in flat and different bending configurations validate the suitability of the antenna for flexible devices and body-centric wireless communications.A conformal tri-band antenna tailored for flexible devices and body-centric wireless communications operating at the key frequency bands is proposed. The antenna is printed on a thin Rogers RT 5880 substrate, merely 0.254 mm thick, with an overall geometrical dimension of 15 × 20 × 0.254 mm Wearable and flexible electronics have gained significant attention in recent years in the industrial and academic worlds as they provide wide support for personal, Internet of Medical Things (IoMT), sports, military, and other applications ,2,3. IoMIn recent years, microstrip patch antennas have drawn considerable attention from researchers for wearable applications owing to their characteristics of light weight, low profile, ease of integration with electronic circuits, and less design complexity. Lately, several patch antenna designs have been reported in the literature for wearable devices covering the ISM  band ,13,14,15The rapid development of wireless technology requires communication systems to operate over multiple frequency bands supporting diverse applications. However, as compared to single-band and UWB antennas, it is challenging to design multiband antennas, as they require a specific impedance bandwidth within the desired frequency bands. Considering the importance of multiband antennas, several recent works reported wearable antennas operating at multiple frequency bands and utilized various substrates and design methodologies ,22,23,24x- and y-axis ascertains good performance of the antenna, which proves the suitability of the proposed antenna for wearable devices.It is observed that the flexible antennas discussed above for wearable devices operating at dual- and triple-frequency bands have large dimensions as well as complex and multilayered structural designs. It is desirable that body-worn antennas are simple and compact in size. The large size of these antennas hinders their application for several body-worn devices. Considering the limitations exhibited by the works reported earlier, this work proposes a simple, compact, and flexible CPW-fed patch antenna for on-body and off-body wearable devices. This proposed antenna is realized on Rogers RT 5880 flexible substrate with 0.254 mm thickness and operates over the triple bands of 2.45/3.5/5.8 GHz for the ISM and WiMAX frequency bands. As body-worn antennas operate in close proximity to the human body, it is necessary to analyze the effects of electromagnetic wave exposure on the human body. For this purpose, a Specific Absorption Rate (SAR) analysis is also carried out for the proposed antenna, which demonstrates that SAR values meet the standards set by the FCC and European Union of a maximum acceptable SAR range of 1.6 W/kg for 1 g of tissue and 2 W/kg for 10 g of tissue. Also, a conformal analysis in both The design and simulation analysis of the proposed antenna were conducted using CST Studio. The proposed tri-band flexible antenna was modeled using the Rogers RT 5880 with a 0.254 mm thick substrate with dielectric constant of 2.2 and loss tangent of 0.0009. The antenna’s geometry was constructed on the topside of the substrate using standard copper cladding. A CPW-fed rectangular patch was modified using two semicircular slots etched from the top-right and bottom-left corner of the patch. Afterwards, a meandered line stub was loaded at the top-right corner to achieve the lowest band of 2.45 GHz while another stub was loaded at the middle of the patch to achieve a resonance of around 3.5 GHz. Impedance mismatch occurred due to the incorporation of the stubs, which was mitigated by optimizing the various dimensions of the antenna. The final design along with its optimized dimensions are shown in SL) for any desired frequency (df) can be estimated by using the following relationship:c denotes the light speed in free space, t denotes the fraction of free-space wavelength at the central frequency, while HS refers to the thickness of the substrate material, while WS represents the width of the substrate.The design methodology adopted to develop the proposed work is illustrated in The total physical length of the inserted stub is 30 mm which corresponds to the quarter wavelength at the resonating frequency of 2.45 GHz. The shift in lowest resonance as well as the impedance mismatching at the higher resonance needed further optimization to cover the targeted bands and to achieve the desired results. For this purpose, a parametric sweep was carried out by tunning the total length of both stubs one by one, as shown in Likewise, the parametric analysis of the shorter stub shows that changing the overall length of the stub resulted in a slight shift in the middle resonance from 3.5 GHz towards higher frequencies along with a decreased impedance-matching performance at the highest resonance. Meanwhile, the 2.45 GHz band remained unaffected while tuning the shorter stub, as illustrated in 11| < −10 dB impedance bandwidths of 2.37–2.5 GHz, 3.37–3.65 GHz, 5.2–7.7 GHz, respectively, as depicted in In this way, the antenna is optimized for the targeted band spectrum of 2.45 GHz, 3.5 GHz, and 5.8 GHz having |Sx- and y-axis, at two radii of 10 and 20 mm, as shown by the simulation setup in 11| results for bent configurations at different axes and radii in As the proposed antenna is targeted to wearable devices supporting body-worn applications, it is therefore compulsory to verify the performance of the antenna in different conformal conditions . ConsideIn order to validate the results of the proposed antenna, experimental investigations were performed on the fabricated prototype, as illustrated in 11| results due to the fact that the simulation model was excited using a 3D SMA-connector. The proposed antenna offers three resonances around 2.45, 3.5, and 5.8 GHz having an |S11| < –10 dB impedance bandwidth of 2.37–2.5 GHz, 3.37–3.65 GHz, and 5.2–7.7 GHz, which correspond to the percentage bandwidths of 5.3%, 8%, and 43%, respectively. It is important to note here that the proposed work covers the band spectrums of ISM-band (2.45/5.8 GHz), 5G sub-6-GHz (3.5 GHz), WLAN (2.4/3.6/4.9/5/5.9/6 GHz), Wi-Fi (2.4/5 GHz), and Wi-Fi 6E (6 GHz). Thus, this makes the proposed work a potential candidate for heterogenous applications requiring a multi-band antenna of compact size.A strong correlation is observed between the estimated and measured |SThe far-field behavior of the proposed tri-band antenna is observed at the selected frequencies of 2.45 GHz, 3.5 GHz, and 5.8 GHz, as depicted in The gain of the proposed antenna was also measured in an RF-isolated chamber by placing the antenna in front of a reference wideband horn antenna with standard spacing of 2 m, as shown in y- and x-axis, as depicted in To analyze the performance of the antenna for wearable devices, a conformability test was also carried out. The antenna was bent along pieces of Styrofoam having radii of 10 mm and 20 mm in both the 3. The antenna as placed with a gap between it and the tissue model, and the order of the layers along with their respective thicknesses are illustrated in As wearable devices work close to the human body, the absorption of electromagnetic radiation by human tissue is hazardous. In order to ensure the safety of individuals, there are established standards for regulating the exposure of human tissues to electromagnetic (EM) radiation. These standards have been set forth by reputable organizations such as the International Commission on Non-Ionizing Radiation Protection (ICNIRP), the Institute of Electrical and Electronics Engineers (IEEE), and the Federal Communication Commission (FCC). These organizations play a crucial role in defining and enforcing guidelines to minimize the potential health risks associated with EM radiation exposure, thereby safeguarding human well-being. According to these regulatory bodies, human exposure at lower frequencies is evaluated in terms of SAR. The permittivity and conductivity of the various layers are provided in In The flexible antenna proposed in this work is compared with recently published wearable antennas to further elucidate the merits of the proposed antenna. This comparison is summarized in 3. The primary goal of this antenna is to serve flexible and wearable devices within body area networks. To ensure its compatibility with such applications, the antenna’s flexibility is assessed through bending tests along both the x and y axes, at various radii. Additionally, an SAR analysis is conducted using a four-layered human tissue model to ensure safety. The obtained SAR values for the proposed antenna design at frequencies of 2.45 GHz, 3.5 GHz, and 5.8 GHz are determined as follows: 1.48 W/kg, 1.26 W/kg, and 1.1 W/kg for 1 g of tissue, and 1.52 W/kg, 1.41 W/kg, and 0.62 W/kg for 10 g of tissue, respectively. Importantly, all these values remain below the safety thresholds established by the FCC and European Union, which are 1.6 W/kg and 2 W/kg for 1 g and 10 g of tissue, respectively. Furthermore, the antenna’s radiation characteristics and performance are evaluated both in its flat and in various bending configurations. These assessments confirm the antenna’s suitability for wearable wireless communication devices, underscoring its effectiveness and real-world performance.This paper introduces a compact and flexible antenna capable of operating in three frequency bands: 2.45 GHz, 3.5 GHz, and 5.8 GHz. These bands are significant for ISM  and WiMAX applications. The antenna design incorporates a slotted rectangular patch configuration with a coplanar waveguide feedline. It is fabricated on a 0.254 mm thick Rogers RT 5880 substrate, with overall dimensions measuring 15 × 20 × 0.254 mm


A wideband superdirective array, composed of a two-element circular monopole configuration, is introduced. The monopoles are placed in close proximity, facing each other on a metal ground. To ensure good matching at high frequencies, two pairs of elliptical patches are added to the sides of the monopoles, enhancing the surface current of the circular patch for wideband performance. To achieve equal amplitude excitation and the desired phase difference, a wideband power divider with a phase shifter is designed to feed the antenna array. Simulation and measurement results demonstrate that the proposed wideband antenna array, operating within the frequency range of 2.94–7.93 GHz, exhibits a maximum directivity of 8.36–10 dBi, with an antenna efficiency ranging from 47.86 to 83.18% across the bandwidth. The proposed array has the advantages of miniaturization, high directivity and wideband operation and can be widely used in various portable wireless communication systems, including WLAN (5.05–5.9 GHz), ISM (5.725–5.875 GHz), 5G communication (3.3–3.8 GHz), etc. Superdirective antenna arrays can be designed to achieve higher levels of directivity compared to conventional antenna arrays, improving communication distance and quality [Communication performance is limited by fading channel and spectrum resources ,2,3. For quality ,9,10. Es quality ,12.Although many superdirective antennas/antenna arrays have been reported, a tradeoff can be gained among antenna/array size, gain/directivity and bandwidth. For example, the superdirective antenna/antenna arrays in ,14,15,16In this study, a two-element, closely spaced superdirective antenna array loaded with parallel elliptic-shaped patches is designed for wideband operation. The circular monopole provides wideband impedance characteristics at 3.04–5.82 GHz (within the low-frequency band). To further expand the bandwidth, two pairs of parallel elliptic-shaped patches are added to the monopoles. These loaded patches enhance the surface current of the circular patch at high-order modes, enabling resonance at higher-frequency bands. Additionally, a wideband power divider with a phase shifter is designed to achieve the desired excitation. Through careful optimization of the spacing and phase difference between elements, the proposed antenna array achieves a high directivity of 8.36–10 dBi across a wider frequency bandwidth of 2.94–7.93 GHz. Compared to the state-of-the-art designs, our design exhibits miniaturization, high directivity and wideband operation and can be widely used in various portable wireless communication systems, including WLAN (5.05–5.9 GHz), ISM (5.725–5.875 GHz), 5G communication (3.3–3.8 GHz), etc.The superdirective microstrip antenna is arrayed as shown in To achieve high directivity over a wide bandwidth, the antenna impedance with different spacing between the elements is investigated, as displayed in The phase difference between the two elements is studied while the amplitude of the excitations is kept constant over the impedance bandwidth. It can be seen from It is well known that radiation properties and impedance bandwidth can be improved by loading arms or plates . AccordiThe simulated reflection coefficient |S11| of the two-element monopole array with and without loaded patches for different values of element spacing S is shown in To feed the superdirective array, a wideband Wilkinson power divider with a phase shifter printed on the same TP substrate with a thickness of 1 mm is designed for the desired excitation, as shown in 2 of the loaded patches, as shown in 2. Despite the frequency shift, the overall trend remains basically the same. Although a discrepancy in the reflection coefficients exists between the simulation and measurement, the simulation results are in reasonable agreement with the experiments. The radiation performances of the wideband superdirective array are measured in the anechoic chamber. The simulated and measured radiation patterns are shown in The proposed superdirective array with loaded patches is fabricated as shown in The antenna array with a feeding network can work at the broadband of 2.94–7.93 GHz (|S11| < −10 dB) while maintaining a good radiation pattern in the frequency band below 7 GHz. It is worth noting that |S11| is the amplitude rather than the phase. With the introduction of a close-spaced array, the phase difference ΔΦ of the feeding network at the two ports changes and is different from that without an antenna array, especially at 7 GHz. To investigate this, we simulated the surface current distribution of the feeding network with and without the monopole array, as displayed in 2 + 2ka, where k is the wave number and a is the radius of the smallest sphere that encloses the antenna. The measured directivity of the antenna array is higher than the Harrington normal directivity limit when the frequency is lower than 4.87 GHz with a bandwidth of 49.42%, indicating that superdirectivity characteristics are obtained by the proposed array.The measured maximum directivity of 8.36–10 dBi and antenna efficiency of 47.86–83.18%, which are slightly lower than those in the simulation  over the wide frequency operation, but the tendency of the two results is basically consistent. Regarding the decrease in the antenna efficiency at 7 GHz, it may be caused by the appearing grating lobe with the additional feeding network. The normal directivity as defined by Harrington [The design guidelines of the proposed broadband superdirective antenna array are summarized as follows.Step 1: Wideband array element design. To achieve broadband, a circular monopole with wideband characteristics is chosen as the array element, as displayed in Step 2: Broadband superdirective array design. The two monopole elements are arrayed face to face, as shown in Step 3: Wideband feeding network design for the broadband array. A power divider with a phase shifter is designed to feed the broadband superdirective array for the same amplitude and different ΔΦ. Then, the superdirective array with the feeding network is established, as shown in The performance of this work is fair compared with some recently published wideband superdirective antenna arrays, as listed in In this study, we have presented a wideband superdirective monopole array. To achieve the desired excitation, a wideband power divider and phase shifter with a T-shaped open stub is used to feed the broadband array. By incorporating elliptic-shaped patches, we are able to further expand the bandwidth of the antenna array, achieving good matching across the frequency range of 2.94–7.93 GHz  while maintaining high directivity and good antenna efficiency. Overall, the proposed array has the advantages of wide bandwidth, high directivity and good efficiency, and it can be widely used for mobile devices, sensors and small-sized communication devices.


The antenna’s key characteristics are compact size, wide-band sub-GHz operation, dual sense CP, polarization bandwidth reconfigurability and good MIMO performance. Thus, it is a suitable candidate to be utilized in CubeSats applications in sub-GHz bands.In this letter, a compact, planar circularly polarized (CP) sub-GHz slot-based multiple-input-multiple-output (MIMO) antenna with dual sense CP along with polarization bandwidth reconfigurability is presented. The pentagonal reactively loaded slot is fed by two folded tapered feedlines to achieve CP. The antenna offers left-hand-circular polarization (RHCP) with the as well as right hand circular polarization (LHCP). The antenna exhibit linearly polarization (LP) by exciting two ports simultaneously. Moreover, the antenna CP resonance can be reconfigured by varying the capacitance of the varactor diode. The antenna has a wide −10 dB operating frequency band from 578–929 MHz. while the axial ratio (AR) bandwidth ranges from 490–810 MHz. Moreover, the two elements MIMO are optimized and placed on compact dimensions 100 × 100 × 0.76 mm Slit-slot and microstrip patch-based antennas offer bidirectional CP radiation, where the rotating senses of CP in the front and back sides are presented in9.Circular polarized (CP) antenna offers many advantages over the linearly polarized (LP) antennas due to their decreased polarization mismatch losses, and ability to provide freedom of antenna orientation. Therefore CP antennas are desired for satellite applications, WiMax, WLAN, RFID-tags, and 5G applications. Several efforts have been put in the literature to design CP antennas10. The CP senses can be reconfigured in single port antennas by switching the states of at least two p-i-n diodes16 and MEMS17. These kinds of antennas need additional complex biasing circuitry to control the diode, which increases the cost and reduces the antenna efficiency and CP bandwidth. Alternatively, CP sense reconfigurable antennas utilizing dual-ports have also been presented due to their wide-band characteristics20.Moreover, various methods have been employed to achieve polarization reconfigurable antennas that offer right-handed circular polarization (RHCP) and left-handed circular polarization (LHCP). The ability to operate with both senses (RHCP and LHCP) enables frequency reuse and doubles the capacity of the communication system23. In these designs, varactor and p-i-n diodes are utilized to achieve continuous frequency-polarization agility. At the same time, the switching between three polarization states and tunable working frequency bands are accomplished by microfluidic injection in24.Most importantly, if the antenna can be switched between two senses of CP (RHCP and LHCP) and LP, as well as the operating frequency, it will allow the user to roam to virtually any existing communication network system. Therefore, some works have been reported to designing the frequency and polarization reconfigurable antennas27, they do not offer polarization bandwidth reconfigurability. Additionally, multiple-input-multiple-output (MIMO) configurations are essential for high data rates with seamless connectivity. None of the aforementioned CP antenna with reconfigurability has MIMO capabilities. The proposed polarization bandwidth reconfigurable antenna is compared with state-of-the-art CP antennas. The proposed design is compared with other reference CP designs as shown in Table It is worth noting that most of the CP antenna designs as discussed are operating at frequency bands above 1 GHz band. The design of CP antenna with the features of frequency-polarization reconfigurability with wide-band and compact size characteristics is always challenging at the sub-GHz spectrum. Although many CP antennas have been presented at the sub-GHz band for RFID tags, the internet of things, CubeSat, and several other applicationsCP antennas have several advantages in sub-GHz wireless communication systems, particularly in CubeSat applications. These antennas improve signal propagation, reduce the impact of orientation, mitigate polarization mismatch, and find use in RFID, IoT, and satellite communication systems. The proposed antenna design is suitable for CubeSats operating in sub-GHz bands with CP characteristics. CP antennas are beneficial for satellite communication systems in the sub-GHz frequency range as they enhance signal reception and transmission, compensating for polarization mismatch caused by satellite orientation and ground station antennas. The specific frequency range of 578 to 929 MHz is mentioned as a potential range where circularly polarized antennas are commonly used. This range includes VHF and UHF bands, which are utilized by CubeSats, wireless microphone systems, UHF RFID systems, and certain satellite communication systems. Circularly polarized antennas help maintain consistent signal quality and improve performance in these applications.This letter focuses on the design of compact circularly polarized antenna for CubeSat applications. Most of the sub-GHz antenna for CubeSat operation are 3D structure which needs additional deployment mechanism. Thus, adding more complexity to the CubeSat design. However, the proposed antenna is low profile and planar and doesn’t need any additional deployment structure.This work is based on dual-port slot-based antenna with multiple polarization . A single antenna structure is being utilized to obtain 3 different types of CP at sub-GHz operation.The proposed antenna design is optimized in MIMO configuration with good MIMO performance metrics.The proposed antenna design has the 3-dB axial ratio (AR) bandwidth that can be tuned from 490–810 MHz by utilizing a single varactor diode. This is the prominent feature of the antenna design to provide switching flexibility between narrow-band and wide-band CP operation in wide-band antennas structure.The 2-element MIMO antenna offers a good isolation within a compact size and stable radiation patterns.To the best of the author’s knowledge, this design is the first of its kind that combines the advantages of compact size (100 mm × 100 mm × 0.76 mm), polarization bandwidth reconfigurability, narrow & wide bands CP configuration, and MIMO operation at sub-GHz bands.The distinguishing features as described above show the uniqueness and appropriateness of the proposed antenna design to be utilized in CubeSat applications operating at sub-GHz bands.This letter focuses on the design of a dual-port slot-based MIMO antenna with multiple polarization  along with polarization bandwidth reconfigurable MIMO antenna. The novelty and unique features of the proposed antenna design are given below: The schematic diagram of the proposed dual-feed single-element antenna is shown in Fig. The single element was then rotated along the axis with 360-degree for the MIMO configuration while keeping the parameters same of the antenna, as shown in Fig. For the proposed antenna design, a sub-GHz band communications was selected due to its numerous advantages. The slot-based antenna design are quite popular due to its ease of manufacturing, integration with other circuit components, planar structure, wide-band attributes, and omni-directional radiation patterns, all of which are well suited for the proposed antenna design.The proposed slot has been configured in a meandering closed loop pattern to achieve compact size, resembling a non-uniform pentagonal shape as depicted in Fig. Furthermore, a parametric analysis are performed to enhance performance in terms of size reduction and bandwidth expansion. The slot antenna is coupled with a capacitor (Cap), as shown in Fig. The E-field distribution on the antenna for two ports is investigated to explain the CP mechanism of the MIMO antenna Fig. . For porTo understand the circular polarization (CP) mechanism of the MIMO antenna with two ports, the distribution of the electric field (E-field) on the antenna is thoroughly investigated. This investigation aims to analyze the behavior of the E-field and its impact on the radiation characteristics of the antenna. Figure In contrast, when port-2 is excited, the E-field exhibits a counter-clockwise rotation. This counter-clockwise rotation of the E-field generates left-hand circularly polarized (LHCP) radiation. LHCP polarization involves the rotation of the electric field vector in a left-handed circular pattern as the wave propagates. One crucial observation made during this investigation is that during the excitation of ports 1 and 2, there is a negligible effect on antenna 2. In other words, no significant E-field is detected on antenna 2 when both ports 1 and 2 are excited simultaneously. This negligible effect leads to a high level of isolation between the MIMO elements. Isolation is an essential characteristic in MIMO (Multiple-Input Multiple-Output) systems, as it ensures that the signals transmitted from one antenna element do not interfere with the signals received by the other elements. The high isolation achieved in this MIMO antenna configuration helps minimize cross-interference and improves the overall performance and reliability of the system.By investigating the E-field distribution and the resulting polarization characteristics of the MIMO antenna, a comprehensive understanding of the CP mechanism is obtained. This knowledge is valuable for designing and optimizing MIMO antenna systems in various applications, such as wireless communications, radar systems, and satellite communications, where efficient signal transmission and reception are essential.iiThe simulated and measured . The antenna gives RHCP when port-1 (left port) is excited, while it gives LHCP when port-2 (right port) is excited. The simulated AR as a function of the frequency is shown in Fig. The proposed antenna design is capable of radiating in three different polarization modes: LP, RHCP, and LHCP. Unlike other antennas, this design doesn’t require any switching circuitry like p-i-n diodes or MEMS to switch between polarization modes. Typically, polarization reconfigurable antennas use switching mechanisms involving active components, which adds complexity and requires additional circuitry and control mechanisms.In this antenna’s case, LP radiation is achieved by simultaneously feeding both ports (port-1 and port-2) with the desired signals. This results in LP radiation with an Axial Ratio (AR) value exceeding 40 dB. The Axial Ratio measures the circular polarization of the antenna’s radiation, with lower values indicating better circular polarization. By configuring the feeding of the antenna ports appropriately, LP radiation is achieved without the need for switching circuits. This sets it apart from other antennas that rely on additional components for polarization switching.Overall, this antenna design offers the advantage of radiating in different polarization modes  without the complexity and potential drawbacks of switching circuitry. It provides versatility in terms of polarization selection and bandwidth reconfiguration, making it suitable for various applications.The antenna also has the additional advantage of AR bandwidth reconfigurability. The central frequency of the AR can be tuned by varying the capacitance of the diode. The resonance of AR can be shifted from 490 to 810 MHz by changing the capacitance values of varactor diode As shown in Fig. The gain of the antenna for port-1 and port-2 (LHCP and RHCP gain) plots are shown in Fig. The radiation patterns for each polarization scenario are also shown in Fig. The proposed MIMO antenna is also investigated for co-pol and cross-pol patterns at 650 MHz as shown in Fig. The proposed antenna design exhibited omni-directional radiation patterns and providing 360-degree coverage pattern in the azimuth plane, thus enabling communication with multiple ground stations without requiring precise pointing or alignment. This can be particularly beneficial for CubeSats operating in low earth orbit, due to limited resources or maneuvering capabilities.The diversity of the MIMO antenna in terms of envelope correlation coefficient (ECC) is calculated to show how much antenna elements are independent in their performance. The values are found to be very low, less than 0.02, ideal for the MIMO operation.24 do not offer sub-GHz operation and only a few designs24 offer frequency reconfigurability. Although most of the antennas are offering polarization reconfigurability, none of them have the extra advantages of the MIMO configuration. Thus, the proposed antenna outperforms its competitor designs with its operating capability in the sub-GHz band, polarization bandwidth reconfigurability, and MIMO configuration in a single design.The proposed polarization-frequency reconfigurable antenna is compared with state-of-the-art antennas in terms of their typical functionalities. Most of the antennasA low-profile 2-elements MIMO antenna at sub-GHz with polarization-frequency agility is presented. The primary single-element antenna consists of a pentagonal slotline having a varactor diode in its center to obtain a compact antenna design. The unique capacitive loaded slot-line with the folded feedlines gives a broadband impedance bandwidth of 46.58% (578   929 MHz), and a 3dB AR bandwidth from 490 to 810 MHz. The proposed dual-feed antenna offers RHCP as well as LHCP radiations. Additionally, the AR bandwidth can be reconfigured by varying the capacitance of the varactor diode. The AR can be tuned over the operating bandwidth of 490 to 810 MHz. Furthermore, the antenna gives a high RHCP and LHCP gain of 1.079 dBi, and 1.078 dBi, respectively. The two-element MIMO antenna is realized on compact board size of 100 mm × 100 mm. The attractive features of this design are compact size and wide operating bandwidth in the sub-GHz band, polarization-bandwidth reconfigurability, and good isolation.


High isolation (>15 dB) was attained with thorough testing without employing a decoupling scheme in the design. Laboratory measurements resulted in a peak gain of 3.49 dBi and an efficiency of around 80% in the entire operating band. The evaluation of the presented MIMO multiband antenna was carried out in terms of the envelope correlation coefficient (ECC), diversity gain (DG), total active reflection coefficient (TARC), and Channel Capacity Loss (CCL). The measured ECC was less than 0.04, and the DG was well above 9.50. The observed TARC was also lower than −10 dB, and the CCL was below 0.4 bits/s/Hz in the entire operating band. The presented MIMO multiband antenna was analyzed and simulated using CST Studio Suite 2020.This research work presents a compact design of a Multiple-Input Multiple-Output (MIMO) multiband antenna along with high-isolation characteristics. The presented antenna was designed for 3.50 GHz, 5.50 GHz, and 6.50 GHz frequencies for 5G cellular, 5G WiFi, and WiFi-6, respectively. The fabrication of the aforementioned design was undertaken using FR-4 (1.6 mm thickness) substrate material with a loss tangent and relative permittivity of about 0.025 and 4.30, respectively. The two-element MIMO multiband antenna was miniaturized to 16 × 28 × 1.6 mm In MIMO antennas, it is critical to maintain high isolation between the radiating elements for good radiation performance of the MIMO system. A simple approach to achieve isolation of less than −10 dB using ferrite skin was presented in [In today’s wireless communication technology and 5G era, in which wireless devices are evolving at a very fast pace, the improvement of data transmission with large capacity and link reliability is greatly needed . Researcented in . It was ented in  for WiMAented in , in whicented in . An AI-bented in . An overented in . A novelented in . In thisented in . A detaiented in . The perented in . A novelented in . A widebented in .The research efforts in this paper demonstrate a compact multiband 2 × 1 MIMO antenna that operates at 3.50 GHz, 5.50 GHz, and 6.50 GHz, catering to 5G, WLAN, and WiFi-6 devices, respectively. Initially, a single antenna element was introduced, and subsequently, the design was replicated orthogonally for a 2 × 1 MIMO configuration. The notable features of this antenna include its higher gain, enhanced radiation efficiency, superior isolation, improved diversity performance, and reduced antenna size in comparison with the antennae in the existing literature. The primary objective behind the design of the proposed MIMO antenna was to offer a compact design with simplified fabrication, suitable for devices operating within the 5G frequency bands that require compact antennas.3, and it was fabricated on an FR4 (1.6 mm thickness) substrate (εr = 4.30 and tan(δ) = 0.025). It was fed through microstrip lines of distinct lengths and widths to achieve good multiband characteristics. On the back side of the substrate, there was a partial ground structure and a flipped L-shaped strip, which provided a wide impedance bandwidth for the 6.5 GHz frequency. Its simulated reflection coefficient (S11) along with the design steps are presented in In this section, a single element of the MIMO multiband antenna design is explained. Its geometry is presented in Firstly, a simple 5G-shaped strip with a modified G-shape (embedded within it) along with a microstrip feeding line was designed on the uppermost side, and a partial or semi-ground structure was etched on the lowermost side of the substrate. The 5G-shaped strip was tuned to radiate at 3.5 GHz. At its optimized version, the reflection coefficient for the 3.5 GHz band was well below −10 dB. In step 2, an inverted L-shaped strip was added to acquire the categorization of a dual-band, i.e., 3.50 GHz and 5.50 GHz. The feeding structure was modified for a better impedance bandwidth (<−10 dB); these modifications shifted the 3.5 GHz dip, which was rectified in step 3. For the proposed antenna design, two L-shaped strips were added, one at the bottom side of the substrate and the other at the bottom edge of the 5G-shaped strip. The microstrip feeding structure was tailored and resulted in triple band characteristics .6 = 3.1 mm and W11 = 7 mm resulted in variations, as presented in 7 was evaluated at different values, as shown in 7 = 4.9 mm. The L-shaped strip, which was embedded to obtain the 6.5 GHz frequency band, made the other design parameters more sensitive to any change, as can be seen in 4, its finest value was observed to be L4 = 1.1 mm. Moreover, L14 and W15 were fine-tuned at 8.8 mm and 6.5 mm, respectively, to obtain a wide impedance bandwidth at the same frequency, as presented in 11) of simulated and laboratory-measured single-element antennas is illustrated in The physical dimensions of the presented MIMO multiband antenna design are tabulated below in To design a two-element MIMO multiband antenna, a single-element antenna was duplicated and marked as Ant 1 and Ant 2, as depicted in 2. Furthermore, Ant 2 was shifted into three different locations—the top  was observed when Ant 2 was placed in the bottom position.In the MIMO design, the minimum distance between the centers of two antennas should be λ/2 in order to obtain good isolation, but this distance would make the physical area of the MIMO design too large for a compact antenna design. The best possible alternative for this problem is to place the second element in a different orientation. An optimal distance D1 of 3.3 mm was set, which gave us the total dimensions of our proposed MIMO antenna, which were 16 × 28 mm—the top a, center—the top b, and bo—the top c positioThe simulated maximum surface current density at the resonance frequencies of a two-element multiband MIMO antenna at port 1 is depicted in 12 and reflection coefficient S11 of the presented MIMO multiband antenna are shown in 12) were below −15 dB, and the presented antenna showed good isolation in the entire operating band. This was achieved because of the inherent property of polarization diversity.The laboratory-measured and simulated isolation coefficient SThe radiation pattern of the antenna was measured by utilizing an anechoic chamber, as illustrated in i and j indicate port numbers. DG is an important aspect that certifies the diversity performance of MIMO systems. It is calculated using the following equation [The diversity performance of the proposed two-element MIMO multiband antenna was assessed with the help of the parameters DG (diversity gain), ECC (envelope correlation coefficient), and TARC . It considers the radiation pattern, its polarization, and the antenna element’s relative phase. The ECC between two antenna elements in a MIMO is calculated with the help of the following mathematical model ,20:(1)ECequation ,22:(2)DGIn i denotes an incident wave and bi denotes a reflected wave. It is evident in For calculating TARC, the laboratory-measured S-parameters of a two-element MIMO multiband antenna were used in the following mathematical relationship ,24:(3)TAThe channel capacity loss (CCL) is another essential diversity parameter that defines the maximum attainable limit of the communication transmission rate. The CCL of the proposed MIMO antenna was measured using Equation (4) from :(4)ClossIts simulation results were compared with the measured results shown in 3. The laboratory-measured findings are in good agreement with simulated results, indicating that the presented antenna is suitable for 5G, WLAN, and WiFi-6 applications.This research effort presents a compact MIMO multiband antenna. The initial design architecture comprised a single element consisting of a 5G-shaped microstrip, two L-shaped strips, and a partial/semi-ground structure designed to radiate on the 3.50 GHz, 5.50 GHz, and 6.50 GHz frequency bands. For the MIMO, a duplicate design was placed in an orthogonal position with respect to the first element. The peak gain was recorded at 3.49 dB at 5.50 GHz, and the radiation efficiency was above 80% throughout the operating bands. ECC was observed below 0.04, DG was higher than 9.5 dB, and TARC was below −10 dB in all the bands of operation. In the presented research effort, the MIMO multiband antenna was miniaturized to a size of 16 × 28 × 1.6 mm


The article represents the design of two port-based printed MIMO antenna structures that have the advantages of low profile, simple structure, good isolation, peak gain, directive gain, and reflection coefficient. The performance characteristics are observed for the four design structures by cropping the patch region, loading the slits near the hexagonal-shaped patch, and adding and removing the slots in the ground area. The antenna provides a least reflection coefficient of −39.44 dB, a maximum electric field of patch region of 33.3 V/cm, a total gain of 5.23 dB, and good values of total active reflection coefficient and diversity gain. The proposed design provides nine bands’ response, a peak bandwidth of 2.54 GHz, and a peak bandwidth of 26.127 dB. The four proposed structures are fabricated using a low-profile material to support mass production. The comparison among simulated and fabricated structures is included to check the authenticity of the work. The performance assessment of the proposed design with other published articles is carried out for the performance observation. The suggested technique is analyzed over the wideband of frequency region 1 GHz to 14 GHz. The multiple band responses make the proposed work suitable for wireless applications in S/C/X/Ka bands. The use of wireless communication technology and the ongoing quest for small portable devices have undergone remarkable development during the last decade. There is potential for new wireless systems to enable a wide variety of applications and numerous users simultaneously. However, the need for an enhanced data rate is severely hampered by the limited bandwidths available due to spectral congestion. Therefore, technologies that may be used in wireless devices to perform antenna operating at several frequency bands are in high demand .The ability to send data rapidly gives it a lot of leeway when it comes to using the spectrum, which might be helpful for mobile computing devices in the future. The broad bandwidth and rapid data transfer speeds are ideal for various applications such as short-range radars, surveillance devices, and medical imaging ,3,4. WidIn addition, the metamaterial property helps to achieve better antenna performance. For MIMO technology to function, it is essential that the antenna components used by the transmitter and receiver be independent of one another. In order to measure the performance of an antenna array in MIMO systems, numerous metrics have been proposed in .The miniaturization of the MIMO structure has been shown to reduce the isolation between antenna components in . The decIn , the antThere is a need for high-gain, low-cost, multiband antenna to be designed, which can operate over a wide range with multiple bands. This manuscript presents the 1 × 2 MIMO antenna, which is analyzed for the L, S, C, and X frequency bands. The main objective of the proposed work is to achieve the multiband response with healthy gain and sufficient wideband response to target multiple applications for S, C, X, and Ka bands. The presented manuscript represents the novel concept of the addition of complementary split rings and slots in the patch area and the ground region. The modification in the patch structure and ground structure helps to achieve the targeted goals. The presented work shows the path to achieving a broad bandwidth, lower E.C.C., and good gain using the low-profile substrate material, which helps to achieve satellite communication applications. 2. The lumped port was applied to the structure for the excitation. The patch and ground layers were designed by considering them as copper material. The thickness of the patch and ground layers was 0.35 mm. The substrate height was 1.6 mm. The substrate material was chosen to be FR-4 epoxy, as it is inexpensive and easy to fabricate. FR4 has dielectric constant of 4.4 [Four designs were considered to analyze the reflection/transmission coefficient: peak gain, directivity, electric fields, and radiation patterns. t of 4.4 . The novThe proposed MIMO antenna design incorporates CSRR metamaterial, which offers unique electromagnetic properties, such as negative permeability and permittivity. This integration enables the antenna to exhibit enhanced performance characteristics, including improved bandwidth and radiation efficiency.The antenna design incorporates a defected ground structure to mitigate surface wave propagation and improve the isolation between antenna elements.The proposed MIMO antenna design achieves a low profile and compact form factor, making it suitable for various space-constrained applications. In addition, the antenna’s reduced size and planar structure contribute to its versatility and ease of integration into modern wireless communication devices.The antenna design offers wideband operation across the S/C/X/Ka frequency bands, covering various communication applications. In addition, the CSRR metamaterial and DGS implementation contribute to the achieved broad operating bandwidths, ensuring compatibility with multiple frequency bands.A sufficient reflection coefficient, more than 1 GHz of bandwidth, and a healthy gain are achieved for all of the proposed designs.Design and fabrication were carried out using FR4 dielectric material to check the reliability of the structure. The FR4-based low-profile material was used to achieve targeted goals, which helped with cost reduction.The four design structures were analyzed based on modifying the DGS and slit around the patch region to check the performance of different forms.The proposed design is compared with the other existing methods to identify the improvement in the structure.The simulated and measured results are compared to judge tolerance among them. Lower tolerance among both results is observed.Overall, the proposed antenna design exhibits excellent characteristics for MIMO applications. It demonstrates low mutual coupling between antenna elements, high isolation, good radiation efficiency, healthy gain, and proper diversity parameters. These attributes contribute to enhanced channel capacity, increased data rates, and improved system performance in MIMO systems.The different antenna structures were examined by inserting CSRR in the patch with slits outside of the patch and defective ground structure. The insertion of a CSRR in a patch antenna can have several effects on the performance of a MIMO antenna, as follows: The CSRR can alter the resonant frequency of the patch antenna. Due to a unique metamaterial property, it is possible to shift the resonant frequency of the patch antenna to a desired frequency. This adjustment can be beneficial in achieving better frequency selectivity and matching with other antennas in the MIMO system. The CSRR can widen the bandwidth. By introducing the CSRR, the effective electrical length of the antenna structure can be modified, resulting in a broader frequency bandwidth. This expanded bandwidth can be advantageous for MIMO systems that require simultaneous operation over multiple frequency bands. In MIMO antenna systems, mutual coupling between the individual antennas can degrade performance by interfering with signal transmission and reception. The CSRR can help to reduce mutual coupling between the antennas by creating isolation between them. It can act as a decoupling element, decreasing the coupling effects and improving the isolation between the antennas. The insertion of a CSRR can alter the radiation pattern of the patch antenna. The CSRR can affect the current distribution and electric field distribution on the patch, leading to changes in the radiation characteristics. Properly designed CSRR structures can be used to shape the radiation pattern and improve the antenna’s performance in terms of gain, directivity, and radiation efficiency. MIMO systems often benefit from polarization diversity, where different antennas are designed to transmit and receive signals with different polarizations. The CSRR can help to enhance the polarization diversity of the patch antenna by modifying its polarization properties. By carefully designing the CSRR, it is possible to achieve improved polarization isolation and better performance in terms of cross-polarization discrimination.The incorporation of a defective ground structure (DGS) in a antenna can have several effects on its performance, as follows: Cross-polarization refers to the undesired reception or transmission of signals with a polarization orthogonal to the desired polarization. DGSs can help to suppress cross-polarization in MIMO antennas. By introducing appropriate DGS patterns or slots in the ground plane, the radiation characteristics and polarization properties of the antenna can be controlled, leading to improved polarization isolation and reduced cross-polarization. DGSs can be used to shape and control the radiation pattern of MIMO antennas. By incorporating specific DGSs, such as slots or patches, in the ground plane, the current distribution and electromagnetic field distribution on the antenna can be manipulated. This enables control over the antenna’s radiation pattern, beam steering, and directivity, which can be advantageous in MIMO systems to optimize coverage, increase gain, or mitigate interference. DGSs can help to mitigate interference from external sources or neighboring antennas by introducing appropriate DGSs.Using structures in the ground plane, such as slots or fractal patterns, it is possible to suppress the effects of nearby interfering signals or electromagnetic noise, improving the signal quality and overall performance of the MIMO system.The antenna design technique is analyzed in four essential parts in order to explain the working concept of the planned MIMO antenna. Electromagnetic isolation: In a MIMO system, each antenna element should ideally radiate and receive signals independently without interfering with each other. However, due to the close proximity of multiple antennas, there can be coupling between them, resulting in interference and the degradation of system performance. By introducing DGSs in the ground plane, the coupling between the antenna elements can be minimized. The DGS creates an electromagnetic barrier between the antennas, reducing the mutual coupling and enhancing isolation.Surface wave suppression: DGSs can also suppress surface waves propagating on the ground plane. Surface waves can lead to increased coupling between antennas and reduce isolation. By incorporating DGSs in the ground plane, the surface waves can be attenuated, reducing their impact on the antenna elements and minimizing the coupling between them.Impedance matching: DGSs can be designed to provide additional degrees of freedom in tuning the impedance of the antenna elements. By appropriately designing the shape and size of DGSs, the impedance seen by each antenna element can be adjusted. This helps in achieving a better impedance match between the antenna elements, minimizing reflections and optimizing the overall system performance.Enhanced radiation efficiency: The presence of DGSs in the ground plane can also improve the radiation efficiency of the antenna elements. By reducing the coupling and surface wave effects, the energy that would otherwise be lost in coupling or surface wave propagation is efficiently radiated by the antennas. This leads to improved overall system performance and better signal quality.DS-2 has two hexagonal-shaped MIMO structures without a circular shape. Two slits are also placed on the outer side of the hexagonal-shaped patch region. Two arrays of lines and one plane region are available in the diffracted ground region. DS-3 has two hexagonal-shaped MIMO structures with a circular-shaped portion engraved in the patch area. There are not any slits in the outer place of the hexagonal-shaped patch region. Two arrays of lines and one plane region are available in the diffracted ground region. DS-4 has two hexagonal-shaped MIMO structures with one circular-shaped portion engraved in the patch area. In addition, there are slots available in the outer place of the hexagonal-shaped patch region. The ground region is simple without array lines. The DGS is a technique used in the design of antennas and R.F. circuits to reduce electromagnetic coupling and improve isolation between antenna elements in MIMO systems. The DGS in the ground plane helps in reducing coupling in MIMO antennas through the following mechanisms:The fabrication of the proposed structures is carried out for analysis purposes, and indifferent proposed structures are shown in W of the patch is calculated using Equation (1) [C0 is the light speed, W is the patch width, 𝑒𝑓𝑓) is determined using Equation (2).The circular-shaped ring is cut in the hexagonal-shaped patch area. The width tion (1) .(1)W=C0L. The change is calculated using the following Equation (3).h is the substrate’s height. The exact dimension of the patch can be calculated using Equation (4).The fringing effect changes the effective area of the patch. Therefore, variation in the area is represented by Δ11 of −18 dB and the second band is shown at 8 GHz with an S11 of −21 dB.Each design was numerically analyzed using the full-wave higher-frequency structural simulator (HFSS). A variety of performance metrics such as return loss, radiation pattern, total gain plot, field distribution, gain for the different frequencies, directivity, TARC, and directivity gain were analyzed. The reflection and transmittance response must be analyzed for the high-frequency antenna structure . The ini11 is less than −10 dB in the range of 1 to 2.485 GHz. The second band provides the S11 of −17.507 dB at 10.5 GHz; this band is observed in the range of 3.165 GHz to 3.825 GHz. Therefore, a peak bandwidth of 2.485 GHz is attained in this configuration. The MIMO structure’s unusual form primarily aids in attaining higher isolation (<−28 dB) during measurement as per DS-1. Two bands of reflection response are observed in 11 of −23.172 dB at 4.5 GHz; this band is observed in the range of 4.15 GHz to 4.775 GHz. The third band is shown in the range of 5.145 GHz to 6.19 GHz with an S11 of −16.572 dB at 5.5 GHz. The fourth band is shown in the range of 10.215 GHz to 11.05 GHz with an S11 of −31.457 dB at 10.5 GHz. Finally, the fifth band is shown in the range of 13.76 GHz to 14 GHz with an S11 of −10.69 dB at 13.9 GHz. In this mode, a maximum bandwidth of 1.045 GHz is achieved.DS-2 shows the five bands with less than −10 dB value. The first band represents reflection coefficients of −10.16 dB at 3 GHz; this band is observed in the range of 3.005 GHz to 3.14 GHz. The second band provides an SDS-3 shows the eight bands of reflection coefficients with less than −10 dB value. The first band represents reflection coefficients of −23.425 dB at 3 GHz; this band is observed in the range of 1 GHz to 3.54 GHz. The second band represents reflection coefficients of −39.44 dB at 3.8 GHz; this band is observed in the range of 3.69 GHz to 3.865 GHz. The third band represents reflection coefficients of −15.075 dB at 5.2 GHz; this band is observed in the range of 5.19 GHz to 5.52 GHz. The fourth band represents reflection coefficients of −25.319 dB at 6.5 GHz; this band is observed in the range of 6.325 GHz to 6.675 GHz. The fifth band represents reflection coefficients of −21.620 dB at 7.8 GHz; this band is observed in the range of 7.555 GHz to 7.915 GHz. The sixth band represents reflection coefficients of −10.147 dB at 8.9 GHz; this band is observed in the range of 8.765 GHz to 8.985 GHz. The seventh band represents reflection coefficients of −15.075 dB at 5.3 GHz; this band is observed in the range of 5.19 GHz to 5.52 GHz. The eighth band represents reflection coefficients of −15.497 dB at 10 GHz; this band is observed in the range of 9.86 GHz to 11.215 GHz. The ninth band represents reflection coefficients of −10.759 dB at 13.8 GHz; this band is observed in the range of 13.655 GHz to 14 GHz. Therefore, in this mode, a maximum bandwidth of 2.54 GHz is achieved. 11 of −14.420 dB at 3.1 GHz; this band is observed in the range of 3.065 GHz to 3.215 GHz. The second band represents an S11 of −20.308 dB at 3.55 GHz; this is observed in the range of 3.485 GHz to 5.655 GHz. The third band represents an S11 of −16.715 dB at 7.6 GHz; this band is observed in the range of 7.2 GHz to 7.97 GHz. The fourth band represents reflection coefficients of −19.805 dB at 11.8 GHz; this band is observed in the range of 9.45 GHz to 11.8 GHz. The fifth band represents reflection coefficients of −13.914 dB at 12.57 GHz; this band is observed in the range of 12.55 GHz to 12.585 GHz. In this mode, a maximum bandwidth of 2.35 GHz is achieved.DS-4 shows the five bands of reflection coefficients with less than −10 dB value. For example, the first band represents an SThe radiation pattern for the simulation and measurement of the proposed antenna structure is represented in The three-dimensional total gain plot of the proposed MIMO structures is presented in The gain over the 1 to 14 GHz span is presented in The directivity and −3 dB down directivity plots of the proposed four structures are shown in The value of ECC should be zero. The isolation between the individual antenna nodes improves as the ECC increases. Using the scattering parameter, the ECC can be calculated as per Equation (5). The simulated result represents the ECC response of the proposed DS:1, DS:2, DS:3, and DS:4, as represented in The improvement in the SNR of a multiple element system over a one element system is referred to as diversity gain (DG). The DG is calculated using the following Equation (6).The diversity gain of the planned four design structures is illustrated in The power received by a diversity antenna relative to that received by an isotropic antenna is the definition of a fading environment. The mean effective gain (MEG) is determined by solving Equation (7).A high gain is a need for any respectable MIMO system. The proper MEG response indicates that its diversity performance has been enhanced. Evaluating TARC is the best way to assess radiation performance and frequency response. TARC considers both mutual coupling and accidental signal pairings between ports. The waves that are reflected and incident are used to form Equations (8) and (9), which may be used to obtain the value in terms of the S parameters/The measured and simulated MEG response of the proposed MIMO antenna is represented in j and aj stand for reflected and incident waves, respectively. Channel capacity loss (CLL) is another crucial factor to consider while assessing the projected antenna’s MIMO performance. CCL specifies the maximum possible data transfer rate that suffers significant degradation. To prove lossless data transmission, a well implemented MIMO system should achieve a rate of 0.5 bits/s/Hz. Simply said, CCL alerts the user to the point at which further communication cannot be conducted without risk. CCL may be computed using either Equation (10) or Equation (11), respectively.In the above equations, the phase of the incident wave is θ. bHere, the main goal behind these four phases is to achieve a higher gain, which was achieved using the proposed MIMO structures. The simulated and measured channel capacity loss response for all of the proposed design structures is represented in The TARC values for the proposed four MIMO antennas are shown in The comparison of the presented MIMO structure with the other structures is shown in 11 less than −10 dB is attained in DS-3. The maximum electric field of 33.3 V/cm was attained in DS-3. Overall, the addition of slots around the patch area and DGS improves the performance of the antenna. The presented design targeted multiple wireless communication applications for the range of 1 GHz to 14 GHz. Therefore, the presented design structure has potential applications in satellite communication, vehicle speed monitoring, the military, and other 5G/6G applications requiring high data rate transmission.An FR4, substrate-based, two-port, loaded MIMO antenna has been proposed. Four types of design structures have been analyzed by cropping the patch region, loading the slits near the hexagonal-shaped patch, and adding/removing the slots in the ground area. The proposed design was analyzed in terms of peak gain, DG, reflection coefficient, total gain, electric field, directivity, and E-field pattern. The proposed structure provides an isolation of 35 dB, DG of 10 dB, and peak gain of 20.44 dB. The reflection response of −39.44 dB with the nine band responses having an S


S × LS = 12 mm × 9 mm. Three designs of two-port MIMO configurations are derived from the reference unit element antenna. In the first design, the antenna element is placed parallel to the reference antenna, while in the second design, the element is placed orthogonal to the reference element of the antenna. In the third design, the antenna elements are adjusted to be opposite each other. In this study, we analyze the isolation between the MIMO elements with different arrangements of the elements. The MIMO configurations have dimensions of 15 mm × 26 mm for two of the cases and 15 mm × 28.75 mm for the third case. All three MIMO antennas are made using similar materials and have the same specifications as the single element antenna. Other significant MIMO parameters, including the envelope correlation coefficient (ECC), diversity gain (DG), channel capacity loss (CCL), and mean effective gain (MEG), are also researched. Additionally, the paper includes a table summarizing the assessment of this work in comparison to relevant literature. The results of this study indicate that the proposed antenna is well-suited for future millimeter wave applications operating at 28 GHz.In this paper, a compact and simplified geometry monopole antenna with high gain and wideband is introduced. The presented antenna incorporates a microstrip feedline and a circular patch with two circular rings of stubs, which are inserted into the reference circular patch antenna to enhance the bandwidth and return loss. Roger RT/Duroid 6002 is used as the material for the antenna, and has overall dimensions of W The recent advancements in communication systems  have brought about numerous changes and revisions. These changes have been made to meet the requirements, which are a basic necessity for operating over future wireless communication spectrum ,2. TheseHigh gain and wideband antenna have gained increasing attention in recent years due to the evolving demands in communication systems ,9. DevicThe implementation of multiple-input multiple-out (MIMO) antenna technology increases the capacity and reduces interference ,16. SomeIt is mentioned in  that a tEtching slots from the ground plane can also increase an antenna’s bandwidth and gain if it has a defective ground structure (DGS) . In 27]27], a wiA compact sized 24 mm × 20 mm, wideband 33–44 GHz, and transparent antenna is reported in . The antFor 28 GHz applications, a two-port MIMO antenna is suggested in . The antCompact design with low structural complexity;Analysis of the MIMO antenna under various element orientation;Low mutual coupling and ECC from all three MIMO designs;High gain antenna without using additional layers.Three sections make up the remaining portion of this study. The unit element of an antenna is examined in the second section, along with its findings. The three scenarios of the recommended MIMO antenna are examined in The suggested antenna’s layout is illustrated in 3 and R4. This procedure significantly reduced the antenna’s return loss, enabling wideband operation. The resulting antenna, shown in The three steps of the design evaluation for the antenna are illustrated in 11 parameter of the recommended antenna single element is given in 11 < −10 dB). The proposed design offers two resonant frequencies around 28 GHz and 31.75 GHz with a return loss of 28 dB. From the figure it can also be seen that the S11 plot generated from the software and tested is quite similar with minor distortions. Additionally, The comparison between the predicted and tested SThe suggested antenna’s radiation pattern at specific frequencies of 28 GHz and 32 GHz is shown in The gain vs. frequency plot of this design is provided in This section studies and discusses the idea for the antenna’s two-port MIMO architecture. In order to analyze the MIMO characteristics of closely spaced elements, three separate scenarios are used. In the first scenario, both parts are positioned side by side. One MIMO antenna component is orthogonal to the other in the second scenario. The element is parallel but positioned on the opposite side in the third instance. The hardware prototypes are fabricated and tested for all aforementioned cases. To measure the reflection and transmission coefficient of the antenna, a vector network analyzer (VNA), 220 ZVA by Rohde & Schwarz, is used. To measure the far field, the antenna is placed in a newly designed shielded millimeter-wave anechoic chamber and we utilize a multi probe array technique, which provides accurate result for a spanning angle of 180° .1 = 4.75 mm between the second element and the reference element. The entire size of the MIMO antenna in this instance is MXI × MY1 = 15 mm × 26 mm. The same substrates and other design criteria were employed for one element only. To validate the simulated results of the antenna, the hardware prototype is created.In In Rψ refers to the below matrix for receiving antenna correlation.Our study uses the MIMO parameters, including the ECC, DG, CCL, and MEG. To examine the performance of single unit element in MIMO configuration, ECC is studied. DG is the study of losses in the form of transmission, and CCL is the examination of correlation losses in MIMO systems. The MEG is analyzed to study the power received in a fading area. The mathematical equations below, which are used to calculate these MIMO parameters, are well explained in ,38,39,40The ECC and DG of the suggested parallel element placed design are shown in 2 = 4 mm. The entire size of the MIMO antenna in this instance is MX2 × MY2 = 15 mm × 28.75 mm. The same substrates and other design criteria were employed for one element only. To validate the simulated results of the antenna, the hardware prototype is created.In Similarly to the above antenna presented in 3 = 5.2 mm. The MIMO antenna in this example has the same overall size as the antenna in the previous scenario, which is MX3 × MY3 = 15 mm × 26 mm. The location of the antenna with the reference antenna is the only distinction between cases 3 and 1. To validate the simulated results of the antenna, the hardware prototype is created. In As in the previous two designs, the important MIMO parameters of this antenna are also analyzed. In For 5G applications, a wideband, small, simple, high gain, and high radiation efficiency antenna is suggested in this research. For 28 GHz applications, a circular patch antenna was first developed. Later, to acquire the wideband, circular rings are placed into the antenna. The antenna provides a high gain of 11.25 dBi and a wide band of 8.25 GHz. Afterward, three MIMO antenna were adopted from initial reference antenna. The aim when designing a MIMO antenna under various arrangements of elements is to analyze and study the performance of the antenna. It is studied and concluded from the results that the antenna offers wideband and acceptable value of isolation in all three cases. The results in terms of MIMO parameters are also acceptable values, as the antenna offers ECC around 0.001, DG 9.9 dB, CCL 0.001 bits/Hz/sec, and MEG around 6.38 dB. The performance of MIMO antenna defined in three cases is concluded in


The envisioned application is collision avoidance in aid to visually impaired people at a medium-long distance.Three metasurfaces (MTS) are designed to be combined with a series end-fed 1 × 10 array antenna with a modified Dolph-Chebyshev distribution for imaging applications in the millimeter frequency range, 24.05–24.25 GHz. A reduction in secondary lobes and an increase in FTBR can be achieved while preserving gain, radiation efficiency, SLL and size using an MTS–array combination. Moreover, as a result of each single-layer MTS–array combination, operation bandwidth is widened, with gain and radiation efficiency enhancement. The overall devices’ size is 86.8 × 12 × 0.762 mm In recent years, the literature has been enriched with plenty of research works using metamaterials, especially metasurfaces for various applications, such as RCS reduction, antenna enhancement, wavefront transformation and unconventional waveguiding and scattering ,7,8,9,10Moreover, the use of increasingly higher frequencies, such as millimeter-wave frequencies, is spreading, due to the large bandwidths that can be allocated, with corresponding advantages in throughput for 5G and other applications, such as radar.The aim of this work is to investigate whether it is possible to improve all the properties of an antenna at once, both its impedance-matching bandwidth and its radiation properties, without increasing its overall size, using metasurface-based techniques. This approach contrasts with the traditional methods carried out so far, in which either artificial magnetic conductors (AMCs) are used under the antenna at some distance (or replacing the ground plane of the antenna) increasing the thickness, or EBGs with numerous cells surrounding the initial antenna (increasing its size), or employing frequency selective surfaces, polarizers or partially reflecting surfaces over the antenna at some distance to modify its radiation pattern, always increasing the thickness. All of the above methods increase the cost and complicate the antenna manufacturing process. In addition, these techniques improve only some of the antenna parameters, but not all at once (or considerably improve some at the expense of certain deterioration of others). Other recent works including  focus onAnother important aspect to highlight is that most of the works that involve the combination of an antenna with a metasurface, usually derive in a modification of some of the dimensions of the metallization of the antenna to achieve that the antenna–metasurface assembly works conveniently, even when a certain distance is left between both. Therefore, the comparison of the antenna alone with the metasurface–antenna combination loses rigor, since it is not the same antenna. Consequently, the conclusions drawn from the point of view of the usefulness of the metasurfaces or the physical phenomena involved are more than debatable, even if an operational device is achieved.Concerning the state-of-the-art on wearable antennas with metasurfaces, there have been significant advances for biomedical applications , as wellThe aim of this work is to design an antenna with improved performance for a specific radar application in the millimeter frequency range, specifically in the ISM band from 24.05 GHz to 24.25 GHz, due to the suitability of such frequencies for high-resolution detection in foggy, smoky and dusty environments. It is intended for use in a collision-avoidance-based system to assist visually impaired people and therefore must be compact and lightweight, efficient and radiate as little as possible in the direction of the human body (which translates into a high front-to-back ratio). The antenna has a major impact on radar performance. A compromise solution between range and coverage area has to be adopted. In addition, it must have an appropriate directivity to be able to detect obstacles at the intended distance and it also influences the angle determination of the target.Regarding the design and analysis of the metasurface to be combined with the antenna, it is not only intended to be operative in the frequency band of interest (24.05 GHz to 24.25 GHz), but also to analyze its complete angular stability, that is, taking into account both the variation of the electric field polarization angle, Phi, from 0 to 90 degrees, and the angle of oblique incidence, Theta, from 0 to 60 degrees, both for TE- and TM-polarized incident plane waves. It should be noted that most angular stability studies only consider the Phi = 0 case, leaving relevant cases unanalyzed, which is not convenient when intending to combine metasurface with antennas. The angular stability in TM can be improved by reducing the thickness of the metasurface dielectric, although this reduces the operating bandwidth, so a compromise must be found between both parameters, as well as considering the availability of commercial substrates.The paper is organized as follows: first, r = 3.0 and tan δ = 0.0013) [A recently published series end-fed 1 × 10 array antenna on polypropylene  will be  0.0013)  is used 11(dB) results obtained in simulation for the optimized array antenna design, the operation bandwidth with suitable impedance matching is 23.98–24.42 GHz, being optimal in the intended 24.05 GHz to 24.25 GHz ISM band. The radiation characteristics of the antenna obtained in simulation, in terms of peak realized gain (G), peak directivity (D), radiation efficiency (η) and front-to-back ratio (FTBR), are indicated in From the SPhi0° = −16 dB, HPBWPhi0° = 12°, SLLPhi90° = −28 dB and HPBWPhi90° = 64°. All of these results are suitable for the envisioned collision-avoidance application. However, while this antenna can be said to meet the requirements in the commercial 24 GHz band, and there is scarce margin for improvement within it (perhaps try to increase somewhat radiating efficiency and FTBR and reduce secondary lobes), it is well known that if the operational bandwidth of the antenna is widened, this results in a higher resolution and improvement in the detection of obstacles, which is the main objective of this application. Therefore, by using a wider bandwidth radar transceiver, end-user performance would be improved. Thus, if it is possible to widen the antenna’s operating frequency band while preserving (or even improving) the characteristics of the radiation pattern, without increasing the size and neither complicating nor making manufacturing more expensive, it would be a significant achievement for the aim pursued.In addition to the high G (>14.0 dBi) and D (>14.5 dB) levels achieved in the whole band, it is noteworthy the levels of both the high radiation efficiency (>90%) and the FTBR (>20 dB), which are critical in a wearable application. The side-lobe level (SLL) and the half-power beam width (HPBW) at the center frequency of the band (24.15 GHz) are SLLThree high-impedance metasurfaces (MTS)  are desiIt can be observed that under normal incidence, MTSsquare resonates at 23.66 GHz, very close to the antenna’s operating frequency band, with in-phase operation from 20.07 GHz to 27.32 GHz, whereas MTS and MTSsquare-scaled resonate at a higher frequency, 26.58 GHz, with in-phase reflection operation from 20.07 GHz to 30.9 GHz, therefore still exhibiting in-phase reflection and increasing surface impedance in the antenna’s operating band. The three metasurfaces can be considered high-impedance surfaces (HIS) in view of their surface impedance shown in The angular stability of the metasurfaces has been analyzed. The incidence angle θ is varied from 0° to 60° in steps of 15° for each polarization angle of the incident electric field φ which is, in turn, varied from 0° to 90° in steps of 15°. The metasurfaces are arranged around the antenna, as shown in The current distribution at the center frequency of operation (24.15 GHz) for the array antenna alone and combined with the metasurfaces and the metallic parasite is shown in Analyzing the effect of the aforementioned combinations with regard to the frequency band with proper impedance matching, it can be seen in As for the radiation properties within the 24.05–24.25 GHz frequency band and given in The most significant advantage comes from the fact that the highlighted increase in bandwidth is accompanied by an improvement in gain and radiation efficiency, and preservation of the radiation pattern shape and the FTBR (which is improved in some cases).As an advantageous example of the inclusion of a metasurface, it can be observed in Conventional micromachining is used to fabricate the prototypes, and an SMA connector operating up to 26 GHz is soldered by hand to feed them (see The measured reflection coefficient for the fabricated antennas is depicted in The radiation pattern cuts for Phi = 0° and Phi = 90° at the center frequency of the band (24.15 GHz) were measured in an anechoic chamber see , and theFairly good agreement is achieved between simulation and measurement results, especially for the CP components. The resulting HPBW level is almost identical to the one obtained in simulation for both Phi = 0° and Phi = 90° cuts, whereas the SLL slightly worsens in measurement for Phi = 0°. All the antennas under analysis exhibit an asymmetry in the Phi = 0° plane due to the feeding method, which is already observed in the simulation results. This makes Phi = 0° pattern cut more sensitive to everything that is actually located in the feeding part of the antenna. In the measurement results, both the tilt and the regrowth of the side lobes observable on one side of said pattern cut are attributable to the effect of the connector and especially of the bend (right-angled adapter or 90-degree bend) between the cable and the connector, which are not considered in the simulation. Both the connector and the bend enlarge the effective length of the antenna and modify the current distribution at its input. In addition, the movement of the cable when rotating the positioner to measure said pattern cut further contributes to modify the current distribution and exerts mechanical tension pulling the antenna, which may cause slight differences in positioning between antennas.Again, it should be noted that the connector was soldered by hand on each antenna. Both the cable and the connector can severely perturb the current distribution on the small antenna and be also responsible for the observed XP levels in measurement.The gain transfer method, which involves inter-comparison of the array antenna prototype (antenna under test (AUT)) with a Flann Microwave Standard horn 20240-25 (probe antenna of known characteristics), was used to conduct the measurements of the peak realized gain. It has to be taken into account that, in addition to the aforementioned effects of the cable and the connector, the gain measurement can be disturbed by other effects : multipaThe most significant finding of this work is that the operating bandwidth of an antenna can be widened, without degrading or even improving its radiation characteristics, without the need to increase its size, using metasurfaces with a reduced number of unit cells.To explain the broadening of the impedance-matching bandwidth, the impedances of the antenna and the metasurfaces have been analyzed. The reactance of the antenna alone  are obtained and analyzed for further insight into the antennas under analysis.In view of the results shown in The Ey level is slightly higher for the MTSsquare-scaled–array, which agrees with a higher value of the cross-polar component observed in the radiation pattern of r =3) should be considered. Otherwise, the values of impedance-matching bandwidth and antenna dimensions will not be comparable. Neither would be those radiation parameters affected by potential surface wave propagation, which increases for high εr (and even more so the thicker the substrate). Therefore, the range will be limited to εr values between 2.9 and 3.66.To assess the relevance of the antennas presented in this work, a comparison with other state-of-the-art antennas operating in the 24 GHz radar band is presented in terms of size, bandwidth and radiation properties see . There aIt is also noteworthy that most of the works in the literature do not provide the results of radiation efficiency and FTBR.r values or as close as possible to that of the antennas presented in this work [r values, which are slightly higher than 3 and lower than 3.6, [r. Finally, Refs. [r, making them less advantageous in terms of compactness. It should be clarified that [Regarding the antennas on substrates with identical εhis work ,44,45,46his work  shows a his work  for the han 3.6, ,48,49,50han 3.6, ,48,50, ay, Refs. ,52,53, wied that  uses a 1Provided that millimeter-wave energy penetrates the stratum corneum easily but is rapidly absorbed within the deeper epidermis and dermis and does not propagate further into the body  and thatIt has been proven that the operating bandwidth of an antenna can be widened, without degrading or even improving its radiation characteristics, while maintaining both its size as well as its ease and cost of manufacturing, using metasurfaces with a reduced number of unit cells. It has been shown that a metallic parasite of the same size and located at the same distance does not provide the same performance, but rather degrades the antenna. An explanation has been given for the phenomena that make these improvements possible.3.Three metasurfaces (MTS) have been designed and combined with a series end-fed 1 × 10 array antenna with a modified Dolph-Chebyshev distribution. As a result, three fully operational prototypes for imaging applications in the millimeter frequency range 24.05–24.25 GHz have been obtained, with an overall size of 86.8 × 12 × 0.762 mm


This paper presents the design and realization of a simple and low-profile, four-port multiple-input-multiple-output (MIMO) antenna operating in a mm-wave band supporting 5G communication technologies. As part of the design methodology, the initial stage involved the development of a conventional monopole patch antenna optimized for operation at 26 GHz, which was matched to a 50 Ω stepped feed line. Afterward, a square-shaped defected ground structure (DGS) with semi-circle slots on the edges was placed on the ground to improve the isolation, and the circular and rectangular slots were incorporated as DGSs to optimize the antenna impedance bandwidth. Etching semi-circular-shaped slots on the ground plane achieved more than 34.2 dB isolation in the 26 GHz operating band. In addition, an arrangement of four symmetrical radiating elements was positioned orthogonally to minimize the antenna’s physical size and improve the isolation. The proposed MIMO antenna’s overall dimension was 25 × 25 mm Wireless communication systems have become essential in various areas, such as health, agriculture, finance, education, the Internet of Things, media, and entertainment ,2,3,4,5.The usage of MIMO antennas in mm-wave applications is particularly valuable due to their low cost, low profile, and compact size. The appropriate design of four-port MIMO antennas can result in strong isolation levels and optimal radiation characteristics, which are crucial for achieving high-performance wireless communication systems. The proposed antenna was designed with a stepped line feed, rectangular and circular slots implemented as defected ground structures for achieving wider bandwidth, as well as optimal radiation characteristics. Moreover, it has been determined that the optimal radiation characteristics of the antenna can be achieved by incorporating semi-circular slots at the edges of the ground surface of the antenna elements, and by employing an orthogonal arrangement of the four antenna elements to achieve a strong isolation of at least 34.2 dB.The unit cell of the proposed antenna comprises DGS and an antenna radiator, developed in three steps, incorporating various geometrical shapes, such as rectangular and circular slots, and the design steps of the unit cell illustrated in PThe width  as another key component. Optimizing the diversity gain is crucial for achieving a high-performance MIMO antenna system. If the envelope correlation coefficient (ECC) exceeds 0.5, the system fails to provide the diversity gain, potentially degrading the system’s performance. A value of |ECC| ≤ 0.3 is considered sufficient to achieve the diversity gain [The envelope correlation coefficient (ECC), diversity gain (DG), and total active reflection coefficient (TARC) are metrics that provide crucial information about the antenna’s ability to provide stable, high-quality, and diverse signals, thereby ensuring optimal system performance. Although MIMO antenna systems can theoretically increase a system’s capacity, the system’s performance can be negatively impacted if the signals received at different antenna elements are correlated . ECC meaity gain . The DG ity gain .(10)DG=The total active reflection coefficient (TARC) is an important parameter to consider when designing a MIMO communication system. TARC is calculated by taking the ratio of the square root of the total reflected power to the total power in the system, and is estimated by using arbitrary signal combinations and measuring excessive couplings between antenna ports. It is crucial that the TARC value does not exceed 0 dB, as this could lead to distorted received results and erroneous data transmission. The voltage standing wave ratio (VSWR) is an essential metric that indicates the degree of impedance matching between the antenna and transmission line. It is defined as the ratio of the amplitudes of the forward and reflected waves along the transmission line. When the VSWR value is equal to 1, it signifies that all power is transferred to the antenna with no reflection, thereby ensuring optimal antenna performance. The simulated VSWR value of the antenna is 1.2; indicating that the impedance of the antenna matches the transmission line’s impedance, allowing maximum power to be transferred from the source to the antenna.Another critical metric in the antenna design is directivity gain, which is the ratio of the radiation intensity of an antenna in a particular direction to the average radiation intensity of the antenna over all directions. Antennas with higher directivity gains are more efficient, allowing for better transmission of signals over longer distances or reception of weaker signals. Therefore, the directivity gain is a key factor to consider when designing and optimizing antennas for various applications, particularly in wireless communication systems. In recent years, 5G technology has gained critical significance as a communication service. This is largely due to its ability to provide higher multi-Gbps peak data speeds, ultra-low latency, increased network capacity, heightened availability, greater reliability, and a more uniform user experience for a larger number of users. Moreover, there is a growing trend towards the development of lighter and thinner portable gadgets that demand powerful processing capabilities. This manuscript presents a novel, compact, and low-profile 4-port MIMO antenna design for millimeter-wave applications operating at 26 GHz. The antenna has dimensions of 25 × 25 × 0.787 mm