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Abstract This paper introduces a novel design and development of a compact circularly polarized antenna operating at two distinct frequency bands, 2.5 GHz and 6 GHz. The innovation lies in using a modified circular patch and a uniquely designed feeding element, which optimize circular polarization performance while maintaining a low-profile form factor. Fabricated on an FR4 substrate with an overall thickness of 1.6 mm and dimensions of 62 mm × 62 mm × 1.6 mm, corresponding to approximately 0.5167λ × 0.5167λ × 0.0133λ at 2.5 GHz, the antenna achieves a compact configuration well-suited for space-constrained applications. Extensive simulations and measurements show close agreement, with axial ratios below 3 dB across both operational bands, ensuring effective circular polarization. The antenna achieves peak gains of 1.8 dBi at 2.5 GHz and 3.8 dBi at 6 GHz, highlighting its ability to combine compactness, dual-band functionality, and strong radiation characteristics. This proposed design provides a groundbreaking solution for Industrial, Scientific, and Medical (ISM) and Vehicle-to-Everything (V2X) communication systems, addressing the increasing demand for efficient, dual-band, and space-saving wireless communication solutions.

Abstract A network created in places inaccessible to humans is known as the wireless sensor network. A sensor must detect data/information before it sends this data to a base station. Data can be routed between just one node to a base station using a variety of routing protocols. The hierarchical routing method is one of the routing protocols that hierarchically distributes sensed data. Using clustering to arrange the network into an interconnected hierarchy has shown to be a successful strategy. Bio‐inspired particle swarm optimisation is combined with the Enhanced LEACH protocol to overcome the shortcomings of conventional protocol like overall consumption of energy, the total number of survival nodes, and packets being delivered during the network's life. Metaheuristic approach of particle swarm optimisation which explores alternative paths during optimisation, leading to more adaptive and efficient energy dissipation. Enhanced LEACH with the bioinspired protocol makes it more efficient for real‐time applications. Simulation results show that the proposed protocol has a greater advantage over the conventional and Enhanced LEACH.

This article presents a four-port multiple-input-multiple-output (MIMO) flexible antenna design for Ultrawideband (UWB) application using Polydimethylsiloxanes substrate. The single UWB antenna structure consists of modified circular radiator with elliptical slot and partial ground plane. To achieve high isolation among antenna element, a Defective Ground Structure (DGS) is adopted to decouple the low frequency band. DGS comprises a centrally placed square shape slotted structure connected by horizontal and vertical lines, achieving isolation below −20 dB. The proposed antenna has a dimension of 56×41×1.6mm3(0.41λ×0.3λ×0.0117λ), bandwidth of 3.4–11.8 GHz (110.5 %) and 4.3 dBi peak gain. Simulation and measurement findings show that the antenna performs well under bending situations with different curvature radius. At 4 GHz and 8.1 GHz, W/kg specific absorption rate (SAR) for 1/10 g tissue is examined. A low value of SAR is obtained at both the resonating frequencies. Additionally, an analysis is conducted on the diversity parameters, and the antenna demonstrates comparatively good results having mean effective gain (MEG) ratio 9.99 dB, implying that it's appropriate for wearable MIMO applications.

The aircraft collision avoidance system (ACAS) is a surveillance system required in all airplanes to ensure a safe flight. To locate the presence of any surrounding aircraft that may pose a threat, the ACAS sequentially scans the fixed sectors (0° to 90°, 90° to 180°, 180° to 270°, and 270° to 360°) for a specified duration. As a result, ACAS necessitates a highly directional multi-element antenna capable of spanning the whole 360° azimuth plane. Over the past decade, studies on the performance enhancement of ACAS antennas have become increasingly prevalent due to the numerous limitations of the typical ACAS antenna, which include poor gain ( $3.6dB$ ), larger size ( $>\lambda$ ), high side-lobe level ( $-7dB$ ), challenges with beam tuning, etc. The main focus of this study is to improve the gain and bandwidth (BW) of an ACAS antenna using the notion of MTM, hence a novel U and L-shaped compact negative index MTM structure is developed. Because the ACAS antenna broadcasts the signal in the entire azimuth region, the proposed MTM should be competent to endure its double negative behavior for all incident angles; thus, the MTM cell is evaluated for different incident angles ranging from 0° to 360° in the azimuth plane with a 60° offset, and for all incident angles, the proposed design exhibits DNG behavior in the required frequency spectrum. The effective medium ratio (EMR) of 12.86 at 1.06GHz demonstrates the efficacy and compactness of the proposed design. The proposed MTM cell is expanded as a $5\times 4$ array, and placed as a superstrate at a fixed distance from the proposed ACAS antenna, and the MTM-loaded antenna outperforms the conventional antenna in terms of overall gain by 2.09dB and BW by 27.2MHz. Further, for the experimental verification, the MTM cell and its array structures are fabricated, and in both designs, the fabricated results correspond well with the simulated results.

This article presents a flexible four-element MIMO antenna designed on a thin polyimide substrate to operate at 30.50 GHz for wearable applications. The antenna element structure is a combination of circular rings combined with T-shape stubs. The MIMO antenna isolation is enhanced through a novel decoupling structure consisting of a rectangular stub, four open-ended horizontal strip lines, and two vertical strip lines connecting the ground plane. It provides 22.5 dB isolation, even with conformal orientation. A metamaterial (MTM) reflective surface of $9\times9$ unit cells is designed and placed below the antenna to improve its radiation characteristics, bandwidth, gain, and specific absorption rate (SAR). The metamaterial unit cell provides a double negative property with a wide stop-band range of 26.27–36.49 GHz. With an MTM surface, the measured bandwidth is 25.2–33 GHz with a maximum broadside gain of 8.90 dBi. The resulting SAR is reduced from 1.71 W/kg to 0.86 W/kg by incorporating MTM reflective surface. The MIMO antenna with MTM improves the bandwidth by 56%, gain by 32.5%, and radiation from bidirectional to broadside, compared to the MIMO antenna without MTM. Also, the antenna is measured and validated for diversity parameters such as Envelope correlation coefficient (ECC), Diversity gain (DG), Channel coefficient loss (CCL), Total active reflection coefficient (TARC), and Mean effective gain (MEG). The reduced SAR and improved antenna performance suggest the proposed MIMO antenna with an MTM reflective surface is suitable for on-body wearable applications in IoT devices, smart watches, headwear, and footwear devices.

This article presents a compact six-element multiple-input multiple-output (MIMO) antenna for vehicle-to-vehicle (V2V) communications. The proposed structure is planar, having a dimension of $1.28\lambda _{0} \boldsymbol {\times } 2.04\lambda _{0}$ , where $\lambda _{0}$ is the wavelength at 25.5 GHz, resonating in n258 and n260 bands. The antenna is a modified rectangular patch designed to generate TM10 and TM02 modes, resulting in vertical and horizontal polarization at 25.5 GHz and 38 GHz. The measured bandwidth of the MIMO antenna is 800 MHz at the n258 band, i.e., 25.1-25.9 GHz, and 850 MHz at the n260 band, i.e., 37.75-38.6 GHz. The isolation between the elements is improved by incorporating a novel open-ended decoupling structure. It resulted in an isolation that surpass 20 dB at both bands. The antenna has a broadside radiation pattern with a maximum realized gain of 9 dBi and 7.2 dBi at 25.5 and 38 GHz, respectively. The antenna is validated for various diversity parameters such as envelope correlation coefficient, diversity gain, channel capacity loss, and mean effective gain. Also, the performance of the proposed antenna for V2V communication is analyzed for line-of-sight and non-line-of-sight conditions. The estimated channel capacity, path loss, and link budget for the above-said condition suggest that the antenna is suitable for V2V communications.

The wireless communication system is steered towards the millimeter wave spectrum to achieve low latency and high-speed data rate. The MIMO antennas aid in attaining a higher data rate. The prominent spectrum at millimeter wave is Ka-band, suitable for short-range communication. The $\vert \text{S}$ -parameter $\vert $ response and radiation pattern of the existing MIMO antenna at this band are relatively unstable. Hence it encouraged to design and develop a four-element MIMO antenna operating at Ka-band. The antenna is a circular ring shape with two concentric rings with a plus-shape stub overlayed on circular rings. The structure is developed in four-stage with the comprehension of characteristic mode theory (CMA). The proposed structure generated Mode 2 as an efficient mode, with minor Modes 3 and 5 contributing for resonance out of five modes. The overall antenna profile is $3.27\lambda _{0} \times 3.74\lambda _{0}$ (where $\lambda _{0}$ is the wavelength at a resonance frequency of 28 GHz). The novel decoupling structure has improved the isolation to 30 dB and increased the bandwidth. The antenna has an operating bandwidth of 24.1-30.9 GHz, with a maximum gain of 6.5 dBi. The $\vert \text{S}$ -parameter $\vert $ from all the ports has an exact and stable response. The proposed antenna has resulted in bidirectional radiation tilted at an angle of $334^{0}$ and $210^{0}$ in the XZ plane. In the YZ plane, it has a triple beam. The radiation pattern is also stable throughout the bandwidth. The proposed MIMO antenna has a symmetrical design, demonstrating the possibility of expansion to n-element MIMO through a six-element MIMO antenna design. The article also presents the channel capacity, path loss, and link margin calculation for designed antenna line-of-sight (LOS) communication. The antenna has been evaluated with diversity parameters such as ECC, DC, CCL, TARC, and MEG.

A double-stub matching technique is used to design a dual-band monopole antenna at 28 and 38 GHz. The transmission line stubs represent the matching elements. The first matching network comprises series capacitive and inductive stubs, causing impedance matching at the 28 GHz band with a wide bandwidth. On the other hand, the second matching network has two shunt inductive stubs, generating resonance at 38 GHz. A Smith chart is utilized to predict the stub lengths. While incorporating their dimensions physically, some of the stub lengths are fine-tuned. The proposed antenna is compact with a profile of 0.75λ1×0.66λ1 (where λ1 is the free-space wavelength at 28 GHz). The measured bandwidths are 27–28.75 GHz and 36.20–42.43 GHz. Although the physical series capacitance of the first matching network is a slot in the ground plane, the antenna is able to achieve a good gain of 7 dBi in both bands. The proposed antenna has a compact design, good bandwidth and gain, making it a candidate for 5G wireless applications.

This article presents a planar, non-angular, series-fed, dual-element dipole array MIMO antenna operating at 28 GHz with the metasurface-based isolation improvement technique. The initial design is a single-element antenna with a dual dipole array which is series-fed. These dipole elements are non-uniform in shape and distance. This dipole antenna results in end-fire radiation. The dipole antenna excites the J1 mode for its operation. Further, with the view to improve channel capacity, the dipole array expands the antenna to a three-element MIMO antenna. In the MIMO antenna structure, the sum of the J1, J2, and J3 modes is excited, causing resonance at 28 GHz. This article also proposes a metasurface structure with wide stopband characteristics at 28 GHz for isolation improvement. The metasurface is composed of rectangle-shaped structures. The defected ground and metasurface structure combination suppresses the surface wave coupling among the MIMO elements. The proposed antenna results in a bandwidth ranging from 26.7 to 29.6 GHz with isolation improvement greater than 21 dB and a gain of 6.3 dBi. The antenna is validated with the diversity parameters of envelope correlation coefficient, diversity gain, and channel capacity loss.

Broadband antennas with a low-profile generating circular polarization are always in demand for handheld/ portable devices as CP antennas counter multipath and misalignment issues. Therefore, a compact millimeter-wave antenna is proposed in this article. The proposed antenna structure comprises two circular rings and a circular patch at the center. This structure is further embedded with four equilateral triangles at a 90° orientation. The current entering the radiator is divided into left and right circular directions. The equilateral triangles provide the return path for current at the differential phase of ±90°, generating circular polarization. Structural development and analysis were initially performed through the characteristic mode theory. It showed that Modes 1 to 4 generated good impedance matching from 20 to 30 GHz and Modes 1 to 5, from 30 to 40 GHz. It also demonstrated the summation of orthogonal modes leading to circular polarization. The antenna-measured reflection coefficient |S11| > 10 dB was 19 GHz (23–42 GHz), and the axial ratio at −3 dB was 4.2 GHz (36–40.2 GHz). The antenna gain ranged from 4 to 6.2 dBi. The proposed antenna was tested for link margin estimation for IoT indoor conditions with line-of-sight (LOS) and non-line-of-sight (NLOS) conditions. The communication reliability with co- and cross-polarization was also studied under these conditions, and the results proved to be satisfactory.

The demand for wide bandwidth has driven us to focus on the higher spectrum, the millimeter wave (mmWave) spectrum. The mmWave offers several advantages, such as wide bandwidth, low latency, and higher data rate, but the signal at these frequencies significantly suffers high atmospheric attenuation and path loss. To overcome these issues, array antennas are used as it provides high gain, better directivity, and bandwidth. This paper presents a detailed review of array antennas based on feeding methods and discusses reconfigurable arrays. The series-fed array results in a narrow bandwidth, high return losses, and low gain that can be resolved with tapered structures, coplanar waveguide (CPW), and SIW technique. Similarly, transmission losses, coupling losses, and high side lobe-levels in parallel-fed arrays can be addressed with multilayer structures, coplanar strips with CPW, pill-box transmission, ridge gap waveguide (RGWG), graphene material, and magnetoelectric dipoles. The merits of series and parallel feed methods are availed with the hybrid feed method. These feeding methods with arrays are extended to reconfigurable array along with active RF switch with stubs, continuous or discrete phase shifter, butler matrix, and/or graphene nanoplates. The parameter performance metrics of these arrays are summarized and concluded with the future scope.

The increased traffic in e-commerce, cloud-based processing, social media, and online video streaming demands higher data rates. The current 4G has reached the bottleneck, due to which it may not be able to fulfill the high data demand, so the focus is drifting toward millimeter wave (mmWave). The mmWave spectrum ranging from 30 to 300 GHz offers wide bandwidth with low latency, which finds its application in various communication fields, including 5G cellular. Despite its atmospheric attenuation and non-line-of-sight (NLOS) propagation, most countries are currently adopting mmWave 5G at the 28/38 GHz band due to less atmospheric attenuation, low path loss exponent, and low signal spread at these bands. The single-element patch antenna is a compact solution for mmWave applications, but its performance is inferior in terms of bandwidth, gain, and radiation efficiency. The array antennas have overcome these demerits, as it has shown a significant increase in bandwidth, gain, and radiation efficiency. Still, it has a limitation on data rate support. As a result, Multiple-Input-Multiple-Output (MIMO) technology can increase the data rate to 1000 times through spatial diversity and multiplexing techniques. So, to refine the performance further, there is a need to comprehend the MIMO antenna structures designed so far at mmWave. This paper presents the planar MIMO antenna structures developed so far, categorized here as slot, coplanar waveguide, defected ground structures, tapered/Vivaldi, meta-surface/metamaterial, dielectric resonator, and flexible antennas. The performance of these designs is compared based on bandwidth, gain, isolation, efficiency, and radiation pattern. This article also discusses the effects of slots, partial ground, and decoupling structures on impedance matching, bandwidth, and isolation levels. Also, a thorough discussion of the design issues and future work to be undertaken is discussed in this here.

Advancement in smartwatch sensors and connectivity features demands low latency communication with a wide bandwidth. ISM bands below 6 GHz are reaching a threshold. The millimeter-wave (mmWave) spectrum is the solution for future smartwatch applications. Therefore, a compact dual-band antenna operating at 25.5 and 38 GHz is presented here. The characteristics mode theory (CMT) aids the antenna design process by exciting Mode 1 and 2 as well as Mode 1–3 at their respective bands. In addition, the antenna structure generates two traverse modes, TM10 and TM02, at the lower and higher frequency bands. The antenna measured a bandwidth (BW) of 1.5 (25–26.5 GHz) and 2.5 GHz (37–39.5 GHz) with a maximum gain of 7.4 and 7.3 dBi, respectively. The antenna performance within the watch case (stainless steel) showed a stable |S11| with a gain improvement of 9.9 and 10.9 dBi and a specific absorption rate (SAR) of 0.063 and 0.0206 W/kg, respectively, at the lower and higher bands. The link budget analysis for various rotation angles of the watch indicated that, for a link margin of 20 dB, the antenna can transmit/receive 1 Gbps of data. However, significant fading was noticed at certain angles due to the shadowing effect caused by the watch case itself. Nonetheless, the antenna has a workable bandwidth, a high gain, and a low SAR, making it suitable for smartwatch and IoT applications.

In this current era of communications and networking, The Internet of things plays the main role in the making of smart communication and networking. In this article, we have focused on the literature survey on wireless sensor networks which are energy efficient. Various standard protocols are reviewed along with some enhanced protocols which makes the network energy efficient. The comparison of the standard and enhanced protocols with respect to various applications in wireless sensor networks is thoroughly done in this article. The outcomes of the enhanced protocols are also briefly discussed. For easier analysis to future researchers, a comparative table which lists the enhanced protocols which are compared with standard counterparts along with the factors for energy efficiency of the protocols. This article also comments on the issues and challenges of the protocols which can be further analyzed for making the wireless sensor network more energy efficient.

In this current era of communications and networking, The Internet of things plays the main role in the making of smart communication and networking. In this article, we have focused on the literature survey on wireless sensor networks which are energy efficient. Various standard protocols are reviewed along with some enhanced protocols which makes the network energy efficient. The comparison of the standard and enhanced protocols with respect to various applications in wireless sensor networks is thoroughly done in this article. The outcomes of the enhanced protocols are also briefly discussed. For easier analysis to future researchers, a comparative table which lists the enhanced protocols which are compared with standard counterparts along with the factors for energy efficiency of the protocols. This article also comments on the issues and challenges of the protocols which can be further analyzed for making the wireless sensor network more energy efficient.

In this current era of communications and networking, The Internet of things plays the main role in the making of smart communication and networking. In this article, we have focused on the literature survey on wireless sensor networks which are energy efficient. Various standard protocols are reviewed along with some enhanced protocols which makes the network energy efficient. The comparison of the standard and enhanced protocols with respect to various applications in wireless sensor networks is thoroughly done in this article. The outcomes of the enhanced protocols are also briefly discussed. For easier analysis to future researchers, a comparative table which lists the enhanced protocols which are compared with standard counterparts along with the factors for energy efficiency of the protocols. This article also comments on the issues and challenges of the protocols which can be further analyzed for making the wireless sensor network more energy efficient.

In this current era of communications and networking, The Internet of things plays the main role in the making of smart communication and networking. In this article, we have focused on the literature survey on wireless sensor networks which are energy efficient. Various standard protocols are reviewed along with some enhanced protocols which makes the network energy efficient. The comparison of the standard and enhanced protocols with respect to various applications in wireless sensor networks is thoroughly done in this article. The outcomes of the enhanced protocols are also briefly discussed. For easier analysis to future researchers, a comparative table which lists the enhanced protocols which are compared with standard counterparts along with the factors for energy efficiency of the protocols. This article also comments on the issues and challenges of the protocols which can be further analyzed for making the wireless sensor network more energy efficient.