A new isolated out-of-phase filtering power divider/combiner with high selectivity is presented. Based on a single-layer microstrip, required coupling strength and phase difference are achieved by two pairs of three-line coupled feeding structures. To realize desired bandpass responses, a specific resonator is located between the input and output coupled structures. By connecting a half-wavelength transmission line which is centrally loaded with a grounded resistor between two output ports, high port-to-port isolation is attained. Based on the impedance matrix of coupled three-line structure, theoretical filtering responses are predicted with the specified bandwidth and return loss. For demonstration, a prototype is designed, fabricated, and tested. Both simulated and measured results are displayed to verify the design mechanism.

Substrate integrated waveguide (SIW) technology represents a good solution for the design of couplers. Coupler structures proposed in most relevant reports cannot achieve excellent performance in the case of weak coupling. This work proposes a new weak coupling coupler architecture, similar to the branch line coupler. The metal via arrays is used to reshape the electric field distribution of the SIW structure, making the overall structure achieve weak coupling characteristics. The even-odd mode decomposition method analyzes this structure's equivalent transmission line model. For this purpose, a systematic design procedure is deployed to achieve several coupling values over a wide frequency bandwidth. A novel half-mode substrate integrated waveguide (HMSIW) coupler with a 29 dB coupling is designed and fabricated for verification based on the proposed method. Good agreements between the calculated and simulated results are observed. The proposed coupler has the advantage of high directivity within the broadband and can be used for SIW-based circuits and power detection in the Ku-band.

The design of a low-cost, flexible, miniaturized, and a high code density chipless radio-frequency identification (RFID) tag is presented as a solution for tracking the transportation of biomass fuel pellets. The performance of the tag is presented and demonstrates the applicability of the design for different material systems, while maintaining a compact size of 5.06 cm2. The tag consists of nested concentric hexagonal elements and a central spiral resonator suitable for ID encoding. The tag presented demonstrates code density of 3.6-bits/cm2, possesses angular stability up to 60°, and high radar cross section (RCS). The tag performance was also observed for tracking 5 kg of fly-ash biomass. Additionally, as the tag mass mostly consists of FR4, PET, or Taconic TLX-0 with a minute mass of either copper, gold, or silver, the tag can be easily combusted and disposed of during biomass combustion. The novel features of this tag are the combination of hexagonal and spiral shape slots for maximum space utilization thereby achieving high RCS signatures along with high code density. All these properties of the proposed chipless RFID tag provide a pioneering pathway for a real-time biomass tracking application.

A five-slot hexagonal shape chipless RFID tag is designed, simulated, and manufactured on FR4 substrate. The designed tag's copper geometry was replicated on a wide range of dielectric substrate to quantify the impact on resonance quality factor (RQF) and resonating frequencies. The tag's performance was assessed in three configurations. First, a hexagonal shape tag's radar cross section (RCS) was studied over different dielectric substrates. The various dielectric substrate effects were investigated over the maximum read range, resonant frequencies and RQF. In the second evaluation, the physical geometry of the tag was adjusted to achieve the spectral signatures in 2–7 GHz frequency band with high RQF. In step three, the optimized tag geometry was manufactured on FR4, Roger Duroid 5880, and polyethylene naphthalate (PEN) substrates. Denford milling machine for PCB engraving and inkjet printing for silver nanoparticles deposition were used for tags manufacturing. During tag manufacturing, copper and silver were used as conducting materials for RCS backscattering. The tag RCS response was measured by vector network analyzer with bi-static antenna setup. The analysis of different dielectric substrate provides a pathway of designing a novel substrate by using various nanomaterials.

In this article, an innovative design of a broadband circularly polarized compact planar antenna for RFID (radio frequency identification) receivers is presented. The suggested structure used the concept of slots loaded parasitic element printed underneath a coplanar waveguide-fed radiator to achieve the circularly polarized (CP) radiation and size reduction. A parasitic element loaded with F-slot and L-slot, and a window-type slotted ground plane was used to achieve resonance at 2.45 GHz with right-handed circularly polarized radiation for RFID handheld reader application. Experimental results confirmed that the designed antenna of size 16.5 × 14.8 × 1.6 mm3 attained a −10 dB impedance bandwidth of 15.8% (from 2.330 to 2.716 GHz) and 3 dB AR bandwidth of 3.43% (from 2.410 to 2.494 GHz). An axial ratio of 0.43 dB was achieved in the boresight of the antenna at the 2.45 GHz RFID band. The concept of electric field distribution on the antenna was used to elaborate the excitation of CP radiation in the antenna.

In this paper, a ring patch antenna with 1 GHz of bandwidth and three resonances is designed for the RF energy harvesting application. Two complementary split-ring resonators (CSRR) is etched out from the patch, designed at 6.8 GHz, near feed to enhance the bandwidth from 267 MHz to 1 GHz. Due to the introduction of CSRR, two additional resonances at 6.4 and 7.4 GHz are obtained with gain 5.5 and 0.2 dB respectively. The gains at 6.8 and 7.4 GHz are enhanced to 8.3 and 2 dB respectively by cascaded circular mushroom electromagnetic bandgap structures (EBGs) with circular symmetry designed around the antenna. This ring patch antenna with cascaded EBG is integrated with a rectifier circuit based on voltage doubler topology (rectenna). The results of the rectenna show that, for an input power of 7 dBm to the rectifier circuit, a DC output voltage of 1.2 V with a power conversion efficiency of 40% is achieved.

In this research article, a metasurface (MTS)-loaded high-gain and broadband circularly polarized (CP) monopole antenna is reported. The proposed antenna configuration consists of a symmetric Y-shaped radiating monopole over a partial ground plane with extended twin parasitic conducting strips (PCS) loaded with a MTS reflector. To achieve left-hand circular polarization characteristics, a metallic copper strip is utilized to short the partial ground plane with one of the twin PCS [PCS(L)]. By using the grid-slotted sub patches on a rectangular MTS a reflector of 2λfa × 1.65λfa × 0.02λfa is placed just below the monopole radiator at a height of 0.33λfa, which provides broadened impedance (IBW) and 3 dB axial ratio bandwidth (ARBW) responses with high gain. The proposed prototype with an volumetric dimension of 1.33λfa × 0.9λfa × 0.02λfa at fa = 5 GHz is designed and characterized. It exhibits a measured IBW of 48.45% (3.57–5.89 GHz), ARBW of 25.25% (4.21–5.42 GHz), and CP gain of > 8.35 dBic with the antenna efficiency of > 75% in the desired operating frequency bands. The obtained performances of the proposed MTS antenna confirm its suitability for RF energy harvesting application.

A printed ultra-wideband (UWB) multiple input multiple output antenna with isolation enhancement and pattern diversity is presented, which is based on characteristic modes (CM) for upcoming wireless mobile platforms. The presented antenna is composed of dual UWB antenna elements (AEs) – an antipodal Vivaldi and a circular-shaped monopole positioned on the two opposite edges of the antenna's rectangular-shaped ground surface. The two AEs are capable to stimulate distinct modes in the ground surface and therefore obtaining the required distinct radiation (far-field) patterns and highly isolated AEs without introducing any decoupling structure (DS). The measured results demonstrate that both AEs are UWB with an operating frequency from 2.6 to 11 GHz. The measured isolation is more than 30.1 dB with a maximum value of 55 dB in UWB. The peak realized gain is over 0.83 dBi. The radiation efficiency is greater than 75%. The Envelope Correlation Coefficient is <0.08.

This paper presents a miniaturized, tunable, high-power, eight-port hybrid coupler, based on a lumped element hybrid coupler topology. The 90° hybrid couplers are ubiquitous elements used for feeding RF coils in quadrature in magnetic resonance imaging (MRI) systems. Due to the low Larmor frequency (64 MHz) of 1.5 T MRI, distributed elements are too large for practical circuits to drive multi-port RF coils. Thus, miniaturization with MRI-compatible, non-magnetic, and high-power components is necessary. First, a miniaturized hybrid coupler is proposed for MRI systems with non-magnetic variable capacitors. Afterwards, the miniaturization methodology is applied to develop an eight-port coupler, capable of supporting both a transmit and a receive quadrature RF coil system. The high-power (up to 1 kilowatt) extended coupler measures 10 cm × 6 cm. Test results show that each port has a return loss of more than 16 dB, each input-isolated port is isolated by more than 24 dB, and each output has an insertion loss of less than 2.5 dB and output phases of 0.0°, 90.8°, 182.1°, and 278.7°.

In this paper, a simple and compact coplanar waveguide (CPW)-fed four coexisting inverted-L-shaped radiating elements based wideband (ILW) multi-input and multi-output (MIMO) antenna has been presented for wearable wireless applications. The 50 Ω CPW feed structure has been used to excite the four inverted-L-shaped radiators in a wide square slot of the proposed four-port wearable MIMO antenna. The four diagonal strips are extended from the corners of the wide square slot to form a square split ring structure in the center, which suppresses the coupling of electromagnetic waves among coexisting compact quad inverted-L-shaped radiators. The proposed ILW wearable MIMO antenna achieves a wide −10 dB simulated impedance bandwidth of 58.18% (3.9–7.1 GHz) and more than 30 dB isolation along with much reduced envelope correlation coefficient (<0.0002), high diversity gain (>9.9 dB), and specific absorption rate lower than 1 W/kg in the entire operational band. The simulated results are in good agreement with the measured ones. This ensures its utility for several wearable wireless applications in the C-band that comprises WLAN (5 and 5.2 GHz), Wi-MAX (5.5 GHz), and ISM band (5.8 GHz) applications.

Multiple input multiple output (MIMO) systems, which use multiple antennas to deliver faster data rates, are one of the promising methods in 5G services. 5G is a popular issue among the world's main telecom firms currently. The sub-6 GHz band for 5G applications in various countries lies between 3 and 5 GHz. The sub-6 GHz 5G bands are 3.4–3.8 GHz in Europe, 3.1–3.55 GHz in the USA, and 3.3–3.6 GHz and 4.8–4.99 GHz in China. This paper presents a two-element slotted octagon-shaped antenna operating in the sub-6 GHz band (3.1–4.5 GHz) for 5G applications. A T-formed isolation structure is placed at a ground plane to minimize mutual coupling between MIMO antennas. The proposed MIMO antenna has physical dimensions of 55 × 38 mm2 and an envelope correlation coefficient or correlation of 0.0004 over the entire operating band. The antenna operates at 3.6 GHz, with a return loss of 40.8 dB at the resonance. An antenna prototype has been investigated and proven to be of excellent quality in terms of performance like isolation >20 dB, efficiency >80%, and mean effective gain <−3 dB over the full operating band.

A flexible and high gain saw-tooth-shaped boundary fractal wearable antenna is anticipated for WBAN applications. A square shape is converted to the saw-tooth-shaped boundary by sequentially rotating the square at an angle of 45° and overlaid on one another and repeated the same for three iterations. The geometry is built on a flexible jeans substrate, and the total size of the anticipated antenna is 0.583λ × 0.583λ × 0.02λ at 2.4 GHz. A prototype has been made to test its suitability for wearable applications' gain, bandwidth, and efficiency. The results revealed that the proposed antenna provides an impedance bandwidth of 4.2% and a peak gain of 5.76 dBi at the 2.45 GHz ISM band. The anticipated antenna is wrapped around foam cylinders of radii 70 and 50 mm to test the stability and deformability of its performance. The full ground plane isolates the antenna from the human body contributing to the lower SAR values. The simulated maximum achieved specific absorption rate (SAR) of 0.3025 and 1.16 W/kg for 10 and 1 g of average tissue for the input power of 100 mW, respectively, found within the standards as per FCC. The peak gain, compact size, and SAR permanence made the anticipated antenna a worthy candidate for wearable on/off body applications.

An 8 × 8 butler Vivaldi beam-steering antenna array is introduced for X-band application. Circularly polarized array is made of eight Vivaldi elements with 1×8 platform and an 8×8 beam feeding network. Vivaldi radiating element is used in array form to enhance impedance bandwidth and overall efficiency such as gain. Using microwave passive components, for instance, 3 dB branch line couplers and crossover help to have orthogonal modes for feeding each of the elements, hence circular polarization property is achieved. Extracted results show that the array has an impedance bandwidth over 7–12.7 GHz (~58%) for VSWR≤2 and an axial-ratio bandwidth of 3.15 GHz that is between 8.15 and 11.3 GHz (~33%). The peak gain of antenna array is 18 dBi at 10 GHz. The proposed beam-steering antenna with compact size and good operation is capable to cover an angle range from −42 to 55 degree in whole operation frequency.

The meta-surface is introduced into the design of band-notched monopole antennas with single/dual/triple notched bands around 3.6/5.5/7.2 GHz for ultra-wideband (UWB) communication. We show that strong coupling effects between the meta-surface and the antenna would be created through tuning the orientations of the meta-cells according to the field distributions on the antenna surface, and eventually, filter the unwanted radiations of the antenna. Especially, by simply attaching different meta-surfaces on the back of the antenna, the band-notched properties are shown to be changeable without hurting the original structure of the antenna. Both simulation and measurement results demonstrate the monopole with replaceable meta-surfaces has achieved an UWB operating band ranging from 3 to 9 GHz for voltage standing wave ratio (VSWR) ≤ 2 with single/dual/triple notched bands covering frequencies around 3.6/5.5/7.2 GHz. Such a design would obtain significant notched performances for the desired bands and maintain the good radiations in the operating frequency, which make it a suitable candidate for UWB applications.

This article concerns the design of a new quad-band monopole antenna with a specific configuration and reduced size. The antenna design is based on a single step meander-line parasitic structure, parasitic stubs, and double inverted L-shaped defected microstrip structure units on the top side of the substrate, and double rectangular-shaped defected ground structure units along with partial ground plane on the bottom side. This design procedure allows to achieve quad-band characteristics and improve the impedance matching. The fabricated antenna prototype with overall dimensions of (0.24λ0 × 0.17λ0 × 0.013λ0) is operating at 2.55, 3.65, 4.65, and 5.8 GHz with fractional bandwidth about 7.7, 12.84, 9.23, and 12.8%, respectively. The antenna exhibits an omnidirectional and a monopole like radiation patterns in the H- and E-planes, respectively, with suitable gains between 2.1 and 3.75 dBi. The measurement results are in good agreement with simulation values, which indicates that the proposed antenna design is suitable for WiMAX, LTE B7 (2.52–2.75 GHz), Sub-6 GHz 5G (n77) (3.39 – 3.87 GHz), Sub-6 GHz 5G (n78) (4.4–4.86 GHz), and WLAN, LTE B46 (5.1–5.82 GHz) wireless applications.

An innovative and simple method for attaining broadband frequency reconfigurable antenna structure is presented using low-cost materials and a compact design. The frequency reconfigurability is attained by the “OFF” and “ON” mechanisms of three PIN diodes. Many performance observations are carried out such as reflectance coefficient, bandwidth (BW), resonance frequency, electric field, feed position variation, and gain among different configurations. The suitability of the presented work for the different applications lies in the X frequency band. The resonating frequency for all switch OFF modes is achieved at 13.2 GHz and one switch ON mode at 10.7 GHz. The proposed design yields the frequency tunability behavior over the broadband of 2.5 GHz. The design yields the directivity of 5.58 dB, the minimum reflectance coefficient of −17.27 dB, and a total gain of 3.87 dB. This design offers an electric field of 46 558 v/m, a BW of 340 MHz, and a normalized directivity of 87° using low-cost substrates. The results of the presented design were also fabricated and related to simulated results. Performance observation of the proposed work with previously published research work is also included. The presented design provides the solution of the low-cost, compact, reconfigurable antennas, which can be used for next-generation communication systems.

The major component for the effective functioning of airborne unmanned aerial vehicles (UAVs) is an antenna. The unified integration of an antenna with UAVs without disturbing the other components of UAVs, compact size, thin, and wideband antennas are preferred. Based on these prerequisites, a low-profile and a wideband antenna is presented for autonomous UAVs for C-band applications. The antenna is designed on an extremely thin substrate of size 0.01λ0 without compromising the antenna's broad bandwidth. The antenna structure comprises a corrugated U-shaped patch, a few parasitic patches along with the partial ground. Initially, only a single mode was excited using a U-shaped patch, then further on introducing a few more parasitic patches along with corrugations in a U-shaped patch other modes were also coupled leading to enhancement in the bandwidth of the antenna. The proposed antenna along with a 2 × 2 quad-port multiple input, multiple output (MIMO) antenna is designed and fabricated. The measured results are found to be in good agreement with the simulated results. A broad bandwidth of 74.5% (4.1–9.0 GHz) with 5.02 dBi peak gain at 6.6 GHz is exhibited by the fabricated MIMO antenna. Thus, the designed antenna proves to be an effective candidate for UAV-based C-band applications.

This study presents a dual-circularly polarized (CP) Fabry–Pérot (FP) antenna, employing a novel receiver–transmitter (RT) metasurface (MS). The RT-MS unit cell consists of two identical neighboring substrates, with a three-layer metal coating printed on their surfaces. The bottom patch is adopted as a receiver to transfer electromagnetic waves to the top-corner cut patch, passing through the coupling cross-slot sandwiched in the middle. The RT-MS has high reflectivity to achieve a high gain. Through energy and a cross-slot, high aperture efficiency can be realized. A conventional corner cut patch can excite a CP mode of equal magnitude and a 90° phase difference. The RT-MS is arranged in 12 × 12 unit cells and used as a superstrate for a dual-CP antenna. Two orthogonally etched slots fed by two branch-matched orthogonally arranged feed lines are used as feeders to produce perpendicular linearly polarized waves. To enhance the bandwidth and improve the gain, double identical stack substrate patches are placed at the top side of the slot with no air gap, for a wide impedance band and high gain. Two wide CP bands, left-hand circular polarization and right-hand circular polarization, of 12.21–13.1 GHz (7.03%) and 12.35–13.1 GHz (5.89%), respectively, have maximum high gains of 16.5 and 15.97 dBic at 12.58 and 12.7 GHz, respectively, with a compact size of 2.6λ0 × 2.6λ0, suggesting better properties than recent antennas. The aperture efficiency can reach 63.2%. Thus, the RT-MS-based FP antenna is a good candidate for commercial and military communication systems.

Dielectric resonator antennas (DRA) are advantageous due to their small size, high radiation efficiency, ease of excitation, and minimal metallic losses. Enhancing the gain of DRA will make them suitable for a variety of applications such as wireless communication technology. High-gain antennas are required for wireless communication to overcome the difficulties of high path loss. The researchers have presented numerous techniques, including stacked DRA, DRA array, and higher-order mode excitation for enhancing the gain. This article provides a comprehensive summary of the state of art techniques and geometries including stacking, frequency-selective surfaces superstrate, electromagnetic band gap structures, hybrid techniques, arrays, etc., for increasing DRA gain. Techniques to improve other parameters like bandwidth and radiation efficiency along with gain are also discussed. The challenges in each technique and the possible applications are presented. Based on the studies, it is observed that the DRA array technique is suitable for obtaining gains up to 19 dBi for radar-based applications. For low-profile DRA requirements, hybrid DRAs prove prolific in enhancing gain up to 16 dBi.