We present a higher-order spectral element method (SEM) for analyzing eccentric anisotropic multilayered waveguides. The formulation uses conformal transformation optics to map the original eccentric waveguide to an equivalent concentric one. This transformation is extended to handle anisotropic non-reciprocal media characterized by non-symmetric and non-Hermitian tensors. In the transformed concentric domain, higher-order two-dimensional basis functions associated with the zeros of the completed Lobatto polynomial are used to expand the fields. To simulate radially unbounded problems, we employ a complex-stretched perfectly matched layer to mimic open space. The method is validated with radially bounded two-layer and three-layer waveguides and radially unbounded three-layer waveguides. Comparisons with the finite element method (FEM) demonstrate that our SEM approach requires significantly fewer degrees of freedom than FEM.

Reconfigurable intelligent surfaces (RISs) enhance the performance of wireless communication networks, particularly within millimeter wave (mm-Wave) bands. When a line-of-sight link is not strong enough or is fully blocked. The location of RIS has a significant impact on the RIS’s wireless channel and system performance. A wireless communication model has been proposed in a mm-Wave environment supported by RIS. The proposed model contains one transmitter and five users at the receiving end. Due to the small distance between users, there is interference between them and the received signal-to-interference-plus-noise ratio (SINR) decreases. Three RISs separated by different interspace distances were proposed to serve users at various distances from the transmitter to reduce inter-user interference. The simulation results showed that increasing the distance between the RIS site and the TX-User line served a larger number of users, and the three heights of the RIS provided a coverage range domain complementary to each other for different user sites. The improvement percentages in SINR for the second and third RIS are 48.46% and 77.38%, respectively. Enlarging the size of the RIS only increases the signal capacity and does not affect the coverage range domain of the single RIS.

This paper introduces a three-substrate layered transmitarray design that avoids the use of vias, aiming to produce broadband orbital angular momentum (OAM) vortex beams within the Ka-band. The suggested element configuration accomplishes a full 360∘ transmission phase while upholding a 1-dB transmission loss, with an overall thickness measuring 3.4 mm (equivalent to 0.34λ0 at 30.0 GHz). Its balanced unit cell arrangement amplifies its effectiveness in applications involving dual polarization. We examine the transmitarray behavior across four OAM modes (+1, +2, +3, +4 ), unveiling notable mode purity at operating frequency. Specifically, a broadband OAM vortex beam is achieved for the +1 mode during simulation. A square aperture transmitarray fed by a horn antenna is fabricated and measured to validate these simulated findings. Experimental results confirm the successful broadband vortex beam generation for $l = +1$ mode across the frequency spectrum from 27.0 to 40.0 GHz, approximately 43.3%. Additionally, the proposed transmitarray achieves a peak gain of 21.7 dBi, accompanied by an 11.8% aperture efficiency. Noteworthy is the consistent maintenance of mode purity above 86%.

In this article, we present a simulation tool for modeling a quasi-optical bench for material characterization. The model uses a Gaussian beam expansion and tracking analysis, together with a modal analysis to enable a comparison of the simulated reflection and transmission S-parameters to the ones measured with a 4-port vector network analyzer. A Thru-Reflect-Line calibration is performed to de-embed the simulated S-parameters of a dielectric slab located between two lens antennas, showing good agreement with the analytical slab model used for experimental permittivity extraction.

This paper presents a comprehensive analysis of interference events in automotive scenarios based on radar systems equipped with communication-assisted chirp sequence (CaCS). First, it examines the impact of interference on radar and communication functionalities in CaCS systems according to the orientation of the investigated nodes. For this purpose, a graph-based approach is employed with MATLAB simulations to illustrate the potential occurrence of interference on the graph for communication functionality compared with their counterparts on radar. Second, the paper delves into the impact of interference on the synchronization between two communicating CaCS nodes. It extends a previous study to match the frequency of current radar sensors, where chirp estimation, an adjusted version of the Schmidl & Cox algorithm, and correlation are adopted to synchronize the transmitter and receiver of two CaCS communicating nodes in the time-frequency plane. The proposed synchronization method is finally verified by measurements at ${79}\,\mathrm{GHz}$ with a system-on-chip, where the resulting correlation metric and mean square error are illustrated as validation factors.

A compact 8-port eighth-mode substrate-integrated waveguide (EMSIW) multiple-input multiple-output (MIMO) antenna is presented in this paper. It consists of eight EMSIW cavities placed side by side sharing their open-ended edges which are separated by rectangular slots. High isolation (>22 dB) between the antenna elements over the entire operating band is obtained by the strategic placement of rectangular slots and vias. The open-ended region of EMSIW cavity resonator and the edges of the diagonal slots help in the excitation of TE110 mode at 5.5 GHz. The simulated bandwidth of MIMO antenna is 180 MHz, while the measured bandwidth is 220 MHz. The proposed MIMO antenna system has potential applications for sub-6 GHz communication systems.

This invited, extended, paper compares and contrasts a number of different near-field (NF) to far-field (FF) transformation algorithms that can be used for the purpose of processing NF data acquired using multi-axis industrial robots. The merits and limitations of these various, commonly encountered algorithms are highlighted with comparison FF data presented across a frequency range spanning 3–15 GHz. Crucially, the paper explores the viability of using mixed mode acquisition geometries when performing antenna gain measurements where, prior to this work, several of the transforms yielded different transform gains, and electrical lengths. Here, we verify that at 8 GHz and above, where truncation effects were minimal, for a circa 30 dBi gain (at 8 GHz) test antenna the FF peaks were in agreement to better than ±0.02 dB, at 3σ irrespective of the acquisition geometry and transform algorithm used. In this invited, extended work, the existing simulation results are augmented with experimental results obtained from planar and spherical NF measurements of a pyramidal horn taken using a dual robotic antenna measurement system and a consistent distributed RF subsystem.

In this paper, a novel polarization conversion metasurface (PCM) is proposed. Compared with the conventional receiver-transmitter metasurface units, two metallized via holes are set up to correct the current. It can achieve better polarization conversion from linear to circular and maintain a high reflectivity performance. A patch antenna with an L-probe feed is used as a feeder. The circularly polarized Fabry–Perot resonator antenna (CP-FPRA) consists of the PCM with a 5 × 5 array and a feeder. The measurements indicate a 3 dB axial ratio (AR) bandwidth of 8.6% (22.3–24.3 GHz). And it achieves a maximum gain of 14.2 dBic at 24 GHz, compared to the feed antenna has a gain enhancement of 5.5 dBi (from 8.7 dBi to 14.2 dBic). The proposed CP-FPRA has high gain, a wide AR, and a relatively low profile, providing ideas for subsequent antenna designs.

In this paper, a large, compact array antenna that can be expanded in the 2-D plane is proposed for near-field radio frequency identification applications. By the introduction of the fractal structure and corner joint method, the array is easy to expand in the 2-D plane. An antenna element can be divided into a dozen or so loops, and traveling wave distribution makes sure that every loop is excited in a time period. So that a strong and uniform magnetic field could be generated in a large area. As a proof of concept, array antennas with $1 \times 8$, $2 \times 4$, and $3 \times 3$ elements are designed, fabricated, and measured. The measured bandwidth of the antennas covers the entire Chinese standard. Reading distances of the proposed large array antennas achieved up to 57 mm. Results show that the proposed antenna could realize flexibility and extendibility in a large area with stable and uniform magnetic field distribution.

A long-range, compact, wideband Ultra High Frequency (UHF) radio frequency identification (RFID) tag is proposed. The design features a triangular patch, designed to achieve wideband impedance matching. The tag exhibit −10 dB bandwidth from 753 to 917 MHz with a substrate thickness of 1.575 mm. The overall size of the tag is 121 × 30 × 1.575 mm3. The gain and bandwidth for varying substrate thickness is investigated. Further, two parasitic strips are designed alongside the patch to operate the tag more effectively at Federal Communications Commission (FCC) band in simultaneous with European Telecommunications Standards Institute (ETSI) band with increased read range. With parasitic strips, the tag exhibits −10 dB bandwidth from 766.5 to 896.5 MHz and 905 to 943.6 MHz which covers the bandwidth requirements in Europe, Asia, and North America. A prototype of the proposed tag is fabricated and experimentally tested with the RFID Higgs 4 IC. The tag provides a read range of over 14 m in 865–870 MHz and 13 m in 902–928 MHz when the tag is operated in free space and a read range of over 10.2 m in 865–870 MHz range and 10.3 m in 902–928 MHz range when mounted on a metallic surface of size of 200 × 200 m2 . The performance parameters of the designed tag are also compared with some commercially available designs.

A single-layer polarization converting metasurface (PCMS) with wideband is presented for polarization conversion and radar cross-section (RCS) reduction. The proposed PCMS is composed of metallic biconic shape resonator imprinted on a metal-backed F4BM dielectric substrate of relative permittivity 2.2 and loss tangent 0.001. The unit cell has a compact size of 0.16$\lambda_\mathrm{o} \times 0.16\lambda_\mathrm{o}$. A comprehensive parametric analysis, angular sensitivity study, bistatic and monostatic RCS analysis are conducted by illuminating the proposed PCMS using linearly polarized (LP) plane waves. The PCMS converts LP electromagnetic waves to their orthogonal polarization state in the frequency band of 8.7–24.8 GHz resulting in polarization conversion ratio over 90% with a fractional bandwidth of 96%. Additionally, the developed structure is applied in chessboard configuration, using phase cancellation techniques for RCS reduction, that achieve 10 dB RCS reduction across a wideband of 7.9–23.4 GHz. The unit cell and its rotated version has a cross-polarization reflection phase difference of (${180}\pm {37}{^\circ}$) in the operating band, which fulfill the criteria for RCS reduction. The chessboard configuration exhibits a scattering pattern with four strong lobes that deviates from the normal incident path because of the phase cancellation in normal direction. The experimental results are in good agreement with the simulated result. Applications for the developed structure include antenna design (gain enhancement and beam steering), target hiding, imaging, and microwave communications.

This work describes the design process of a single-pole double-throw (SPDT) microwave switch operating at Ka-band. It is tailored to a tunable reflective termination design that can be used in tunable power amplifier configurations. A high electron mobility transistor and a resonating network are employed in shunt configuration to enhance the performance in the output port’s active and inactive conditions. The small and large signal measurements showcase a 2 GHz bandwidth with an insertion loss and isolation better than −1.8 dB and −25 dB, respectively, and handling power levels of up to 3 W at 30.5 GHz. The load-pull measurements across the entire Smith chart offer comprehensive insights into the behavior of the SPDT when operating with complex and reactive loads, fulfilling the purpose of tunable reactive termination.

In order to recover the magnetic energy that leaks from an induction cooktop, this study suggests a straightforward and cost-effective magnetic-field energy harvester (MFEH) circuit. With the aid of the intended circuitry, the acquired magnetic energy is transformed into DC electrical energy. We harvested the magnetic-field energy (MFE) from the induction cooktop at various heights and locations of the energy harvesting coil. With a load resister of 46.6 Ω and a capacitance of 1 mF, the developed MFEH circuit, when positioned 2 cm beneath the cooktop, can capture an average DC power of 1936 mW. Placing the energy-harvesting coil 7.5 cm beneath the induction cooktop lowers the power output to 142 mW. For a range of load resister values, the MFE gathered at various locations along the energy harvesting coils is examined. Batteries can be used to store the gathered energy for later use. Additionally, the suggested device is shown to be capable of wirelessly powering low-power Internet of things devices and charging mobile devices. The suggested device differs from the previously published magnetic harvesting circuits due to the induction cooktop’s superior performance and capacity to gather MFE.

A novel microstrip filtering antenna with slot coupling feed is presented in this work. An asymmetric interdigital coupling structure is used for the feed to excite the patch antenna through gap-fed coupling. Introducing a U-shaped slot on the patch surface modifies the current path to attain different resonant modes. The asymmetric coupled fingers in the low-frequency band generate a radiation null of −64 dBi, while additional resonances introduced in the high band broaden the bandwidth from 4.9 to 5.3 GHz. A horizontally shorted microstrip branch produces another null point of −28 dBi between the bands, enabling steeper roll-off and further improvement in frequency selectivity. The proposed filtering antenna provides a flexible filter response without requiring extra filtering circuits, with appreciable peak gains (6.1 dBi and 7.1 dBi) and stable radiation characteristics. This makes it suitable for WLAN (Wireless Local Area Network) and ISM (Industrial Scientific Medical) systems applications.

In this article, a coupled line diplexer (operating at 2.4 GHz and 3.5 GHz) which can be used as single-band filter with tunable attenuation characteristics in the pass band has been designed. Multilayer graphene (MLG) pads are used to achieve tunable features in this circuit. The graphene pads are placed at each branch of the diplexer. Single-band tunable attenuation characteristics are achieved by applying bias to graphene pads placed at optimum locations on the filter. The proposed tunable coupled line attenuating diplexer is realized on FR-4 glass epoxy substrate of thickness 1.58 mm with a total size of 45 × 75 mm2. By varying the bias voltage (0 V –6 V) of MLG pads the resistance of graphene pad placed in the circuit gets decreases thereby attenuating/controlling the transmission power to the other port in the required band. In lower pass band (2.28–2.55 GHz) the signal is attenuated from 3 to 10.8 dB and in higher pass band (3.2–3.58 GHz) signal is attenuated from 5 to 13 dB. Simulations of the structure with and without graphene pads have been carried out and are in good agreement with measured results.

A compact microstrip eight-channel diplexer based on quad-mode stepped impedance resonator (QMSIR) is proposed in this paper. The proposed diplexer is composed by two second-order quad-band bandpass filters (BPFs) and common-port distributed coupling matching circuit. Each quad-band BPF is formed by two coupled-QMSIRs controlling the passband characteristics. By introducing multiple coupling paths between input and output ports, the isolation between the eight channels is performed. For demonstration, an eight-channel diplexer based on QMSIR is designed and fabricated with microstrip technology. The use of the QMSIR can lead to significant size reduction for the multiplexer, this is because the required resonator number is reduced. As a result, the diplexer occupies a compact size of 0.083λ2, which is smaller than most of the eight-channel diplexers that have been proposed. And the 3 dB fractional bandwidth is 97% (2.5–7.2 GHz). Measurement results correlate well with the simulated predictions, showing that a good isolation of better than 20 dB and upper stopband of better than 10 dB.

A four-port ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna with three notch bands is proposed in this work. The antenna uses ultra-thin flexible material liquid crystal polymer (LCP) as the substrate. Four identical monopole radiators are designed in this proposed antenna system. The notch bands of the antenna are generated by adding complementary split ring resonator (CSRR) and ring branch. A cross-shaped stub is set in the center of the four antenna units to enhance the isolation. The measured bandwidth of the antenna is 2.54–10.69 GHz, filtering out three notch frequency bands of 2.81–3.85, 5.11–5.98, and 7.34–8.69 GHz. The isolation in the entire working frequency is better than 22 dB. The bent performances of the MIMO system and the specific absorption rate (SAR) value are analyzed. The low SAR values, low envelope correlation coefficient (<0.05), high diversity gain (>9.999), and stable gain of the proposed antenna indicate that in UWB-MIMO systems and wearable Internet of Things applications, it can be widely used.

In this paper, a novel dual-mode spherical resonator is proposed. By rotating the coupling irises, perturbations are generated to split the degenerate modes. The proposed filter is cascaded by a dual-mode resonator and two single-mode resonators, which are placed in a rotary way. Four poles appear in three resonators by only changing the rotation angle (φ) without any additional design. It forms the BSCT, generating a transmission zero (TZ) at the upper stopband. Furthermore, by adjusting φ further, the CT coupling topology can be obtained, resulting in a TZ at the lower stopband. Finally, slots are etched on the surface of the resonators for spurious response suppression. With the above methods, the out-of-band selectivity and suppression are greatly improved. For the fast validation, the filter is 3-D printed and measured. As a result, the measured results match well with the simulated ones.

In this paper, on–off switching digitization of a W-band variable gain power amplifier (VGPA) with above 60 dB dynamic range is introduced for large-scale phased array. Digitization techniques of on–off switching modified stacking transistors with partition are proposed to optimize configuration of control sub-cells. By the proposed techniques, gain control of a radio frequency variable gain amplifier (VGA) could be highly customized for both coarse and fine switching requirements instead of using additional digital-to-analog converters to tune the overall amplifier bias. The designed VGA in 130 nm SiGe has achieved switchable gain range from −46.4 to 20.6 dB and power range from −25.0 to 15.7 dBm at W band. The chip size of the fabricated VGPA is about 0.31 mm × 0.1 mm.