This work delves into advancements in wireless power transfer (WPT) and radiofrequency (RF) energy harvesting (EH), focusing on on-demand beamforming and wide-dynamic power range technologies. These innovations promise significant improvements in efficiency and adaptability for wireless energy systems. For transmitting RF power, on-demand beamforming enhances power delivery precision by accurately targeting specific devices, minimizing energy waste, and maximizing received power. This technology is particularly useful in dynamic environments with constantly changing device positions, ensuring stable power levels and effective real-time power transfer, such as for mobile device charging. For receiving RF power, wide-dynamic power range implementation allows EH and WPT systems to adjust power output across a broad spectrum, optimizing energy use and extending device lifespan. This capability supports scalability, accommodating devices with varied power needs, also enabling new applications in consumer electronics, healthcare, smart homes, and cities, and enhancing energy management in smart infrastructures. Additionally, this study explores three-dimensional (3D)-printable antennas and RF circuitries for battery-free applications. The versatility of 3D printing allows the creation of complex, efficient, and customizable RF components, fostering innovative battery-free solutions. Integrating on-demand beamforming and wide-dynamic power range technologies in EH systems promise improved energy transfer efficiencies, reduced losses, and sustainable, cost-effective wireless power systems.

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.

This manuscript presents a novel design for an absorptive bandpass filter for mm-wave applications, specifically the commercial FR2 spectrum. Three bands have been selected to be properly input matched with only one of them being the passband, where the insertion loss is minimized. The proposed approach relies on a multiplexer topology implemented through microstrip lines and on thin-film manufacturing process on alumina to shrink the footprint. Cascades of half-wavelength C-shape open-ended resonators are exploited to create the matched bands and define the filter’s selectivity. The selected passband spans from 26.5 to 28.5 GHz, with a measured maximum insertion loss of 3.05 dB for a −3 dB fractional bandwidth of 7.3%. Two absorptive bands are realized to match signals at 24 and 32.25 GHz. The alumina die footprint is 5500 × 3440 µm2, compatible with immediate integration within a mm-wave lineup.

Development of large-scale quantum computing systems will require radio frequency (RF) and microwave technologies operating reliably at cryogenic temperatures down to tens of milli-Kelvin (mK). The quantum bits in the most promising quantum computing technologies such as the superconducting quantum computing are designed using principles of microwave engineering and operated using microwave signals. The control, readout, and coupling of qubits are implemented using a network of microwave components operating at various temperature stages. To ensure reliable operation of quantum computing systems, it is critical to ensure optimal performance of these microwave components and qubits at their respective operating temperatures, which can be as low as mK temperatures. It is, therefore, critical to understand the microwave characteristics of waveforms, components, circuits, networks, and systems at cryogenic temperatures. The UK’s National Physical Laboratory (NPL) is focussed on developing new microwave measurement capabilities through the UK’s National Quantum Technologies Programme to address various microwave test and measurement challenges in quantum computing. This includes the development of various measurement capabilities to characterize the microwave performance of quantum and microwave devices and substrate materials at cryogenic temperatures. This paper summarizes the roadmap of activities at NPL to address these microwave metrology challenges in quantum computing.

A 3D-printed polarization-independent low-cost lightweight and flexible frequency selective surface based dual-band microwave absorber is presented in this paper. Two concentric square loops fabricated at different heights using 3D printing technology are responsible for exhibiting dual-band responses at 3.32 GHz (S-band) and 5.46 GHz (C-band) with more than 97% absorptivities. The corresponding full widths at half maximum bandwidths are observed as 230 MHz (3.21–3.44 GHz) and 450 MHz (5.27–5.72 GHz). The proposed topology is polarization-insensitive owing to the four-fold symmetry. The absorption phenomenon is explained with the analysis of current distributions at the surface and impedance curves at the frequencies of resonance. Further, the performance has been evaluated for both planar and curved surfaces with different angles of curvature, and the good agreement between the measured and simulated responses confirms the flexible behavior of the proposed structure.

This paper presents a methodology designed to leverage multitemporal sequences of synthetic aperture radar (SAR) and multispectral data and automatically extract urban changes. The approach compares results using different radar and optical sensors, describing the advantages and drawbacks of using SAR data from the COnstellation of small Satellites for the Mediterranean basin Observation (COSMO)/SkyMed, SAtélite Argentino de Observación COn Microondas (SAOCOM), and Sentinel-1 constellations, as well as nighttime light data or Sentinel-2 images. Multiple indexes obtained from multispectral data are compared, too, and results obtained by an unsupervised clustering procedure are analyzed. The results show that using different datasets it is possible to obtain consistent results about different types of changes in urban areas (e.g., demolition, development, and densification) with different levels of spatial details.

Adrenal gland-induced hypertension, also known as secondary hypertension, is a medical condition caused by an underlying adrenal pathology, most typically adrenocortical adenomas. Current clinical practices involve pharmacotherapy or surgical resection to treat adrenal gland diseases that cause hypertension. However, due to the limitations of these treatment options, microwave ablation (MWA) has emerged as a promising minimally invasive alternative. An accurate understanding of the dielectric properties of adrenal glands would support the further development and optimization of MWA technology for treating adrenal tumors. Only a few studies have examined the dielectric properties of both human and animal adrenal glands, and the sample sizes of these studies have been relatively small. Therefore, further dielectric data of human and animal adrenal glands are warranted. This paper presents the ex vivo dielectric properties of the ovine adrenal glands (medulla and cortex) and summarizes the published literature on dielectric data of adrenal glands from porcine, bovine, ovine, and human samples in the microwave frequency range to analyze the consistency and reliability of the reported data. The dielectric properties of the ovine adrenal glands (N = 8) were measured using an open-ended coaxial probe measurement technique at frequencies ranging from 0.5 to 8.5 GHz. This study also investigated the temperature-dependent dielectric properties of the ovine adrenal medulla ranging from 37 to 64°C at frequencies ranging from 0.5 to 8.5 GHz. The dielectric properties of the ovine adrenal medulla measured in this study were found to be consistent with the literature. Moreover, the review suggests that variations exist in the dielectric properties of the adrenal medulla and cortex among species. The study also found that the dielectric properties of the adrenal medulla decrease with increasing temperature, similar to other tissues for which temperature-dependent dielectric data have been reported. This summary of dielectric data of adrenal glands and the temperature-dependent dielectric properties of the ovine adrenal medulla will accelerate the development of MWA technologies for hypertension treatment.

This article presents the design of a dielectric lens antenna that utilizes the concept of a stepped Fresnel lens for focusing electromagnetic millimeter waves. Based on the quasi-optical properties of these waves, a Cartesian Oval is optimized and employed as a focusing lens. Multiple such lenses are combined to two different Fresnel-based lens antennas. We survey these newly designed lens antennas and compare them with a focusing lens antenna based on a Cartesian oval and a far-field lens antenna. Simulations and measurements with a frequency-modulated continuous-wave (FMCW) radar validate the effectiveness of the new design, demonstrating an even improved focus size while significantly reducing the size and weight of the lens antenna by up to 53% and by nearly 48 %, respectively. Additionally, the Fresnel-based lens antennas reveal a frequency dependency, enabling frequency-based steering of the focal length over a wide relative tuning range of 177%, which we thoroughly investigate for various bandwidths and center frequencies.

We introduce a systematic approach for designing ultrathin, flexible, and polarization-insensitive metasurface absorbers (MSAs), suitable for aviation applications, such as radar cross-section reduction of unmanned aerial vehicles. Metal-backed resistive patches are arranged on a flexible polyethylene terephthalate substrate of thickness about 1/100 of the operating wavelength, classifying the absorbers as ultrathin. The ultralow weight of the proposed MSAs is crucial for the targeted aviation applications, to ensure airworthiness. A narrowband uniform MSA is designed to achieve maximum absorption and serves as a starting point to synthesize a broadband and polarization-insensitive $3 \times 3$ absorber supercell. The non-uniform absorber is systematically designed by a fast semi-analytical method. The proposed absorbers have been fabricated and experimentally tested both on flat and cylindrical curved surfaces, with measurements being in very good agreement with the corresponding simulations, and corroborate the high absorption and broadband behavior of the proposed non-uniform ultrathin and flexible absorber.

Radar absorption structures made of an active frequency selective surfaces (AFSS) have enormous potential in the aviation, naval, and other industries. In this research paper, a systematic review (SR) is carried out in the field of the AFSS to bring uncertainties, obstacles, challenges, classifications, applications, and design issues that arrive in the development of the sub-6 GHz architecture. To bias the AFSS component, as per the signal requirements, a unique set of circuits (PIN diode) is required, with ON and OFF state and a transmission zone. The bandwidth of which is determined by the bias voltage supplied. It can behave as a complicated hybrid impedance structure by providing ON and OFF biasing voltage to a PIN diode embodied in an FSS structure. Higher manufacturing costs of AFSS components, more significant complexities involved, a large amount of power consumption, and reactive impedance losses are some common limitations faced while implementing and designing an AFSS. Many envisioned problems are corrected with the AFSS design, current or creative implementations, and processing parameters are investigated progressively. It implies that new AFSSs will be an alternative to regular FSSs in the future. This paper is based on Kitchenham’s three-phase review procedure and supplements it with results, views, and recommendations from other leading experts in the field.

In this study, a novel, cost-effective miniaturized tag antenna was developed for applications on the human body. To achieve impedance matching with the complex conjugate impedance of the Monza-4 tag chip (7.17–j74.22 Ω at 915 MHz), the proposed structure was configured by coarsely tuning the positions of vias and fine-tuning the small gaps of its coupled patches. For further reducing the profile and dimensions of the antenna, a design technique based on the three-dimensional dipole antenna current distribution was used. The proposed antenna configuration was not only miniaturized but also achieved a long stable reading distance (>5.0 m) and a wide impedance bandwidth of 71 MHz or 7.65% (covering the ultrahigh frequency radio frequency identification ranges in most regions), regardless of the location of the tag on the human body. Experiments were conducted to validate the simulated results, and adequate agreement was found between the simulated results and the measured results.

A broadband ±45° dual-polarized base-station antenna based on crossed-dipoles with parasitic elements has been presented in this study. This antenna consists of printed dipoles fed by integrated baluns, parasitic elements, and a ground plane below them. As a result of using parasitic elements on the dipoles and creating a suitable coupling between them, the antenna’s impedance bandwidth has been improved, and its dimensions have been reduced. The experimental results show that the proposed antenna can cover the frequency band of 1.58–2.73 GHz with |S11| < −15 dB and isolation better than 17 dB. The measured peak gain for the proposed antenna in the frequency band is reported as 7.8 dB. Also, the antenna’s half-power beamwidth equals 62.15° ± 1.45°. The proposed antenna is fabricated with overall dimensions of 0.67λ0 × 0.67λ0 × 0.17λ0 and has been measured in the antenna laboratory.

Radar-based hand gesture recognition is a potential noncontact human–machine interaction technique. To enhance the recognition performance of hand gesture, a multidomain fusion-based recognition method using frequency-modulated continuous wave radar is proposed in this article. The received raw echo data of gestures is preprocessed to obtain the range–time matrix, Doppler–time matrix, and range–Doppler–frame tensor. The obtained three-domain radar data corresponding to each gesture are input into the three-channel convolutional neural network for feature extraction. In particular, the extracted features from three-domain data are fused with learnable weight matrices to obtain the final gesture classification results. The experimental results have shown that the classification accuracy of the proposed multidomain fusion network based on learning weight matrix-based fusion is 98.45%, which improves the classification performance compared with the classic average-based fusion and concatenation fusion.

The main interest in using synthetic aperture radar (SAR) technology in automotive scenarios is that arbitrarily long arrays can be synthesized by exploiting the natural motion of the ego vehicle, enabling finer azimuth resolution and improved detection. All of this is achieved without increasing the hardware complexity in terms of the number of physical antennas. In this paper, we start by discussing the application of SAR imaging in the automotive environment from both theoretical and experimental perspectives. We proceed by describing an efficient processing workflow and we derive the rough number of operations required to focus an image proving the real-time imaging capability of the system. The experimental results are based on open road data acquired using an eight-channel radar at 77 GHz, considering side-looking SAR and forward SAR. The results confirm the idea that SAR imaging can be successfully and routinely used for high-resolution mapping of urban environments in the near future.

This paper proposes an air-filled substrate integrated waveguide (AFSIW) bandpass filter with a miniaturized non-resonant node (NRN). The NRN structure is introduced between the three resonators, and its size is smaller than the resonator size, which can realize the NRN structure’s miniaturization and reduce the model’s size. The NRN size of this filter is 41% of the NRN size of the existing AFSIW filter. This filter also introduces a transmission zero (TZ) above the passband. The measured results show that the filter’s center frequency is 20.73 GHz, and the bandwidth is 0.86 GHz. The insertion loss in the passband is 0.95 dB, and the return loss is better than 23 dB. Due to the TZ in the upper stopband, the AFSIW filter obtained good selectivity.

In this article, a new microstrip dual-narrowband bandpass filter employing parallel-coupled transmission lines and open stubs is presented, investigated, and fabricated. The proposed dual-narrowband bandpass filter is analyzed and its exact scattering parameters are calculated, simulated, and measured. So, calculated scattering parameters offer a deep inside view of the performance of the proposed filter. To analyze the proposed filter, the even- and odd-mode excitation are utilized. The input impedance under even- and odd-mode excitation are achieved by transmission line theory and inserted in the scattering parameter equations. Finally, the accurate scattering parameters are derived and compared with simulation results. Simulation results prove the theoretical results. Then, an optimized proposed filter is fabricated and matched with simulation results. The center frequency bands are 4.5 and 6.8 GHz. The optimized filter occupies 0.12 $ \times $ 0.096$\lambda _g^2$, which is small. Its fractional bandwidth of the first and second passband are 1.5 and 2.5%, respectively. Furthermore, wide and strong rejection levels in the stopbands are offered. The structure of the proposed filter provides many freedoms to design. There is an agreement between experimental and simulation results.

Frequency-modulated continuous-wave radar systems profit from increasing the absolute bandwidths of the generated frequency chirps to improve range resolution. As the relative bandwidth of SiGe-voltage-controlled oscillators (VCOs) is limited to about 80%, increasing the center frequency fundamentally or via frequency multiplication is the most direct way to increase that absolute bandwidth. However, as some applications require penetration depth, which dramatically decreases with frequency, other solutions are necessary. Therefore, state-of-the-art concepts rely on the down-conversion of generated frequency chirps via two separately stabilized frequency sources. This article implements a novel architecture, offering relative bandwidths of >100% within a single phase-locked loop (PLL). Therefore, two VCOs at different center frequencies are fed into a down-conversion mixer, whose output is directly stabilized via that PLL with one loop filter generating both tuning voltages. Those circuit blocks can be summarized as one equivalent VCO, offering a higher relative bandwidth and a significantly more linear tuning curve. Thereby, a solution to limited relative bandwidths with high VCO gain variation of single VCO synthesizers is offered while substantially reducing the hardware and implementation effort compared to the state-of-the-art.

Liquid crystal microwave phased arrays (LC-MPAs) are regarded as an ideal approach to realize compact antennas owing to their advantages in cost, size, weight, and power consumption. However, the shortcoming in low radiation deflection efficiency has been one of LC-MPAs’ main application limitations. To optimize the steering performance of LC-MPAs, it is essential to model the channel imperfections and compensate for the phase errors. In this paper, a phase error estimation model is built by training a neural network to establish a nonlinear relationship between the near-field phase error and the far-field pattern, hence realizing fast calibration for LC-MPAs within several measured patterns. Simulations and experiments on a 64-channel, two-dimensional planar antenna were conducted to validate this method. The results show that this method offers precise phase error estimations of 3.58° on average, realizes a fast calibration process with several field-measured radiation patterns, and improves the performances of the LC-MPA by approximately 4%–10% in deflection efficiency at different steering angles.