A 24 GHz radar, useful for remote vital signs monitoring, operating in linear/circular polarization (LP/CP), was developed to analyze vital signs detection performance in the presence of environmental clutter. The radar system, equipped with horn antennas with and without suitable LP-CP field polarization converters, is designed for monitoring respiratory and cardiac activity in domestic and hospital environments. A geometric optics (GO) model is introduced to show the advantages of circularly polarized (CP) radar sensors over linearly polarized (LP) ones in discriminating vital signals in scenarios where vibrating metallic panels simulate environmental disturbance (worst case). An analytical model, numerical computations, and experimental investigations highlighted the robustness of CP radar sensors in detecting vital signs in the presence of environmental clutter.

This paper addresses the importance of performing an accurate electrical characterization of dielectric materials for 3D printers before using them in the design of printed components for microwave applications. The great versatility of 3D printers has enabled their use for easy and cost-effective manufacturing in the field of electromagnetic systems, particularly in the areas of antennas (dielectric and lens antennas) and RF sensors. In these fields, for a correct design of the printed RF devices, using the nominal value of the dielectric permittivity provided by the vendor is not enough, as the electrical specifications may be affected by various factors, such as the print filling, color, print quality, etc. This study addresses this problem with reference to the design of a linear-to-circular field converter for radar systems used to detect vital signs. Since the performance of polarization converters is highly sensitive to the actual electric characteristics of the material used for their manufacturing, it is shown that only making preliminary measurements of the permittivity of the adopted dielectric allows to meet the design constraints. Finally, measurements taken on test RF converter prototypes confirm the effectiveness of the design procedure.

A novel class of lightweight, single, and multirefractive dielectric stacked-disk lens (SDL) designs with excellent electromagnetic performances, exhibiting reduced cost and enhanced simplicity compared to their massive counterparts, is presented. Design guidelines, derived from an accurate frequency-dispersive uniaxial tensor model of periodic dual-layer planar stacks, allow optimization of the key lens parameters-disk number, permittivity, thickness, diameter, and interdisk spacing. Several SDLs designed at 5 GHz using these guidelines, exhibiting stopband thresholds between 6.6 and 13 GHz, -3-dB fractional gain lens-antenna bandwidths ranging from 46.6% to 81.1% and aperture efficiencies between 44.5% and 58.8%, are illustrated. Full-wave simulations, employing a locally conformal finite-integration technique (FIT), show interdisk field propagation, diffraction, field focusing, and stopband behaviors. A novel time-domain metric, the proportional-derivative energy fidelity factor (PDEFF), is introduced to quantify received field waveform quality at the lens focus region, achieving 95%-99% for ultrawideband through-the-wall imaging and 92%-94% for ultrawideband (UWB) waveforms (for lenses having stopband thresholds near and above 10 GHz). An antenna prototype incorporating an SDL design approaches a peak gain of 17 dBi (approximately 10 dB attributable to the lens) and an 87% weight reduction relative to a conventional massive lens, confirming numerical simulation predictions.

Short-range radars are widely used for automotive applications, through-the-wall imaging and to monitor the elderly activity into their home. Given the relevance of such applications, some imaging algorithms are reformulated using a consistent formalism applied with a very efficient region-of-interest grid step. Moreover, to reduce the computation time, the numerical procedures are structured in such a way to employ fast Fourier transform or inverse fast Fourier transform algorithms whenever it results possible, while to reduce the artifact effects due to antenna and wall reflections, the time domain gating technique is implemented. The adoption of the combined technique (reconstruction algorithms and time gating) allows to remove the artifacts on both simulated and measured data with performances comparable with those of the background subtraction technique. The reported test cases evidence the ability of the considered algorithms to operate with different fractional bandwidth and with very fast execution times.

A new modulation scheme for frequency-modulated continuous-wave (FMCW) radars with millimeter-level target motion detection capability is presented. The proposed radar scheme is free from the synchronization constraint and exhibits low sensitivity to internal parasitic mutual coupling, thus significantly reducing its design complexity without worsening its performance in terms of accuracy and operating ranges. Alternatively to canonical FMCW radars, which exploit chirp signals with triangular or sawtooth-like frequency variation, a radar based on a sinusoidal frequency modulation, which does not require specific synchronization procedures to achieve accurate motion detection even at a short distance from the radar, was developed. Both numerical and experimental results, performed with a 24 GHz radar, have shown the suitability of the proposed modulation scheme for monitoring very small target movements, consistent with those typically exhibited by the human thorax during basic vital activities (heartbeat and respiration). This makes the proposed radar scheme a suitable solution for contactless heart and breath rate monitoring.

(1) Background: An optical simulator able to provide a repeatable signal with desired characteristics as an input to a photoplethysmographic (PPG) device is presented in order to compare the performance of different PPG devices and also to test the devices with PPG signals available in online databases. (2) Methods: The optical simulator consists of an electronic board containing a photodiode and LEDs at different wavelengths in order to simulate light reflected by the body; the PPG signal taken from the chosen database is reproduced by the electronic board, and the board is used to test a wearable PPG medical device in the form of earbuds. (3) Results: The PPG device response to different average and peak-to-peak signal amplitudes is shown in order to assess the device sensitivity, and the fidelity in tracking the actual heart rate is also investigated. (4) Conclusions: The developed optical simulator promises to be an affordable, flexible, and reliable solution to test PPG devices in the lab, allowing the testing of their actual performances thanks to the possibility of using PPG databases, thus gaining useful and significant information before on-the-field clinical trials.

The technology transfer of terahertz wireless communication from research laboratories to commercial applications is a global strategic achievement currently pursued to match the ever-increasing demand for high-speed communication. The use of commercial integrated electronics for the detection of THz waves is an intriguing challenge which has enticed great interest in the scientific research community. Rapid progress in this field has led to the exploitation of THz direct detection using standard CMOS technology based on the so-called self-mixing effect. Our research, stemming out of a collaboration between Sapienza University of Rome and STMicroelectronics company, is focused on the complete design process of a THz rectifier, realized using 50 nm ST B55 CMOS technology. In this paper, we report the optimization process of a case-study receiver, aimed to demonstrate the feasibility of direct demodulation of the transmitted OOK signal. A relatively limited bandwidth extension is considered since the device will be included in a system adopting a radiation source with a limited band. The design refers to a specific technology, the 60 nm MOS in B55X ST; nevertheless, the proposed optimization procedure can be applied in principle to any MOS device. Several aspects of the rectification process and of the receiver design are investigated by combining different numerical simulation methodologies. The direct representation of the rectification effect through the equivalent circuit of the detector is provided, which allows for the investigation of the detector–amplifier coupling, and the computation of output noise equivalent power. Numerical results are presented and used as the basis for the optimization of the receiver parameters.

A linear-to-circular polarization converter, consisting of a suitable arrangement of thin periodic airdielectric slabs, is presented. The uniaxial nature of the periodic structure, highlighted by the field propagation matrix representation, is exploited to perform the required field polarization conversion. The performances of the proposed polarizer are illustrated by analyzing the radiative performance in terms of impedance bandwidth, axial ratio and realized gain of a horn and a wideband planar slot lens antenna.

A frequency-modulated continuous-wave radar for short-range target imaging, assembling a transceiver, a PLL, an SP4T switch, and a serial patch antenna array, was realized. A new algorithm based on a double Fourier transform (2D-FT) was developed and compared with the delay and sum (DAS) and multiple signal classification (MUSIC) algorithms proposed in the literature for target detection. The three reconstruction algorithms were applied to simulated canonical cases evidencing radar resolutions close to the theoretical ones. The proposed 2D-FT algorithm exhibits an angle of view greater than 25° and is five times faster than DAS and 20 times faster than the MUSIC one. The realized radar shows a range resolution of 55 cm and an angular resolution of 14° and is able to correctly identify the positions of single and multiple targets in realistic scenarios, with errors lower than 20 cm.

An antenna operating between 300 MHz and 700 MHz, designed to be used on a ground penetrating radar installed on an Unmanned Aerial Vehicle (UAV) for the exploration and characterization of the buried ice deposits on Mars, is presented.To this end, a lightweight, high-gain Vivaldi antenna having compact dimensions and high operating bandwidth has been taken into consideration.This antenna, equipped with circular-loaded rectangular slots etched on its radiating arms, exhibits improved performance in terms of size, return loss, gain, and fidelity factor with respect to a conventional antipodal Vivaldi antenna.Experimental measurements performed on a prototype of the Vivaldi antenna with slots showed a return loss lower than -12 dB with realized gains between 4 dBi and 6.5 dBi in the 300-700 MHz frequency band.

A wideband high-gain circularly polarized (CP) shaped dielectric horn-lens antenna (SDHLA) operating in the frequency band between 6.7 and 18.2 GHz [fractional impedance bandwidth (FIBW) of 92.4%] with a 3-dB axial-ratio in the frequency range from 8.1 to 16.3 GHz [fractional axial-ratio bandwidth (FARBW) of 67.2%], is presented. The antenna, composed of a suitably shaped dielectric horn, integrated with a super-ellipsoidal-axicon dielectric lens made out of stacked thin dielectric disks, is mounted on a printed circuit board (PCB) where a microstrip line terminated with a wideband radial stub is used to excite a S-shaped slot through which the circular polarization is achieved. Parameterized 3D Lamé curves, describing the horn and lens profile, are used to optimize the antenna design. The antenna features a peak realized gain exceeding 13.1 dBi that is beneficial in a variety of applications, such as digital video broadcasting (DVB), remote sensing, weather monitoring, satellite communications, and air traffic control. The full-wave electromagnetic solver CST Studio Suite $^{\mathrm{ TM}}$ , based on a locally conformal finite integration technique (FIT), was employed to design and characterize the antenna whose performances were found to be in good agreement with the experimental measurements.

High gain, compact and lightweight antennas are highly requested in nowadays communications. To this end, a novel stacked-disk dielectric lens useful for converting linear to right-hand circular polarization (RHCP) or to rotate the field polarization towards a slant-45°, while increasing gain and limiting the overall antenna weight is presented. The lens, of simple manufacturing, is based on a new construction technology which allows an overall weight lens reductions of the order of 90%. Numerical results concerning the gain enhancement and polarization conversion of the field radiated by a horn antenna equipped with the proposed lens are presented and analyzed in detail.

CMOS technology can easily handle the modulation/demodulation process of a RF signal, but cannot reach, at least today, bands toward 1 THz. For this reason, there has been great interest in evaluating the intrinsic rectification process that occurs in a metal oxide semiconductor field effect transistor (MOSFET) structure. Recent results showed that high detection velocity can be achieved using a device with proper detection architecture. These results, combined with the intrinsic low cost of the CMOS technology, allows to foreseen this as a suitable solution for the large market of the future 6G communication systems. The MOSFET detector is studied by TCAD simulations, and its characteristics explained following the self-mixing model. Numerical results show a new picture of the device functioning, giving a new description of its external response. The knowledge of the self-mixing mechanisms allowed to achieve an accurate design of an array of 64 rectifiers, each one integrated with a suitable printed antenna.

A wideband high-gain dielectric horn-lens antenna (DHLA) for wireless communications and ultra-wideband (UWB) applications, featuring 122% fractional bandwidth, is presented. The antenna, consisting of a dielectric horn equipped with a spherical-axicon dielectric lens that increases and equalizes the gain within the operating band, is placed on a PCB featuring a wideband eight-shaped slot fed by a tapered microstrip line employed to excite the radiating system. The lens, integrated within the body of the dielectric horn, has the property of focusing the RF energy near its vertex on an extended frequency band, thus ensuring compact size, excellent field coupling with the antenna excitation system, and a significant increase of antenna gain. The antenna operates in the 3–12.4 GHz frequency range with relatively flat group delay, making it suitable for operating with narrowband, broadband, and impulsive UWB signals. The antenna can work properly without or with a metal reflector useful to enhance gain (up to 19 dBi), improve the front-to-back ratio, and reduce the impact of the antenna characteristics from the installation site. CST Studio Suite, implementing a full-wave locally conformal finite integration technique (FIT), was employed to design and characterize the antenna. The antenna characteristics estimated numerically were found to be in good agreement with the experimental results performed on an antenna prototype.

The electromagnetic performance of a class of lightweight stacked disk dielectric lenses, useful for reducing the cost and construction complexity of a radiating system, are compared with the massive counterpart in order to highlight their strengths and weaknesses. A full-wave numerical technique to include field propagation and diffraction effects taking place between the different disks forming the considered lenses, was employed. Information useful to exploit the electromagnetic characteristics of the analyzed lenses are suggested.

The performances of a multiple-input multiple-output (MIMO) radar, employing 16 equivalent antennas, and multiple-input single-output (MISO) radar, employing 10 antennas, for through-the-wall imaging applications are analyzed. In particular, imaging algorithms based on the Fourier transform (FT) and the multiple signal classification (MUSIC) available in the literature are compared with the FT-MUSIC hybrid algorithm recently developed by the authors. Three different investigations have been performed. The first, performed analytically, refers to a scenario in which a point scatterer is placed in free space, and the second, addressed numerically using the CST full-wave software, refers to a scenario in which two targets are present, while the last was executed in a real scenario where a metal panel is placed behind a tuff wall. All the algorithms and radar configurations were found to be suitable for accurately reconstructing the position of the investigated target. In particular, applying the FT technique, the MISO configuration has a lower cross-range half-power beamwidths (HPBW) than the MIMO one, while the range HPBW is the same for the two radar configurations. Despite the different number of elements present in the two radar configurations, similar range and cross-range HPBW are obtained for both configurations when MUSIC and FT-MUSIC techniques are employed. The field of view for FT and FT-MUSIC is about 45°, while it is less than 15° for the MUSIC algorithm. The HPBWs obtained with the experimental setup are very close to those obtained in the analytical study. Finally, the proposed experimental MISO radar acquires the data in half the time required by the MIMO one. The numerical results, confirmed by the experimental measurements, seem to indicate in the FT-MUSIC technique the one that provides the best performance for the considered radar configurations.

Microwave imaging is an active area of research that has garnered interest over the past few years. The main desired improvements to microwave imaging are related to the performances of radiating systems and identification algorithms. To achieve these improvements, antennas suitable to guarantee demanding requirements are needed. In particular, they must operate in close proximity to the objects under examination, ensure an adequate bandwidth, as well as reduced dimensions and low production costs. In addition, in near-field microwave imaging systems, the antenna should provide an ultra-wideband (UWB) response. Given the relevance of the foreseen applications, many UWB antenna designs for microwave imaging applications have been proposed in the literature. In this paper, a comprehensive review of different UWB antenna designs for near-field microwave imaging is presented. The antennas are classified according to the manufacturing technology and radiative performances. Particular attention is also paid to the radiation mechanisms as well as the techniques used to reduce the size and improve the bandwidth.

Extensive literature demonstrate that CMOS technology can be suitable for the realization of THz detector, with very high sensitivity. Recent results showed that a high detection velocity can be achieved using a proper detection architecture. These results, combined with the intrinsic low cost of the CMOS technology, allows to foreseen this as a suitable solution for the large market of the future 6G communication systems. In this paper the MOS detector is studied by TCAD simulations, in the light of the self-mixing model. Results show a new picture of the device functioning giving a new description of external response. This approach dramatically improves the comprehension of THz rectification performed by the MOSFET, for many years limited to the plasma waves model. The achieved comprehension of the interaction of the radiation with the silicon structure, offers a new possible approach to the system design. The knowledge of the self-mixing mechanisms allowed to perform an accurate design of the detection system presented in this paper. In particular, the THz detector, composed of an array of 64 rectifiars, each one integrated with an antenna, made in LFiS110 technology, is presented.

A fast low-cost through-the-wall radar imaging (TWRI) system, based on a vector network analyzer (VNA), a couple of switches and an array of Vivaldi antennas, has been designed, realized, and tested. To solve the TWRI inversion problem, an original theoretical modeling for a class of TWRI techniques whose basic functions are the cross-range Fourier transform (FT) of the scattered field and its covariance operator has been proposed. Using these functions, four conventional algorithms, namely the delay and sum (DAS), the FT, the multiple signal classification (MUSIC), the hybrid DAS-MUSIC and a new algorithm, the hybrid FT-MUSIC, have been derived. All these techniques have been implemented and their accuracy and field of view have been tested on canonical scatterers. Then, the algorithms have been applied to measured data collected in different scenarios constituted by a metallic bar or a human subject in the absence and in the presence of a wall between the antenna and the considered targets. Using the proposed TWRI system, it has been possible to detect a subject located up to 5-m away from the radar antenna array through a tuff wall. The proposed FT-MUSIC algorithm has evidenced performances similar to those of the DAS-MUSIC but with significantly lower execution times. Finally, FT-MUSIC performances in terms of field of view and immunity to disturbances are better compared to those of the MUSIC algorithm.

A class of lightweight spherical-axicon-like dielectric lenses suitable for enhancing broadband antennas performances is presented. The proposed lenses, sized according to a reference massive lens, are formed by thin dielectric sheets spaced equiangularly on the azimuth plane (petal-shaped lens), or regularly orthogonally arranged along the lens axis (disks-shaped lens), thus yielding construction simplicity and significant weight reduction. While petal-shaped dielectric lenses are shown to yield mild gain increase, lenses made by thin dielectric disks, orthogonally periodically arranged along the lens optical axis, offer performances much closer to those achieved by comparable massive refractive spherical-axicon dielectric lenses. The time-domain and the focusing characteristics of the proposed lenses are investigated. Then, a Floquet’s mode-based model is proposed to describe the stop-band characteristics of stacked-disk lenses, illustrating the mechanism underpinning their sudden performance degradation observed at the stop-band onset frequency. Full-wave analyses, based on a locally conformal finite integration technique (FIT), implemented in CST Studio Suite™ and validated by measurements or highly accurate FEM simulations, illustrate the excellent characteristics of the proposed lenses to operate with narrowband as well as ultra-wideband (UWB) waveforms.