Discriminating and locating multiple closely spaced targets in dense environments remains a fundamental challenge for radar sensing, especially in the context of indoor scenarios like smart home or automotive in-cabin applications. Recently, the integration of relatively large aperture metasurface-type reflectors such as frequency selective surface (FSS) or RIS with radar systems was proposed as a promising approach to overcome this limitation. In this work, we present ultra-thin mm-wave absorbers with strong frequency-selective characteristics and we deploy them for integrated tests with a FMCW radar system. By placing the frequency selective absorber (FSA) into the scene, we demonstrate that targets in obscured regions can be successfully detected when operating outside the absorption frequency range. Additionally, using the distinct frequency response, it can help to reveal additional information from the scene, for instance distinguish targets located in line-of-sight (LoS) and non-line-of-sight (NLoS) paths. Several FSA samples are designed and fabricated using screen printing on flexible PET films, i.e., very low-cost processes that enable the creation of large apertures. The experimental characterization includes microscopy and profilometric measurements as well as measurements of the reflection coefficient using a waveguide setup. In the absorption region, a reduction in reflection magnitude is observed down to $-30 similar to $ with a -10 dB bandwidth on the order of 1.2 GHz. Bistatic VNA measurements are used to estimate the reflection properties of the FSA under nonnormal incident angles, which is important for the deployment. Using a relatively large aperture FSA of the size $430{\,} imes {\,}380 similar to ext {mm}<^>{2}$ , we can distinguish two targets that appear in the same range but are placed in LoS and NLoS paths, respectively. To this end, two radar subbands were defined: one is inside with the FSA's absorption range and one at frequencies where the FSA is strongly reflective. When switched to the absorption subband, the strength of the signal that comes from reflection via the FSA aperture is reduced by about 22.5 dB compared to that in the reflective band.

In-car vital signs monitoring using millimeter-wave (mmWave) radar sensors has attracted growing interest due to its nonintrusive, real time, and privacy-preserving nature. Leveraging the high-resolution capabilities of mmWave radar technology, this article proposes noncontact detection of human presence and measurement of vital signs such as respiration and heartbeat within a vehicle cabin. A range of spectral analysis techniques-both parametric and nonparametric-are employed, including fast Fourier transform (FFT), periodogram, correlogram, estimation of signal parameters via rotational invariant techniques (ESPRIT), multiple signal classification (MUSIC), nonlinear least squares (NLS), and the iterative adaptive approach (IAA), to extract the vital sign information with high precision. For real-time measurements, we utilize three radar platforms operating in the 60-GHz band: the BGT60LTR11AIP (pulse Doppler), the BGT60TR13C [single-input-multiple-output (SIMO)], and the IWR6843ISK [multiple-input-multiple-output (MIMO)]. Experiments were conducted in diverse environments, including laboratory settings, on-road scenarios, and a custom test setup using a pump-embedded dummy to simulate infant vital signs. To distinguish living beings from inanimate objects, statistical features, such as variance, entropy, and the Kolmogorov-Smirnov (KS) test, are extracted and used as input to support vector machine (SVM) and $K$ -nearest neighbors (KNNs) classifiers. The proposed solution is low cost, privacy preserving, and robust against environmental interference, making it ideal for integration into next-generation intelligent transportation systems.

Radar plays a key role in Vital Sign Monitoring (VSM) for applications such as smart homes and in-cabin security. In this context, dense environments with Non-Line-of-Sight (NLoS) propagation are common. The combined use of radar systems and dedicated passive reflectors or Reconfigurable Intelligent Surfaces (RIS) offers a promising solution for improving NLoS. In this paper, we explore an NLoS indoor sensing scenario with a metal plate reflector and demonstrate the feasibility of metasurface-aided FMCW radar for vital sign detection. We employ a millimeter-wave FMCW radar operating at 60-64 GHz in conjunction with a dummy target that mimics the chest movement of a 2–5-year-old child. Using this setup, we demonstrate precise breathing rate estimation in LoS and NLoS scenarios. Additionally, we show NLoS measurements of an adult subject to demonstrate feasibility in real-world scenarios where non-periodic body movements are involved.

In this work, we present a meta-atom synthesis approach for passive metasurfaces of subwavelength thickness using the description in terms of surface impedance. The approach includes an optimization procedure that aims for broadband single resonance structures with 2 pi phase coverage which are easy to fabricate. The motivation of this work is to develop a passive, large aperture, multi-resonator phase-gradient metasurface for sensing applications in V band (60-64 GHz).

Recent advancements in radar technology have transformed vital sign monitoring in healthcare, providing non-intrusive alternatives to conventional methods, such as electrocar-diography. While previous works have explored various aspects of radar-based vital sign analysis, including signal processing and peak detection algorithms, a clear performance comparison remains challenging due to variations in hardware and operating frequency across studies. This paper focuses on the simultaneous measurement capability of two FMCW radar systems operating at different frequencies (24 GHz and 60 GHz) with an children dummy as measurement target. The flexibility of the measurement setup is highlighted, allowing the dummy's torso displacement functions via a pressure pump. Both radar systems exhibit the capability to capture stable and high-quality signals, facilitating a subsequent comparative analysis of two heartbeat estimation algorithms.

Characterizing material at millimeter wave frequencies can be challenging. Test engineers have to make a proper choice between many different measurement configurations depending on the sample shape and nature of the interaction with the electromagnetic wave. Many existing experimental setups are optimised for dielectric materials and the determination of the complex permittivity but only few are handling strong losses in transmission which are typical for EMI materials. In this paper, the material characterization of EMI absorbers using a waveguide-based sample fixture around 60 GHz is discussed. The attenuation constant and the wave impedance are determined under consideration of the reflections at the material interfaces of the bulk material. Best practices for RF material characterization can be deduced for this particular case.

In this paper we examine the feasibility of using ultra-wideband impulse-radar for vital sign detection, like breathing and heartbeat frequencies, while complying with the IEEE 802.15.4 standards for ultra-wideband devices. To achieve this, we use an ultra-wideband transceiver that enables secure ranging and radar applications. We examine the system on a human test subject with two sets of different antennas. We measure the vital signs by analyzing both the amplitude and demodulated phase of the carrier signal over slow time and compare the results to ground truth data which was recorded separately. The results show, that accurate vital sign estimation is possible under laboratory conditions, while complying with IEEE 802.15.4 regulations, which heavily limit the bandwidth of the radar application.

Flexible printed circuits show great potential for applications due to additive-type fabrication, ease of integration into devices and cost effectiveness. In the microwave frequency range, they mostly suffer from significant losses and cannot compete with their rigid PCB counterparts. In this work, we assess the performance of transmission lines from two different low cost flexible printed circuit fabrication techniques at frequencies up to 40 GHz. We show that transmission lines from laminated aluminum on PET show significant performance advantages compared to more conventional screen printed lines made using silver ink.

This paper explores the reflection and propagation of EM waves by Reconfigurable Intelligent Surfaces (RIS) using Gaussian beam models. We characterize the impinging waveform by experimentally determining the beam width and phase distribution. Furthermore, we detail the experimental setup and show accurate curve-fitting using the Gaussian beam model. Finally, we solve an electrically large RIS problem semi-analytically. In particular, we use the experimentally obtained impinging wavefront to analyze the far-field behavior of large aperture metasurfaces in V band application. This method is applicable for RIS and passive reflect metasurfaces with nearly full phase coverage.

Despite their great potential in communication and sensing applications, printed leaky-wave antennas have rarely been reported at mm-wave frequencies. In this paper, tapered leaky-wave antennas operating at 80 GHz are designed, fabricated and experimentally characterized. While most continuous leaky-wave antennas use subwavelength strips or other comparably small elements, in this work, the surface impedance is discretized very coarsely using only three square patches per period. With this architecture, a wide range of surface reactance can be achieved while maintaining a minimum feature size of the metallic pattern that is feasible for printed circuit fabrication. As the analytical solution for the bandstructure of sinusoidally modulated reactance surfaces is inaccurate for coarse discretization, we find it using full-wave simulation. In order to control side lobes effectively, we use a tapered aperture illumination according to the Taylor one-parameter distribution. A comprehensive experimental demonstration is presented, including near-field and far-field measurements. Therewith, we verify the designed aperture illumination and we reveal the origin of spurious far-field features. Side lobes are effectively suppressed and spurious radiation is reduced to -18 dB compared to the main lobe.

Metasurfaces have demonstrated a wide range of functions, including guided scattering and localization of electromagnetic waves, and they are a promising platform for replacing bulk components in the Terahertz frequency range. Nevertheless, precise local control of the scattered wave on a very subwavelength scale remains a challenge, with high magnitude and full 2π phase tuning being difficult to achieve with a single resonance structure. Here, we present a simple deeply subwavelength (λ/16) metamaterial unit cell based upon the coupling of dipole and quadrupole resonances, which achieves complete phase coverage as well as high reflection magnitude. We numerically demonstrate high efficiency anomalous reflection (81%) at an angle of 45°, avoiding almost all other spurious diffraction orders (0.15%). We also design a structure with a lower reflection angle of 25°, where efficiency is expected to degrade due to the presence of higher order Floquet modes. However, we observe that efficiency remains high (∼ 85%) with 0.58% spurious reflections. Hence, our design can be used in a variety of applications such as beam forming and scanning antennas, reflectarray antennas, phase shifters and beam splitters.

Metasurfaces have emerged as a promising technology for the manipulation of electromagnetic waves within a thin layer. In planar ultrathin metasurfaces, there exist rigorous design methods, based on the equivalent surface impedance of patterned metallic layers on dielectric substrates. In this work, we derive a limit on bandwidth achievable in these metasurfaces, based on constraints that their meta-atoms should be passive, causal and lossless and that they should obey the time-bandwidth product rules of a single resonance structure. The results show that in addition to elementary design parameters involving variation of the surface impedance, the bandwidth is critically limited by the dielectric substrate thickness and permittivity. We then propose a synthesis method for broadband ultrathin metasurfaces, based on an LC resonance fit of the required surface impedance and experimentally verify a broadband dispersive structure at millimeter-wave frequencies. This results in a bandwidth enhancement of over 90%, relative to a reference metasurface created with the narrowband design process.

We investigate near-infrared photodetectors based on subwavelength Ge nanoparticles. While the photodetector size guarantees a high-bandwidth device, the high quantum efficiency is possible by the localization of the optical energy.

Switched beam antennas provide an efficient and cost effective alternative to complex phased array and digital beam forming techniques. In this paper, we present the design and characterization of an electrically thin Huygens metasurface lens operating at 83GHz with high transmission efficiency and we demonstrate its applicability to switched beam antenna applications. We accurately characterize the 3D scattered field distribution and determine the focal length and transmission efficiency of the lens by near-field scanning. A model based on ideal Huygens sources is used to predict the focal performance, allowing the geometrical measurement parameters to be determined in advance. Finally, the ability of the lens to operate as a switched beam antenna is experimentally verified by exciting it with an omnidirectional waveguide antenna placed at different positions. The experimental results are in very good agreement with numerical simulations, showing steering angles up to 12° while keeping the side lobe level below -15dB.

We introduce a method of bandwidth enhancement on metasurfaces based on a more insightful analytical approach of LC resonances. In obtaining L and C parameters, we use surface impedance model considering not only a single frequency matching but also its first frequency derivative. We show that broadband anomalous reflection can be obtain with simple geometries involving dipole and inverse dipole structures. We verified our method using experiment in millimeter-wave frequencies and show that the broadband metasurface achieves a significant increase of bandwidth (more than 80% increase) compared to a single frequency design, with minimum -3dB power is maintained in the desired reflection and maximum -10dB for all other diffraction orders.

Printed circuit metasurfaces have attracted significant attention in the microwave community for their versatile wavefront manipulation capability. Despite their promising potential in telecommunications and radar applications, few transmissive metasurfaces have been reported operating at millimeter-wave frequencies. Several secondary effects including fabrication tolerances, interlayer near-field coupling, and the roughness of conductors are more severe at such high frequencies and can cause significant performance degradation. Additionally, very accurate experimental techniques are required in order to characterize these effects. In this article, we present highly efficient refracting metasurfaces operating at 83GHz. We use a synthesis technique that minimizes performance degradation due to effects such as interlayer near-field coupling and conductor roughness. Our experimental characterization includes an accurate determination of the intensity of all forward propagating Floquet harmonics in a broad frequency range. The experimental data show very good agreement with full-wave simulation and verify our synthesis method.

One of the most promising metasurface architectures for the microwave and terahertz frequency ranges consists of three patterned metallic layers separated by dielectrics. Such metasurfaces are well suited to planar fabrication techniques and their synthesis is facilitated by modelling them as impedance sheets separated by transmission lines. We show that this model can be significantly inaccurate in some cases, due to near-field coupling between metallic layers. This problem is particularly severe for higher frequency designs, where fabrication tolerances prevent the patterns from being highly-subwavelength in size. Since the near-field coupling is difficult to describe analytically, correcting for it in a design typically requires numerical optimization. We propose an extension of the widely used equivalent-circuit model to incorporate near-field coupling and show that the extended model can predict the scattering parameters of a metasurface accurately. Based on our extended model, we introduce an improved metasurface synthesis algorithm that gives physical insight to the problem and efficiently compensates for the perturbations induced by near-field coupling. Using the proposed algorithm, a Huygens metasurface for beam refraction is synthesized showing a performance close to the theoretical efficiency limit despite the presence of strong near-field coupling.

In this paper, we present preliminary results of the permittivity estimation of dielectric rough surfaces using fully-polarimetric bistatic measurements in specular direction at Wband frequencies. These results are the first validation of this method for very rough surfaces. Indeed, three Gaussian dielectric surfaces with a roughness of <0.0 mm (smooth), 0.3 mm (medium rough) and 2.0 mm (rough) have been generated and fabricated. The complex permittivity of the dielectric material has been measured as a reference value using the SwissTo12 Material Characterization Kit (MCK). Well calibrated measurements of the specular reflection at different angles have been carried out in the IEE bistatic polarimetric facility which is located in an anechoic chamber. The independence of the copolarized ratio of the specular reflection from the roughness has been validated with both measurement and numerical simulation. Finally, first results of the permittivity estimation of the rough surfaces are presented and analyzed.

This talk will give an overview of developments in the field of metasurfaces, followed by a presentation of our recent contributions to the field. Metasurfaces have recently emerged as one of the most promising forms of metamaterials for applications at frequencies ranging from RF to visible. Building on the established concepts of reflect-arrays and frequency-selective surfaces, they promise to become a key technology for the manipulation of microwave radiation. As shown in Figure 1, the anomalous refraction effect underlies many metasurface devices such as lenses and generators of vortex beams. Other applications include low-profile antennas, absorbers, diffusers, polarizers, retro-reflectors, polarimeters and hologram generators.