Vincent Laur

This letter introduces a mask-based surrogate modeling framework that generalizes the feature-based paradigm for efficient microwave design exploration using a limited number of full-wave simulations. Unlike feature-based approaches, which extract scalar performance metrics, the proposed method reconstructs the complete $S$ -parameter boundary contour within the operating band. This preserves each design’s parametric signature and enables on-demand evaluation of multiple performance specifications without model retraining. The framework employs two vector-valued kernel ridge regression models to predict boundary coordinates from geometric parameters. Validation on a three-port circulator demonstrates accurate reconstruction and rapid exploration of thousands of candidate designs.

Three-dimensional printing of microwave absorbers gave rise to new shape optimization possibilities. However, new printable materials dedicated to this application have to be developed. In this study, an iron flake polymer composite was elaborated and characterized. Scanning Electron Microscopy observations demonstrated that iron flake particles are naturally aligned along the flow direction in the nozzle without applying a magnetic field during the 3-D printing process. Magnetic anisotropy was demonstrated with both vibrating sample magnetometer and microwave measurements. Indeed, at 5.4 GHz, a ratio of 1.67 between parallel and perpendicular susceptibility was observed. These measurements demonstrated a self-induced magnetic anisotropy that can be used to control the angular robustness of free-space microwave absorbers. Simulations show that an induced planar anisotropy allows for improving better stability of absorption at oblique incidence compared to an isotropic absorber.

A new multi-materials’ compact topology, allowed by multi-materials 3-D-printing, is proposed to design microwave terminations in rectangular waveguide technology. The concept is based on a Salisbury-like resonant absorber constituted of a 3-D-printed polylactic acid (PLA) dielectric spacer and a lossy dielectric in the form of a pad whose dimensions allow controlling the frequency of maximum absorption. The flange of the termination is also printed with a lossy dielectric to achieve a compact and low-weight component that can be directly connected to a standard metallic waveguide. The concept has been validated experimentally in K-band (18–26 GHz) and X-band (8–12 GHz) with relative bandwidths (RBWs) around 20% for both frequency bands. These components demonstrate a quite stable temperature behavior. The power-handling capability (PHC) has been investigated through simulations and experiments. It was demonstrated that the maximum power than can support the termination is around 4–5 times lower than a more standard and bulky termination made with the same materials.

With the multiplication of electronic devices in our daily life, there is a need for tailored wideband electromagnetic (EM) absorbers that could be conformed on any type of surface-like antennas for interference attenuation or military vehicles for stealth applications. In this study, a wideband flexible flat electromagnetic absorber compatible with additive manufacturing has been studied in the X-Ku frequency bands. A multilayer structure has been optimized using a genetic algorithm (GA), adapting the restrictions of additive manufacturing and exploiting the EM properties of loaded and non-loaded filaments, of which the elaboration is described. After optimization, a bi-material multilayer absorber with a thickness of 4.1 mm has been designed to provide a reflectivity below −12 dB between 8 and 18 GHz. Finally, the designed multilayer structure was 3D-printed and measured in an anechoic chamber, achieving −11.8 dB between 7 and 18 GHz. Thus, the development of dedicated materials has demonstrated the strong potential of additive technologies for the manufacturing of thin wideband flexible EM absorbers.

This article deals with the design and fabrication of compact self-biased Q-band circulators. A pre-oriented substituted strontium hexaferrite that has a high anisotropy field was used to design a planar circulator at 38 GHz. The component has been measured in the 32-40 GHz frequency band at different temperatures. Measured insertion loss as low as 0.45 dB together with a high isolation level has been demonstrated. Moreover, the component demonstrated a quite stable temperature behavior. However, a slight shift toward low frequencies is observed compared to the simulation. Retro-simulations showed that small variations of the metallic strips' dimensions induce significant S-parameters changes. These experimental results demonstrate that substituted strontium hexaferrites have a high potential for the realization of ultra-compact Q-band circulator but also that the metallization process resolution is crucial for performance reproducibility.

This article presents the design and fabrication of a microstrip C-band microwave isolator using additive technology. Such fabrication requires multimaterial 3-D printing (dielectric and absorber) and the embedding of ferrite ceramic inside the substrate. First, our design demonstrated that an efficient microstrip load can be made by 3-D printing in a single process: not only the dielectric but also the absorber, which takes the form of steps in the substrate. Then, a circulator with a composite dielectric/ferrite resonator was designed. Finally, circulator and load were integrated in a single component to optimize the isolator. The device was then 3-D-printed with a pause during the fabrication to allow the ferrite disk to be embedded. Microwave measurements demonstrated that minimum insertion losses of 0.76 dB were achieved, together with a -15 dB bandwidth of 11.9%. To our knowledge, this is the first demonstration of the feasibility of fabricating a non-reciprocal microwave component by additive technology.

This study deals with the development of a printable composite material based on a polyphenylene sulfide (PPS) matrix and carbonyl iron (Fe) particles, with controlled electromagnetic performance. More specifically, materials were simultaneous melt mixed and shaped under the form of filament with a diameter suitable for Fused Deposition Modeling. After reminding the potentialities of the printable PPS matrix, especially in terms of temperature resistance, microwave characterizations were performed on toroidal samples. The measured electromagnetic properties were compatible with absorption applications and compared to those of a commercial iron-filled PolyLactic Acid (PLA). Rectangular waveguide microwave loads were designed and fabricated by Fused Deposition Modeling with both materials. The PPS-Fe load has a volume that is 7 times lower than the PLA-Fe load due to a higher permittivity-permeability product and losses. Heat treatments demonstrated that no degradation is observed for the PPS-Fe load up to 180 ℃ while the PLA-Fe load is totally melted at 150 ℃. In the same time, it was observed that the maximum power supported by the PPS-Fe load is three times higher than the one supported by the PLA-Fe load. Finally, the temperature stability of the electromagnetic response of the PPS-Fe composite was demonstrated by measurements in the −70 ℃ to 140 ℃ temperature range. This new high temperature printable composite paves the way to the development of efficient, low-cost, low-weight, power and temperature stable absorbers for microwave applications.

The knowledge of the magnetocrystalline anisotropy is often essential and its determination remains an ambitious challenge. In this article, we propose a new way to measure the anisotropy magnetic field of some substituted lithium ferrites. To achieve that, we have measured the complex permeability versus frequency of several substituted lithium-based ferrites, in which, the anisotropy field is related to the gyromagnetic resonance frequency. However, the gyromagnetic resonance frequency is hidden by the domain wall motion resonance. To solve the problem, we have sintered the ferrite at different temperatures to control the microstructure of the ferrite, thus separating the magnetization mechanisms. We have obtained results of anisotropy fields comparable to those of the literature between 270 and 460 Oe.

In this article, we propose to apply 3-D printing technology to the design and realization of honeycomb microwave absorbers. First, printability of simple honeycomb structures, made of a lossy dielectric material, was evaluated and validated by measurement in the 2–18 GHz frequency band. Effective dielectric properties of honeycomb structures with different dimensions are also discussed. Then, the interest of 3-D printing technology was highlighted by the design, fabrication, and measurement of a multiscale honeycomb absorber that can significantly increase the relative bandwidth. The 3-D printed honeycomb absorbers are compared with others technologies. The potential of this technology was also illustrated by the fabrication and measurement of a pyramidal honeycomb absorber that demonstrates an absorption level of more than 22.9 dB over the 2–18 GHz frequency band.