This work presents the design of a broadband frequencyselective surface absorber (FSSA) using laser-induced graphene (LIG). This type of resistive material composed of porous graphene has been analyzed, and its performance has been modeled through different cases of FSS based on LIG patches. The results of this analysis, both simulated and experimental, show the existence of periodic frequency bands of absorption ranging from a few GHz to 50 GHz. Taking advantage of this absorbing behavior, a single-layer FSSA, including a circular LIG patch at the center of the unit cell, is proposed. A circuit model for this FSSA unit cell is introduced, which allows us to efficiently improve the absorption over a large bandwidth by including an ad-hoc dielectric layer. The FSSA is fabricated and measured: the experimental results show a good agreement with the simulated ones, providing absorption of more than 0.9 from 14 GHz to 48 GHz (109.7% relative bandwidth). In addition, the proposed design exhibits robust behavior to both TE and TM oblique incidence up to 45º.

A document by Pablo Mateos-Ruiz . Click on the document to view its contents.

This paper presents the extraction of an equivalent circuit model for PIN diodes and varactors in the millimeter waves. This circuit model is handy for new communication applications involving, for example, electronic beam reconfigurability, where diodes are commonly used. For parameter extraction, the proposed model includes the effects of the device, the pads, and the gap underneath the device. From the measurement at various bias points (varactor) or states (PIN diode), it is possible to extract an equivalent circuit to properly model the behavior of these devices when using them to design reconfigurable elements. The results are validated experimentally, obtaining an excellent agreement between the measurements and the equivalent circuit models in a large frequency band up to 67 GHz.

This paper presents a metal-only reflectarray based on a 3D unit cell with dual-band capability. The 3D unit cell is a square waveguide whose vertical walls include resonator elements with independent frequency performance. Different resonator geometries are analyzed to obtain a reflected phase variation in the target frequency band and to be feasible for 3-D manufacturing. C-shaped, triangle-shaped, and circle-shaped resonators are selected to obtain the required phase shift in reflection. A reflectarray (RA) prototype is designed, including pairs of resonators where the C-shaped resonator controls the low-frequency band, and the triangle resonators do so for the high-frequency band. The main reflected beam directions for each frequency band are different to show the independent phase tuning of the resonators. The prototype is manufactured using stereolithography (SLA) with a subsequent silver coating. Measured results show realized gains of 21 dBi in the 18 GHz band and 24 dBi in the 26.5 GHz band, with a high radiation efficiency and good agreement with the simulated results.

This document presents different phase shifter designs implemented in contactless air-filled substrateintegrated waveguide (CLAF-SIW) technology. The phase shifters are based on unit cells that avoid leakage losses in the assembled layers and produce the desired phase shift along the waveguide. The unit cells are represented by waveguides whose lateral walls have double-mushroom structures, and phase-shifting elements are in their propagation zone. The proposed phase-shifting elements are integrated cavities, integrated cavities with patches, and integrated cavities with mushroomlike patches. Transitions are needed to match impedances between the waveguides loaded with phase-shifting elements and a reference CLAF-SIW. Other transitions are designed to measure the prototypes, from coplanar waveguide to SIW and SIW to CLAF-SIW. Finally, some prototypes are manufactured to provide an experimental validation of the proposed phase-shifting elements. Good agreement is obtained between the simulated and measured results for all the CLAF-SIW phase shifters in the frequency range between 30 and 50 GHz.