This work outlines the procedure to establish a system for assessing materials' complex permittivity and permeability within the 110-170 GHz frequency range. We present the employed methodology and the calibration procedure for the system, incorporating a dual approach using TRL (Thru-Reflect-Line) and GRL (Gated-Reflect-Line) methods. Following that, a smoothing technique is used to enhance the accuracy of the results. Tests were conducted on a HIPS sample to validate the system's performance, demonstrating the results' reliability across the entire measured bandwidth.

This paper describes the process followed to implement a system to characterize the complex permittivity of materials in the 10-330 GHz frequency band. Firstly, the method used and the system's calibration process are shown, consisting of a double calibration TRL (Thru-Reflect-Line) and GRL (Gated-Reflect-Line). Subsequently, a smoothing technique is used to improve the accuracy of the results. Finally, a test is performed on quartz and glass fiber samples, showing that the results are quite reliable over the entire measured bandwidth.

This work presents the complete design process of a 3D fully-metallic reflectarray. The structure is composed of two unit cells with resonators in z direction that are phase-shifted 180 • and distributed in the x-y direction. Here will be shown how those unit cells have been designed and how we determine their distribution. Furthermore, a tolerance analysis is carried out with the aim of obtaining a prototype that is robust to manufacturing imperfections. Lastly, that protoype is measured to verify the results.

The presented study introduces a semi-analytical circuit model to fully characterize a dielectric unit cell structure made by blocks instead of applying the effective medium theory. A fitting strategy os used to a obtain a better understanding of the properties of the unit cell. Then, the approach is used to develop a semi-analytical model, demonstrating a good agreement with the scattering parameters extracted from a full-wave simulator. By tuning the dielectric material, our model can describe the behaviour of unit cells with different permittivities and electrical sizes. Finally, as an application for the analyzed dielectric unit cells, transmitarrays to generate orbital angular momentum (OAM) waves are designed and 3D-printed. The measured results show good performance for different number of OAM modes at 31 GHz.

Wireless communications based on reconfigurable intelligent surfaces (RIS) have upsurged as a key research topic in the communication-theoretic arena. Recently, the RIS concept has evolved into the so-called simultaneously transmitting and reflecting intelligent omni-surfaces (STAR-IOS, or simply IOS), which extend the RIS functionality by incorporating transmission (in addition to reflection) capabilities. To realize STAR-IOS, full and independent reconfiguration capabilities for both reflected and transmitted waves are crucially required. However, such full independent reconfiguration has thus far been hurdled by the intimate coupling between the transmission and reflection behaviour of the IOS elements. To overcome this challenge and realize the full potential of reconfigurable IOS-aided systems, we here advocate for the use of the polarization domain in the design and operation of STAR-IOS. Thanks to the polarization- dependent features of IOS elements, fully independent (reconfigurable) transmission and reflection modes can be delivered, thus bringing key performance improvements and opportunities for new communication scenarios.

This paper presents a metal-only reflectarray that enables the control  of incident orthogonal polarizations in a large bandwidth. The proposed  reflectarray is based on a unit cell whose tuning elements allow the independent control of the reflection phase value for the vertical and  horizontal impinging polarizations. Due to the symmetry of the unit  cell, the same performance is produced by each polarization when its reflection phase response is modified. The proposed unit cell provides a  fairly linear phase response along the frequency with a phase variation  in the orthogonal polarization of ±1°. The performance under oblique  incidence and the frequency limitation of the unit cell are also  investigated. From this unit cell, a metal-only reflectarray that  produces circular polarization from a linear polarization is designed.  The reflectarray presents a simulated directivity greater than 27 dBi  with an axial ratio below 1.5 dB from 32 GHz to 50 GHz (43.9% of  bandwidth). A prototype is fabricated and the measured results agree  well with the simulated ones. The obtained aperture efficiency is  between 56% and 41% in the considered frequency band. The measured  realized gain ranges from 27 dBi to 30.3 dBi where the achieved  radiation efficiency is greater than 97%

This paper presents a versatile full-metal 3D periodic structure based on square waveguides with non-closed resonators perforated on their walls. The complex unit-cell architecture is modelled via accurate equivalent circuits, previously characterized. The circuit model predicts the excitation of phase resonance, which will be used to optimize and design different functionalities, such as polarisation converters or absorption.

This letter presents a gap-waveguide phase shifter based on ridged unit cells with glide-symmetric configuration. The proposed unit cell design provides a higher stable phase shift compared to a conventional ridged  unit cell whose ridge height and waveguide width are tuned to achieve a stable phase shift. Through the insertion of glide-symmetric holes with semi-circle base in the ridged waveguide, a stable phase shift in a wide frequency range is achieved. Depending on the radii of the holes, the operating frequency range of the phase shifter can be tuned. A 90o phase shifter in millimeter-wave range is designed and manufactured to validate the analysis. The experimental results reveal a 85o +- 5o phase shift from 33 GHz to 43 GHz (26.3% bandwidth). In this bandwidth, the reflection coefficient is below -10 dB with a maximum insertion loss of 0.7 dB.

The upcoming high-speed wireless communication systems will be hosted by millimeter and sub-millimeter-wave frequency bands. At these frequencies, electromagnetic waves suffer from severe propagation losses and non-line-of-sight (NLOS) scenarios. A new wireless communication paradigm has arrived to resolve this situation through the use of reconfigurable intelligent surfaces (RIS). These metadevices are designed to reconfigure the wireless environment in a smart way. Traditional RIS designs based on the implementation of 2-D configurations have been considered up to now. However, 3-D structures enable an extra degree of freedom in the design that can be taken as an advantage for the development of improved RIS structures with advanced functionalities. This article proposes the implementation of a novel electronically-reconfigurable RIS based on the use of 3-D graphene meta-atoms. The reconfigurability lies on the graphene conductivity, easily tunable with a biasing voltage. Different conductivity values vary the meta-atom electromagnetic response, modifying the RIS functionality. A multi-objective optimization framework determines the optimal phase state of each meta-atom to accomplish the desired RIS performance. The operation of the RIS as an efficient beam steerer/splitter, absorber and polarization selector is validated with full-wave results.