Percorrer por autor "Ferreira, David"
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- 3-D Mechanically Tunable Square Slot FSSPublication . Ferreira, David; Cuinas, Inigo; Caldeirinha, Rafael F. S.; Fernandes, TelmoWe introduce an innovative 3-D mechanically tunable frequency selective surface (FSS), which is inspired by the classical flat square slot FSS. The proposal improves the performance of classical 2-D FSS designs, and it also represents a novel method of achieving mechanical frequency tuning, despite other 3-D designs that consist of a collection of stacked 3-D layers exist. In our proposal, the rotation of an inner element provides tuning capability to the squared cell structure, consisting of metallic grids with a movable inner element. An aluminum prototype was built, which can be tuned from 2.4 to 4 GHz, and also compared its measured performance and numerical simulations. Some characteristics of the proposed structure are the rejection level at main polarization, up to 20 dB, and the maximum frequency sweep of approximately 50% of the fundamental frequency. The prototype showed a stable frequency response for angles of incidence up to 45°. Since results are in good agreement with simulations, we provide parametric equations to design 3-D structures at desired frequencies.
- Characterization of wireless propagation through traditional Iberian brick wallsPublication . Ferreira, David; Caldeirinha, Rafael F. S.; Fernandes, Telmo R.; Cuinas, InigoThis paper presents the wireless transmission properties for brick walls with different thicknesses and facade finish coatings. The ceramic hollowed bricks under study are traditionally employed in the Iberian residential construction, where the 11, 15 and 20 cm thicknesses are most common in indoor and outdoor walls. For each brick dimension, three different prototypes were manufactured varying in the type of wall finish, i.e.: naked brick, smooth painted plaster and rough painted plaster. The prototypes were measured in an anechoic chamber at frequencies ranging from 680MHz up to 10GHz. Results demonstrate that the brick wall internal heterogeneity, as well as the type of finish, significantly influence the frequency response of the walls.
- A dual-band sine-square FSS designPublication . Ferreira, David; Cuinas, Inigo; Caldeirinha, Rafael; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.This paper proposes a new frequency selective surface (FSS) based on the combination of four sinusoidal-waveform like shapes, each formed by the combination of two semi-circles, for an overall arrangement similar to a square shape. The proposed design was simulated and evaluated through a commercial electromagnetic (EM) simulation software. The sine-square FSS has a relatively simple design, yet provides a dual band frequency response, with narrow bandwidths compared to classical designs, and with a relatively stable angular behavior of up to at least 45◦. It was shown that the second resonance frequency can, within a limited range, be independently controlled from the first resonance frequency band. Both band reject and band pass variants, as well as possible active/tunable implementations of this FSS design, were evaluated and are discussed in this paper.
- A dual-band single-layer frequency selective surface for Wi-Fi applicationsPublication . Ferreira, David; Cuinas, Inigo; Caldeirinha, Rafael; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.This paper proposes a frequency selective surface (FSS) design to be used in Wi-Fi applications as either a reject or pass band single-layer filter. This design consists of a combination of canonical elements, i.e. ring loops/slots, and is tuned for both 2.4 and 5.2 GHz Wi-Fi frequency bands. It has a stable frequency response in the aforementioned Wi-Fi bands for incidence angles ranging from 0° to 45°. Both reject and pass band designs are presented, along with their respective unit cell dimensions and simulated frequency response. Active variants are also proposed and briefly evaluated which should allow for an on-off switching at the 2.4 and 5.2 GHz Wi-Fi bands. © 2015 EurAAP.
- Dual‐band single‐layer quarter ring frequency selective surface for Wi‐Fi applicationsPublication . Ferreira, David; Cuiñas, Iñigo; Caldeirinha, Rafael; Fernandes, Telmo R.This study proposes a frequency selective surface (FSS) design to be used in Wi‐Fi shielding applications as either a band reject or band pass dual‐band single‐layer filter. The proposed design consists of a combination of basic elements, that is, ring loops/slots, and is tuned at both 2.4 and 5.2 GHz Wi‐Fi frequency bands. It has a relatively stable frequency response in the aforementioned Wi‐Fi bands for incidence angles ranging from 0° to 45°. Both band reject and band pass designs are presented, along with their unit cell dimensions. Simulation and model validation through measurements demonstrate the performance of the proposed FSS design. Active variants are also proposed and briefly evaluated, in simulation environment, which should allow for applications where an on–off switching is desired at 2.4 and 5.2 GHz Wi‐Fi bands.
- Estimation of dielectric concrete properties from power measurements at 18.7 and 60 GHzPublication . Feitor, Bruno; Caldeirinha, Rafael; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Ferreira, David; Leonor, NunoIn this paper, microwave and millimetre wave reflection coefficient measurements at 18.7 and 60 GHz are presented for a group of concrete slabs, in which only the thickness of the gravel present inside is varied. An empirical reflection coefficient value is determined using both reflected and LOS power measurements. A concrete slab with no gravel is used as a reference. The measurement results show the dependence of the reflection coefficient with the thickness of the gravel inside the slab. The measurement results allow for the appropriate extraction of dielectric properties of the materials under study.
- Extension of the dRET Model to Forests of Thin CylindersPublication . Leonor, Nuno; Ferreira, David; Caldeirinha, Rafael; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Manuel Garcia SanchezThis paper presents a feasibility study on extending the applicability of the discrete radiative energy transfer (dRET) model to the trunk layer of raised canopy forests. The framework herein presented, proposes a method to predict the scattered signal inside and around a formation of cylinders (mimicking thin tree trunks), which is based on the prior knowledge of the cylinder 2-D reradiation pattern as input parameters to the dRET model. The characterization and modeling of the cylinder 2-D reradiation patterns were performed in an anechoic chamber, so that frequency-dependent effects could be evaluated. The assessment of the proposed method was performed against measurements within a formation of both metallic and dielectric cylinders mimicking thin tree trunks, at micro- and millimeter-wave frequencies, i.e., 9.4, 18.8, and 37.6 GHz.
- Hollow Clay Brick Wall Propagation Analysis and Modified Brick Design for Enhanced Wi-Fi CoveragePublication . Ferreira, David; Caldeirinha, Rafael; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Cuinas, InigoThe radiowave propagation through hollow clay brick walls, which are common in southern European construction, is analyzed. The brick walls have thicknesses of 11, 15, and 20 cm, common in both interior and exterior walls, and are bound with a portland cement, water, and sand mixture. For each brick dimension, three prototypes were assembled, varying in the type of wall finish, i.e., exposed brick, smooth painted plaster, and rough painted plaster. A 10 cm concrete wall was also included for comparison purposes. Penetration loss metrics were evaluated in an anechoic chamber at frequencies ranging from 680 MHz to 10 GHz. Results demonstrate that the brick wall internal heterogeneity, as well as the type of finish, significantly influences the propagation phenomena and thus the frequency response of the walls, with relatively high penetration losses observed at some relevant commercial frequency bands. Finally, an alternative brick design, with reduced penetration losses, is also proposed and evaluated under simulation environment only.
- Hybrid FSS and Rectenna Design for Wireless Power HarvestingPublication . Ferreira, David; Sismeiro, Luis; Ferreira, Adelino; Caldeirinha, Rafael F.S.; Telmo R. Fernandes, Telmo R.; Cuinas, Inigo; Caldeirinha, RafaelThis communication presents a hybrid frequency selective surface (FSS) and rectenna design for wireless power harvesting. To this extent, the center of the original FSS structure acts as a probe-fed patch antenna and an additional layer was added to the rear of FSS structure to mimic the required antenna ground plane, while minimizing its impact on FSS performance. The proposed hybrid solution yields a bandpass frequency response at GSM 900-MHz frequency band, with band reject behavior at 2.4 GHz and, on the other hand, act as a energy harvesting system through the antenna element at the 2.4-GHz ISM band, e.g., for recycling ambient Wi-Fi energy. The developed prototype, consisting of an array of 12 unit cells, was able to power a 56-mW LED at a 2-Hz rate from an incident power level of−10 dBm, resulting in an average output power of 300 μW and an unit cell RF–DC conversion efficiency of 50%.
- A Multilayer EM Simulation Tool to Assess RF Transparency Control of Building Wall StructuresPublication . Caldeirinha, Rafael; Ferreira, David; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Cuinas, InigoThis paper presents a multilayer electromagnetic (EM) simulation tool to assess the radiofrequency transparency control of building wall structures, developed in Matlab environment, that allows one to explore different layer arrangements and determine the overall frequency response. In particular, the deployment of frequency selective surfaces, either passive or active (electronically controlled), as well as the assessment of EM properties of different types of building walls commonly used in building construction, combined into one complete EM tool, is proposed.