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- 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%.
- Square Loop and Slot Frequency Selective Surfaces Study for Equivalent Circuit Model OptimizationPublication . Ferreira, David; Caldeirinha, Rafael; Cuinas, Inigo; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.This paper presents a parametric study of square loop and square slot frequency selective surfaces (FSSs) aimed at their equivalent circuit (EC) model optimization. Consideration was given to their physical attributes, i.e., the unit cell dimensions and spacing, substrate thickness and dielectric properties, for several frequencies and plane wave incident angles. Correlation analysis and evaluation of the influence of physical related input parameters on the FSS performance are presented. Subsequent optimization factor for the square loop classical EC model is analyzed, and a novel EC model formulation for the square slot FSS is proposed. The performance of the proposed EC model was assessed against results obtained from appropriate electromagnetic (EM) simulations, based on a root-mean-square error (RMSE) criteria. Results demonstrate the validity of the optimized EC model, in which good estimations of the frequency response of FSS structures were obtained. Significant reduction of the resonant frequency offsets, in the order of 650 (from 910 to 260) and 460 (770 to 310) MHz, was obtained for square loops and square slots, respectively. The models were further validated against measurements performed on two physical FSS prototypes inside an anechoic chamber at 2.4 GHz. Relatively good agreement was obtained between measurements of real FSS prototypes and results obtained with the EC model. Finally, this work is sought to provide the necessary refinement of elementary models for further studies with more complex and novel FSS structures.
- 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.
- 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.
- 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.
- 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.
- A simple scattering model for tree trunksPublication . Leonor, Nuno; Ferreira, David; Caldeirinha, Rafael; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Ferreira, DavidThis paper presents the work carried out in an effort of modelling the scattering and absorption effects of single tree trunks with application to the discrete Radiative Energy Transfer (dRET) based model applications. The assessment of the proposed empirical model was performed against measurements of both metallic and dielectric cylinders, mimicking tree trunks, at several micro- and millimetre wave frequencies, i.e. 9.4, 18.8 and 37.6 GHz. These were carried out in a controlled environment, i.e. an anechoic chamber, so that frequency dependent effects could be evaluated.
- 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.
- Real‐time high‐resolution radio frequency channel sounder based on the sliding correlation principlePublication . Ferreira, David; Caldeirinha, Rafael F. S.; Leonor, NunoA channel sounder based on the cross‐correlation properties of pseudo‐noise random sequences, known as swept time delayed cross‐correlation channel sounder, is presented. This sounder enables the characterisation of doubly selective channels, providing both amplitude and Doppler spectra information. It performs real‐time measurements of specific radio channels with a high resolution of 2 ns for adjacent multipath component, and a current maximum Doppler spread of ±100 Hz, but that can go beyond ±1 kHz, if some components are upgraded. The amplitude measurements given by the sounder, presented as power delay profiles, have a dynamic range of about 35–40 dB. This study provides detailed information on the most relevant components used to develop the sounder, and presents some of the calibration measurements performed on the bench, as well as specific radio frequency measurements at 18.7 and 60 GHz in a controlled environment. Finally, improvements that are feasible to be implemented in the near future are proposed.
- Extension of the dRET Model to Include Scattering from Tree Trunks in Microcell Urban Mobile ScenariosPublication . Caldeirinha, R. F. S.; Fernandes, T. R.; Leonor, N.; Ferreira, D.This paper proposes a framework for extending the applicability of the discrete RET (dRET) model to accommodate radiowave scattering from tree trunks, particularly in microcell urban mobile scenarios, at micro- and millimetre wave frequencies. This framework aims to provide accurate modelling of the signal emanated inside and around isolated blocks of tree trunks, for instances, in raised canopy forests or in urban street canyons like scenarios. Model validation against measurement results inside an anechoic chamber for a double line of regularly spaced metallic and dielectric trunks at 18.8 GHz, as well as recommendations for a more encompassing model, are presented.