David Ferreira

Square Loop and Slot Frequency Selective Surfaces Study for Equivalent Circuit Model Optimization

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.

A 2D Ray-Tracing Based Model for Micro- and Millimeter-Wave Propagation Through Vegetation

A novel two dimensional model to characterize the electromagnetic behavior of trees has been developed with the purpose of being used in ray-tracing based simulation platforms. This model uses various point scatterers with specific radiation characteristics to describe the effect of the trees present in the radiowave propagation path. The method to extract the parameters of the point scatterers from measurements is presented. The performance of this novel formulation is assessed in a tree formation scenario against measurements results obtained in a controlled environment, inside an anechoic chamber, at 20 and 62.4 GHz. Additionally, a comparison analysis with a discretized radiative energy transfer (dRET) approach is conducted, where a relatively good agreement has been found. The absence of readily plug-in models considering propagation through and/or around vegetation makes this new tool interesting for radio planning purposes.

Development and performance assessment of a real time high-resolution RF channel sounder

This paper presents a swept time delayed cross-correlation (STDCC) channel sounder. The proposed topology employs the real-time sliding correlation technique of pseudo-random (PN) sequences, enabling amplitude and Doppler spectrum measurements and analysis of individual multipath components. It makes use of recent technologies, specifically in the generation and transmission of PN sequences, which allows for an easy adjustment of the main specifications of the sounder. The intermediate frequency (IF) stage of the system presented here has been bench-tested and validated by using a PN sequence at 1GHz chip rate, which allows for a time-delay resolution of consecutive multipath components of 2ns. The radio frequency (RF) stage as also been bench-tested, and will be followed by rigorous measurements in an anechoic chamber. The dynamic range of the system was measured to be better than 40dB.

Wearable Textile Antennas: Examining the effect of bending on their performance.

This article presents a study on the effect of bending on the performance of a rectangular textile-patch antenna operating at a 2.4-GHz industrial, scientific, and medical (ISM) band. The substrate of the antenna was made from denim textile, and the conducting layers were made from a copper and nickel plated polyester fabric. A parametric study was made to determine the influence of an antenna bending around its length and width on its performance parameters in chest, leg, arm, or wrist integration for wireless body-area network (WBAN) scenarios. Results were obtained from bench and anechoic chamber measurements and compared with simulation results. The prototype presents a maximum gain of approximately 4 dBi and 70° of half-power beamwidth (HPBW) in the flat position. When subjected to a wrist equivalent bending, the gain decreases by 2 dB, HPBW has an increase of about 25°, and front-to-back radiation ratio decreases. Mean and standard deviation parameters as a function of bending curvature were calculated from parametric simulations.

Hybrid FSS and Rectenna Design for Wireless Power Harvesting

This 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%.

3-D Mechanically Tunable Square Slot FSS

We 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.

Extension of the dRET Model to Include Scattering from Tree Trunks in Microcell Urban Mobile Scenarios

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.

Estimation of dielectric concrete properties from power measurements at 18.7 and 60 GHz

In 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 Cylinders

This 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.

Development and Implementation of a Real Time High-Resolution Channel Sounder - IF Stage

This paper presents an intermediate frequency (IF) stage of a swept time delayed cross-correlation (STDCC) channel sounder. The proposed topology employs the real-time sliding correlation technique of pseudo-random (PN) sequences, enabling amplitude and Doppler spectrum measurements and analysis of multipath components. It makes use of recent technologies, specifically in the generation and transmission of PN sequences, which allows for an easy adjustment of the main specifications of the sounder. The system presented herein has been tested and validated by using a PN sequence at 1GHz chip rate, which allows for a time-delay resolution of consecutive multipath components of 2ns. The dynamic range at the input of the IF stage was measured to be better than 40dB.