Scielo RSS <![CDATA[Journal of Microwaves, Optoelectronics and Electromagnetic Applications]]> vol. 14 num. 1 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[A Compact Dual-Band Octagonal Slotted Printed Monopole Antenna for WLAN/ WiMAX and UWB Applications]]> AbstractA compact (20 x 20 mm2 size), coplanar waveguide (CPW) fed, octagonal slotted, dual-band antenna is presented in this paper. The proposed antenna has a simple structure consisting of an octagonal slot, L-shape stub, and a two stepped rectangular patch. The introduction of the L-shaped metallic stubs in the ground plane generates a lower resonance frequency at 2.42 GHz. The proposed antenna is fabricated on a low cost FR4 substrate having thickness of 1.6 mm. The measured impedance bandwidth of the proposed antenna is from 2.40 to 2.46 GHz and 3.2 to 6.2 GHz. It is also shown that the proposed antenna has stable radiation patterns of almost dumb-bell shape in the E-plane and omni-directional shape in the H-plane. The effects of various design parameters on the impedance bandwidth are also studied and presented in detail. The proposed antenna can be used for 3.5/5.5 GHz worldwide interoperability for microwave access (WiMAX), 2.4/5.2/5.8 GHz wireless local area network (WLAN) and 3.1-6.0 GHz Direct-Sequence Code Division Multiple Access (DS-CDMA) / Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) ultra wideband (UWB) applications. <![CDATA[Analysis and Design of a GaAs Monolithic Tunable Polyphase Filter in S/C Bands]]> AbstractThis paper proposes a new topology of a 2-Stages Active Polyphase Filter (APPF) in MMIC GaAs technology. This new topology, named “radial”, allows a greater balancing of output signals phases and amplitudes, over the actual APPF’s at the state of the art. In this paper an ex-novo study is shown and synthesis formulas for the APPF are provided. The MMIC simulations in 1-5GHz bandwidth show a tunable Image Rejection Ratio (IRR) greater than 40dB, a worst case input/output matching of 7.8dB and a maximum insertion loss of 10.8dB. By comparison with the tandem topology, as will be shown in the following, the radial one allows a significant improvement in the electrical performances. <![CDATA[Long Period Bragg Grating in Coaxial Transmission Lines]]> This work shows the utilization of a coaxial cable for the fabrication of a long period Bragg grating. The grating is fabricated removing the dielectric in short pieces of the cable so that the discontinuities account for the variation in the medium refractive index. Simulated and experimental results of the grating resonances are shown as a function of the scattering parameters S, demonstrating the feasibility of the technique. By using the sensor in the MHz frequency range cheaper electronics can be employed, which reduce the overall cost of the sensing system. <![CDATA[Design and implementation of a Quarter Mode Substrate Integrated Waveguide (QMSIW) cavity filter]]> AbstractIn this paper, a novel Quarter Mode Substrate Integrated Waveguide (QMSIW) cavity filter is presented. A prototype at 5GHz for the proposed filter has been simulated using CST Microwave Studio, and fabricated using standard Printed Circuit Board (PCB) process. The fabricated filter has been measured using a Vector Network Analyzer (VNA). The measurement results are compared with the simulation results, good agreement is observed between simulation and measurement results concerning the pass band, the selectivity and the out-of-band rejection. <![CDATA[Etched FBG written in multimode fibers: sensing characteristics and applications in the liquid fuels sector]]> AbstractIn this work, a refractometric fiber sensor for the liquid fuels sector is described. The use of etched FBGs produced in multimode fibers is proposed to overcome the drawbacks associated with sensing high refractive index samples employing fiber transducers. The transducer sensitivity can be tailored by adjusting the final diameter of the etched Bragg grating. Due to a trade-off between the signal-to-noise ratio and the sensitivity, operational parameters of the sensor must be designed to match the expected refractive index dynamic range for specific applications. Metrological properties of the sensor are determined, resulting in resolution from 5.6 % v/v to 0.4 % v/v for refractive indexes ranging from 1.4562 to 1.4729. Specific applications regarding the quality assessment of biodiesel and conformity analysis of diesel-biodiesel blends are discussed. <![CDATA[Radio-Wave Propagation Model for UHF Band in Different Climatic Conditions with Dyadic Green’s Function]]> AbstractThis article proposes a deterministic radio propagation model using dyadic Green's function to predict the value of the electric field. Dyadic is offered as an efficient mathematical tool which has symbolic simplicity and robustness, as well as taking account of the anisotropy of the medium. The proposed model is an important contribution for the UHF band because it considers climatic conditions by changing the constants of the medium. Most models and recommendations that include an approach for climatic conditions, are designed for satellite links, mainly Ku and Ka bands. The results obtained by simulation are compared and validated with data from a Digital Television Station measurement campaigns conducted in the BelĂ©m city in Amazon region during two seasons. The proposed model was able to provide satisfactory results by differentiating between the curves for dry and wet soil and these corroborate the measured data, (the RMS errors are between 2-5 dB in the case under study). <![CDATA[An improved UWB Patch Antenna Design using Multiple Notches and Finite Ground Plane]]> AbstractAn UWB antenna design with finite ground plane approach and multiple notches is reported. The antenna design provided a 10 dB bandwidth in excess of 8 GHz with better return loss characteristics. The antenna implemented in FR4 substrate, is compact and cost effective. The measured antenna parameters are in agreement with simulation. <![CDATA[Semi-analytical formulation for a Unified Time and Frequency Antenna Characterization]]> AbstractThe definition of parameters that characterize the radiation of electric and magnetic fields for antennas in the time and frequency domain on an unified representation is proposed. The formulation uses a straightforward semi-analytical formulation that can be subsequently applied on the analysis of excited antennas for an arbitrary source with temporal behavior. The effective height is a parameter for antenna analysis defined for quantities in far field region and can be used as a transfer function of the antenna. This transfer function can be described through the antenna singularities which can be obtained by singularity expansion. The Singularity Expansion Method (SEM) is capable to model an electromagnetic quantity with the singularities extracted by the current densities of an arbitrary object. This work proposes that the singularities are extracted by the Matrix Pencil method applied on the current densities. The current densities are obtained numerically through the method of the Finite Differences in the Time Domain (FDTD) for wired log-periodic antenna and, after the singularities are obtained, the formulation of the semi-analytical effective height equation is written. To validate the presented method, a formulation of the time-domain radiation pattern is presented and a corresponding frequency-domain radiation pattern is also presented using Parseval’s theorem. <![CDATA[Maximum Power Transfer versus Efficiency in Mid-Range Wireless Power Transfer Systems]]> AbstractThe condition for maximum power transfer of 2-coils wireless power transfer (WPT) system is derived from circuit analysis and discussed together with the respective WPT system efficiency (η). In the sequence, it is shown that a 4-coils WPT system (which can be divided in source, two communication and load circuits) without power losses at the two communication circuits (ideal 4-coils WPT system) presents, from maximum power transfer and efficiency point of view, a performance similar to those of a 2-coils WPT system. The exception is the influence of coupling coefficient (k): in 2-coils system η increases as k approaches one, while in ideal 4-coils WPT system η increases as k between the two communication coils approaches zero. In addition, realistic 4-coils WPT systems (with power losses at the two communication circuits) are also analyzed showing, for instance, that η presents a maximum as a function of k of the communication coils. In order to validate the presented theory, 4 coils were built, and a setup to perform 2-coils and 4-coils WPT systems has been carried out. Practical results show good agreement with the developed theory. <![CDATA[Inversion Method for Obtaining Electrical Parameters for Soil in the Rural Region in Brazil]]> AbstractThis paper presents an inversion methodology through weighted least squares to obtain the electrical parameters for the soil of a typical mid-western region in Brazil using the model based in the formalism of the parabolic equations to calculate the electric field intensity received. To validate this methodology, the results of the radio signal measurement campaign conducted in six radial routes leaving the city of Brasilia, Federal District, where the transmitter was located, were used. The measurements were compared to computer simulations and, thus, the optimal values for the electric conductivity and relative permittivity for the soil of the region could be estimated. Finally, a quantitative analysis of these parameters was performed with the values found in the literature, which demonstrated the effectiveness of the proposed methodology. <![CDATA[An efficient method of EM field modification around transmitting stations]]> AbstractAn efficient practical method for the electromagnetic (EM) fields reduction around transmitting stations has been developed. The project has been stimulated by the necessity to reduce the field levels in the vicinity of transmitting stations, including FM, UHF, cellular base stations and other radiocommunication services, to obey regulations concerning human protection against electromagnetic radiation. The main constraint in the optimization procedure is the requirement to preserve the radiated power ERP of a service in operation. The method joins accuracy with high efficiency, and is based on 3 stages procedure using the antenna numerical models of different levels of approximation. An example is given of its application to a real case of transmitting stations in which the levels of measured EM exceeded the safety limits.