Scielo RSS <![CDATA[Journal of Microwaves, Optoelectronics and Electromagnetic Applications]]> vol. 16 num. 4 lang. es <![CDATA[SciELO Logo]]> <![CDATA[Magnetic topology with axial flux concentration: a technique to improve permanent-magnet motor performance]]> Abstract This article presents a different topology for the magnetic circuit of permanent-magnet motors, using the concept of Axial Flux Concentration. By means of longitudinal extension of the rotor length beyond the stator core, a substantial increase in the flux per pole is enabled, improving the general performance of the machine. Even in low cost motors with ferrite magnets, this improvement is easily achieved utilizing the proposed scheme. With rare-earth magnets this technique permits a higher torque density, contributing to the compactness of the machine. This configuration is suitable for several magnetic topologies, but particularly for small diameter and low pole number permanent-magnet motors of the embedded-magnet type. The proposed topology is briefly described and the theoretical aspects presented. It is shown that, from the stator point of view, this scheme behaves like a conventional motor with fictitious equivalent magnets with augmented remanent flux density and recoil permeability. A 3D finite element modeling is performed to validate the theoretical analysis permitting some additional conclusions concerning magnetic saturation. The methodology is applied to a prototype DC brushless motor constructed for research purposes, composed of a stator and two rotors of different lengths, both with embedded NdFeB magnets. Various characteristics are compared, concerning flux per pole, torque, power density and efficiency. The improvement in the performance of the axial flux concentration rotor is confirmed by experimental results, in accordance with the theoretical previsions. <![CDATA[Design of Waveguide Structures Using Improved Neural Networks]]> Abstract In this paper, an improved neural networks (INN) strategy is proposed to design two waveguide filters (Pseudo-elliptic waveguide filter and Broad-band e-plane filters with improved stop-band). INN is trained by an efficient optimization algorithm called teaching–learning-based optimization (TLBO). To validate the effective of this proposed strategy, we compared the results of convergence and modeling obtained with a population based algorithm that is widely used in training NN namely Particle Swarm Optimization (PSO-NN). The results show that the proposed INN has given better results. <![CDATA[Energy Efficient Rate Adaptation Algorithm for FiWi Access Network]]> Abstract Similar to any telecommunication network, energy efficiency is a desirable feature for fiber wireless (FiWi) access networks. These networks have optical back end and wireless front end. Both ends may contribute for energy efficiency. This work focuses on front end of FiWi access network, which is IEEE 802.11a wireless local area network (WLAN). For energy saving WLAN uses power saving mode (PSM), in which sleeping opportunity of a station is increased. During sleep time, station remains switched off and results in reduction in energy required. However, it is also observed that during active period of transmission considerable energy is consumed, which is the function of rate of data transmission. More data rate results in more active energy consumption but less transmission delay and vice versa. In order to reduce active and hence total energy consumption, we tried to transmit the data at lower data rate, while maintaining transmission delay in tolerable limit. This paper presents an Energy Efficient Rate Adaptation Algorithm (EERAA) for the front end of fiber wireless access networks. Simulation results compare the energy efficiency and transmission delay of EERAA and various existing fixed data rate schemes. Proposed scheme offers good trade-off between energy efficiency and transmission delay. <![CDATA[Optimized Limited Size Printed Bowtie Antenna for UHF RFID Readers]]> Abstract A novel design of a printed bowtie antenna without a ground plane and with size below a quarter wavelength for RFID readers is presented. Due to the lack of a ground plane, the designed antenna has omnidirectional characteristics and is insensitive to small permittivity deviations. For matching purposes without introducing gain distortion, a shorting strip connecting the radiating arms and four triangular shaped extensions of the radiating arms are added to the antenna design. Measurements show that operation in the 915 MHz band is met with a bandwidth of 13% (S11 &lt; -10 dB) and gain around 2 dBi, while presenting an omnidirectional radiation pattern. Also, the proposed antenna design can be easily modified to operate at neighboring frequency bands. <![CDATA[Multiband FSS with Fractal Characteristic Based on Jerusalem Cross Geometry]]> Abstract This paper proposes a new Frequency Selective Surface (FSS) with modifications on the Jerusalem Cross (JC) geometry to provide a multiband response. Fractal levels are created by introducing concentric self-similar JC copies in the original unit cell. This modification results in new dipoles at the structure which allows for the appearance of new bands proportional to these diploles’ lengths, while maintaining total unit cell size. Three fractal levels of the JC FSS are studied in this paper, and third level presents five resonant frequencies which are f1 = 1.78 GHz, f2 = 6.42 GHz, f3 = 8.69 GHz, f4 = 10.94 GHz and f5 = 12.98 GHz. Simulation results for the insertion loss (S21), surface current distribution and measurement results are presented. <![CDATA[A Direct Approach for Coupling Matrix Synthesis for Coupled Resonator Diplexers]]> Abstract Diplexers are used to separate or combine two bands of frequencies and they are designed by combining channel filters using distribution circuit or alternatively designed based on coupled resonator circuits. Design parameters of coupled resonator circuits such as coupling coefficients, self-resonant frequencies and external couplings are done using optimization techniques. However, optimization techniques are not always efficient particularly for large set of parameters and hence more efficient approaches are required. In this paper, numerical solution instead of optimization is proposed to synthesize coupled resonator diplexers and obtain coupling coefficients directly from equations. The proposed direct approach achieves the desired characteristics for any given number of resonators and some numerical examples are given to demonstrate the validity of the proposed approach. One of the given examples is compared to a diplexer in literature synthesized using optimization technique and implemented using cavity resonators. <![CDATA[Microwave Measurements of Dielectric Properties of Corn Vegetation at C-Band and Comparison with Debye- Cole Dual Dispersion Model]]> Abstract Microwave irradiations from vegetation canopy are strongly influenced by its dielectric properties and give useful information for microwave remote sensing -interpretation and applications. This paper reports on the laboratory measurements of dielectric properties of corn vegetation at C-Band frequency. Von Hipple method is used to measure complex dielectric properties using automated C-Band microwave bench set up. The least square fitting technique is used to calculate dielectric constant (ε′), dielectric loss (ε″) and errors in their measurements. The measured data is compared with Debye-Cole Dual Dispersion model. Emissivity and radiometric brightness temperature are theoretically estimated from measured dielectric properties at different angles of incidence for dry and moist corn leaves using Fresnel equations. <![CDATA[A Novel Design of an H-shaped Microstrip Diplexer]]> Abstract In this paper, a new microstrip diplexer using modified H-shaped resonators with a compact size and high isolation is presented. This circuit is designed for the ISM band at 2.4 GHz and GPS at 1.575 GHz. The proposed diplexer is formed by connecting two band-pass filters designed by using the modified H-shaped resonator and tapped microstrip Input/output (I/O) lines. An analytic study of the H-shaped resonator and an equivalent LC model of the proposed H-shaped band-pass filter were introduced. Two cross-slots are etched in the ground plane of the proposed microstrip diplexer in order to enhance its electrical performances and to set the pass bands at low frequencies. The circuit with defected ground structure is able to operate at the GPS L1 and L2 bands. The electrical performances of the diplexer are investigated numerically by using Momentum integrated in ADS from Agilent technologies and verified with another electromagnetic software CST microwave Studio. <![CDATA[Improved PLL-based MUSIC Algorithm for Single Receiver DF Systems]]> Abstract In this paper we propose an improved version of a single receiver DF system that combines a PLL scheme with the MUSIC algorithm to obtain both azimuthal and zenithal angles. The new approach uses a more efficient algorithm to remove the typical ambiguities of PLL-based schemes. We carry out several experiments to assess the algorithm performance. Among them, we consider complex environments, such as wireless communication channels where the received signal is corrupted by noise and suffers from distortion, interference, and multipath effects. The new scheme is tested for BPSK and QPSK signals under these conditions. The simulated results show superior performance of the proposed PLL algorithm when compared to previous techniques. <![CDATA[Shielding Effectiveness Measurements of Coaxial Cable and Connectors Using Compact Open Area Test Site]]> Abstract: This article presents a set of measurements and simulations that evaluate the technical feasibility of a Compact Open Area Test Site (COATS), for the frequency range of 200 MHz to 2 GHz. Its main goal is to provide shielding effectiveness measurements. The described test setup is used for the tests in both semi anechoic chamber and COATS. These experimental and computational results offer orientations and guidelines about the best method to measure radiated emission in a COATS. These measurements results can be quantified through the Feature Selective Validation technique. <![CDATA[A Small Quad-Band Monopole Antenna with Folded Strip Lines for WiMAX/WLAN and ITU Applications]]> Abstract In this article, a small Quad-Band microstrip patch antenna with folded strip lines for WiMAX/WLAN and ITU Applications is presented. This antenna consists eight symmetrical rectangular folded strip lines with 2 PIN diodes, ground plane and dimensions of 15×20×1.6mm3. Using PIN diodes integrated within the antenna configuration, the antenna would be able to operate in two modes. The operational frequency bands of antenna are WLAN (2.4/5.2/5.8GHz), WiMAX (3.5/5.5GHz) and ITU-Region 1 allocation (4.3GHz) when diodes=on. In this design approach of antenna, omnidirectional radiation pattern and S11-10dB bandwidths are obtained for all frequency bands. This antenna is fabricated on FR4 substrate with permittivity of 4.4 and loss tangent of 0.024.