نشریه الکترومغناطیس کاربردی
سال یازدهم شماره 1 (پیاپی 26، بهار و تابستان 1402)

Excessive sensitivity of light waves emitted within optical fiber to environmental changes has led to the widespread use of optical fibers as sensors of environmental parameters such as temperature, strain, stress, humidity, density, and so on. Among the fiber optic sensors, the fiber Bragg grating (FBG) sensor has found many applications in various civil and industrial fields due to its high accuracy, reasonable sensing length and low price, but some limitations of this sensor has led to a significant reduction in performance. One of the most important of these limitations is the problems in measuring several environmental parameters at the same time, especially temperature and strain. This limitation has led to the suggestion of many methods for measuring temperature and strain at the same time, using the FBG sensor. Most of these methods have require more than one uniform FBG, and other methods use one special FBG to measure temperature and strain at the same time. Each of these methods have disadvantages such as requiring more than one uniform FBG, complexity in setup and overused of spectral sources. Our purpose in this paper is to numerical simulation as well as design a suitable setup for calculating temperature and strain changes at the same time, by a single uniform FBG. Based on the simulation results obtained from MATLAB software, the sensitivity of this sensor is 14 pm/℃ for temperature and 0.678 pm/με for strain.

Flight systems, have extensive applications in various scientific, industrial, and commercial fields. One component utilized in flight systems' structure is radar. In various applications of these systems, it is required to track specific targets and directions in a narrow angular region. This feature is achievable by utilizing the narrow beam-width antennas. Array antennas, besides providing the demanded gain, can fulfill such requirement. Also, the beam-width should be tunable in an acceptable range of different directions. Such a tunability can be realized using the phased array antennas. The capability of change in main lobe direction of these antennas is provided using the active phase shifting components as feeders of the phased arrays, such as PIN diodes and ferrite devices. However, using the passive Butler matrix components is considered as simpler and cheaper tool to realize the approach. Utilizing the Butler matrix with more input-output ports, leads to narrower beam-width radiation pattern. In this paper, a simple design of 32×32 Butler matrix for X-band frequency spectra is proposed, and the simulation results of its performance are presented. The simulations are carried out via Comsol software which is based on finite element method. Finally, after applying the appropriate waves to two specific input ports and connecting the Butler matrix structure to the microstrip array, the beam-width of 3.5 degrees is achieved. The achievement to narrow beam width radiation realized by a microstrip antenna array fed with a 32×32 Butler matrix and only based on a single layer board, is the main purpose of the research.

High-frequency power transformers are available in electronic power converters for many applications such as power transmission, renewable energy systems, and power supplies. Magnetic materials production technologies such as ferrite, amorphous, and high-frequency nanocrystals are used to miniaturize transformers and optimize the design. In this paper, the design of a high-frequency transformer based on the finite element method (FEM) is proposed, and a frequency transformer above 5 kV and a power of 100 watts with a ferrite core is designed for use in high voltage charges in pulse power technology. After the calculation step, the three-dimensional model of the transformer with the nonlinear core is created with Maxwell finite element analysis software and then the simulations of the electromagnetic model of the transformer with electronic power converter circuit are implemented with the help of Simplorer software for operating conditions. Also, the efficiency of the transformer, the exact equivalent circuit of the transformer, and the flux distribution in the transformer core are obtained. In addition, transformer samples have been fabricated and tested. The data obtained from the finite element method are compared with the analytical and laboratory methods. The results show that the finite element method seems more accurate compared to the analytical methods. Due to the importance and complexity of designing high-frequency transformers, using this method can provide advantages and simplicity for transformer designers for parameter calculation and optimal design.

Asphaltene is a component of crude oil that creates a variety of problems in the oil industry, including reservoir wettability alteration,, corrosion in the pipelines, and pore plugging. In this paper, asphaltene samples collected from four oil wells in southwest Iran were characterized using Raman spectroscopy to investigate their molecular structures. The recorded spectra were analysed using the integrated intensities of the observed G and D1 modes, utilizing Tunistra and Koenig's proposed model. The analyses result in an estimation of the aromatic sheet diameter (La) of asphaltene samples in the range of 1.3-2.5 nm. The obtained results are consistent with those previously reported for asphaltene samples from other parts of the world. Furthermore, spectroscopic studies of recorded FT-IR spectra of samples allow estimating the structural parameters of asphaltene's Aliphatic, Aromatic, Long-chain, Substitution 1, and Substitution 2 indices with average values of 0.20, 1.36, 0.056, 0.32, and 0.28, respectively.

Permanent magnet brushed DC motors are widely used in the automotive industry due to their low cost and simple speed control. Sensorless speed and position estimation are interesting topics for car manufacturers and researchers. Usually, the armature current ripples are used for senseless speed estimation. Therefore, this paper aims to present a model to precisely predict the behavior of this motor, especially in the prediction of armature current ripples. First off, the previous models of DC motor are studied, and then a new test is proposed for measuring armature resistance to increase the accuracy of the model. Besides, the effect of cogging torque was investigated and the model was modified to consider the cogging torque. Finite element analysis used for different studies and verifying the performance of the proposed improved model. Comparison between the results obtained from the proposed model, previous model, and finite element method shows the superiority of the proposed model.

The dispersion relation and the growth rate of the hybrid electromagnetic waves propagating in the two types of combined waveguides with complete conductor walls including isotropic cold plasma column that powered by relativistic electron beam, have been studied.The conducting wall cross section of the first waveguide is elliptical that plasma column with circular cross section is located in its core and the rest of interior regions is filled with dielectric material from the plasma boundary to the waveguide boundary. But the second waveguide has acircular cross section that elliptical plasma column is situated in its center. Electron beams as source energy are injected inside it coaxially with plasma column. By calculation of electromagnetic fields components in each of regions in the considered configurations, applying the boundary conditions, operating frequency and growth rate are obtained. The results of numerical computations are graphically presented.

Radial linear slotted array antennas are slotted wave-guide antennas. These antennas are highly efficient and are used to produce circular polarization. In this paper, a slow wave structure is used to increase the tolerable power of the antenna. The use of this structure creates a two-level radial linear slot array antenna. On the slow wave structure, there is a slot plate that is fed by this structure. Both slots perpendicular to each other (to create circular polarization) form a unit of radiation. Radiation units are distributed on a spiral path at regular intervals. This antenna is designed, simulated, and built to operate at a frequency of 9.45 GHz and a radius of 270 mm. According to the simulation results, the reflection coefficient is -18.93 dB, the radiation efficiency is more than 93% and the aperture efficiency is 28.5%. The evaluation of the value of the reflection coefficient for a sample made in the range of 9.385 GHz to 9.417 GHz is less than -10 dB.

In this paper, a comprehensive study was performed to investigate the effect of the absorbent layer thickness on the tapered fiber sensor sensitivity, which leads to the highest sensitivity for the sensor. For this purpose, a staircase method was introduced to investigate the effect of layer thickness on the sensitivity of fiber optic sensors. According to the results obtained from the simulations performed using this method, it was found that the type of variation in the sensitivity of the tapered fiber optic sensor depends on the refractive index of the layer. To investigate the dependence of sensitivity on the layer thickness and refractive index of the layer, two different general cases have been considered. If the refractive index of the layer is smaller than the refractive index of the fiber, it can be seen that the sensitivity increases with increasing the layer thickness. On the other hand, when the refractive index of the layer is greater than the refractive index of the fiber, there are optimal points in the sensitivity plot versus the layer thickness, which demonstrate the highest sensitivities.

Light propagating in the hollow-core photonic crystal fiber is based on the photonic band-gap (PBG) structures. Triangular and honeycomb structures are sub-structure of the alternating hexagonal structure. In this paper, several geometric factors such as structure type, air-filling factor, and core size, are investigated and compared on the gap map of are triangular and honeycomb photonic crystal fiber. The basic configuration has a cylindrical shape with an air-hole in the silica surroundings. The propagation beam is assumed to have hybrid mode polarization. Simulation of triangular and honeycomb structures in three dimensions has been shown that there is no band-gap structure for longitudinal transverse modes. The assumed input energy exhibits in the C band. The geometrical parameters include lattice period, air-filling factor considers to have the same values in both structures so that the structures are comparable. The results of this paper have been performed using R-soft photonic band-gap software.

Magnetic anomaly detection (MAD) is a passive method for airborne detection of subsurface objects. Light, radar and sound waves are unable to pass through the open air into the sea water environment and penetrate deep into the water, weaken or return to the air environment. On the other hand, the magnetic field force lines on this boundary are unchanged. MAD method is based on measuring the smallest changes or anomalies caused by the earth's magnetic field due to the passage of a ferro-magnetic object and the magnetic bipolar field generated around it, and especially in shallow waters, is one of the most efficient methods. Due to the rapid reduction of the magnetic field by increasing the distance, the magnetic disturbance generated by the magnetic target in the distance, usually buried in magnetic noise, in other words, the signal-to-noise ratio (SNR) decreases. In this paper, in order to improve the detection performance of magnetic impairment in low SNR, a hybrid method of MAD based on empirical mode decomposition (EMD) and minimal entropy method is proposed. In order to evaluate the performance of the method, an electronic measuring device has been constructed and magnetic data have been fieldly harvested from caspian sea in Anzali port area. These data impregnated with environmental magnetic noise have been investigated by entropy method. According to the entropy feature, magnetic anomaly is detected whenever entropy degrades below the defined threshold. In this way, the proposed method for detecting weak magnetic anomalies is also effective. The test results indicate a high probability of detection of the proposed method for low input SNR. Compared to the original signal SNR with -10 dB, the reconstructed signal SNR has improved to 8 dB. In addition, the total time of updating the parameters of the probability density function (PDF ), of noise is about 0.075s obtained.

In this paper, a new electromagnetic fault current limiter with rotary motion is presented. The features of this limiter are its simple structure, low construction and maintenance cost, optimal performance speed, and easy and quick return to initial conditions after the error is corrected. This limiter consists of two spherical air core coils with the ability to rotate freely around the radial axis. In normal conditions, due to the presence of negative mutual inductance between two coils, the total inductance of the limiter has the lowest value. When a fault occurs due to the forces created between two coils, these coils are automatically rotated relative to each other, and the limiting inductance increases rapidly, and the fault current is limited. In this article, for the optimal design of the limiter, the effect of all the limiting parameters such as radius, height, thickness, number of turns and the initial angle of the coils has been investigated and in order to achieve the highest final inductance and the highest operating speed, a suitable value is determined. The results of the simulation show that the designed limiter has a better performance than the initial design and has a good performance speed and final inductance.