نشریه الکترومغناطیس کاربردی
سال چهارم شماره 4 (پیاپی 13، زمستان 1395)

In this paper, we propose a model with frequency-dependent lumped parameters to represent the behavior of laminated ferrite -core inductors that are used in the Local Position System. The total power loss in the inductor consisting of the winding resistance loss and the core loss is modeled by a frequency-dependent equivalent series resistance. The magnetic field distribution in the core is derived from Maxwell's equations for long solenoid. The complex permeability of the ferrite core is introduced in the electromagnetic field equations. The behavior of the inductors showed that the effects due to the resistance in the laminated ferrite core and the turn-to-turn and turn-to-core stray capacitances become significant in different frequency ranges. A critical point in the design of laminated ferrite-core inductors is the evaluation of the inductance behavior versus frequency near 100 KHz and the additional core losses. The purpose of this paper is to explore the behavior of ferrite core inductors and present a circuit model valid at high frequencies.

Most of high power microwave generators have a TM01 mode in their circular waveguide output which makes them improper due to its doughnut shaped pattern. To overcome this problem, Coaxially Beam Rotating Antenna (COBRA) is used which transforms TM01 mode to TE11. The conventional structure of COBRA, depending on the required polarization type, consists of 1-4 discrete steps. Minimum number of steps to create circular polarization is 3. In this paper, a modified COBRA including a continuous tapering which mimics an infinite number of steps, has been proposed. This structure is implemented using CNC technology. A convex lens is used to increase the gain up to 1dB. Compared to conventional circular polarized COBRA with 3 or 4 steps, this structure has improved side lobe level, significantly. In addition, impedance and axial ratio bandwidths of this antenna are more than 5.5 and 4GHz, respectively. The experimental test results are in good agreement with the computer simulations.

For achieving acceptable level of QoS, network designers rely on the results of propagation path loss models and based on these results they design. In this paper, for prediction the loss power of Tehran city for 3G network, we have applied Standard Propagation Model (SPM). Propagation models are used for path loss calculation to predict the area under coverage in a certain environment, frequency allocation and interference that are main concerns of cellular network planning. In many papers, propagation models presented for different cities, but it must be noted that all of their cities don’t have specifications of the Tehran city and furthermore, there is not any research paper for Tehran city yet. Also, the number of considered cites in this paper is more than the other papers, in spite of in those papers, maximum three of four cites were considered. Tehran, due to its position to Alborz mountains, have a continuous slope from North to South. In the north part of Tehran that is considered to be foothill clutter types and is suburban. Buildings in the north are taller than the buildings in the south. The official organizations are almost in the center of this city. The east of Tehran is an almost flat area with buildings with medium heights. These Tehran specifications make Tehran be a special city and as a result, its network design will be hard. The plan of this research, using drive test data, is done for 2100 Mhz frequency band for almost 400 cites. First, the SPM model and simulations with Atoll and MATLAB is done for three regions of Tehran, and finally we have proposed a propagation model for Tehran.

Bandwidth of patch antennas has always been a challenge for designers. One of the simplest methods of ‎increasing its bandwidth and gain is to use a thicker patch antenna substrate. Using this method in a truncated ‎circular polarized patch antenna causes a frequency shift between impedance bandwidth and axial ratio ‎bandwidth. In this paper, we were able to compensate this frequency shift by transforming the square truncated ‎patch antenna into a rectangle truncated patch antenna. Using analytical analysis, the factor the main cause of ‎this frequency shift is shown and then its effect compensated. The performance of the proposed method is ‎successfully verfied by applying it to a truncated patch antenna in UHF frequency band designed with air and ‎FR4 substrate. Before applying this method, the total antenna bandwidth namely the common band between the ‎impedance and the axial ratio bandwidth of the antenna is less than 1% while by applying the proposed method, ‎the bandwidth of the antenna is enhanced by about 5%. The manufacture antenna’s measurement results are in ‎good agreement with the simulation results.‎

Corona losses are one of the main issues in the design of HVDC transmission lines. A lot of research has been done in the calculation of corona losses in DC transmission lines in various line voltages. In bipolar transmission lines, positive ions, negative ions and ion-ion particles are produced under the influence of corona. Due to the presence of various particles in the corona, the complexity of the calculations will be higher than unipolar fields. In this article, the bipolar relations governing the field are solved and after calculating the positive and negative load flow density, corona losses are calculated.
In this paper, finite element methods (FEM) have been used to calculate corona losses in a DC high voltage transmission line. Simulations of the FEM method have been performed by ANSYS software. Given that the algorithm provides a lot of calculations in the analysis of the results of simulations and reloading the results in the simulation, then the MATLAB software has been used to analyze the ANSYS outputs. The results obtained in this paper include distribution of electric field strength vectors, electrical flux density, potential and also free loads within the corona range. In addition to the above, corona flow and casualties are the final results of the calculations made in this article.

In the manufacture of porous piezoelectric ceramics (Pb0.99 (Zr0.95Ti0.05)0.98Nb0.02O3) is usually used polymethyl methacrylate (PMMA) granules for the production of porosity. Creating porosity causes changes in electromechanical properties and reduces or increases some piezoelectric parameters. Adding PMMA from 0.6% to 2.2% reduces piezoelectric constant from 78.1 pC / N to 68 pC / N. It also provides a dielectric constant for polar components from 400 to 307. By increasing the porosity, the piezoelectric voltage constant increased from 9.96 ×10-3 V. m / N to 22.3×10-3, and then decreases. To the samples, a mechanical shock was applied to generation the voltage. The highest voltage achieved was 57.5 KV for samples containing 1% PMMA. The structural analysis of samples by TEM indicates the presence of coarse and regular crystals.