نشریه الکترومغناطیس کاربردی
سال چهارم شماره 2 (پیاپی 11، تابستان 1395)

In this Article, no-load magnetic flux density and Back-EMF of the redial flux Air-Cored permanent magnet electrical machine (RFAMP) is provided with meddle stator using the subdomain analysis. First, Analytical relationships of no-load magnetic flux density and Back-EMF of this machine are presented with maxwell equations and, then, these relationships are validated by finite element numerical results in Maxwell 16.02 software. This comparison shows that the difference between analytical relations and finite element results is 3% about. In this article, to calculate the Back-EMF resulting from the permanent magnet, all the components of winding conductor density distribution are used. It should be noted that in the investigations relating to this case, only the first radial component of the magnetic flux density is used. It is also shown that magnet width coefficient plays an essential role in the non- sinusoidal of the no-load magnetic flux density in the stator nominal radius and reduction of the number of pairs of poles in the magnetic saturation of yokes and induced voltage reduction. It is also shown that increasing the length of the air gap can have the greatest role in zero harmonic distortion of Back-EMF.

This paper presents a novel omnidirectional circularly polarized antenna with simple and low cost configuration designed by a combination of tilted semi loops antenna for unmanned aerial vehicle such as drone and quadcopter. Length of each semi loop is one wavelength that has been repeated around antenna axis with a specific angle. The 90° phase difference between two equally orthogonal polarized components is inherently provided by semi-loops and without the need for exciting semi-loops 90° out of phase. Symmetric configuration antenna yields in circular polarization in both the azimuthal and elevation plane. The measured results show that the 10-dB impedance bandwidth is 390MHz (range from 2.1GHz to 2.5GHz) at frequency of 2.45GHz and axial ratio bandwidth for less than 3dB is 400MHz (range from 2.1GHz to 2.5GHz). Also this antenna has a right hand circular polarization whose difference between the measured left hand circular polarization and the right and circular polarization gain is about 20 dB. The right hand circular polarized gain is about 1.5dBi.

Most of microwave sources, such as electromagnetic tubes, generate circular TEM or circular TM01 mode. The best radiating mode of cylindrical waveguides is circular TE11 mode. Therefore, mode transducers are used to convert circular TEM or circular TM01 to circular TE11 mode. In this paper, designing and fabricating of a mode transducer that convert circular TEM to circular TE11 mode are investigated. The combination of this mode transducer with an open-ended waveguide antenna makes it sutible for compact usages. The mode transduser antenna is designed to handle high power and therefore its internal area is closed by the help of a dielectric window to become vacuum. Mode conversion is carried out by changing the phase velocity of each mode-transducing section. The maximum achieved gain of this antenna is 8.8 dB in 1.48 GHz frequency. An optimized dual dielectric window at the end of the waveguide improves the antenna matching. The measured results are in a good agreement with simulation results.

In this paper, the effects of wall structures on the parameters of realistic indoor wireless channel using two dimensional finite difference time domains (FDTD) method are investigated. Two types of wall are considered for the environment. In the first case, the walls are a type of concrete block with air gaps as inhomogeneous walls and, in the second case, the walls are a type of homogeneous oneswith effective constitutive parameters. For two cases, field coverage maps, average path loss, power delay profile and RMS delay spread are extracted and compared. Power delay profiles show that inhomogeneous walls create stronger multipath effects than homogeneous walls and that these effects are severe in none line of sight conditions. On average, the RMS delay spread for environment with inhomogeneous walls, in LOS conditions 5 ns and in NLOS conditions 8 ns is more than the environment with homogeneous walls.

In this paper, a marine magnetohydrodynamic thruster is simulated three dimensionally and its operating parameters are obtained. In this simulation, both electromagnetic and fluid flow fields are considered in three dimensions and the effects of magnetic field non uniformity on the operating parameters of the thruster are investigated. To do this, a typical saddle shaped MHD thruster with specific dimensions is selected and its geometry is implemented in the software environment. The electric current of superconducting coils is selected using a trial-and error method, in such a way that the average magnetic flux density in the channel becomes 15 T. An analytical model is used to validate the numerical results. It is shown that some of the operating parameters of MHD thruster, such as fluid velocity, are not affected by the end current of the channel. These parameters are functions of the effective electric current of the thruster and can be calculated analytically. On the contrary, other parameters, such as efficiency, are strongly influenced by the end effects of the channel. These operating parameters are functions of the overall electric current of the thruster and should be calculated numerically.

Interferometer is the one of tools to determine the plasma characteristics. In this paper, Firstly the regime of propagation mode for an infrared laser interferometer in plasma medium of Damavand tokamak is determined by considering the tokamak magnetic field range and the propagation angle. At the same time, the role of laser wavelength of diagnostic system in changing the propagation mode is examined. Results indicate, in a certain magnetic field, by increasing the laser wavelength the limitation of the quasi- longitudinal propagation mode becomes smaller, conversely that of the quasi-transverse propagation mode becomes larger. The introduced error in determining the refractive index with comparison between the refractive index of the ordinary mode and averaged refractive index from the Altar- Appleton- Hartree dispersion relation depends on the wave propagation angle and also the strength of the magnetic field. In a certain laser wavelength, increment of magnetic field increases the error value in phase measurement. On the other hand, in a specified magnetic field, by increasing of the laser wavelength, the error of phase measurement increases. Also, in a given propagation angle, by increasing the laser wavelength the error value in phase measurement significantly increases.