نشریه الکترومغناطیس کاربردی
سال دوم شماره 3 (پیاپی 4، پاییز 1393)

This paper presents an approach to calculate the initial equivalent inductance and resistance of helical flux compression generators and their dependence on frequency accurately. This approach is based on the 2-D filamentary method in frequency domain. By using this method, it is possible to consider both the effects of the armature presence and frequency on the inductance and resistance of helical flux compression generators. In order to consider the eddy current effects on the equivalent resistance and inductance of the helical coil, we introduce an eddy current coefficient using the filamentary method. The calculation results show that the presence of the armature not only decreases the equivalent inductance of the HFCG, but also increases its equivalent resistance. The latter effect has not been discussed previously in the open literature. To verify our derived equations, a small scale HFCG was built and its inductance, resistance and impedance were measured at different frequencies. It is shown that the calculation results demonstrate a good agreement with measurement results.

Millimeter wave systems have a wide frequency band. Reflector antennas are a good choice for use in these systems, due to the broad frequency coverage, high efficiency and gain. In the present paper parabolic reflector antenna characteristics and their design principles have been provided. For this purpose, first geometry of the parabolic reflector is inspected, and then design process of Cassegrain antenna and its feed have been discussed. Based on it, the design of a Cassegrain antenna has been done with its feed. Due to the high sensitivity of Cassegrain reflector to the placing of phase center of the feedhorn antenna at focusing point of the reflector, it has been tried to obtain equal phase centers at E and H-planes. After the simulation of antenna by CST MWS, the gain of the antenna is obtained 37.27dB. In the current study a broadband Cassegrain system designed and implemented by using designed feed antenna that operate well in 30-40GHz.

An analytical model for prediction of air gap flux density and cogging torque in slotted Surface PM (SPM) machines have been presented. Cogging torque has been calculated by integrating the Maxwell stress tensor inside the air gap. Two different magnetization patterns (radial and parallel) have been considered and their results have been compared with each other. The obtained model has been verified with Finite Element Analysis (FEA). Using the proposed model, the cogging torque and low-order harmonics of the magnetic flux density have been optimized. The cogging torque and air-gap flux density THD are very sensitive to the pole-shaping, thus changing the pole-shaping optimization can be obtained. Three methods (optimal pole-arc, magnet pole-shifting and mixed materials) have been used to design pole-shaping in SPM machines. A weighted normalizing method has been applied with direct search method to find the optimum solution. Finally, the validity of the proposed model and the obtained results has been verified with FEA.

Each Channel of the phased array antenna innately has the amplitude and phase errors. The effect of mechanical errors on the performance of the active phased array antennas is of great importance. In this paper two kinds of mechanical errors, namely, antenna structural distortions and random errors in element positions is studied and the relationship between the electromagnetic performance and the mechanical errors of the antenna is proposed. The mechanical errors can create phase errors and reduce the antenna performance, for instance the gain reduction, sidelobe level increase, and inaccurate beam direction. In order to enhance the performance of the antenna in the presence of mechanical errors, a phase compensation method for error correction using Particle Swarm Optimization (PSO) is proposed. The simulation results show that the proposed method can greatly correct the phase errors and after correction the overall radiation pattern is recovered close to ideal radiation pattern without error.

The most common application of millimeter wave is the imaging from buried metals. 94 GHz is one of the best choices due to the signal loss curves. In this paper the antenna of a millimeter wave imaging system is studied. The main purpose is to provide an image with 2 cm × 2 cm resolution at 4 m distance (near-field). The power at this distance with 1 cm offset should be half of the power on the antenna axis without offset. Therefore the antenna diameter should be about 70 cm. The feed antenna has the -12dB beamwidth of 24° and its phase center is approximately placed on a single point for both e-plane and h-plane patterns. Different sub-reflectors are used to obtain the best efficiency for the near-field focusing antenna. Finally the effect of rotating the sub-reflector of antenna system is indicated for three angles on the form of field distribution and the efficiency of antenna system.

In this paper, a smart antenna with arbitrary down tilt is proposed for 3G wireless communication. In the third communication generation, the antenna downtilt is a common method used to adjust the interference conditions especially in urban scenarios with high base station density. The proposed antenna is able to tilt the antenna pattern in the desired direction as well as null in the interference directions. In the design procedure, the antenna element is designed and optimized to achieve the desired properties for 3G networks. Then a proper algorithm is used to minimize the noise variance in the antenna output. Finally, some simulations are performed to verify the proposed method. In these simulations, the antennas down tilts are considered as 0, 5 and 10 degrees with arbitrary interference directions. Moreover, some antenna with different numbers of interferences are designed and simulated. These results verify the ability and capability of the proposed method.