فصلنامه رادار
سال سوم شماره 2 (پیاپی 8، تابستان 1394)

A compact and sharp-rejection ultra-wideband (UWB) microstrip bandpass filter (BPF) is presented. The proposed filter is constructed with multiple-mode resonator (MMR) and split ring resonator (SRR). Moreover, proposed structure consists of two doublets parallel cou-pling gaps at each side of a microstrip ring. In comparison with some other filters, this struc-ture shows a significantly wider passband due to the introduction of a cross-coupling be-tween the feed lines (input and output) which generate four pairs of attenuation zeros in the passband. In order to control the transmission zeros in the pass band and adjust the transmis-sion zeros near the lower edges of the filter, a Short Stub, Loaded on microstrip Rectangular Ring Has Been Used. Also, In order to sharpen the upper edge of the filter, we used SRRs in both side of the doublets parallel coupling gaps that leads to the addition of one extra trans-mission poles at the upper edge of the filter. Consequently, a compact four-zero and one pole ultra-wide bandpass filter is designed which exhibits extremely sharp rejection skirts around the target passband. The proposed filter has a passband covers 2.8 to 10 GHz and its meas-ured 3 dB fractional bandwidth is about 112.5%. Furthermore, rejection level better than 20 dB in upper stopband is extended to around 18 GHz. To our knowledge, the size of proposed ultra-wideband filter is more compact in comparison with known similar filters.

Controlling the electromagnetic waves propagation using optical transformation is a topic that finds many applications in recent years. One of the most important applications of these transformations is hiding the objects from Radar waves، i. e. cloaking. Hiding under the car-pet or carpet cloak is a special type of cloaking that hides the objects situated between a met-al sheet and the ground. In this paper we propose a simple way to design a carpet cloak for a circular sheet. The main advantage of this method is its simplicity and using closed form transformer. Also a new method named “Modified Hausdorff distance” is proposed to ana-lyze، compare and check the similarity of the backscattered waves. Finally a carpet cloak structure is proposed to hide a hangar with the help of PSO algorithm to achieve optimal di-mensions. Simulations are based on Finite Difference Time Domain (FDTD) method using MATLAB software in the frequency range of 50MHz to 2GHz and the results are accepta-ble.

One of the problems in the ground-based circular synthetic aperture radar is the dependence of the azimuth resolution to the antenna rotation radius and operational limitation of increas-ing it. To enhance the image quality in azimuth، model-based image formation through three matched filter، inverse filter and optimum filter is suggested and analyzed. For the purpose of comparison، three quality measures for the noise، distortion and total error levels in the output image are extracted and it is showed that each of the three suggested image for-mation methods is optimum in one of these measures. For increasing the speed of the algo-rithm، the coefficient matrix of the proposed model is diagonalized by the replaced rows Fourier transform matrix and using this property، it is showed that despite of accuracy، the proposed algorithm is faster than the well known Back-Projection algorithm.

High resolution range profile (HRRP) is being known as one of the most powerful tools for radar target recognition. The main problem with range profile for radar target recognition is its sensitivity to aspect angle. To overcome this problem, consecutive samples of HRRP (or feature vectors extracted from them) were assumed to be independent identically distributed (iid) in small frames of aspect angles in most of the previous works. In this work, an alterna-tive method based on dynamic system is proposed to overcome the limitations of the recent methods such as the independency assumption. Two different feature extraction schemes using spectral features and PCA coefficients are utilized. When the first scheme is employed, modeling is done in short-term, whereas long-term modeling is done while employing the second scheme. Simulation results show superiority of our model over common models us-ing either approach to feature extraction.

Detection of stealth targets calls for employment of high-power radars which itself relies on usage of high-gain antennas like large parabolic reflectors. In the past few years، using of an array with many small reflectors instead of a large reflector antenna has been considered. This method provides enhanced EIRP by a factor of N2 where N is the number of array an-tennas. Regarding improvement of EIRP، signals radiated from each antenna should be co-herently added in the destination and phase calibration between array elements is a critical issue. In this paper، calibration opportunities using satellites have been investigated. Loca-tion of phase center of array elements and location of satellites are taken into account and their effect on arrays with different sizes has been analyzed.

In this research، active capacitance circuit and the feature of negative resistance are analysed in details. Also، a new active filter is presented based on active capacitance circuit. For simulations، we used MATLAB and Advanced Design System (ADS). proposed active filter (RF) is a narrow band filter. The results of simulation shows some aspects of sustainability and low noise feature. The circuit has negative resistance at frequency range of 2-7 GHz، in which negative resistance is adjusted with neural network. The central frequency is 5. 50 GHz، and band width is 278 MHz. In the central frequency the gain is 1 dB and quality factor (Q) is 24. The active filter noise figure in central frequency is 1. 51 dB. Generally، K factor (stability factor) is greater than one meaning circuit is completely stable in the considered frequency range. The results from regulations with neural network has an error less than 1% in central frequency.