فصلنامه رادار
سال هشتم شماره 2 (پیاپی 24، پاییز و زمستان 1399)

This paper focuses on improving the signal-to-noise plus interference ratio on multi-input multi-output radars. Here, an algorithm is proposed, whereby the waveform of the transmitter and the receiver filter coefficients are designed simultaneously to better detect the targets in the presence of the dependent interference signal. The proposed algorithm is a convex optimization-based sequential algorithm, in which each iteration optimizes the covariance matrix of the transmitted signals to concentrate the antenna radiation pattern on the target and also attempts to eliminate the maximum number of interferencein the receiver. The problem is in addition to limiting the use of identical RF circuits to all transmitting antennas, with the limitation of minimum interference power for each target. In previous research this scheme was designed to intercept a target, but in this study the covariance matrix is designed to maximize the signal-to-noise and interference ratio and the antenna transmit power at the maximum position of all targets and at the interference position as low as possible. The simulation results also show that the proposed method can achieve the maximum signal-to-interference plus noise ratio in all targets. This value can also be increased by increasing the number of antenna elements.

This article studies the design of a power modulator for driving a magnetron tube. In this design thepower modulator works with the half-bridge structure, series resonator and phase shift control method.In this scheme, by controlling the switching phase shift,the balance condition of magnetization current issatisfied and the maximum amount of core saturation current is reduced. As a result, the current stress of switches is reduced and switching and core losses are reduced. So, the size, weight and the cost of thecore of the power transformer will reduce. Also, using leakage inductance of the power transformer, the series-resonance circuit is provided soft switching condition is guaranteed. Suggested power modulator,is able to control the peak and average output power with the minimum loss. The maximum power of a microwave oven is about 1000 w and its average power is 250 W which is controlled proportionally to theinverter on-state periods. Other advantages to the proposed design are simplicity of the power circuit,decrease in the number of switching elements, decrease in the switching losses, and decrease in the leakage inductance of the power transformer. The design results have been verified through simulated ofthe PSCAD software and laboratory tests.

Using of compressive sensing in radar systems caused to eliminate the matched filter from receiver, and to reduce the required receiver analog-to-digital conversion bandwidth in radar systems. One of compressive sensing parameters is measurement matrix. Measurement matrix for radar systems is usually random matrix. Although exact recovery of signal using random matrices is possible with high probability and this matrix is incoherent with every basis matrix but implementation of that is impossible in practice. So it is useful to use deterministic matrices as measurement matrix. One of these matrices is Alltop matrix that obtained from Alltop sequence. There are limitations in use of this matrix for compressive sensing. We not only will resolve These limitations in this article but also will present a suitable alternative for matched filter block based on compressive sensing that has better performance in comparison to matched filter and can reconstruct radar targets with lower error than matched filter.

Microdoppler is a new subject in radar signal processing with a research history of about twenty years. Radial velocity of targets relative to radar position, causes doppler frequency shift of return signal to radar. In addition to radial velocity of target fuselage, many targets consist of components which have small scale motions which results in frequency modulation around the main doppler frequency shift. Vibrations of the motor components and fuselage of airplanes and blade rotation of helicopters are some examples of these types of small scale motions which produce periodic doppler shifts known as microdoppler. Extraction of microdoppler shift and its properties is very important in target classifications and estimating many parameters of the target. In this study we focus on microdoppler properties of return signal of radar from helicopter. Estimation of helicopter parameters such as number of blades, their length and velocity of angular rotation and finally identification of helicopter type are aims of this research. For this purpose, the previous methods for estimating the helicopter parameters are reviewed and classified.

Multi-target ISAR imaging is one of the most important and challenging issues in radar imaging. Detection and imaging of targets at low altitudes and near the surface due to the presence of surface clutter is also a research topic in this area. If air targets are flying at low altitudes in the sea environment, the sea clutter will greatly affect the radar signals received from these targets and this problem will increase the difficulty of ISAR imaging. In this paper, that extracted from the results of a research project, an efficient algorithm is presented that can perform ISAR imaging of multiple targets at low altitude in the presence of sea clutter, despite the aforementioned challenges. In this algorithm, called target grouping, based on space-doppler adaptive processing (SDAP), the effect of sea clutter is reduced and then by compensating the translational motion of each target group, its image is formed. Target clustering (grouping) based on the similarity of translational motion parameters and image formation of each group in a frame are the most important parts of this algorithm. The results of software implementation and simulation show that it is possible to effectively reduce the effect of sea clutter using the SDAP method and successfully form a group image of targets in clusters with the proposed algorithm. This result is very important in practical issues such as detection and identification of air targets in the sea environment.

Concealment and hiding of military equipment from the enemy is one of the most important issues in the design of this equipment. This hiding generally performed against microwave frequencies, and the quantity that minimized is the radar cross section (RCS). In order to measure the mentioned quantity, it is necessary to expose the target in an environment without reflection to electromagnetic waves and then to measure the reflected waves from it and thus determine the RCS. In most cases, RCS of an object is the desired in the absence of the ground effect. However, the object is exposed to the ground effect due to its position on the pylon, and in addition to measuring the real object, its image also be seen. One way to reduce the ground effect is to use metal fences. However, the edges diffraction and the large side lobe level of the reflected wave from these fences has made their use difficult. As shown, the use of resistive tapered fences solves both of the above problems. In this paper, for the first time, using 7 fences with tapered resistance and designing a desired arrangement for them, this effect has been removed in the form of five examples. Since the length of the fences is several meters and several tens of wavelengths, here the whole structure considered two-dimensional and therefore a two-dimensional moment code (2D-MoM), in addition to the asymptotic solution of CST software, used to analyze the structure.

This paper deals with the design and simulation of a waveguide rotary joint using gap waveguide technology for millimeter wave applications. It contains two TE10 to TM01 mode converters and rotary junction between two circular waveguides. The mode used along the circular waveguide for rotational symmetry purpose is TM01.The most important advantage of using gap waveguide technology is its ability to decrease complexity and cost of fabrication especially in millimeter wave frequencies because there is no requirement of contact between the different parts of the rotary joint structure. The Simulation results show that at different rotation angles the insertion and return loss are better than 0.4 dB and 10 dB over 58.5 GHz to 61 GH frequency band.

The purpose of this article is design and development of an aperture coupling microstrip array antenna with high gain, high bandwidth, low side-lobe level. work with optimally design of element started. Then, the complete 512 element array was fed to all the elements by feeding and distributing the same power and gain was obtained over 30 dB. To reduce the side-lobe level, the feed network is designed with Taylor distribution so that the specific range required for each element, which is different from the other elements, is obtained and also, all elements of the array are fed in phase. The elements that are in the middle of the array receive the most power and the elements that are in the besides of the array receive the least power. This structure is designed with 512 elements in an array of 32 × 16, and the operating frequency of the antenna is 12.5 to 14.9 GHz, equivalent to 17% of bandwidth in the Ku band. The gain of this antenna is 29 dB and gain variations over the entire frequency range is low, which is desirable and the side-lobe level also varies between -22 dB and -29 dB on both of the E-plane and H-plane. Finally, a metal reflector plate was used to lower the Backlobe level, reaching F/B levels above 35 dB. The overall dimensions of the antenna are 576 mm × 288 mm.

In recent years, different researches have been done in the context of the sidelobe level suppression for the military and civilian porposes. In these studies, various methods have been considered to reduce the sidelobe level, such as: edging the antenna in different cases, surface impedance tuning, placing metal pills on the reflector surface, placing resonant and non-resonant elements on the edges around the antenna. In this article, the effect of focal length and diameter of parabolic reflector antenna on the suppression of the sidelobe level has been investigated and obtained by optimization without the use of complex and costly structures. Depending on the need for the maximum gain, the small size or the minimum sidelobe level, the antenna can be designed. According to the purpose of the minimum sidelobe level, the proposed antenna has a working frequency of 12.5GHz, gain of 39.07dBi, 4.967λ (11.9 Cm) depth, 39.24λ (94.11 Cm) diameter, 19.375λ (46.47 Cm) focal length, -44.91dB and -43.5dB Sidelobe level in E and H-Plane respectively. This antenna has sidelobe levels of -30.76 and -47.15 at frequencies of 6.5 and 18.5GHz.

Frequency Diverse Arrays (FDAs), are kind of arrays in which a unique waveform is transmitted from each element by a small carrier frequency offset across the array elements. This small carrier frequency offset in frequency diverse array (FDA) radars leads to the interesting range-angle-time dependent beampattern of FDA radars which is different from the angle only dependent beampattern of phased arrays. In this paper, the received signal from a linear frequency diverse array radar is modeled and simulated and it is shown that this signal is a function of range and angle. Then by investigating the received signal from FDA radars more deeply, the possibility of target localization using one element receiver in these arrays is proved. Finally, based on the intrinsic coupling between the range and angle in FDA radars, the algorithm for target localization by one element receiver in one-target and multiple-target scenarios is proposed. The simulation results show the validity of the proposed algorithm.

Acquiring acceptable image qualitiy in microwave imaging is one of the challenging task in this field. This paper aims to investigate and compare modified level set method (MLSM) with four s‎tate-of-the-art methods i.e., distorted born iterative method (DBIM), contrast source inversion (CSI) method, linear sampling method (LSM) and multiple signal classification (MUSIC) method in image reconstruction and focuses on ‎quantitative ‎comparison ‎of them. Furthermore, three important criteria of image quality, namely accuracy, resolution and contrast, are quantitatively investigated for the aforementioned methods. The three quantitative methods of DBIM, CSI, and MLSM start with an initial guess of the scatterer profile and changing during an iterative process close to the actual scatterer. In spite of high amount of numerical computation in these methods, the resolution and contrast are high compared to the other two methods. As will be seen, the DBI and CSI methods are capable of completely resolving the two objects at 0.4λ and MLSM at 0.07λ. And also, MLSM provides more accurate reconstructions than the two others since the investigation domain is deformed to a smaller one.

In this paper a modified Gaussian pulse stimulus is employed to improve the accuracy of tumor detection inside breast phantom for 2D reconstructed image results in cylindrical setup of the microwave imaging system. This pulse shaping puts more energy at higher frequencies in contrast with conventional Gaussian in impulse radar. Hence a wider bandwidth is available to achieve higher accuracy for precise spatial localization. In the following a simulated cylindrical setup of the microwave imaging system with a modified stimulated pulse will be generated. The main purpose of this paper is employing a new stimulating pulse to detect a tumor from a biological phantom for 2D visualization of time-domain results. In order to achieve the goal the advantages of generating a novel confocal image-reconstructing algorithm based on back-projection method will be employed. The advantage conferred by “high resolution imaging” is that more energy is used at reflected signal than with conventional confocal imaging, subsequently a relatively lower spatial resolution in identifying the reflected signal is achieved. Simulated results are presented to validate the effectiveness of the proposed method for precisely calculating the time-dependent location of targets.