Calculation of scattering characteristics of aerial radar objects of resonant sizes based on iterative algorithm

The paper considers a method for calculating the scattering characteristics of aerial radar objects of resonant sizes and complex shape, the surface of which can be assumed perfectly conducting. This method is based on applying an iterative algorithm for solving the magnetic field integral equation. The developed method allows obtaining stable results in the case of large-dimensional matrices of integral equation and provides for the elimination of internal resonances caused by the idealization of mathematical model. Peculiarities of the developed numerical algorithm are discussed. The calculation results of the effective cross-section of test object obtained by different methods were compared. In addition, the calculated range profiles of cruise missiles in the VHF band are presented.     

Near-field detection of buried dielectric objects

The plane-wave scattering-matrix method is used to compute the response of a detector to a buried dielectric scatterer. The Born approximation is used to derive the scattering matrix for scatterers of small dielectric contrast, but the general theory is not limited to such cases. Specific numerical results are generated for a UHF dipole detector swept over a buried dielectric cube. The maximum response is obtained when the detector is located at the soil interface, and the response decays rapidly with the detector height. The sweep curves are symmetrical in the horizontal direction and have a null when the detector is directly over the object. An experimental curve for a free-space environment has the same qualitative features     

On the determination of resonant modes of dielectric objects using surface Integral equations

Although surface integral equations have been extensively used for solving the scattering problem of arbitrarily shaped dielectric objects, when applied to the resonance problem, there are still some issues not fully addressed by the literature. In this paper, the method of moments with Rao-Wilton-Glisson basis functions is applied to the electric field integral equation (EFIE) for solving the resonance problem of dielectric objects. The resonant frequency is obtained by searching for the minimum of the reciprocal of the condition number of the impedance matrix in the complex frequency plane, and the modal field distribution is obtained through singular value decomposition (SVD). The determinant of the impedance matrix is not used since it is difficult to find its roots. For the exterior EFIE, the original basis functions are used as testing functions; for the interior EFIE, the basis functions rotated by 90/spl deg/ are used as testing functions. To obtain an accurate modal field solution, the impedance matrix needs to be reduced by half before SVD is applied to it. Numerical results are given and compared with those obtained by using the volume integral equation.     

Method for detection of ground objects and recognition of their types from radar images of the earth surface

A method for detection of ground objects and recognition of their types against the background of the earth surface from objects’ radar images (RIs) is proposed. The method is based on a priori formation of template RIs of the objects in free space and scanning the RI of the earth surface with two types of rectangular windows. The dimensions of the window of the first type take into account the shadows of the objects, and the dimensions of the windows of the second type take into account only the outer dimensions of the objects. The results of the simulation of the detection characteristics and of the parameters of recognition of a KAMAZ truck and the control station mounted on the same chassis, which were located on a standard earth surface, are presented.     

Polarimetric detection of objects buried in snowpack by a syntheticaperture FM-CW radar

Presents experimental results of polarimetric detection of objects buried in a natural snowpack by a synthetic aperture FM-CW radar. First, the principle of polarimetric imaging in the Co- and Cross(X)-pol radar channels is outlined based on the scattering matrix and the characteristic polarization states for a specific target. Then, polarimetric measurements were carried out to detect objects buried in a natural snowpack 230 cm deep. The targets included two orthogonally placed metallic plates, an ice layer within the snowpack, and a human body. It is shown that the polarimetric FM-CW radar clearly enhances target signatures and that it serves to significantly improve detection in snowpack. Several polarimetric detection results are displayed, demonstrating the potential capability of characteristic polarization imaging and the usefulness of FM-CW radar     

A multifrequency adaptive radar for detection and identification of objects: Results on preliminary experiments on aircraft against a sea-clutter background

An experimental model of a simple multifrequency continuous wave (CW) radar system has been developed. Preliminary experiments have been performed in order to illustrate the potential of this radar concept in regard to detection, course ranging, and also identification of low flying aircraft against a sea-clutter background. The radar can be organized so as to measure ocean wave height, wave direction, and wave velocity simultaneously for 15 different ocean wavelengths ranging from 150 m to approximately 6 m. An example of such a directional wave intensity spectrum is given. To illustrate the ranging and the identification potential of the technique in regard to airplanes, two different airplane categories, namely a propeller aircraft (Cessna 172) and a rigid jet aircraft (F-16), were flown through the radar beam. The signatures of these two airplanes are show in two signature domains: spatial distribution of scattering centers and motion pattern. The experimental results confirm theories presented in earlier contributions. It remains to introduce two more domains, namely the space/time coherence properties of the target and its polarization characteristics.     

Doppler signatures of radar backscattering from objects with micro-motions

Radar backscattering from objects with micro-motions is subject to Doppler modulations that help determine dynamic properties of objects and provide useful information about the objects. Doppler modulations represented by joint time-frequency analysis provide useful time-varying Doppler characteristics and, thus, add additional time-dimension information to exploit motion characteristics. The author discusses how to simulate radar backscattering from objects with rigid body motions and objects with non-rigid body motions, and how to analyse, interpret and characterise Doppler signatures of objects that undergo these micro-motions. Precession heavy top and human locomotion are used as examples of rigid and non-rigid body motions, respectively. Radar micro-Doppler signatures derived from these motions illustrate the potential of the joint time-frequency analysis for exploiting kinetic and dynamic properties of objects.     

Theory of Fabry-Perot resonators with dielectric medium

A Fabry-Perot resonator filled with a dielectric slab is investigated using the Vainshtein method. Simple and explicit formulas are obtained for the diffraction loss, phase shift, and field distributions for both strip and circular mirrors. It is found that the presence of the dielectric reduces the diffraction loss, and causes mode confinement to certain degrees.     

Microwave detection and dielectric characterization of cylindrical objects from amplitude-only data by means of neural networks

In this paper, we propose a new microwave technique for the localization and the dielectric characterization of physically inaccessible cylindrical objects from amplitude-only data. By means of a neural network used to solve the inverse scattering problem; this technique allows to directly achieve the object retrieval, avoiding the drawbacks related to the measurement of the phase distribution of the field that generally represent a critical point, especially at high frequency. The efficiency of the proposed technique in the reconstruction of both the position and the dielectric properties of a circular cylindrical body from amplitude-only information is illustrated and compared with the reconstruction performances of a neural network imaging technique that makes use of both amplitude and phase of the scattered field. The presence of noisy data is also taken into account, showing the dependence of the reconstruction accuracy on the signal-to-noise-ratio.     

Effect of resonant cavity in wafer-bonded Green InGaN LED with dielectric and silver mirrors

The green InGaN-based resonant cavity light-emitting diodes (RCLEDs) on Si substrates were fabricated using laser liftoff and wafer bonding techniques. Five-pair TiO/sub 2/--SiO/sub 2/ distributed Bragg reflectors (reflectivity: 85%) and an Ag metal layer (reflectivity: 99%) were employed as the top and bottom mirrors, respectively. The light output power of the RCLED at room temperature is 1.5 times the magnitude of a similar structure without a resonant cavity at an injecting current density of 600A/cm/sup 2/. The mode spectrum exhibits a line width of approximately 5.5 nm at the dominant peak wavelength of 525 nm, which indicates a quality factor of 100. Under various injection current densities, a low thermally induced red shift in the 525-nm emission peak was observed. This indicates that the resonant microcavity effect contributes to the stability of electroluminescence emission wavelength.     

有限元方法计算介质腔谐振频率和品质因子

采用有限元方法计算了二维方形介质腔和二维微盘的谐振频率和品质因子,并给出了两种腔中磁场、电场振幅分布图,介质腔中的谐振问题对应亥姆霍兹波动方程的本征值问题,本征值的实部和虚部与谐振腔Q值关联;比较了近似解析解、FDTD解、FEM解的结果,对于谐振频率比较,FEM和解析解更接近,对于品质因子比较,低Q值结果FDTD和FEM结果相近,高Q值结果两者相差较大。通过比较知道,FEM方法比时域有限差分方法计算更准确,求解速度更快。     

Imaging of objects buried in a finite-sized dielectric background

The authors propose a simple algorithm to reconstruct the image of objects embedded in a finite-sized dielectric background. The algorithm is based on the concept that an undistorted image may be reconstructed if the true range profiles of the embedded object can be obtained. Neglecting the phenomena of multiple reflections and the bending effect of wave propagating in the dielectric, they propose a simple algorithm to compensate the electrical length of the wave traveling in the dielectric. Although the assumptions made are not precise, nevertheless, experimental results show that the proposed method is very time-efficient, and is effective in reconstructing a recognizable image of the object embedded in a background having moderate dielectric constant and smooth air-dielectric boundary shape     

Precise computations of resonant frequencies and quality factorsfor dielectric resonators in MICs with tuning elements

In this paper, the highly accurate results of resonant frequencies, field distributions, and quality factors of the TE_01δ mode for the cylindrical shielded dielectric resonator (DR) in microwave integrated circuits (MIC's) with a practical tuning element, such as the metallic tuning screw and the dielectric tuning device, are presented. By using the newly developed FD-SIC method, numerical results can be calculated accurately and efficiently. The DR structures with tuning elements can be more easily modeled by the present approach than the other methods using approximate solutions or the mode-matching (MM) methods. Numerical results in the literature are compared to the present FD-SIC results for the DR without tuning elements and detailed discussions on these results are given. In addition, design curves are also presented for the DR with the metallic tuning screw and with the dielectric tuning device. These design curves are helpful for designing DR systems with tuning elements in MIC applications     

Effective dielectric constant of a medium with spherical inclusions

The Maxwell-Garnett theory is frequently used to predict the effective, or the average, dielectric constant of a mixture composed of spherical inclusions embedded in a host medium. This effective medium theory assumes that the volume fraction occupied by the spherical inclusions is small and that the size of the inclusions is small compared to the wavelength. Experiments using controlled samples have shown that the Maxwell-Garnett theory is applicable up to an inclusion volume fracton of 0.2. At higher volume fractions, the effective dielectric constant appears to be dependent on the inclusion sizes     

The RCS of a metal plate with a resonant slot

A numerical method based on the thin-wire electric field integral equation is used to calculate the radar cross section (RCS) of a wire grid with a resonant slot. The numerical results are in good agreement with experimental data for a wire grid, where the antiresonance is found to occur at a slightly lower frequency than a similarly slotted plate.     

Complex radar objects and polarisation dependence of received backscattered echoes

Herein, selected experimental results concerning complex radar-object backscattering and polarisation-dependent receiver responses are presented; in particular the optimum responses that are obtainable. The objects are a boresight tower and an apartment-block conglomerate. The experiments are related to a study of the applicability of adaptive polarisation for the improvement of target detection in clutter.     

常见定标体的雷达截面计算与仿真

针对雷达截面(Radar Cross Section,RCS)测量技术中的常见定标体,分析了雷达散射截面计算理论,总结推导了几种雷达截面计算方法,获得了常用定标体的RCS精确解.计算结果与测量值相当吻合.运用Matlab编程语言绘制了其仿真结果图.通过比较得出一致性绕射理论在平板RCS测量计算中的合理性.给出了三面角对线极化入射波产生扭转极化和圆极化反射波的方法.计算了某型战斗机的RCS.     

金属介质混合目标电磁特性的快速分析

将自适应积分算法与基于体面混合积分方程的矩量法相结合快速分析任意结构金属/介质混合目标的电磁散射和辐射特性.通过将传统矩量法的阻抗矩阵分为两部分且采用不同的方法进行处理计算,提高了矩量法的计算速度并大幅度缩减了需要的计算机内存占用量.最后,分别用传统的矩量法与结合自适应积分快速算法的矩量法计算了三个典型例子,通过比较充分说明了文中方法的有效性.     

Role of structure sizes in determining the characteristics of the resonant tunneling diode

Using the Wigner function formalism, we study the effects of structural parameters on the DC I-V characteristics and on the large-signal transient response of the resonant tunneling diode. We model two types of structures of GaAs/Al_xGa_1-xAs; first, with symmetric barriers ranging from 3 to 8 nanometers in thickness separated by a 5 nanometer well, and second with a well varying from 3 to 8 nanometers between 3 nanometer barriers. Experimental variation of the barrier thickness and height changes the peak-to-valley ratio in the I-V curve, as predicted by elementary models of tunneling structures. This stems directly from the changes in tunneling probabilities. For the DC studies, we show that the peak-to-valley ratio in the I-V curve is a function of the resonance width, which depends both on the well and barrier thickness. The location of the peak on the I-V curve depends on the location of the resonant energy, which is a function of the well width. Transient switching behavior is compared to earlier numerical studies of tunneling times of wave packets. Charge storage stabilizes the position of the resonant state, thus damping the transients. Consequently, wider barriers yield faster transient settling times, in agreement with the tunneling time results which predicted longer charge storage times for thicker barriers.     

Design and analysis of resonant cavity-enhanced quantum ring inter-subband photodetector with resonant tunneling barriers for terahertz detection

The effect of resonant cavity structure on the performance operation of In As/Ga As quantum ring intersubband photodetector is studied for detection of terahertz radiations range.In order to confinement of optical field w ithin active region and consequently enhancement in responsivity of device,tw o periods of Al2O3/Ga As distributed bragg reflectors are used as bottom dielectric mirror and a thin layer of Au material as top mirror of device.For further improvement in detectivity,Al0.3Ga0.7As/In0.3Ga0.7As resonant tunneling barriers are included in absorption layers to reduce dark current of device.Proposed photodetector show s a peak responsivity of about 0.4(A/W)and quantum efficiency of 1.2%at the w avelength of 80μm(3.75 THz).Furthermore,specific detectivity(D*)of device is calculated and results are compared to conventional quantum ring inter-subband photodetector.Results predict a D*of~1011(cm.Hz1/2/W)for device at T=80 K and V=0.4 V w hich is tw o orders of magnitude higher than that of conventional QRIPs.