In order to ensure that SAR scene matching aided navigation system can acquire the position errors and yawing errors simultaneously, we propose an image matching algorithm based on Scale Invariant Feature Transform (SIFT). However, the SIFT is proposed for optical image, and its performance degrades when used in SAR image. To enhance the adaptability of SIFT, two ways are employed. One is the application of a preprocessing on image pairs before matching. The other is the establishment of a scale and rotation restriction criteria on tie-points after SIFT matching. Compared with other matching methods, experiment results show that the proposed method is much more suitable for SAR image and successes in matching performance improvement. Furthermore, the method can meet the real-time requirement.
The integration of the Iterative Multi-Scaling Multi-Region (IMSMR) procedure and the Inexact-Newton method (INM) is proposed within the contrast-field formulation of the inverse scattering problem. Thanks to its features, such an implementation is expected to effectively deal with the reconstruction of separated objects. A selected set of numerical results is presented to assess the potentialities of the IMSMR-INM method also in comparison with previous INM-based inversions.
The Kelvin image theory for a conducting sphere is extended to the case of a conducting oblate spheroid in uniform motion along its axis of revolution (a Heaviside ellipsoid) using the well-known method provided by Special Relativity. The results derived are checked in various ways.
The downlink beamforiming technology plays a key role in a cognitive radio network (CR-Net). It can be used to reduce transmission power and interference to other users, etc. This paper presents a robust downlink beamforming method with power control for a multiuser multiple-input-single-output (MISO) CR-Net. In this proposed approach, the beamforming optimization problem is formulated as the second-order cone programming (SOCP). The presented method can not only minimize the transmitted power but also guarantee that the received signal-to-interference-plus-noise ratio (SINR) is strictly above the prescribed quality-of-service (Qos)-constrained threshold at each secondary user (SU) and the the interference power (IP) is strictly below the prescribed threshold at the primary user (PU). Simulation results are presented to verify the efficiency of the proposed method.
In this work, a metasurface consisting of an array of circular holes in a metal conducting sheet with a sub-wavelength periodicity is considered. The surface partially reflects the incident field according to the shape and geometrical dimensions of the inclusions and, due to this property, is widely employed in antenna systems to improve the radiation pattern of regular radiators. Since the reflection properties of the metasurface are determined by the current density distribution on the metal, we inspect this distribution and coherently develop a new, easy, and accurate analytical model to describe the grid impedance of the metasurface. In order to validate the model, we compare the reflection coefficient of the array obtained through our approach to the one resulting from full-wave numerical simulations and to other accurateanalytical methods available in the open technical literature.
This aricle reports the effect of vapour chopping technique on the properties of vacuum evaporated poly (3-methyl thiophene) thin films such as surface morphology, optical transmittance, band gap, refractive index and optical transmission loss. Vapour chopping gives smooth surface morphology with smaller grain size reduced roughness than non chopped thin films, while the transmittance of the thin film increases with simultaneous decrease in the refractive index, band gap and optical transmission loss decreases due to vapour chopping.
This paper presents a novel stochastic microwave method for the detection, location and reconstruction of electric properties of breast cancer in a simplified breast phantom. The method is based on the inversion of time domain data. The problem is recast as an optimization one by defining a suitable cost function which is then minimized using an efficient evolutionary algorithm. Selected numerical simulations of a simplified three dimensional breast model and a realistic numerical phantom based on magnetic resonance images (MRIs) are carried out to assess the capabilities of the method. The results obtained show that the proposed method is able to reconstruct the properties of a tumor-like inclusion to a reasonable degree of accuracy.
It is highly desirable to use advanced sensor networks to continuously monitor the structural health of an aircraft. It would be advantageous if the network was wireless to avoid the need for additional wire bundles and associated interconnects but the reliability of a suitable wireless channel in low loss enclosed structures needs to be understood. This paper reports on work undertaken testing the 2.4 GHz ZigBee wireless protocol in a mode stirred and mode tuned reverberation chamber. The results show that even for very low loss enclosures wireless communications is possible but only under very specific conditions. A higher loss chamber has more reliable communication channels, but even with loading there are large variations in packet error rates even between adjacent ZigBee channels.
The distribution of fields travelling in the laminated structure with assumed values for the tangential components of the magnetic field intensities on the top and bottom surfaces of the structure, has been obtained using linear electromagnetic field theory. The treatment takes cognizance of interlaminar capacitance inherently present in a laminated structure. Analysis presented in this paper assumes identical field distribution in each lamination and a given current sheet as the source for the travelling electromagnetic fields. It has been concluded that convection currents are developed at the interface between iron and insulator regions.
This paper proposes a hybrid higher order finite difference time domain (FDTD) scheme that combines the classical FDTD scheme and the higher order FDTD scheme with second order accuracy in time and fourth order accuracy in space for analyzing the three-dimensional electrically large scattering problems. The classical FDTD stencils were used as buffers in the scattered field region to make the higher order FDTD stencils not intrude inside the absorbing boundary condition's regions. The superior performance of the hybrid higher order FDTD scheme has been compared with the classical FDTD one. Numerical results demonstrate that the proposed scheme would improve the accuracy and save the computer resources significantly compared to the classical FDTD scheme involved in the radar cross section (RCS) calculation. The obtained computational efficiency allows this proposed scheme to model the RCS of electrically large targets using the number of higher order FDTD cells which are much less than that of the classical FDTD cells required by three-dimensional FDTD scheme.
While a radar target is illuminated under the condition of spherical wave, two-dimensional ISAR image can be obtained in near field, and the wavefront curvature must be compensated. A novel two-dimensional mathematical model is set up, and a 2D-ESPRIT super-resolution algorithm based on matrix pencil is applied to estimate the accurate locations of the scattering centers in near field. Numerical simulations are conducted in different distances as well as with different SNRs. It is proved that the method can revise the spherical wavefront curvature with a high accuracy. Finally, near field ISAR imaging experiments were done outdoor, and raw data were processed with this super-resolution method, which verify that 2D-ESPRIT algorithm based on matrix pencil can compensate the spherical wavefront curvature effectively in near field.
By means of a three-step linear optical transformation method, material parameters of a three-dimensional diamond-shaped electromagnetic concentrator composed of tetrahedral homogeneous blocks has been derived in this paper. The performance of the concentrator has been confirmed by full-wave simulation. The designed concentrator can operate in a wide bandwidth due to the line transformation. It represents an important progress towards the practical realization of the metamaterial-assisted concentrator.
The radiated electromagnetic (EM) fields from a rotating current-carrying circular cylinder were numerically simulated in two dimensions using the method of characteristics (MOC), and the numerical results were presented in this paper. To overcome the difficulty of the grid cell distortion caused by the rotating cylinder, the passing center swing back grids (PCSBG) technique is employed in collaboration of MOC in a modified O-type grid system. In order to have clear demonstration of radiated EM fields, the circular cylinder is set to be evenly divided in radial direction into an even number of slices that are made of perfect electric conductor (PEC) and non electric non magnetic material, alternatively. The surface current is assumed to have a Gaussian profile and to flow uniformly along the axial direction on the PEC surface. The radiated electric and magnetic fields around the cylinder were recorded as functions of time and reported along with the corresponding spectra which were obtained through proper Fourier transformation. Several field distributions over the whole computational space are also given.
This paper presents a novel contribution for the analysis of skin effect phenomena in inhomogeneous tubular conductors. For homogeneous tubular geometries the skin effect diffusion equation has an analytical solution described by a combination of Bessel functions, but, when the conductivity and magnetic permeability of the tubular conductor arbitrarily depend on the radial coordinate an analytical solution cannot be found. However, this work shows that closed form solutions for the electromagnetic field and conductor internal impedance do exist, provided that a specific type of radial variation of medium parameters is considered --- tubular structures like these are coined here Euler-Cauchy Structures (ECS). Analytic and computation results concerning general and particular ECS are presented, validated, and discussed. Besides its intrinsic theoretical importance, the simple closed form solutions that we have found can be of interest as benchmark tools for testing the accuracy and performance of EM field software solvers.
The Alfven-spin and helicon-spin waves are analyzed in both sinusoidal periodic and layered periodic structures. These periodic structures are composed of a single composite medium having the properties of both magnetic and semiconducting materials. Numerical analysis of the dispersion relations presented for these periodic structures shows band-gap effects. The idea of these band-gap effects could be utilized in the design of periodic structures operating at microwave frequencies. Extreme cases for the decoupled independent modes in the absence of magnetization or carriers are also discussed.
A new type of waveguide based on the gap waveguide concept is here proposed and called gap-groove waveguide. Its design is based on the realization of a groove on a metal, facing an artificial surface which creates a high impedance surface (HIS) boundary condition. This condition is achieved here by employing a structure of closely packed metallic pins, known as bed of nails. The type of modes that can propagate in the gapgroove waveguide are similar to the ones of a standard waveguide but in this case there is no need of electrical connection. This is a potential advantage, especially when working at high frequencies. The dispersion characteristic of the gap-groove waveguide is derived by solving an eigenvalue problem, settled through a resonance condition at the interface between the groove and the bed of nails. The eigenvalues are associated with the modes propagating in the waveguide, and their dispersion characteristic is analyzed and compared with full wave simulations. A procedure to maximize the bandwidth is also provided, based on an appropriate choice of the geometrical parameters. Furthermore, the field distribution and the modal impedance of the fundamental mode are investigated.
Arbitrarily polarized plane-wave diffraction equations for semiinfinite periodic rectangular grooves (RG) in a perfectly conducting plane are approximately proposed. To obtain diffraction equations for semi-infinite periodic RG, we utilize an overlapping T-block method as proposed for the analyses of finite and infinite numbers of RG, and the subtraction technique with infinite periodic solutions. The proposed semi-infinite solutions are then applied to finite periodic RG with very large number of diffracting elements. For verification of our approach, we performed numerical computations for finite periodic RG and compared our solutions based on semi-infinite equations with previously published analytic solutions, thus obtaining favorable agreement and proving computational efficiency.
Reflection from a perfect electric conductor (PEC) plane of infinite dimensions embedded in a second order nonlinear medium is studied. The reflected wave has two parts, one due to linear behavior, the other due to nonlinear behavior of the medium. The expressions of the reflection coefficients for parallel and perpendicular polarization cases are obtained. The reduction of reflection coefficient to a linear medium case is also reported. Dependence of the said coefficients on incident electric field intensity and the angle of incidence is also plotted.
The radio waves propagating through the earth atmosphere will be attenuated due to the presence of atmosphere particles, such as water vapor, water drops and the ice particles. Meanwhile, the atmospheric gases and rain will both absorb and scatter the radio waves, and consequently degrade the performance of the link. The results of various studies conducted in temperate and tropical regions have been published in research papers. This paper presents the summary of comparative studies on different rain attenuation prediction methods for terrestrial microwave links tropical regions. Basically the models described in this paper include those of the ITU-R, revised Moupfouma, revised Silva Mello and Lin model. The objective of this study is to reveal the most suitable rain attenuation prediction model for the Malaysian tropical region. This paper will provide useful information for microwave engineers and researchers in making decision over the choice of most suitable rain attenuation prediction for terrestrial links operating in a tropical region. Even though the ITU-R model underestimates the rain attenuation at higher frequencies, the test results have clearly indicated that it is most suitable for predicting terrestrial rain attenuation in tropical Malaysia, compared to others.
For array beampattern synthesis, it is possible to simplify the model and reduce the computational load by formulating it to be a Quadratic Programming (QP) problem. The QP method is conceptually simple and imposes no restriction on array geometry. In the QP method, a key component is the template function which describes the desired beampattern as a deterministic function of direction. In this paper, the template functions in the form of Hypergeometric Function corresponding to Legendre arrays and Dolph-Chebyshev arrays, namely Legendre Hypergeometric Function (LHF) and Dolph-Chebyshev Hypergeometric Function (DCHF), are derived and the synthesis procedures are also presented. The simulation results show that the DCHF and LHF work in the QP method and provide the exactly synthesized beampattern. Moreover, another synthesis method using a Gegenbauer polynomial to synthesize the beampattern of a Uniform Linear Array, is proposed. This method gives rise to the Gegenbauer arrays. Gegenbauer arrays are very generalized and Legendre arrays and Dolph-Chebyshev arrays are considered as its special cases. Using the Gegenbauer array synthesis method, one is able to further adjust the beam efficiency and the directivity when the Side-Lobe Level and the element number are specified. As well as Dolph-Chebyshev arrays and Legendre arrays, its template function for the QP method, Gegenbauer Hypergeometric Function, is also derived.