The calibration target is a vital instrument for calibrating space-borne microwave radiometers, and its emissivity performance must be accurately determined before usage. Based on the Kirchhoff's law of thermal equilibrium, the emissivity of a calibration target can be determined from its electromagnetic reflectivity, which is defined as space integration of scattering. However, due to the general shape of periodic coated sharp pyramids, the scattering from calibration targets shows Floquet mode properties with scattering lobes in upper space. That phenomenon must be considered in the reflectivity measurement of calibration target, especially in the mono-static backscattering configuration. To support such backscattering-based reflectivity measurement, the Floquet mode and scattering patterns from periodic unit and finite-sized array are investigated by numerical simulations, more specifically, by the finite-difference time domain (FDTD) algorithm. The investigations include the scattering power distributions among scattering lobes from coated and bare pyramid arrays, and the ratio of total reflection to backscattering in cases of typical parameters. It is found in the millimeter wave region that the scattering power from bare pyramids is still concentrated in the backscattering lobe in the mono-static configuration, while for the coated pyramids the scattering power is distributed around Floquet modes. For the considered geometry and coating parameters, the power ratio of total scattering to backscattering can be more than 10 dB at the cared frequencies. After all, the numerical results provide referencing correction factor for actual measurement studies. It is also validated by numerical results and suggested in practice, to use periodic simulations of low computational burden to evaluate the compensation factor for the mono-static reflectivity measurement.
Array antennas synthesis is one of the most important problems in the optimization of antenna and electromagnetics. In this paper, a recently developed metaheuristic algorithm, known as the Gravitational Search Algorithm (GSA), is employed for the pattern synthesis of linear and non-uniform planar antenna arrays with desired pattern nulls in the interfering directions and minimum side lobe level (SLL) by position-only optimization. Like other nature-inspired algorithms, GSA is also a population-based method and uses a population of solutions to proceed to a global solution. The results of GSA are validated by comparing them with the results obtained using particle swarm optimization (PSO) and some other algorithms reported in literature for linear and planar array. The side-lobe level and null depth obtained from gravitational search algorithm for planar array are improved up to -30 dB and -200 dB, respectively. The results reveal the superior performance of GSA to the other techniques for the design of linear and planar antenna arrays.
In hyperthermia treatment planning (HTP) the goal is to find the amplitudes and phases of antennas in the applicator to efficiently heat the tumor. To do this prior information regarding tumor characteristics such as the size, position and geometry, in addition to an exact model of the hyperthermia applicator is needed. Based on this information, the optimal frequency of operation can be determined. In this paper the optimum frequency for hyperthermia treatment based on the tumor and applicator characteristics, using time reversal as the focusing technique, is studied. As prior information, we consider tumor size and position, the number of the antennas in the applicator and the frequency characteristics. The obtained optimal frequency range is found using hyperthermia quality indicator values calculated from simulations. We also determine the optimum position of the virtual source in the initial step of the time reversal method to increase the quality of the treatment.
We propose an approach to characterize the AC underwater radiation produced by a ship over a shallow water medium using dipole sources distributed over an interior surface to the ship. The proposed approach relies in the accurate and efficient representation of dipole sources over the shallow water medium that characterize the behavior of the electric or magnetic field. The approach is reduced to the solution of the resultant matrix system from the dipole representation. These systems are ill-posed, i.e., if the matrix systems are not solved by special regularization methods, the resultant solution will amplify the measurement noise. The regularization method applied is the least squares QR iterations combined with a new stopping rule that uses a numerical estimate of the measurement noise. Numerically generated data is used to study the validity of the different dipole representations. Finally we validate our methodology using magnetic measurements that result from degaussing coils of a mid-size vessel.
Two techniques are described for the calibration of ground-based (GB) circularly polarized (CP) full polarimetric radars. The techniques are based on the point target calibration approach that uses various types of canonical reflectors with different orientations. Specifically, the calibration methods for linearly polarized (LP) radar proposed by Wiesbeck et al. and Gau et al. are selected and adapted to CP with suitable reflectors. The applicability of the techniques is examined through C-band scatterometric and synthetic aperture radar (SAR) measurements in an anechoic chamber. For the scatterometric mode, comparisons of calibrated channel imbalances with theoretical values show agreement within ±0.3 dB in amplitude and ±5˚ in phase. The crosstalk between the channels is also reduced by ~5 to 30 dB after calibration. For the SAR mode, calibrated scattering matrix of a vertical wire target exhibits significant elimination of distortions between channel amplitudes and phases. The effect of calibration on target parameter retrieval is also investigated through the Cloud-Pottier eigenvector-based decomposition. Both calibration techniques are shown to yield improved accuracy of entropy-alpha (H-α) distributions and orientation angle (β) values.
The diffraction by a finite parallel-plate waveguide with sinusoidal wall corrugation is analyzed for the E-polarized plane wave incidence using the Wiener-Hopf technique combined with the perturbation method. Assuming that the corrugation amplitude of the waveguide walls is small compared with the wavelength and expanding the boundary condition on the waveguide surface into the Taylor series, the problem is reduced to the diffraction by a flat, finite parallel-plate waveguide with a certain mixed boundary condition. Introducing the Fourier transform for the unknown scattered field and applying an approximate boundary condition together with a perturbation series expansion for the scattered field, the problem is formulated in terms of the zero-order and the first-order Wiener-Hopf equations. The Wiener-Hopf equations are solved via the factorization and decomposition procedure leading to the exact and asymptotic solutions. Taking the inverse Fourier transform and applying the saddle point method, a scattered far field expression is derived explicitly. Scattering characteristics of the waveguide are discussed in detail via numerical examples of the radar cross section (RCS).
A portable radio transceiver with rubber ducky antenna emitting at 446 MHz with an output power of 5 W was considered as near-field source of electric (E) and magnetic (H) field components when being used in the proximity of the user's face. By taking into account the significant content of ferromagnetic nanoparticles recently identified to reside in the human brain, we assessed the specific absorption rate (SAR) of energy deposition due to H-component penetrating a presumptive forebrain. H-component SAR contribution to the total SAR is for the first time estimated in such a case, based on an original idea inspired from knowledge on magnetic fluids hyperthermia.
Every generation of mobile communication has been associated with higher data rates than the previous generation. So 5G new spectrum radio access should support data rates exceeding 10 Gbps in most of its applications. An Ultra Wide Band (UWB) ultrahigh data rate full six-port receiver architecture up to 6.7 Gbps for 5G new spectrum is presented in this paper. The proposed structure is constructed using one UWB ultrahigh data rate Wilkinson power divider/combiner and three UWB ultrahigh data rate two-stage branch line couplers which are the essential components of any full six-port structure. The design procedure, optimization and implementation of these two UWB essential components in 21-30 GHz are completely done to achieve the optimum performance of final six-port. The final fabrication results show the average of -14 dB of input matching, -20 dB of isolation of isolated Ports, -4.2 dB of coupling in output ports (considering 2 SMA connectors and transitions in each path), and linear phase variation of outputs in the whole bandwidth of 21-30 GHz. To analyze and qualify the UWB six-port structure in any specific application in 5G and other UWB high data rate applications, a new analytical formulation with a new six-port structure of non-ideal UWB six-port circuit is presented. With this new analytical model and new configuration, there is no need to calibrate the outputs of in-phase and quad phase of the six-port receiver outputs. Based on the final fabricated essential components and the new analytical model, the final full six-port structure is constructed and analysed using UWB-OFDM with QPSK and 16QAM demodulation schemes in its sub-bands. To complete and verify the new analysis and to validate the final constructed UWB six-port structure and its essential components in ultrahigh data rate application in 5G new spectrum, the UWB-IR impulse radio with modulated ultrahigh data rate signal up to 7 Gbps and in 21-30 GHz bandwidth is completely discussed. The results show that all clusters of demodulated constellations are very well positioned and individualized in whole bandwidth in all modulation schemes. Also this new design and configuration of six-port receiver improves the dynamic range of the RF input signals up to 60 dB which is valuable. During the design procedure, a very useful method to choose the suitable laminate based on the time, frequency and two dimensional Wigner-Vile Distribution methods is presented. Also, some practical issues in design and implementation of the UWB microstrip component such as transitions are considered to achieve the best results.
Currently, most full-polarimetric synthetic aperture radar (SAR) systems adopt linear polarization (LP). On the other hand, circular polarization (CP) is also becoming popular due to its various benefits over LP. However, since CP-SAR is an emerging technique, there are not many imaging and polarimetric analysis results in the literature. As a fundamental study on CP-SAR, this paper presents the results of an investigation on the CP properties of ground-based SAR (GB-SAR) echoes from various canonical targets and a rice paddy sample. The C-band data acquired in a laboratory environment are analyzed and interpreted by means of several factors such as calibration performance, experimental verification of theoretical scattering matrices, imaging quality and accuracy of scattering decomposition results. The eigenvector-based decomposition of the coherency matrix is adopted, and the performance of CP in retrieving the targets' dominant scattering mechanisms and physical parameters is evaluated from entropy-alpha (H-α) plane and orientation angle (β) value. Results demonstrate the effectiveness of CP in interpreting and discriminating the SAR image features mainly owing to its distinct advantage of highly reliable received signal strength.
The problem of axially-symmetric TM-wave diffraction from the perfectly conducting sphere-conical cavity is analysed. The cavity is formed by a semi-infinite truncated cone; one of the sectors of this cone is covered by the spherical diaphragm. The problem is formulated in terms of scalar potential for spherical coordinate system as a mixed boundary problem for Helmholtz equation. The unknown scalar potential of the diffracted field is sought as expansion in series of eigenfunctions for each region, formed by the sphere-conical cavity. Using the mode matching technique and orthogonality properties of the eigenfunctions, the solution to the problem is reduced to an infinite set of linear algebraic equations (ISLAE). The main part of asymptotic of ISLAE matrix elements determined for large indexes identifies the convolution type operator. The corresponding inverse operator is represented in an explicit form. The convolution type operator and corresponding inverse operator are applied to reduce the problem to the ISLAE of the second kind. This procedure determines the new analytical regularization method for the solution of wave diffraction problems for the sphere-conical cavity. The unknown expansion coefficients, which are determined from the ISLAE by the reduction, belong to the space of sequences that allow obtaining the solution which satisfies all the necessary conditions with the given accuracy. The particular cases, such as transition from sphereconical cavity to the open hemispherical resonator, as well as the low frequency approximation, are analysed. The numerically obtained results are applied to the analysis of TM-waves radiation through the circular hole in the cavity.