An 8 mm-band passive millimeter-wave imager BHU-2D has been developed by Beihang University. This imager is designed for detecting concealed weapons on human body. The imager adopts two-dimensional synthetic aperture interferometric radiometer (SAIR) technique which avoids the radiation risk to human body. Compared with scanning technique, SAIR technique could obtain images of a larger field of view (FOV) while achieving high imaging rate, which are necessary for security monitoring. In this paper, the imaging principle of SAIR is stated firstly. Secondly, background cancellation method designed for BHU-2D is interpreted. The technique is used to reduce the complexity as well as the dimensional requirements of the receiving elements. Thirdly, system configuration is illustrated in detail. Then external point source calibration method is introduced and discussed specifically for security check applications. Finally, imaging experiments on a person with concealed weapon are conducted by which the design and image calibration algorithms are verified. To conclude, experimental results prove that BHU-2D could be employed in security check applications.
The magnetic energy and inductance of current distributions on the surface of a torus are considered. Specifically, we investigate the in°uence of the aspect ratio of the torus, and of the pitch angle for helical current densities, on the energy. We show that, for a fixed surface area of the torus, the energy experiences a minimum for a certain pitch angle. New analytical relationships are presented as well as a review of results scattered in the literature. Results for the ideally conducting torus, as well as for thin rings are given.
In this paper, small dual layer spirals with several various ground structure are applied in the vicinity of the DDR3 high-speed circuit to achieve noise suppression characteristics up to 3.2 GHz region. For wider noise suppression bandwidth, the dual layer spirals with various ground structure, which provide high self resonance frequency (SRF) as well as inductance value, are implemented. The proposed dual layer spiral with various ground clearance dimension exhibits greater than 9 dB power noise suppression characteristics in the frequency range of interests and achieve about 50% voltage fluctuation reduction in time domain compare to the reference case model. To validate the effectiveness of the proposed model, sample PCB are fabricated and measured. It shows good agreement between the measured and simulated results up to 3.2 GHz.
Efficient and accurate modeling of electromagnetic structures is valuable in antenna analysis and design, and time domain solutions are at a premium over frequency domain in the case of ultra wide band signals or transients. Among the full wave electromagnetic methods in time domain the method of moments in time domain (MoM-TD) is very interesting. Such a method can be implemented, as for frequency domain, either resorting to a thin wire approximation or to a surface patch model. Depending on the structure to be analyzed one or the other is most convenient. For heterogeneous structures both implementations might be needed, and the problem of the attachment between a perfectly conducting thin wire and a perfectly conducting surface is hence relevant. In this paper attachment modes are introduced in MoM-TD. The solution is validated on a test case and against another numerical technique.
A biconical antenna has been developed for ultra-wideband sensing. A wide impedance bandwidth of around 115 % at bandwidth 3.73-14 GHz is achieved which shows that the proposed antenna exhibits a fairly sensitive sensor for microwave medical imaging applications. The sensor and instrumentation is used together with an improved version of delay and sum image reconstruction algorithm on both fatty and glandular breast phantoms. The relatively new imaging set-up provides robust reconstruction of complex permittivity profiles especially in glandular phantoms, producing results that are well matched to the geometries and composition of the tissues. Respectively, the signal-to-clutter and the signal-to-mean ratios of the improved method are consistently higher than 5 dB and 10 dB, corresponding to an average increase in image fidelity of more than 140% compared to conventional radar focusing technique.
This paper investigates the influence of critical raindrop diameters on the specific rain attenuation in Durban (29°52'S, 30°58'E), South Africa. The total rainfall attenuation is evaluated by integrating over all the raindrop sizes and the differential change in the attenuation is observed over a given range of drop size diameters. The major contribution to the specific attenuation for the drop size distribution (DSD) models considered is created by the raindrop diameters not exceeding 2 mm especially at higher frequencies. The three parameter lognormal and gamma distribution models are employed for the purpose of analysis. For the DSD models considered in this work, the total percentage fraction created by raindrops in the diameter range 0.5 mm ≤ D ≤ 2.5 mm and 1 mm ≤ D ≤ 3 mm to the total specific attenuation is found to be critical for the overall and seasonal rainfall attenuation at 2.5 GHz-100 GHz in Durban. It is observed that the total specific attenuation increases with increased frequencies and a higher rainfall rate produces high rain attenuation. In this paper, both the overall and seasonal values of R0.01 determined for Durban are used.
This study builds on the earlier work by Odedina and Afullo on Multipath fading in Durban. Their work was based on multipath measurements in Durban over a 6.73 km Line-of-Sight (LOS) link. This submission uses the geoclimatic factor approach and ITU-R recommendations P530-14 to obtain the multipath fading occurrence in five cities in South Africa, including Durban. Three-year radiosonde data is used in estimating the percentage of time that a certain fade depth is exceeded and hence outage probability due to atmospheric multipath propagation, assuming the given fade depth leads to the received signal falling below the squelch level. We employ the Inverse Distance Square technique to estimate point refractivity gradient not exceeded for 1% of the time in the lowest 65 m above the ground for five locations within South Africa. Standard error of the mean and confidence interval for both annual averages and seasonal averages of point refractivity gradient is calculated to reflect possible deviation in the given readings. These values of point refractivity gradient obtained are used in determining the geoclimatic factor K. The results presented show monthly, seasonal and annual variation of both point refractivity gradient and geoclimatic factor K. The results confirm that the geoclimatic factor K is region based. The percentage of time a given fade depth is exceeded for a single frequency increases rapidly with increasing path length. This is due to the fact that as the path length increases so do the multiple reflections leading to multipath propagation, which can result in either signal enhancement or multipath fading. A comparison of fade depth and outage probabilities is made with the earlier work in Durban and Rwanda in Central Africa.
An accurate numerical model, based on multiconductor transmission lines (MTL) able to evaluate the voltage dynamics across the motor bearings and associated currents of an inverter-fed motor is presented. A full three phase stator winding of the wound type of a high power traction motor is considered in the proposed analysis. The different regions of the motor are modeled as suitable connections of lossy MTL which are then studied in the time domain. The per unit length characteristic matrices describing the MTL are accurately calculated by a FEM based software. The effects of the rise time of the input voltage and the length of the feeder cables are discussed. The reliability of the numerical results achieved by means of the MTL model is checked by performing a comparison with those obtained by considering a lumped parameter equivalent circuit.
In this work, we demonstrate that the LSM and LSE modes formulation is an excellent theoretical tool for determining the refractive index and thickness of the guiding layer in planar optical waveguides with step refractive index profile. Refractive index of transparent materials, capable of being deposited as a solid thin layer on a substrate for confining light, can be evaluated very accurately. The method can be applied to analyze and design monomode and multimode optical waveguides, unlike the methods proposed so far, including cutoff wavelength region. This wave model only requires the experimental evaluation of the effective indices of the guided modes. In order to verify the developed formulation, the commercial software Olympios was used for theoretical comparison. Polymeric planar optical waveguides were fabricated and characterized. A prism coupling method and the Metricon system were used for effective indices measurements and to compare the accuracy. The experimental evaluation of the thickness was carried out by profilometry. In all cases a complete agreement was obtained for refractive index and thickness between theory and experiments.
The Least-Squares Boundary Residual Method is employed in the present paper to develop a computer model of the symmetrical six-port waveguide junction. The analytical formulation is difficult because of the insertion of a metallic post together with a dielectric sleeve into the over-sized cavity of the junction. Computational and experimental tests confirm that the resultant model is able to compute (with numerical accuracies of ±0.001 and ±0.1° for magnitude and phase respectively) the scattering parameters of such a structurally-complicated component.
Traditionally, the transmitter (TX) IQ imbalances distortion and power amplifier (PA) distortion are separately modeled. In this paper, the behavior of the two distortions are unified, and characterized by a single model. Rectangular structured Focused Time-Delay Neural Network (RSFTDNN) is proposed to uniformly model IQ imbalances and PA distortions. As a result, the physical distortions in the analog circuits are further abstracted. It also saves computation resources in simulation. Unlike the polynomial based model, which suffers from condition number effects and inaccuracy for deeply nonlinear system, the proposed RSFTDNN shows high accuracy. Two cases of real experiments are carried out, where RSFTDNN model shows much better performance than the polynomial based model in the sense of model accuracy.
Excitation of extraordinarily polarized azimuthal eigen modes by modulated annular electron beam is shown to be characterized by the increase of instability growth rates compared with the case of non-modulated electron beam. Interaction between the modulated beam and azimuthal eigen modes happens in the range of electron cyclotron frequency in waveguides with metal walls, which are partially filled with cold magneto-active plasma. Non-linear set of differential equations, which describs excitation of these azimuthal modes by an annular electron beam is derived and analyzed numerically. Different scenarios of the beam-plasma interaction depending on relation between azimuthal mode number of the exited waves and periodicity of azimuthal modulation of the beam density, degree and manner of the beams' modulation are studied numerically.
The problem of electromagnetic waves radiation into a space outside a perfectly conducting sphere through a narrow slot, cut in an end-wall of a semi-infinite rectangular waveguide, excited by a fundamental wave of H_{10} type is solved using a rigorous self-consistent formulation. The starting point for the analysis is the one-dimensional integral equation for the equivalent magnetic current in the slot, obtained by using the effective thickness of the slot. The asymptotic solution of the equation was found by the generalized method of induced magnetomotive forces (MMF). The physical adequacy of the constructed mathematical model to the real physical process is confirmed by experimental data. Influence of the sphere radius upon energy characteristics of the slot radiator was investigated numerically. It was shown that at any frequency of waveguide single-mode range, the radiation coefficient of a spherical antenna can be made close to one by choosing the slot length, the sphere radius and the waveguide height. Conditions for correct application of infinite screen approximation for spherical scatterers with sufficiently large radii are formulated.
In this paper, the Raindrop Size Distribution (DSD) modeling and analysis are presented. Drop sizes are classified into different rain types, namely: drizzle, widespread, shower and thunderstorm. The gamma and Lognormal distribution models are employed using the method of moments estimator, considering the third, fourth and sixth order moments. The results are compared with the existing raindrop size distribution models such as the three parameter lognormal distribution proposed by Ajayi and his colleagues and Singapore's modified gamma and Lognormal models. This is then followed by the implementation of the proposed raindrop size distribution models on the computation of the specific rain attenuation. Finally, the paper suggests a suitable raindrop size distribution model for the region with its expressions. The proposed models are very useful for the determination of rain attenuation for terrestrial and satellite systems.
|In this paper, a careful study is made of the magnetostatic potential and eld of a magnetic dipole embedded in, and with dipole moment parallel to, the interface between two magnetic regions. Unlike the case of a magnetic dipole perpendicular to the interface, the detailed position of the current of the dipole relative to the location of the interface has a profound effect on the value of the field produced away from the dipole. As a consequence, the question of dening and determining the magnetic polarizability of a superconducting object partially embedded in a magnetic interface is examined. The results of this paper are important for the proper modeling of arrays of scatterers embedded in an interface, such as frequency-selective surfaces (FSSs) and metalms.
The time-harmonic electromagnetic scattering problem from a random inhomogeneous dielectric medium (here a turbulent plasma wake created by the atmospheric reentry of a vehicle) is considered. The electronic density of the plasma, that gives rise to its dielectric permittivity, has a fluctuating part ε_{f} (r), the variance and correlation function of which are known a priori. Because the electrical dimensions of the wake can be very large, the numerical solution of Maxwell's equations via a full-wave calculation performed with a boundary element and finite element method is prohibitive when statistical quantities such as the mean Radar Cross Section (RCS) and its variance are required, that necessitate a large number of random realizations. To remedy this difficulty, two approximations are considered and illustrated for a 2D scattering problem. First, a Mie series approach is adopted where the medium is discretized with small disks, thus reducing considerably the number of unknowns for a given random realization of ε_{f} (r), and a domain decomposition method is proposed to further reduce the complexity of the numerical solution of the corresponding system. Second, the statistical mean and the variance of the RCS are derived in closed-form from the Born approximation and yield accurate results when, as expected, the statistical mean of the relative dielectric permittivity is close to unity and |ε_{f} (r)| is small. Conversely, it is shown how these expressions can be used to validate the results obtained with the Mie series approximation. Numerical examples are presented that illustrate the potentialities of these techniques.
The arrays of compound-slots (inclined and displaced from waveguide centre) are not so common in the literature and the existing systematic method of design does not take into account the presence of surrounding elements. For this kind of slots, the reasons why the Elliott's procedure cannot be applied are physically explained. A new method based on circuit theory and scattering matrices connection to include the effects of the external mutual coupling in compound slots arrays is presented in this paper. To certify the validity of the complete technique, the performance of various designed arrays are compared to the results given by full-wave commercial solvers.
Most of the current imaging methods in microwave induced thermoacoustic tomography (MITAT) system assume that the heterogeneous sound velocity (SV) and density distribution are given or subject to Gaussian distribution. These situations generally are not satisfied. To improve multi-targets thermoacoustic sources imaging quality in a heterogeneous tissue, an iterative TR adjoint imaging method is proposed. The proposed iterative TR adjoint method can reconstruct thermoacoustic sources from the measured data even if the prior heterogeneous information of the tissue is unknown. This method estimates misfit between synthesized and observed measured signals, and iteratively updates supposed model parameters which give the heterogeneous tissue structure. In this iterative procedure, error kernels of SV, density and the approximate point source position information can be obtained independently. After the time of flight (TOF) convergence criterion is reached, a regular time reversal (TR) method with updated model will give out the final imaging result. The proposed TR adjoint imaging method is based on strictly theoretical derivation, and some simulations are presented to validate the method.
When particle swarm optimization(PSO) technique is used for the inverse scattering problems, it will take unbearably long time for the final solution, especially when the PSO algorithm traps into the premature convergence. To overcome this problem, a hybrid multi-phased particle swarm optimization algorithm (HMPPSO) is proposed. By adopting the small swarm size strategy and the idea of ``sub swarms'' working cooperatively and alternatively with ``optimal swarm'' into the MPPSO, the HMPPSO can converge quickly with much less fitness function evaluation times, thus will reduce the reconstruction time. After the HMPPSO is validated by the numerical simulations on benchmark functions, the wall parameters (permittivity, conductivity, and thickness) together with target shape parameters (approximated by the trigonometric serials) with 20 dB additive Gaussian white noise are successfully reconstructed by HMPPSO using multi-frequency, multi-view/single-illumination scattering fields calculated by MOM.
This paper introduces a novel simulation testbed for investigating WLAN indoor localization systems. This testbed referred to as WiLocSim consists of a novel beacon received signal strength (RSS) simulator which provides realistic modeling of beacon signal characteristics such as multipath propagation, measurement noise and body loss. Each component of the simulator is individually modelled and verified prior to integration. In addition, the capabilities of the testbed are demonstrated using two variants of the nearest neighbour classification based indoor localization algorithm. Unlike conventional measurement based performance evaluation, the proposed testbed provides a reproducible environment for accurate evaluation and analysis of indoor localization systems. More importantly, it significantly reduces the high labour cost typically required in measurement based testbed.