Low frequency scattering by a resistive plate

For a plane wave incident on a resistive plate, the first two terms in the low frequency expansion of the far field are determined. Since the magnetic dipole is not excited if the resistivity is nonzero, the leading term is simply the electric dipole contribution, and is identical to that for a perfectly conducting plate. The resistivity appears explicitly in the next term which can be expressed in terms of potentials. Some implications of the results are discussed, and the method is illustrated in the case of a circular disk.     

On the profile of frequency dependent series resistance and dielectric constant in MIS structure

In this study, the frequency dependent of the forward and reverse bias capacitance-voltage (C-V) and conductance-voltage (G/ω - V) measurements of Al/SiO₂/p-Si (MIS) structures are carried out in frequency range of 10 kHz-10 MHz. The frequency dependence of series resistance (R_s), density of surface states (N_ss), dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tan δ) and the ac electrical conductivity (σ_dc) are studied for these structure at room temperature. Experimental results show that both electrical and dielectric parameters were strongly frequency and voltage dependent. The ε′ and ε″ are found to decrease with increasing frequency while σ_ac is increased. Also, both the effects of surface states N_ss and R_s on C-V and G/ω - V characteristics are investigated. It has been seen that the measured C and G decrease with increasing frequency due to a continuous distribution of N_ss in frequency range of 10 kHz-1 MHz. The effect of R_s on the C and G are found noticeable at high frequencies. Therefore, the high frequencies C and G values measured under both reverse and forward bias were corrected for the effect of series resistance R_s to obtain real MIS capacitance C_c and conductance G_c using the Nicollian and Goetzberger technique. The distribution profile of R_s-V gives a peak in the depletion region at low frequencies and disappears with increasing frequencies.     

Application of the finite-element method for solving a spectral problem in a waveguide with piecewise constant bi-isotropic filling

A spectral problem in a bi-isotropic waveguide filled nonuniformly in the cross section is considered. A generalized formulation of the problem is considered, making possible, while using Lagrange finite elements, a substantial reduction in the number of spurious modes in the spectral region of interest and shifting them to the higher frequency region. An algorithm for calculating the propagation constants and fields in the given waveguide is implemented. A series of calculations of dispersion curves and fields in a waveguide with different cross-section structures are performed.     

Uniqueness of solutions of piecewise linear resistive networks withtwo terminal resistors

Consider a network comprising two terminal linear, piecewise linear resistors and independent sources. Contrary to widely held belief, it is shown that the determinant sign condition (i.e. the Jacobian determinant is nonzero and has the same sign in all the regions) is both necessary as well as sufficient to guarantee uniqueness of solutions of these networks. For this purpose the existence of a specific hybrid matrix is not required     

Transient response of a transistor-tunnel diode hybrid combination with resistive feedback

An approach is suggested for studying the transient behavior of a transistor-tunnel diode hybrid combination with resistive feedback which shows an improved switching speed. The method uses the charge-control model for the transistor and empirical power functions for the tunnel diode. Experimental observations with tunnel diode in combination with slow and gold-doped switching transistors demonstrate that the models used in the analysis are satisfactory for engineering calculations.     

TE characterization of resistive strip gratings on a dielectricslab using a single edge-mode expansion

Closed-form expressions are derived for the TE scattering by a narrow resistive strip array in the presence of a dielectric slab. This is accomplished by introducing a single edge-mode expansion to represent the strip current density followed by a Galerkin's solution of the pertinent integral equation. The accuracy of the derived approximate expressions are examined by comparison with a moment method solution and several curves are shown, illustrating how the different array parameters affect the reflectivity and transmissivity of the array. Particular attention is given to the free-standing resistive strip array where an equivalent uniform resistive sheet is constructed for simulating the original periodic structure. The reactance of this equivalent sheet is compared with published expressions pertinent to the metallic strip array     

Diffraction of plane waves by a resistive strip residing between two impedance half-planes

A uniform asymptotic solution is presented for the diffraction of E_z polarized plane waves by a resistive strip residing between two impedance half-planes. The analysis proceeds from triple integral equations approach which leads to a system of uncoupled modified Wiener-Hopf equations (MWHE). This system is then reduced to two pairs of Fredholm integral equations of the second kind which are solved by successive approximations. Diffracted field expressions are derived up to the third order terms which include the surface wave field effects in a uniform manner.     

基于电阻型频率选择表面的超宽带紧耦合阵列

针对接地板严重影响紧耦合天线阵列(Tightly Coupled Array,TCA)阻抗带宽的问题,设计了一种基于电阻型频率选择表面(Resistive Frequency Selective Surface,RFSS)的多层结构超宽带TCA,分析了电阻膜方阻及单元间耦合对阻抗带宽的影响,并对其扫描及未扫描时的性能进行了仿真分析.结果表明:通过在天线阵列与接地板之间加载开口谐振环结构的RFSS,可减小接地板反射,抑制天线短路点,显著增加其阻抗带宽,其辐射性能在整个带宽内均较稳定;由于RFSS加入了损耗,天线阵列增益有所降低.该多层结构TCA具有带宽宽、体积小、易共形等特点,在超宽带小型化相控阵天线中有良好应用前景.     

Synthesis of tapered resistive strips

A technique for synthesizing a resistive taper that generates desired bistatic scattering and backscattering patterns from a strip is outlined. Antenna synthesis techniques relate the scattered field to the induced surface current density. Physical optics approximations then relate the induced current to the resistivity. The taper is checked by computing the surface current density and scattered fields of the tapered resistive strip using the integral equation formulation and comparing with the physical optics results.     

Measurement of the microwave structure constant profile

Measurements of the microstructure of refractivity at 9.4 GHz were made in Colorado and Florida up to altitudes of about 29000 ft. The structure function parameterC_{n}^{2}was calculated from these data. Examples of the resulting profiles are presented with corresponding profiles of refractivity and temperature. Values ofC_{n}^{2}varied from about10^{-13}to10^{-17}m^-2/3. The heights of occurrence of maximum and minimum values are summarized.     

Performance of the maximum likelihood constant frequency estimatorfor frequency tracking

The performance of maximum likelihood (ML) estimators for an important frequency estimation problem is considered when the signal model assumptions are not valid. The motivation for this problem is to understand the robustness of the hidden Markov model-maximum likelihood (HMM-ML) tandem frequency estimator, where the signal is divided into time blocks, and the frequency in each time block is estimated using the ML approach under the assumption that the signal has a constant frequency in each time block. In order to analyze the sensitivity of ML estimators to the model assumptions, the mean frequency of a discrete complex tone that has a time-varying (ramp) frequency is estimated under the incorrect assumption that it has a constant frequency. In particular, the behavior of the threshold region with respect to different chirp rates is analyzed, and a simple rule is given. The mean squared error above the threshold region is shown to be constant even at very high SNR levels. These results are supported by simulations     

3-D electrical impedance tomography for piecewise constant domains with known internal boundaries

Electrical impedance tomography (EIT) is a badly posed inverse problem, but can be stabilized if one assumes that the conductivity is piecewise constant, with a relatively small number of distinct regions, and that the region boundaries are known, for example from prior anatomical imaging. With this assumption, we introduce a three-dimensional (3-D) boundary element method (BEM) model for the forward EIT map from injected currents to measured voltages, and 3-D inverse solutions for both BEM and the finite element method (FEM) which explicitly take into account the parameterization implied by the known boundary locations. We develop expressions for the Jacobians for both methods, since they are nonlinear, to more rapidly solve the inverse problem. We show simulation results in a torso geometry with the heart and lungs as inhomogeneities. In a simulation study, we could reconstruct the conductive values of some internal organs of a human torso with more than 92% accuracy even with inaccurate internal boundary locations, a randomized rather than constant conductivity profile (with the standard deviation of the Gaussian-distributed conductivities set to 20% of their mean values), signal to measurement noise of 50 dB, and with different meshes used for the forward and inverse problems. BEM and FEM perform similarly, leading to the conclusion that the choice between them should be based on secondary considerations such as computational efficiency or the need to model conductivity anisotropies     

Determination of amplitude levels of the piecewise constant signal by using polynomial approximation

A new approach to determining the amplitude levels of piecewise constant signal has been proposed that is based on using its multiplicative model and solving the problem of polynomial approximation. In case of the absence of noise, the statement of polynomial approximation problem is based on the requirement of exact match of the current signal value with the amplitude value of one of its levels. In case of the presence of ordinary additive noise, the problem statement is based on the least squares criterion, while the solution of problem is presented in the analytical form. For the case of the presence of pulse-type noise, the problem statement is based on the minimum duration criterion, while the problem solution is achieved numerically by an appropriate functional minimization in unknown amplitudes of levels. The case of binary piecewise constant signal is considered in detail. The results of numerical simulation are presented for the cases, where the binary signal is distorted by ordinary additive noise with Gaussian distribution law and the pulse-type noise with the Cauchy distribution law.     

Operation of tunnel diodes above their resistive cutoff frequency

Tunnel diodes have been made to operate above their resistive cutoff frequency by mounting them in tapered waveguides. The output is apparently due to harmonics of a lower frequency oscillation. The fundamental frequency was detected by means of a probe placed close to the diode. Examination of the diode's equivalent circuit indicates that it is unlikely that oscillations can occur above the resistive cutoff frequency.     

New constant frequency PWM dual convertor with synchronous rectifier

A new constant frequency PWM dual convertor which has fully regulated dual outputs is proposed. The proposed convertor is the superposition of a forward convertor and a flyback convertor. The switches in the forward convertor for synchronous rectification provide another degree of freedom in controlling the flyback convertor. Thus, not only forward output but also flyback output can be fully regulated by constant frequency PWM control in continuous conduction mode (CCM).<>     

An error bounded current controller with constant sampling frequency

In this paper, a constantly sampled current controller is proposed such that the controlled current can track the command within a specified error bound to guarantee the desired power quality. The authors further propose a simple two-value inner bound for current error comparison to determine the right time to add the zero-mode control to reduce both the switching frequency and the resulting current error. Moreover, an upper bound of the constant sampling period for the controller to guarantee the desired performance is derived and some design criteria are given for proper coordination among the design parameters. Finally, some simulation and experimental results are given to demonstrate the feasibility of the proposed current controller.     

Attenuation profile for constant power dissipation along a transmission line

The attenuation profile required for uniform power dissipation along a transmission line is derived. Results for the attenuation profiles for various values of the total power dissipated in a unit length are presented graphically.