Input Admittance Research Articles

Effect of internal reflections on the radiation properties and input admittance of integrated lens antennas

This paper begins with the modeling of the reflected waves within integrated lens antennas, which consist of a dielectric lens on which a planar antenna is mounted. It is demonstrated that if the relative dielectric constant of the lens is small (/spl epsi//sub r//spl les/4), the single- and double-reflected waves are sufficient to analyze the effect of the internal reflections. For small angles around boresight, these unwanted reflected fields mainly affect the cross-polar far-field pattern, while for large observation angles, both the co-polar and cross-polar patterns are significantly disturbed. It appears that by neglecting the internally reflected field contributions, the beam efficiency may be overestimated more than 10%. In this paper, two types of matching layers are analyzed in order to reduce these unwanted reflections. It is demonstrated that the radiation performances of the integrated lens antennas with optimum-thickness and quarter-wavelength matching layer are almost equal. Even for low dielectric-constant lenses, the beam efficiency can be increased by over 10%. Finally, it is demonstrated that the internal reflections may also have a strong effect on the antenna admittance, which can only be reduced partly by the use of a matching layer.

Experimental Investigation of a Piezoelectric Ceramic Transformer Using Radial Vibration of Disks Combined with a Coupling Element

In this paper a new type of piezoelectric ceramic transformer using radial vibration of disks, combined with a coupling element is proposed. The specific merit of this piezoelectric ceramic transformer is that its electrical input impedance is effectively reduced by the radial vibration of disks with a high electromechanical coupling factor. First, the structure of the new piezoelectric ceramic transformer is presented. Next, the experimental results of the input admittance characteristics, gain of step-up voltage and power transmission efficiency of the piezoelectric ceramic transformer which were measured by varying the length of the coupling element are presented. The experimental results proved that the newly proposed piezoelectric ceramic transformer possessed a vibration mode that could realize a low input impedance and power transmission with efficiency of not less than 90% for a high-voltage step-up application.

Finite-element simulation of piezoelectric transformers

A three-dimensional finite-element approach to the characteristic prediction of piezoelectric transformers is presented. For a numerical example, a Rosen-type piezoelectric transformer is considered. The electrical input admittance and the transmission or transfer characteristics are demonstrated for the fundamental longitudinal mode operation. Based on these characteristics, the lumped parameters are extracted to form the electrical equivalent circuit. The effect of the loading or termination condition on the output voltage and the conversion efficiency are also examined. The numerical results are then compared with the experimental results. Examination is then extended to the optimum design in terms of efficiency and the output voltage.

Input admittance of a multilayer insulated monopole antenna

An efficient numerical technique based on the modal-expansion method in conjunction with a recursive algorithm is developed for a multilayer insulated monopole antenna fed by a coaxial transmission line. The modal-expansion analysis is facilitated by introducing a perfectly matched boundary (PMB) at a variable height over the ground plane of the monopole. The current distribution and input admittance are computed by finding the expansion coefficients of the electromagnetic field expressions. Numerical results for the input admittance of a dielectric-coated monopole antenna and an air-insulated monopole are compared with experimental ones available in the literature. Good agreement is achieved. Calculated results for the effects of various parameters on the input admittance of an air-insulated monopole antenna are presented and discussed.

Impulses concealed by singularities: Transmission line theory

The input admittance of a loss-free short-circuited transmission line is generally said to be a cotangent function of frequency, a function that is imaginary and odd. The symmetry implies that the impulse response, which has the admittance function of frequency as its Fourier transform, is also odd. However, there can be no response at negative times preceding the applied impulse. Therefore the cotangent formula for susceptance cannot be correct. A real, even function of frequency representing a conductance term must be added to the imaginary, odd susceptance. Likewise, the familiar formula Z = iZ0tan(2πL/λ) is deficient.

Input admittance of a coaxial-line-driven cylindrical cavity with a center dielectric rod

An expression for the input admittance of a coaxially driven cylindrical cavity is presented. Two cases are presented: first, where the coaxial-line transition is located on one of the flat walls of an empty cavity, and secondly where there is a dielectric rod located coaxially between the two flat walls of the cavity. A comparison between the theory and measurements is presented which shows that the model is capable of yielding a high level of accuracy.

Three-dimensional base distributed effects of long stripe BJT's: AC effects on input characteristics

The small-signal voltage and current distributed effects in the polysilicon and intrinsic base regions of long stripe bipolar junction transistors (BJT's) at high frequencies are investigated, and simple analytic equations describing the voltage and current distribution in these regions are derived. It is shown that the frequency-dependent debiasing effects in the polysilicon contacts and intrinsic base region change the current behavior and modulate the input admittance. The current and voltage distributions in the polysilicon region are nonuniform and vary with frequency. Conventional two-dimensional (2-D) device simulations cannot accurately predict this three-dimensional (3-D) effect. A quasi-3D simulation scheme combining a 2-D device simulator and the distributed model is presented to properly and efficiently describe the input characteristics of the device at high frequencies. It is also shown the use of various polysilicon sheet resistances, geometry sizes, and layout structures changes the distributed characteristics and modulates device performance at high frequencies. The impact of the layout structure and geometry size on RF circuit design due to the distributed effects is also studied.

FREQUENCIES, INPUT ADMITTANCES AND BANDWIDTHS OF THE NATURAL BENDING EIGENMODES IN XYLOPHONE BARS

The results are reported of experimental measurements of the response of xylophone bars to a random signal applied at their centre and at one end. Empirical values are obtained with these spectra for the frequencies, input admittances and bandwidths corresponding to the natural blending eigenmodes of xylophone bars. The results explain the acoustical behaviour of the vibrating elements of xylophones and show that it is possible to estimate the vibrational effects of the geometrical shapes of such bars.

Effects of pars flaccida on sound conduction in ears of Mongolian gerbil: acoustic and anatomical measurements

This paper presents evidence on how the pars flaccida of the tympanic membrane affects the acoustic input to the middle and inner ear. Measurements of middle-ear acoustic input admittance and sound-pressure levels in both the ear canal and the middle-ear cavity were made in ears of eight gerbils before and after manipulations of the middle ear and tympanic membrane. The results are interpreted in terms of a model proposed by Kohllöffel [Hear. Res. 13 (1984) 83–88]. The input-admittance measurements show that the pars flaccida of gerbil acts as a resonator with a resonance frequency of approx. 500 Hz. The admittance of this resonator appears in parallel with the input admittance of the pars tensa and its ossicular and cochlear load. At frequencies below the resonance, the pars flaccida admittance is compliance-like and its magnitude is comparable to that of the pars tensa and its load; consequently, the presence of pars flaccida increases the overall middle-ear input admittance and decreases the pressure difference across the tympanic membrane. At higher frequencies, the admittance of pars flaccida is mass-like and small in magnitude, and it has negligible influence on the overall middle-ear input admittance and the pressure difference across the tympanic membrane. These results suggest that the presence of pars flaccida reduces low-frequency acoustic input to the middle- and inner-ear and consequently decreases hearing sensitivity in this frequency range. Our measurements suggest that with a constant sound pressure stimulus, stiffening the gerbil pars flaccida would increase the motion of the pars tensa by 3–10 dB at frequencies below the normal flaccida resonance. © 1997 Elsevier Science B.V. All rights reserved.

VOLMAX: a solid-model-based, transient volumetric Maxwell solver using hybrid grids

This paper provides an overview of the finite-volume algorithm, the dissipative time-integration scheme, boundary condition implementation, grid generation, and a time-step definition suitable for sub-time cycling. To examine the accuracy of VOLMAX operating in either a purely unstructured or a hybrid-grid mode, the resonances of a rectangular cavity, the input admittance of a thick wire antenna, and three simple microstrip applications (microstrip line, patch antenna and low pass filter) with well-known FDTD solutions are examined. The latter three examples demonstrate the accuracy that can be obtained by launching microstrip-guided waves on cubical hexahedral (FDTD) grids and through a dense, unstructured linear tetrahedral region.

Permittivity measurements using a coaxial-line conical-tip probe

A conical-tip probe for the measurement of permittivity is presented which retains many of the advantages of the normal-plane probe including ease of sample preparation and moderate sample volume requirement, but which is mechanically better suited than monopole probes for the measurement of a wide range of biological materials since it does not promote free moisture regions resulting from cell rupture. The probe input admittance is found by modelling the probe as a short partially-sheathed monopole in free space and then a well-known antenna scaling theorem is used to determine the admittance and hence permittivity in the dielectric medium.

Dynamic Power and Energy Capabilities of Commercially-Available Electro-Active Induced-Strain Actuators

A method has been developed to predict the apparent material constants, the input and output power and energy, and the electro-mechanical energy conversion efficiency of electro-active induced-strain actuators under full-stroke, quasi-linear dynamic operation. The effect of the piezo-electric counter electro-motive force on the apparent input admittance is included. The non-symmetric expansion-retraction behavior of the electro-active material under full-stroke dynamic operation is symmetrized using a bias-voltage component and a superposed dynamic voltage amplitude that produce, in the actuator, a static position and a dynamic stroke amplitude, respectively. It is shown that the presence of the bias-voltage operation increases significantly the reactive power amplitude, and a simple formula for estimating this increase is provided. Reaction power values up to three times larger than those for unbiased operation were found. The secant linearization method and vendor data were used to evaluate the full-stroke piezoelectric strain coefficient, d, elastic compliance, s, electrical permittivity, ∈, and electro-mechanical coupling coefficient, x, of the electro-active actuator. Consistency with the basic active-material values was checked, and correction of the actuator full-stroke electro-mechanical coupling coefficient was applied, when required. Maximum power and energy delivery under optimal dynamic conditions (dynamic stiffness match) was studied, and the dynamic energy output capability of several commercially-available actuators were computed. Output energy densities per unit volume, mass, and cost were also calculated. The best electro-mechanical conversion efficiency, which was shown to take place at stiffness ratios slightly different from the dynamic stiffness match, was also computed.

Modal expansion analysis of monopole antennas driven from a coaxial line

A rigorous modal expansion analysis is presented for a monopole antenna fed through an infinite perfectly conducting ground plane by a coaxial transmission line. An infinite perfectly conducting plate is introduced to facilitate the use of the modal expansion method. The surface current and input impedance of the monopole antenna result from finding the expansion coefficients of the electromagnetic field expressions by enforcing boundary conditions at conducting surfaces and regional interfaces. Numerical results for the current distribution and input admittance of a parallel plate antenna and a monopole fed by a coaxial line are presented and compared with available data in the literature. Very good agreement is observed. Also examined is the effect of the junction between the feed line and the monopole on the antenna's admittance.

Theory of a multiply fed and loaded insulated linear antenna in air

Analytic expressions are derived for the input admittance and the current distribution along the multiply fed and loaded insulated antenna in air which is excited across feed gaps of nonzero widths. The Wiener-Hopf type analysis for a center fed insulated antenna is combined with the axial field discontinuity (AFD) method to develop the current and admittance expression. This AFD method considers the metallic surface of the linear antenna as a series combination of longitudinal, electric-field surface functions that exist due to feeding and/or loading. The analysis, which does not employ superposition of even and odd distributions of sources and loads, yields final expressions in terms of the excitation location, its aperture electric field within the feed gaps, impedance locations, and their values. The current distribution for cases of unity dielectric constant shows an excellent agreement with data based on the moment method. The input admittances and current distributions are reported for different multiple excitations and loads and dielectric constants of the insulator.

Spectral domain analysis of single and coupled microstrip open discontinuities with anisotropic substrates

The normalized input admittance for single and coupled microstrip open discontinuities with anisotropic substrates are obtained using a full-wave analysis. Problem is formulated in terms of the field Green's function in the spectral domain. Numerical results are found to be in good agreement with the published theoretical and experimental results for microstrip open discontinuities with anisotropic/isotropic substrates.

A new numerical treatment for Pocklington's integral equation

It is shown that the equation for the current flowing on a cylindrical antenna-Pocklington's integral equation-can be put into 'a standard form' of dual integral equations. A general method for solving such dual integral equations is discussed. It consists of a transformation into a single Fredholm integral equation of the second kind with continuous kernel, and then into a linear system of algebraic equations by using the weighted residual method. Plots of the current along the antenna and of the input admittance are finally given.

A linearized model of bassoon sound production: The role of auxiliary fingerings.

A quantitative theory of sound regeneration in the bassoon helps explain differences in the playing frequency and mouthpiece spectra associated with the use of auxiliary fingerings. The linearized condition for steady-state oscillations at a frequency f is 0=Y(f)+YG(f); the air-column input admittance is Y(f) and the double-reed generator admittance is YG(f), whose real part is negative (Fletcher, 1979; Thompson, 1979). The generator admittance is based upon a simple Bernoulli-type flow model, and the cane reed is modeled as a damped oscillator. Model parameter values are empirically determined and combined with measured input admittances, allowing direct comparison of Y(f) and −YG(f) at frequencies below and above the open tone-hole lattice cutoff frequency (Benade, 1960). Auxiliary fingerings produce significant differences in input admittance magnitude and phase above cutoff, and sound production is stabilized for a fingering such that the linearized condition is satisfied near a harmonic multiple of the playing frequency. The model accounts for intonation shifts associated with changes in auxiliary fingerings. While sound production in the bassoon is highly nonlinear, the fine structure of the linear air-column response at frequencies above cutoff is an essential contributor. [Work partially supported by the International Double Reed Society.]

Coaxial to circular waveguide transition as high-precision easy-to-handle measuring cell for the broad band dielectric spectrometry of liquids

For an abrupt transition from a coaxial to circular waveguide with a dielectric interface in the plane of discontinuity, the input admittance has been calculated as a function of frequency and of relevant geometrical and electrical quantities. The calculations have been performed applying a mode matching technique. A simple lumped-element representation has been derived from the theoretical considerations which was found to provide an excellent description of the input admittance of the discontinuity over a wide range of values of the parameters of interest. Network parameters are discussed for transitions that are most suited as sample cells for dielectric measurements in both the time domain and the frequency domain. Results from test measurements are reported to show the agreement of our numerical predictions with precise capacitance values of other authors and also to demonstrate the sensitivity and accuracy attainable with such cells.

Finite-element modeling of a smart cantilever plate and comparison with experiments

The behavior of a cantilever plate instrumented with a piezoelectric sensor and actuator is described using finite-element modeling. To demonstrate the accuracy of the numerical model, a parallel experimental study was carried out in the laboratory for the same geometric dimensions. The two results are compared and show excellent agreement, demonstrating that finite-element modeling is a good approach for optimized smart structure design. A three-dimensional finite-element formulation is employed in the piezoelectric material region and a small neighboring region of the plate structure on which it is mounted. Shell elements, approximated by many flat-shell elements, are used in modeling the remaining part of the plate structure. Transition elements that connect the three-dimensional solid elements to the flat-shell element are used. For the cantilever plate example, the electrical input admittance as well as the sensor response are found from the finite-element analysis and they are compared with experimental measurements. From this, the accuracy and efficiency of this approach is demonstrated. In contrast to many other modeling techniques used for smart structures which are approximate and hence limited, the finite-element model is applicable to complicated geometries.

Equivalent network approach for guided electron waves obliquely incident on step discontinuities in planar heterostructures

An equivalent network approach is developed for the analysis of the guided electron wave obliquely incident on step discontinuities in planar heterostructures. Stress is placed on network representations to establish physical pictures of the quantum effects and to yield insight; in addition, a systematic: microwave network approach is employed. A coordinate transformation translates an eigencoordinate mode of the guided electron wave propagating at an angle to the step discontinuity into a coordinate mode propagating normally to the step discontinuity, so that the step can be viewed as a transverse discontinuity and a procedure for the normal incidence can directly be exploited; transmission line equations in the direction normal to the step discontinuity, the mode-matching procedure for the boundary-value problem, and an equivalent network for the guided electron wave obliquely incident on the step discontinuities can be obtained. Using this network, the input admittance formulation is carried out, with which one can easily derive the equations that estimate the scattering and waveguiding characteristics of the guided electron wave obliquely incident on step discontinuities. Numerical examples are presented for a three-layered electron waveguide with a stub, and it is shown that this structure operates as a channel waveguide in addition to an angle filter.