EM modeling of aircraft at low frequencies

An efficient computational technique for obtaining scattering cross sections of electrically small aircraft is presented. The technique is based on the wire-grid reaction method. The aircraft shape is approximated by a grid of thin wires, leading to a mathematical representation of the aircraft in the form of an impedance matrix. The inverted matrix yields the scattering data. A variety of wire-grid models were tested. The results show that with proper choice of radius of the wire segments, good agreement with experimental data can be obtained for polarizations parallel to the fuselage axis. For polarizations perpendicular to the fuselage axis, only fair agreement was observed. This computational technique is used to simulate realistic scattering data, which serves as input to information processors for target identification.     

Finite-element time-domain algorithms for modeling linear Debye and Lorentz dielectric dispersions at low frequencies

We present what we believe to be the first algorithms that use a simple scalar-potential formulation to model linear Debye and Lorentz dielectric dispersions at low frequencies in the context of finite-element time-domain (FETD) numerical solutions of electric potential. The new algorithms, which permit treatment of multiple-pole dielectric relaxations, are based on the auxiliary differential equation method and are unconditionally stable. We validate the algorithms by comparison with the results of a previously reported method based on the Fourier transform. The new algorithms should be useful in calculating the transient response of biological materials subject to impulsive excitation. Potential applications include FETD modeling of electromyography, functional electrical stimulation, defibrillation, and effects of lightning and impulsive electric shock.     

Sequential activation of a segmented ground pad reduces skin heating during radiofrequency tumor ablation: optimization via computational models

Radiofrequency (RF) ablation has become an accepted treatment modality for unresectable tumors. The need for larger ablation zones has resulted in increased RF generator power. Skin burns due to ground pad heating are increasingly limiting further increases in generator power, and thus, ablation zone size. We investigated a method for reducing ground pad heating in which a commercial ground pad is segmented into multiple ground electrodes, with sequential activation of ground electrode subsets. We created finite-element method computer models of a commercial ground pad (14 times 23 cm) and compared normal operation of a standard pad to sequential activation of a segmented pad (two to five separate ground electrode segments). A constant current of 1 A was applied for 12 min in all simulations. Time periods during sequential activation simulations were adjusted to keep the leading edge temperatures at each ground electrode equal. The maximum temperature using standard activation of the commercial pad was 41.7degC. For sequential activation of a segmented pad, the maximum temperature ranged from 39.3degC (five segments) to 40.9degC (two segments). Sequential activation of a segmented ground pad resulted in lower tissue temperatures. This method may reduce the incidence of ground pad burns and enable the use of higher power generators during RF tumor ablation.     

Initial results for automated computational modeling ofpatient-specific electromagnetic hyperthermia

Developments in finite-difference time-domain (FD-TD) computational modeling of Maxwell's equations, super-computer technology, and computed tomography (CT) imagery open the possibility of accurate numerical simulation of electromagnetic (EM) wave interactions with specific, complex, biological tissue structures. One application of this technology is in the area of treatment planning for EM hyperthermia. In this paper, we report the first highly automated CT image segmentation and interpolation scheme applied to model patient-specific EM hyperthermia. This novel system is based on sophisticated tools from the artificial intelligence, computer vision, and computer graphics disciplines. It permits CT-based patient-specific hyperthermia models to be constructed without tedious manual contouring on digitizing pads or CRT screens. The system permits in principle near real-time assistance in hyperthermia treatment planning. We apply this system to interpret actual patient CT data, reconstructing a 3-D model of the human thigh from a collection of 29 serial CT images at 10 mm intervals. Then, using FD-TD, we obtain 2-D and 3-D models of EM hyperthermia of this thigh due to a waveguide applicator. We find that different results are obtained from the 2-D and 3-D models, and conclude that full 3-D tissue models are required for future clinical usage.     

Transistor design for low distortion at high frequencies

The design of high-frequency bipolar transistors with very low distortion is described. Simple expressions for distortion are used to select device parameters for the optimization of distortion performance. The effect of epitaxial-layer characteristics on device performance is considered in detail, and the importance of collector depletion in achieving low distortion is shown. The influence of device geometry on distortion is considered, and the degradation caused by MOS capacitance is illustrated.     

Temperature measurement within myocardium during in vitro RF catheter ablation

While most commercial ablation units and research systems can provide catheter tip temperature during ablation, they do not provide information about the temperature change inside the myocardium, which determines the lesion size. We present the details of a flow simulation and temperature measurement system, which allows the monitoring of the temperature change inside the myocardium during in vitro radio frequency (RF) cardiac catheter ablation at different blood flow rates to which the catheter site may be exposed. We set up a circulation system that simulated different blood flow rates of 0 to 5 L/min at 37 degrees C. We continuously measured the temperature at the catheter tip using the built-in thermistor and inside the myocardium using a three-thermocouple probe. The system provides a means for further study of the temperature inside myocardium during RF catheter ablation under different flow conditions and at different penetration depths.     

Ground-wave propagation at medium and low frequencies

General methods of computing tropospheric diffraction fields in atmospheres whose modified refractive index increases monotonically with height are indicated, new ground-wave-propagation curves are predicted, and the effective-Earth-radius transition at medium and low frequencies is given.     

Simplified power supply for electrodischarge machining at low frequencies

Modern electrodischarge-machining apparatus often employs paralleled transistors to switch the higher currents required in the process, while maintaining a fast pulse risetime. This method necessitates complex driving circuitry and the need for careful selection of devices. A simpler circuit, using fast-turnoff thyristors is described, which takes advantage of their high power gain and inherent high current-switching capability. The limitations of the circuitry are explained, and a method for increasing the higher switching-frequency limit of the circuitry is indicated.     

HBT optoelectronic mixer at microwave frequencies: modeling andexperimental characterization

A heterojunction bipolar transistor (HBT) optoelectronic mixer was studied experimentally and theoretically. A detailed large signal π-model and a small signal analysis are described. The frequency dependence of down and up conversion has been analyzed and measured. In terms of conversion gain, the advantage of the down conversion process is clearly demonstrated. The values of parameters employed in the mixing process are also discussed for both large and small signal regimes     

Recession depth in metallic conductors at low frequencies

It is shown that, at low frequencies, for nonmagnetic conductors of thickness l, the recession depth is l/3 and is independent of frequency and of the conductivity of the material. This simple relationship for low frequencies allows a graph to be drawn of the recession depth over the whole frequency range, since, at high frequencies, the recession depth is one-half of the skin depth ?.     

Accurate low complexity modeling of twisted pairs suitable for G.fast frequencies

As the higher and higher frequency bands of existing metallic cables in access networks are being continuously exploited by modern transmission technologies, such as the G.fast, the necessity of providing accurate and suitable modeling of their transmission characteristics is evident. Therefore, this paper is focused on modeling of a propagation constant of twisted pairs and metallic cables at high frequencies up to 250 MHz, and an innovative arsinh model is proposed and described. This new model is based on an idea of adopting inverse hyperbolic sine function for modeling of both secondary line coefficients, attenuation constant and phase constant, and its main motivation is to provide their accurate estimations for G.fast frequencies up to 250 MHz for various types of metallic cables while maintaining a low computational complexity. The proposed model was compared with numerous characteristics measured for various real metallic cables as well as with several existing models in order to illustrate its potential. The results, which are presented within this paper, clearly illustrate that the proposed arsinh model generally outperforms existing standard models based on the equal number of required parameters. Copyright © 2015 John Wiley & Sons, Ltd.     

Digital-filter 2nd-order element with low quantising noise for poles and zeros at low frequencies

A 2nd-order recursive digital-filter element is proposed that gives an appreciable reduction in signal quantising noise and coefficient sensitivity compared with the direct or canonic forms of filter element when the poles or the zeros are near the point Z= 1+0j. This is a modificalion to a previously published 2nd-order element that only gives an improvement when the poles are near the point Z=1+0j.     

An equivalent boundary-condition model for lossy planar periodicstructures at low frequencies

An equivalent boundary-condition model is presented for planar periodic scatterers which, through an effective homogenization, accurately predicts the scattering at low frequencies (i.e., in the absence of higher ordered Floquet harmonics or grating lobes). This new anisotropic resistive boundary-condition model provides accurate wide-angle results for one- (1-D) and two-dimensional (2-D) periodic arrays, provided certain restrictions are satisfied concerning the rotational symmetry and surface resistivity of the target. When applicable, this simulation model provides an enormous reduction in computational costs with virtually no memory storage requirements. The anisotropic nature of the boundary condition arises only when the target possesses a twofold rotational symmetry and, thus, produces significant cross-polarized scattering. A unique feature of this model is that since an equivalent boundary condition is developed, finite arrays are also accurately modeled provided a minimum of approximately five unit cells (five by five for 2-D) are contained in the array     

Transport in semiconductors with low scattering rate and at high frequencies

A rigorous derivation of the hydrodynamic carrier transport equations is obtained by extending the moment method to include the moment equation of the collision free path which contains both the scattering and the generation-recombination-trapping-tunneling events. This leads to an electron (or hole) current equation with a new inductive term $\text{τ}\text{j}{}_{\mathrm{<mtext>N</mtext>}}\text{/?t}$ which becomes important when the signal frequency becomes comparable with the reciprocal average collision relaxation time, $\text{τ}$^?1, and which adds a collision delay inductance to the conduction current lines of the circuit model.     

Modeling of the dielectric logging tool at high frequencies:applications and results

The high-frequency dielectric logging tool is useful in electromagnetic well logging, because, by measuring the dielectric constants of rock formations at high frequencies (1 GHz), the water saturation of rocks can be inferred without knowing the water salinity in the rocks. The results of a theoretical model presented in a previous paper by the author are reported (ibid., vol.26, p.382-7, 1988). Use is made of the theoretical model to study the behavior of such a tool across geological sedimentary beds. The effect of using different polarizations on the measurement is considered, along with the standoff and mudcake effects that reflect on the depth of investigation of such a tool. The use of borehole compensation modes in the measurements is also considered. It has been found that the TE polarization is more robust than the TM polarization in terms of mudcake and standoff effects     

Carrier heating effects in junctions at very low currents

The effects of carrier heating by the electric field are analysed in abrupt $\text{p-n}$ junctions at very small currents. Two features, which are usually neglected, are shown to be of importance. (a) The heat transported by the carriers plays an appreciable part in the energy balance equation, which determines the carrier temperature. (b) The current has, in the junction and in the neighbourhood, two important additional components, one being due to a correction of the diffusion current, and the other being proportional to the gradient in the temperature of the carriers.     

RF measurements and characterization of heterostructurefield-effect transistors at low temperatures

The RF performance of both conventional AlGaAs-GaAs and superlattice AlAs-GaAs heterostructure field-effect transistors (HFETs) has been investigated at 120 K, and the results are compared with room-temperature values. Both the system used for low-temperature RF measurements up to 12 GHz and the procedure used to extract the equivalent circuit from measured S-parameters of the packaged FET are described. The high-frequency performance of the HFETs is strongly improved at low temperatures but is sensitive to light due to the device structure. The problems of low-temperature measurement and the results of RF investigation are discussed. Although the gate lengths of the HFETs investigated are greater than 1 μm, the method and the results of the analysis can be transferred to submicron devices without any restrictions. Therefore, submicron superlattice HFETs may exhibit high power gain at 300 K as well as at lower temperatures both in the dark and under illumination     

A new catheter design using needle electrode for subendocardial RF ablation of ventricular muscles: finite element analysis and in vitro experiments

Radio-frequency (RF) cardiac ablation has been very successful for treating arrhythmias related with atrioventricular junction and accessory pathways with successful cure rates of more than 90%. Even though ventricular tachycardia (VT) is a more serious problem, it is known to be rather difficult to cure VT using RF ablation. In order to apply RF ablation to VT, we usually need to create a deeper and wider lesion. Conventional RF ablation electrodes often fail to produce such a lesion. We propose a catheter-electrode design including one or more needle electrodes with a diameter of 0.5-1.0 mm and length of 2.0-10 mm to create a lesion large enough to treat VT. One temperature sensor could be placed at the middle of the needle electrode for temperature-controlled RF ablation. From finite element analyses and in vitro experiments, we found that the depth of a lesion is 1-2 mm deeper than the insertion depth of the needle and the width increases as we increase the diameter of the needle and the time duration. We showed that a single needle electrode can produce a lesion with about 10-mm width and any required depth. If a wider lesion is required, more than one needle with suggested structures can be used. Or, repeated RF ablations around a certain area using one needle could produce a cluster of lesions. In some cases, a catheter with both conventional electrode and needle electrode at its tip may be beneficial to take advantage of both types of electrode.