Linear Current Distribution Research Articles

On the Inverse EEG Problem for a 1D Current Distribution

Albanese and Monk (2006) have shown that, it is impossible to recover the support of a three-dimensional current distribution within a conducting medium from the knowledge of the electric potential outside the conductor. On the other hand, it is possible to obtain the support of a current which lives in a subspace of dimension lower than three. In the present work, we actually demonstrate this possibility by assuming a one-dimensional current distribution supported on a small line segment having arbitrary location and orientation within a uniform spherical conductor. The immediate representation of this problem refers to the inverse problem of electroencephalography (EEG) with a linear current distribution and the spherical model of the brain-head system. It is shown that the support is identified through the solution of a nonlinear algebraic system which is investigated thoroughly. Numerical tests show that this system has exactly one real solution. Exact solutions are analytically obtained for a couple of special cases.

Effect of the tank main gun on the radiation pattern of the monopole antenna

For telecommunication purposes, a monopole antenna is usually positioned on the tank turret. At low frequencies the whole tank has to be treated as a part of the antenna system. In this paper a method for electromagnetic modeling of metallic structures is presented and applied to the analysis of radiation of a tank monopole antenna. Radiation simulations are performed at the frequency range from 1MHz to 30MHz. A special attention is given to the analysis of the effects of increased elevation of the tank main gun to the radiation pattern of the antenna. The analysis of the radiation of the tank monopole is performed with and without the presence of conducting ground. It is shown that the increase in the main gun elevation at certain frequencies can lead to degradation of uniformity of radiation in the horizontal plane. Introduction Informational technologies and reliable and secure communications are an important part of a modern military doctrine. Regarding telecommunications, armored vehicles and tanks in particular present a specific problem. In this paper, a theoretical basis of the electromagnetic analysis of metallic structures and a modeling technique will be presented. The effect of the tank on the radiation pattern will be investigated both for a tank in free space and above the conducting ground. Method of moments The method of moments (MoM) is a method for an approximate solution of integral equations. This section presents the fundamentals of the MoM and the higher-order quadrilaterals as the basic elements used for geometry modeling. The currents in our method are approximated using high-order two-dimensional polynomials. Modeling of the tank geometry The tank is modeled using only 28 elements, out of which 10 bilinear quadrilaterals and 18 second-order surfaces. The monopole antenna is modeled using one straight wire segment. Results The use of the polynomial current approximation yields a reduction in the number of unknowns required for the analysis, compared to classical methods that use linear current distribution defined on small triangles. The analysis of the tank from 1MHz to 30MHz, with a 1MHz step, requires from 130 to 337 unknowns and about one minute of the CPU time on a Dell Vostro 1015 laptop computer. Tank in free space At low frequencies, the monopole-tank system acts as a dipole antenna and the computed directivity is 1.76dBi, almost equal to that of a short dipole. The elevation of the tank main gun does not influence the radiation pattern much at low frequencies. In the resonant range, the effect of the main gun elevation is much more pronounced. At 17MHz, the difference between the minimum and the maximum of radiation in the horizontal plane is 1.22dB, 1.90dB, and 2.33dB for the main gun elevation of 0°, 13°, and 20°, respectively. The effect of conducting ground The effect of the conducting ground (in this paper we assume the ground is perfectly conducting) is taken into account using the image theory. Since the original currents and their images are electrically close at low frequencies (their distance is small compared to the wavelength), their mutual coupling cannot be neglected. Once again, at low frequencies the whole structure acts as a short dipole antenna and the computed directivity is 4.76dBi; at the same time, the radiation pattern in the horizontal plane is almost uniform. At the frequency of 13MHz, however, the difference between the minimum and the maximum of the radiation in the horizontal plane is almost 8dB for the main gun elevation of 20°. Conclusion This paper presented an electromagnetic modeling and an analysis of the monopole antenna positioned on the tank turret. The antenna was analyzed at frequencies from 1MHz to 30MHz, and the effect of the main gun elevation on the antenna radiation pattern was studied. It was determined that at low frequencies the elevation of the gun does not influence the radiation pattern much, but at higher frequencies, where the tank can be considered as a resonant structure, the detrimental effect on the radiation pattern can be quite substantial. At the frequency of 13MHz, the range of communication in the horizontal plane can be almost cut in half for certain directions if the elevation of the main gun is 20°.

The Estimation of the Current-Density Distribution in a Moving Arc Root Using the ART Algorithm

This paper mainly contributes to the estimation of 2-D current-density distribution in a moving arc root. For this purpose, the multiple-split-cathode-with-different-slit-angle method is used to perform measurements of the linear current distribution of the arc root in seven different directions. On the basis of the measurement results, a large system of linear algebraic equations is treated with the algebraic-reconstruction-technique algorithm to obtain the 2-D current density. The calculation results indicate that the transverse profile of current density in a moving arc root can be characterized by a high-current core with two low-current hysteretic wings and that the maximum current density tends to the fore part of it.

Improved Loading Profile for GPR Antenna Applications

One of the most critical hardware components for the performance of the Ground-Penetrating Radar (GPR) system is the antenna and the resistive loaded antenna is one of common antennas used in the GPR system. The authors extend the Wu-King linear current distribution to the double exponential current distribution, modify the inaccurate part of the Wu-King loading profile and deduce an improved resistive loading profile with high efficiency. The broadband performance of the improved resistive loading profile has been studied; the concept of lengthening coefficient is created and the optimum parameter designing curve of the antenna is provided. Numerical calculation shows that the maximum bandwidth and efficiency of antenna can be achieved with the improved loading profile simultaneously. The impedance bandwidth is as high as 40:1, which is much greater than the 3:1 bandwidth of Wu-King distribution when the voltage standing wave ratio is below 3. It exhibits better efficiency and bandwidth than Wu-King loading profile. The loading profile can reduce the late-time ringing, enhance the pulse radiation characteristics and be widely applied to other resistive loading antenna for GPR applications.

Computer simulation to calculate current distributions and emittance patterns of ion beams for synthesis of electric lenses

In this paper a computerized simulation technique is offered to determine linear current distributions and emittance patterns for beams of charged particles in order to find axially symmetric electrostatic fields, which can serve as lenses for electrons or ions, with very good optical properties. The method is based on the direct simultaneous solution of the general trajectory equations describing the motion of the particles building up the beam in external electric fields. In the model the effects of the spherical and chromatic aberrations are in the scope but the properties of the finite source size and the radial velocity and energy spreads of the particles at the source can be also taken into account. An interactive software system, written in Borland C++ under Windows to run on IBM AT or compatibles, makes the simulation technique practically capable of carrying out valid computations in cases of non-laminar flows of charged particles and of calculating adequate information for synthesis of electrostatic lenses. Some computed results on unipotential focusing systems obtained by using the method and the corresponding lens-design examples are also presented in the paper.

Electromagnetic Missiles Generated by Linear and Surface Current Distributions

Electromagnetic(EM) missile sources have so far been limited to the cases where the spatial distribution of current density is either a straight line or a circular disk or a rectangle. It may then be questioned whether an arbitrary linear current distribution or an arbitrary surface current distribution can be used as a launcher of the EM missiles. In this paper the above question is answered, and we have demonstrated that an arbitrary linear or an arbitrary surface current distribution of finite size has the ability to launch an EM missile, i. e. to realize slow decrease of electromagnetic energy on any axis so long as the axis has an intersection point with the source region, and in the neighborhood of this point the spatial distributions of the current are locally flat and perpendicular to the axis.

On the natural frequencies of a hollow cylinder for acoustical and electrical excitations

An intrinsic significance attaches to resonant systems, which are capable of amplifying a small local disturbance (e.g., the sound produced by mechanical means), so that the discovery or explanation of their characteristics becomes potentially valuable. It is pertinent, as regards the latter objective, to determine the so-called natural (or eigen-) frequencies of individual systems, namely the scale parameters in exponential time factors for states that, once excited, receive no further support from external sources or other mechanisms. If a resonator can lose energy by outward transfer its free excitations subside in the course of time and the spectrum of natural frequencies is complex-valued. Lamb [3] discusses the acoustics of an open pipe and obtains, through separate considerations, both a real frequency equation and an estimate for the rate of decay of free vibrations. A memoir by van den Dungen [8] dealing with circular tubes and other resonator shapes, features the Rayleigh quotient of volume integrals, which define the respective measures of kinetic and potential energies, and presents bounds derived therefrom for real frequencies in particular states of excitation; damping that originates in radiation loss, however, is left out of account. Analogous studies of the quasi-periodic modes in electrical systems, due to Thomson [7], Abraham [1] and Brillouin [2], rely on solutions of the Maxwell field equations in coordinates that suit the chosen geometry (sphere or spheroid), along with properties of the special functions involved; Schelkunoff [6], who surveys and contrasts prior efforts, also sketches independent procedures aimed at approximating to the resonant frequencies and attenuation parameters in the case of thin conducting bodies with nearly linear current distributions. The purpose of this paper is to illustrate, by specific example, a self-contained method for estimating complex natural frequencies; that the underlying formulation and variational basis encompass arbitrary resonator shapes, without an obligation to choose special functions at the outset, suggests its general capability.

Radiation Characteristics of a Conical Helix of Low Pitch Angle

The present work is an extension of the author's prior work on the radiation characteristics of and current distribution in a conical helix, the helix used in the present case having a pitch angle much lower than of that studied previously. In this case the condition of the phase velocity adjusting itself automatically to maintain maximum directivity condition (which enables the radiation to be confined in the axial mode over a large band of frequencies) does not hold. Nevertheless the major fraction of energy could be radiated in the axial direction when the geometry of the helix and the distance of the apex from the ground were properly chosen, the conical helix together with the ground being regarded as tapered transmission lines (with the ground as one and the helix wire—with the distance of its elements from the ground line increasing continually,—as the other line). Measurements of the field intensity and the current distribution were made in the frequency range of 100 Mc/sec to 500 Mc/sec with a conical helix antenna arrangement as follows. The helix was of six turns, the radius increasing uniformly from 5 cm at the apex to 65 cm at the base within an axial distance of 60 cm. The apex of the helix was placed at a distance of 17.5 cm from the ``ground'' provided by a 6-ft square copper netting of close mesh. It was found that within the range of frequency measured the major fraction of the energy was radiated in the axial direction. Theoretical expressions for Eφ and Eθ were derived by assuming a linear current distribution, and the calculated radiation patterns were found to agree with the experimentally determined ones.

Radiation Field of a Conical Helix

It is now well known that a cylindrical helix, when excited at frequencies corresponding to wavelengths comparable to the length of one turn of the helix, can radiate a sharp beam along the axis over a wide frequency range (about one octave). It is shown in the present communication that if the helix be conical instead of cylindrical (the diameter varying along the length of helix), then the axial mode of radiation can be maintained over a much wider band of frequencies. The radiation pattern of a conical helix, 60 cm diameter at the base, tapering linearly to 20 cm at the top in 10 turns within a height of 112 cm (with the ``ground'' provided by brass disk of 100 cm in diameter) has been studied experimentally. It is found that the axial mode of radiation is maintained from 150 Mc/sec to 450 Mc/sec. By increasing the number of turns, the band width can be considerably increased. Assuming a linear current distribution, theoretical expressions have also been deduced for Eφ and Eθ for a conical helix. Some modifications of the simple conical helix, such as may have special applications, are indicated.

Note on the Maximum Directivity of an Antenna

It has been shown by Bouwkamp and deBruijn that the directivity of a linear current distribution of fixed length may be made arbitrarily large. By a slight extension of their arguments, the same conclusion is demonstrated for a two-dimensional current distribution and for a distribution of current on an infinite strip.