Frederick M. Tesche

On the direct calculation of a transient plane wave reflected from a finitely conducting half space

An approach to the computation of the electromagnetic field reflected from a lossy half-space directly in the time domain is discussed. This approach requires first evaluating the impulse response of the half-space and then convolving it with the specified incident field waveform. To obtain the impulsive reflected field, either for vertical or horizontal polarization, approximations to the Fresnel reflection coefficients are made, thereby permitting an analytical expression in the time domain. Several different numerical examples using this technique are presented to illustrate the use of the method and the error contained in the solution. >

Comparison of the transmission line and scattering models for computing the HEMP response of overhead cables

The author discusses the use of transmission line and electromagnetic scattering models for computing the induced currents on long above-ground electrical cables, subjected to a transient plane wave excitation. The pertinent equations for determining the excitation electric field along the cable are summarized, along with the expressions which relate the line current to the excitation field. For the transmission line case, the cable current can be expressed in terms of per-unit-length impedance and admittance parameters which are functions of only the line geometry and the frequency. For the scattering case, similar line parameters can be inferred from the form of the solution for the current, but with the line parameters depending on the angles of incidence. Several numerical calculations of the line responses are provided, and these indicate that the transmission line analysis appears to provide response estimates which are smaller than those obtained using the more accurate scattering theory. >

On the use of the Hilbert transform for processing measured CW data

The Hilbert transform is a commonly used technique for relating the real and imaginary parts of a causal spectral response. It is found in both continuous and discrete forms and is widely used in circuit analysis, digital signal processing, image reconstruction and remote sensing. One useful application in the area of high-power microwave (HPM) technology is in correcting measured continuous wave (CW) transfer function data, so as to insure causality in reconstructed transient responses. Another application of the Hilbert transform is in the area of complex spectral estimation using magnitude-only data. Here, the applications of the transform to several specific spectral filtering and phase reconstruction problems are illustrated. >

The Analysis of Monopole Antennas Located on a Spherical Vehicle: Part 2, Numerical and Experimental Results

Presented are various numerical results illustrating the behavior of thin monopole antennas located on a perfectly conducting sphere. The method of analysis, described in a previous paper, uses an integral equation solution for the unknown wire currents, and a modified Greens function to limit the range of integration to over the wires only. Studies are made of the input quantities, radiated currents and induced sphere currents for various antenna geometries. A comparison of the computed input impedance of monopole on the sphere is made with experimental data and good agreement is noted.

A Coupling Model for a Pair of Skewed Transmission Lines

A coupling model in the form of an equivalent circuit is developed for a pair of skewed transmission lines. The inductive coupling is evaluated in closed form and the capitances are obtained from the solution of a pair of coupled integral equations for the excess-charge distributions along the transmission lines. The coupled integral equations for the excess charges are solved by an approximate analytical method and also by employing the method of moments. The two solutions are seen to be in excelent agreement. The excess-charge distributions are then used in a parametric study of the capacitances in the coupling model. The results are presented in graphical form.

Electrostatic observations of laser‐induced optical damage in LiNbO3

A new technique for observing optical damage is reported. It is based on detecting electrostatic potentials which result from the laser‐induced macroscopic charge separation responsible for optical damage. This new technique has been used to determine, for the first time, the charge distribution of the displaced charges. For uniform circular illumination of diameter D, the charges appear to reside in a thin cylindrical shell also of diameter D, with the negative charge on the positive ferroelectric axis. For a Gaussian illumination, we observe the charge distribution to change with increasing exposure time, approaching a cylindrical‐shell charge distribution, a result which may provide an explanation for ’’optical clean‐up’’. This new technique for probing optical damage is as sensitive as standard optical techniques and is unaffected by temperature variations and mechanical vibrations.

The Analysis of Monopole Antennas Located on a Spherical Vehicle: Part 1, Theory

In this paper, a technique for determining the behavior of thin-wire antennas mounted radially on a conducting sphere is formulated. The method of analysis involves the derivation of an integral equation for the antenna current. By a proper choice of boundary conditions, a modified Greens tensor for the sphere can be defined. This limits the range of the integral equation to over the thin wires only, thereby permitting a relatively simple solution for the antenna currents.

The Transverse Distribution of Surface Charge Densities on Multiconductor Transmission Lines

This paper is concerned with a problem which occasionally arises in the general area of multiconductor transmission line theory. In the past, the method of moments has been applied [1], [2] for the computation of transverse charge distribution and the capacitive-coefficient matrix for electrostatic systems formed by multiconductor transmission lines, with prescribed voltages on each line. But classically, there has been an interest in the related problem of finding the transverse charge distributions, given the net charge on each line [3], [4]. When the net charges are prescribed, conformalmapping techniques have been successfully employed in determining the charge distributions for certain special cases, e.g., the two-wire problem [5] and a planar grating [6]. The integral-equation formulation presented in this note for the charge distributions is applicable to a general system of parallel conductors, not necessarily in the same plane, as long as their net charges are prescribed. Another important application of this method lies in determining the field-coupling parameters when the multiconductor transmission line is illuminated by an external field.

Prediction of the E and H fields produced by the Swiss mobile EMP simulator (MEMPS)

The author discusses a simple analysis for determining the electromagnetic fields produced by the MEMPS simulator. A transmission line solution for the currents flowing on the simulator structure is obtained, taking into account both the resistive loading along the simulator and the dispersive nature of the earth under the simulator. Once the current distribution is determined, the fields at an arbitrary location are found by integrating the fields produced by an electric current element located over the lossy air-earth interface. Using this model, a procedure for estimating the fields at an arbitrary location within the simulator is described. This requires a knowledge of the primary transient E and H field components at a reference point near the simulator, or equivalently, a knowledge of the incident E or H field at this point. Results of this study indicate that it is possible to predict the simulator fields at other points, based on the reference fields and the calculational model. >

On the Effectiveness of Charged Wire Structures for Reducing Electron Backscatter in System-Generated EMP Simulators

Using symmetry conditions and conformal mapping techniques, the two-dimensional problem of potentials due to a periodic assembly of thin line charges is first solved. The three-dimensional problem obtained by placing one and two cross-wire meshes parallel to a ground plane are then analyzed for evaluating the effectiveness of such structures in trapping Compton electrons in the system-generated EMP simulators. The parametric analysis conducted here indicates that such structures can successfully trap electrons with velocities considerably in excess of the escape velocity. By considering only very thin wire structures, the relatively unimportant parameter of direct collision of the electrons with the mesh is removed. Parametric analysis yields data suitable for maximizing trapping efficiency as a function of the kinetic energy of the Compton electrons.