Kenneth C. Chen

Accuracy of approximate transmission line formulas for overhead wires

Errors resulting from two approximations of the transmission line parameters of overhead wires are determined. Sundes simple approximation (1968) is found to be more accurate than the Hankel function approximation. >

Equivalent antenna radius for narrow slot apertures having depth

The problem of electromagnetic penetration of a narrow slot aperture in a thick perfectly conducting plane is reduced to the solution of Hallens integral equation with an equivalent antenna radius. The depth and width of the slot are assumed to be small compared to both the length of the slot and the wavelength. The equivalent radius is evaluated in terms of the solution to the transverse static problem. Simple approximations for the equivalent radius is also given. Hallens integral equation is solved by the Galerkin method with piecewise sinusoidal basis functions. Large slot depth-to-width ratios give rise to vanishingly small equivalent radii and thus large antenna resonance quality factors. A simple correction to the static field distribution in the slot is also given. This correction allows the depth to become somewhat larger relative to the wavelength. >

Slot apertures having depth and losses described by local transmission line theory

The problem of electromagnetic penetration of a narrow slot aperture in a thick conducting plane is considered when the conductivity of the plane (and slot walls) is large but not necessarily infinite and when somewhat lossy gaskets are placed in the slot. The problem is partitioned into a local region, where transmission line theory is used to describe the antenna modes along the slot, and a nonlocal region, where a filament-type integral representation is used. The result is a modified Hallen-type integro-differential equation for the slot voltage (or magnetic current). The local transmission line theory allows the effects of wall loss and gaskets to be included in a simple and intuitive manner. Examples involving a rectangular slot aperture are given. The finite conductivity of the conducting plane, even for good conductors, reduces the penetration when realistic slot dimensions are used. Gaskets, even with relatively small loss tangents, also reduce the penetration when realistic slot dimensions are used. >

Relation between equivalent antenna radius and transverse line dipole moments of a narrow slot aperture having depth

A simple general relationship is shown to exist between the equivalent radius and the transverse line dipole moments of a narrow slot aperture having depth. The cross-sectional dimensions of the aperture are assumed to be small compared to both its length and the wavelength. The general relationship can be put succinctly; the perfect magnetic cylinder, having the equivalent radius of an aperture with depth, has not only the same properties but also the same transverse line dipole moments of the aperture. >

Transient response of an infinite cylindrical antenna

A simple analytical formula derived from Wus exact solution for the transient response of an infinite cylindrical antenna is found to be extremely accurate. Existing results from other numerical schemes are also compared to Wus solution and found to be in good agreement.

Time harmonic solutions for a long horizontal wire over the ground with grazing incidence

A parametric study of the current responses on a long horizontal wire over the ground with frequencies ranging from 10 kHz to 100 MHz, earth conductivities of 10^{-1}, 10^{-2} , and 10^{-3} S/m, and wire heights of 10, 5, and 1 m. These current responses for typical wire lengths are given for a grazing incident plane wave and an incident lateral wave.

A simple transmission line model for narrow slot apertures having depth and losses

A simple transmission line model is given to approximately represent a narrow slot aperture having depth when the conductivity of the conducting plane (and slot walls) is large but not necessarily infinite and when somewhat lossy gaskets are placed in the slot. Slot and gasket dimensions, in addition to the electrical parameters of the conducting plane and gasket, are restricted so that gasket- and wall-loss-effects only need to be included locally (near the slot cross section) and additionally so that the slot voltage is nearly constant locally (however diffusion into the depth of the slot is considered when very lossy gaskets are introduced). Normal incidence is emphasized since this is typically the most important case for slots that are not too long compared to the wavelength. The transmission line model is used to obtain simple approximate formulas for resonant quality factor, slot voltage, penetrant fields, and transmitted power. Low frequency quantities including magnetic dipole moments are also obtained. Gaskets with air gaps are considered and optimum choices for electrical parameters are discussed. >

Transient response of an infinite cylindrical antenna in a dissipative medium

The transient solution of an infinite cylindrical antenna in a dissipative medium caused by an impulse excitation at a delta gap is obtained via an exact solution in the form of definite integrals and a simple asymptotic formula. This formula is used to obtain an integral for calculating the antenna current caused by a double exponential input voltage at the gap; furthermore, the resulting integral reduces to Sundes classic result under the diffusion limit. These results are applied to the transient response of a buried wire subject to an electromagnetic pulse (EMP) or nearby lightning incident wave.

A bound on EMP coupling

A bound on the energy coupled to a load by means of an antenna or aperture subjected to a step function electromagnetic pulse (EMP) is given by the polarizabilities of the antenna or aperture. Examples of coupling bounds for a slender electric dipole antenna, a circular aperture, and a narrow slot aperture with depth are given. It is demonstrated, by use of the bound on the integral of the effective area over all frequencies, that the maximum energy absorbed by an object subjected to a step function EMP is bounded in terms of the static polarizabilities of the object. (The energy spectrum of the step function EMP bounds the spectrum of EMP described by the double exponential over all frequencies.) It is demonstrated that this bound can be used on apertures in infinite ground planes for which Babinets principle fails to apply (apertures having depth and cavity backed apertures), where the requirement is simply to use the polarizabilities on the incident side of the aperture. >

A uniformly valid loaded antenna theory

A quasi-static layered approximation is used to simplify the layered solution for insulated antennas to the solution of a generalized impedance boundary value problem, whose solution is expressed in terms of an integral. This integral applies to insulated antennas imbedded in a dense medium, insulated antennas imbedded in air (dielectric-coated antennas), and impedance-loaded antennas, all referred to as loaded antennas. The branch cut contribution for large distances is given by the Sommerfeld space wave formula. The physical transition of loaded antennas to bare antennas is investigated through the asymptotic evaluation of this integral. Simple uniform formulas for loaded antenna current are derived and generalized to cover the same range of validity as the integral. The direct calculation of the input admittance is consistent with the derived uniform formula for antenna current. For insulated antennas in a dense medium, the complete transmission line theory describes the antenna current through the transition to bare antennas. >