John H. Beggs

Finite-difference time-domain implementation of surface impedance boundary conditions

Surface impedance boundary conditions can be utilized to avoid using small cells, made necessary by shorter wavelengths in conducting media throughout the solution volume. The standard approach is to approximate the surface impedance over a very small bandwidth by its value at the center frequency, and then use that result in the boundary condition. In this paper, two implementations of the surface impedance boundary condition are presented. One implementation is a constant surface impedance boundary condition and the other is a dispersive surface impedance boundary condition that is applicable over a very large frequency bandwidth and over a large range of conductivities. Frequency domain results are presented in one dimension for two conductivity values and are compared with exact results. Scattering width results from an infinite square cylinder are presented as a two dimensional demonstration. >

A two-dimensional time-domain near-zone to far-zone transformation

A 2D version of a time-domain transformation useful for extrapolating 3D near-zone finite-difference time-domain (FDTD) results to the far zone is outlined. While the 3D transformation produced a physically observable far-zone time-domain field, this is not convenient for the 2D case. However, a representative, 2D far-zone time-domain result can be obtained directly. This result can then be transformed to the frequency domain using a fast Fourier transform, corrected with a simple multiplicative factor, and used, for example, to calculate the complex wideband scattering width of a target. If an actual time-domain far zone result is required, it can be obtained by inverse Fourier transform of the final frequency-domain result. >

Finite difference time-domain calculation of transients in antennas with nonlinear loads

Transient currents for an antenna with a nonlinear load are calculated using finite-difference-time-domain (FDTD) methods. The FDTD electric field across the nonlinear load (which determines the voltage across the load) is calculated by solving a nonlinear equation at each time step. This allows the time step to be at the Courant limit, and is more efficient than reducing the time step size in all the FDTD cells to maintain stability in just the cell containing the nonlinear load. The stability of this approach relative to a simpler approach of approximating the diode as a variable resistor is demonstrated. As a validation of the method the transient current in a long dipole antenna with a nonlinear load excited by a pulsed plane wave is computed and compared with calculated results obtained by another method. The approach given here extends the applicability of the FDTD method to problems involving radiation and scattering from antennas including nonlinear loads. >

FDTD calculation of wide-band antenna gain and efficiency

Absolute gain and radiation efficiency are calculated using the finite-difference-time-domain (FDTD) method. The method described utilizes pulsed excitation, so that wideband results are available from a single FDTD computation. The approach is demonstrated for a simple wire antenna geometry, and is validated by comparison with results obtained using the method of moments. >

Wireless network simulation in aircraft cabins

An electromagnetic propagation prediction tool was used to predict electromagnetic field strength inside airplane cabins. A commercial software package, Wireless Insite/sup /spl reg//, was used to predict power levels inside aircraft cabins and the data was compared with previously collected experimental data. It was concluded that the software could qualitatively predict electromagnetic propagation inside the aircraft cabin environment.

Analysis of electromagnetic radiation from shaped-end radiators using the finite difference time domain method

The finite difference time domain (FDTD) technique is a popular method for analyzing electromagnetic scattering, radiation, and penetration problems. Several authors have recently applied the FDTD method to antenna radiation problems. To date, the antenna structures considered have been wire and conical monopole antennas, rectangular waveguides, pyramidal horn antennas, and microstrip antennas. Results from these analysis have been in the form of normalized field patterns and no results showing absolute gain have been presented. The article demonstrates the first staircased application of the FDTD method to the analysis of radiation from circular waveguides and other shaped-end radiators. Results of absolute gain versus angle are shown for a straight-cut circular waveguide and for two different shaped-end radiators. All FDTD analyses are full three-dimensional computations and are compared in each case with measured data. >

Wideband finite difference time domain implementation of surface impedance boundary conditions for good conductors

The authors present a one-dimensional implementation for a surface impedance boundary condition for good conductors in the finite-difference-time-domain (FDTD) technique. In order to illustrate the FDTD surface impedance boundary conditions, a planar air-lossy dielectric interface is considered. Overall, the surface impedance boundary condition implementation works well in eliminating the conductor volume from the solution space. This method has a distinct advantage over other possible implementations because the coefficients of the exponential approximation of the impulse response are independent of the conductivity of the scattering object and do not need to be reevaluated for different conductivities.<<ETX>>

Electromagnetic propagation of wireless networks in aircraft cabins

A commercial software package, Wireless Insite/sup /spl reg//, was used to predict power levels inside aircraft cabins and the data was compared with previously collected experimental data. Two methods of propagation were examined and through basic statistical analysis, it was concluded that the software could qualitatively predict electromagnetic propagation inside the aircraft cabin environment.

Numerical analysis of staircasing effects on cutoff frequencies for FDTD modeling of circular waveguides

The authors develop a method for modeling circular waveguides to minimize the effects of staircasing on the mode cutoff frequencies. A simple method for modeling circular cylinders is developed by examining the feed current for several radiating circular waveguide problems and by analyzing the numerical staircasing effects on the mode cutoff frequencies. The modeling method obtained is simple with no computational overhead.<<ETX>>

Finite difference time domain analysis of electromagnetic radiation from shaped-end radiators

The FDTD (finite-difference time-domain) technique was applied to the analysis of electromagnetic radiation from shaped-end radiators (SERs). A shaped-end radiator is a circular tube with a beveled cut on one end. The antennas and waveguides are modeled in the standard FDTD manner (i.e., staircased). Pattern plots are presented for a straight-cut circular waveguide ( theta =90 degrees ) using a sinusoidal excitation radiating the TM/sub 01/ mode as a reference. Results are also presented for SERs with cut angles theta =30 degrees and theta =50 degrees using sinusoidal excitation and also using an excitation with Gaussian time dependence. All FDTD results computed using sinusoidal excitation are compared with measured data.<<ETX>>