Paul McKenna

Effects of reinforced concrete structures on RF communications

The proliferation of communication systems used in and around man-made structures has resulted in a growing need to determine the reflection and transmission properties of various commonly used building materials at radio frequencies typically used in businesses and residential environments. This paper describes the calculation of reflection and transmission coefficients for reinforced concrete walls as a function of wall thicknesses and rebar lattice configuration over a frequency range of 100-6000 MHz. The transmission and reflection coefficients were calculated using a finite-difference time-domain (FDTD) solution of Maxwells equations. The rebar structures analyzed include both a two-dimensional (2-D) trellis-like structure and a one-dimensional (1-D) structure, where the reenforcing bars are all oriented in the same direction. In general, the results show that the reinforced concrete structures severely attenuate signals with wavelengths that are much larger than the rebar lattice and that the transmitted signal has a complex structure with resonances and nulls that strongly depend upon the geometry of the reinforcing structure and the concrete wall thickness.

A model for predicting the power delay profile characteristics inside a room

Multipath effects in indoor wireless communication systems exhibit a characteristic power delay profile (PDP) and can be a detriment to the systems performance. We present a simplified model for calculating the decay rate of the PDP for propagation within rooms. This simplified model provides a time-efficient means of predicting system performance. Predictions of this in-room PDP model are compared to results obtained from a finite-difference time-domain (FDTD) model. Additionally, comparisons of the IPDP model to measured data are presented. The RMS delay spread is the second central moment of the PDP of a propagation channel and is a measure of the communication link degradation due to multipath. We also show results of the estimated RMS delay spread from this model and show comparisons to the measured data. This IPDP model can be used to investigate the effects of variable room size and properties of the surfaces (or walls) on the decay characteristics of the PDP.

Time-domain modeling, characterization, and measurements of anechoic and semi-anechoic electromagnetic test chambers

We present time-domain techniques for modeling, characterizing, and measuring anechoic and semi-anechoic chambers used for emission and immunity testing of digital devices. The finite difference time-domain (FDTD) approach is used to model and characterize these chambers. In the FDTD model presented here, we discuss methods used to eliminate the need to spatially resolve the fine detail of the absorbing structures; present a differential-operator approach for incorporating both frequency-dependent permittivity and permeability into the time domain; and discuss the effects of gaps and holes in ferrite-tile absorbers on both absorber and chamber performance. Comparisons of the FDTD chamber model with measured data for different chamber sizes are presented. Finally, we discuss and illustrate how time-domain techniques can be used to characterize chambers, predict performance, and diagnose problems with both absorbers and chambers. With time-domain and frequency-domain techniques, we show how the performance of chambers can be significantly altered with only small changes in the type of absorbing structure used, and we illustrate how undesirable modal field distributions can occur inside a chamber when a nonoptimal absorber is used.

The effects of gaps in ferrite tiles on both absorber and chamber performance

In this paper, we present a model that accounts for the gaps between ferrite tiles which occur during the installation of the tiles in anechoic chambers. With this model we investigate the effect of gaps on both the performance of the tiles and the degradation of the chamber performance due to the gaps. It is shown that small gaps can cause large changes (as much as 5 to 10 dB) in the reflectivity, especially at the lower frequency end. It is also shown that small gaps can cause variations in chamber performance, and these variations can have a detrimental effect on the ability of the chamber to meet its performance specification.

Finite-difference time-domain modeling for field predictions inside rooms

In this paper, the authors present a three-dimensional model based on the finite-difference time-domain (FDTD) technique that is used to analyze and design absorber lined EMC/EMI chambers. The FDTD is implemented in a manner such that the frequency-dependent material properties are taken into account. Comparisons of measured and predicted normalized site-attenuation are presented.

Radar target image resolution enhancement via propagation channel equalization

The performance of signal/image processing algorithms used to form radar images and identify targets depends on propagation effects such as time-varying multipath, dispersion, attenuation, etc. In this paper, the effects of multipath propagation conditions that result from terrain or man-made environments and noise are modeled and simulated to determine their effects on radar target signatures. Here, we calculate the radar signatures for a ground vehicle subject to the deleterious effects of both multipath and additive noise. A minimum mean-square-error Wiener equalizing filter is developed and applied to the distorted radar target signatures. The mean-square-error is calculated as a function of signal-to-noise ratio and compared for several cases to demonstrate the potential for improvement in radar image resolution. These techniques are particularly relevant for targets located in mountain or urban canyon environments.