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Radar images of complex targets can be understood as a superposition of the reflected signals from a high number of scattering centers. To model complex targets for radar simulations, the plurality of scattering centers should be reduced to few significant scattering centers in order to minimize computational effort. The scope of this work is to present a technique to generate a significantly simplified RCS model of the vehicle with a limited number of virtual scattering centers, each with its own scattering characteristic, and how to group these scattering centers in a cluster database. The work is based on ray-tracing simulations of complex vehicle models. The ray-tracing simulations have been validated by measurements. The scattering centers may not be physically existing strong scattering centers, but virtual scattering centers representing a certain scattering behavior. In this paper, a technique for extracting such virtual scattering centers from a complex 3D-vehicle-model is presented. It is based on ray-tracing simulations of such models. As an example, the design model of a Ford Focus is used. Index Terms--Ray-tracing, RCS-modeling, scattering center.

Three-dimensional (3-D) ray tracing has advanced to such a degree that it can provide channel parameters like delay spread, Doppler spread, angular spread, and distribution functions of long- and short-term fading with high accuracy for fixed to mobile and mobile to mobile communications. These parameters are absolutely required during the specification phase of future mobile communications systems in order to define the air interface and a variety of other relevant system parameters. This paper describes state-of-the-art ray tracing capabilities. A 3-D ray tracing model developed at the Universitat Karlsruhe is presented. Important characteristic channel parameters are briefly discussed. Based on these parameters the model is verified by wideband nondirectional and directional measurements at 2 and 5.2 GHz, respectively, showing a good agreement. The proposed 3-D ray tracing model can therefore be used in order to extract parameter sets for the specification Of future mobile communications systems and to optimize existing ones. Index Terms--Channel measurements, directional channel modelling, ray tracing, urban environment, wave propagation.

This paper reports the characterization of radio wave propagation in hospitals. In contrast to other buildings with usual walls, some rooms within hospitals are constructed with special walls that include metallic layers. The structure of the walls depends on the purpose of each hospital room. To consider all the different wall types, measurements were performed in all relevant areas of four hospitals. Rooms, such as operating rooms, X-ray and magnetic resonance tomography (MRT) rooms were taken into account. The measurements were performed at eight discrete frequencies ranging from 42.6 MHz up to 5.2 GHz. In order to investigate accurately the wave propagation properties, several antennas and measurement systems were used. Results show that radio wave propagation characteristics vary from room-to-room. It turned out that it is important to consider the room as a whole when determining the attenuation between adjacent rooms, because a small part of the wall is not enough. In addition to usual wave propagation phenomena such as reflection and transmission, slots and openings give rise to coupling effects through walls containing metallic layers in their structure. The measurement data is useful for hospital designers and hospital engineers. Index Terms--Clinics, electromagnetic compatibility (EMC), hospitals, measurements, mobile communications, wall attenuation, wave propagation.

This paper presents broadband multimode antennas for multiple-input multiple-output (MIMO) and diversity applications. The antenna system is not based on spatial diversity, as usual MIMO systems, but on a combination of pattern and polarization diversity. Different modes of self-complementary, thus extremely broadband, spiral and sinuous antennas are used to decorrelate the signals. It is shown that only one antenna is necessary to receive three uncorrelated signals, thus the space required to place the MIMO antenna is very small. Simulation results and measurements of a typical indoor scenario are given. Index Terms--Multimode diversity, multiple-input multiple-output (MIMO), sinuous antenna, spiral antenna.

This paper presents two different ultra-broadband antenna designs for mobile communications and navigation, which can be integrated conformally into the bodywork of a car. Both antennas are based on the frequency-independent spiral structure. One antenna, which is intended to be integrated into the car's window, is an externally fed spiral antenna in order to obtain a flat structure. The second spiral antenna consists of four arms which are fed by a coplanar waveguide transmission line. This enables the excitation of two different radiation patterns for terrestrial and satellite services. With a low cost feed network both radiation modes can be excited simultaneously and independently of each other. Measurement results are presented within and beyond the specifications. Index Terms--Automotive, broadband antenna, car antenna, conformal integration, coplanar waveguide, externally fed spiral, frequency-independent antenna, low-profile, spiral antenna.

A new method to include stochastic rough surface scattering into deterministic ray-optical wave propagation modeling is derived. It can be utilized in conjunction with the concept of ray launching. Similar to the Kirchhoff formulations, the approach is based on a tangential plane approximation, i.e., it is applicable to surfaces with gentle undulations, whose horizontal dimensions are large compared to the incident wavelength. However, in contrast to the Kirchhoff models, which are only valid for either slightly rough or very rough surfaces, the proposed stochastic scattering approach includes both the coherent and incoherent components at the same time. The purely deterministic ray-based modeling is expanded by a 'stochastic' component, allowing for the first time to account for nondeterministic scattering in ray-optical wave propagation modeling. Index Terms--Kirchhoff approximation, ray launching, ray tracing, ray-optical propagation modeling, rough surface scattering.

This paper presents an efficient simulation tool for conventional, microwave, and combined heating. Two heat-transfer mechanisms are included: conductive and radiant heat transfer. The conductive heat transfer is modeled by a finite-difference algorithm. A modeling technique for radiant heat transfer in nonuniform grids has been developed and is here presented for the first time. For the radiant heat transfer a finite-difference scheme is not applicable, as radiation from a material surface is not bounded to the immediate vicinity, as is conductive heat transfer. Therefore, ray optical methods are used. Rays connecting mutually visible surfaces are obtained by a new fast method. Necessary, but acceptable simplifications allow fast computations. The algorithms are integrated conveniently together with an electromagnetic finite-difference time-domain program to one simulation tool. Representative simulations are presented for an oven heated conventionally, by microwaves, and by a combination of both. Index Terms--FDTD, heat radiation, microwave heating.

This paper presents a new method to obtain the material parameters of each single layer in a multilayered structure. The compound structure has to be measured over frequency or for different incidence angles in the microwave frequency range. The genetic algorithm for parameter extraction from the reflection or transmission measurement data is based on a simplified evolution strategy. In this paper, the evolution optimization is described briefly and is verified by measurements performed in the frequency range from 115 to 145 GHz. The parameters obtained by the algorithm show good agreement with reference values gained by other researchers. Index Terms--Genetic algorithm, material parameter extraction, multilayered structure.

The biconical antenna is well known as an easy to build broad-band antenna with omnidirectional radiation. In this paper, the biconical antenna is modified for selective radiation in azimuth direction. By enlarging the diameter of the coaxial feed to the biconical antenna, a set of higher order modes can be excited. In addition to the angular independent TEM mode, higher order TE-modes are also allowed for propagation, which result in an angular dependent far-field pattern. Several features of this multimode excitation are of interest. Applying multiple input feed connectors allows simple rotation of the antenna beam by switching or, when independently operated, transmission and reception for communication in different angular directions for real-time space division multiple access (SDMA). As with conventional antenna arrays, adaptive pattern synthesis can also be realized by using multiple input feed connectors in combination with an appropriate amplitude and phase shifting network. The antenna allows an extremely flexible design of its characteristics at low cost, both requisites for SDMA communication. Index Terms - Biconical antennas, multibeam antennas.

A theoretical description of the scattering of multiport antennas is derived in accordance with the polarimetric radar target characterization. For two-port antennas with dual linear orthogonal polarization, a procedure for the determination of the antenna mode scattering parameters and the structural scattering parameters of the complete polarimetric antenna scattering matrix [Mathematical Expression Omitted] is given. A measurement setup and experimental results of wide-band polarimetric radar cross section (RCS) antenna measurements are presented. For two-port antennas, in total, 24 unknowns have to be determined. The load variation method is used. The polarimetric RCS antenna measurement is especially advantageous over a wide bandwidth; for example 2-40 GHz with only one single calibration. From the antenna scattering matrix [Mathematical Expression Omitted], the antenna parameters like polarimetric gain, optimum gain, gain bandwidth, wave admittance, structural scattering, etc., are derived. Results are shown for an X-band horn antenna and a 2-18-GHz dual linearly polarized horn. The measurement technique and evaluation gives the most complete and very accurate information about antenna radiation, antenna characteristics, and scattering obtainable with one single measurement setup. Some quantities determined here, like the maximum available gain and the polarization characteristics, are difficult to access with standard transmission measurements. No cable from the antenna to the receiver is necessary. This helps avoid some difficulties usually experienced at low signal levels. Index Terms - Antennas, antenna measurements, polarimetry.

An analytical method is presented for the inverse scattering problem of a one-dimensional (1-D) inhomogeneous anisotropic medium in a special case. Using the closed-form formulations for the reflection coefficients derived in the first part of this paper, reconstruction formulas are obtained for the wave impedance profiles, permittivity profiles or permeability profiles of the anisotropic medium, all of which are given in closed form. In the meantime, a partial inverse scattering method for the electric parameters at the interface of the medium with free space is also investigated by using a Wentzel-Kramers-Brillouin (WKB) approximation. Numerical examples show the validity of the methods. Index Terms - Electromagnetic scattering, inverse problem.

An analytical formulation is presented for the direct scattering problem of one-dimensional (1-D) inhomogeneous anisotropic medium in a special case. This method gives nonlinear differential equations for the reflection coefficients of the anisotropic medium. Applying the discontinuity condition of the dielectric parameters at the interface of the medium with free space, approximate closed-form solutions for the reflection coefficients are obtained. Numerical examples show the validity of the method. Index Terms - Electromagnetic scattering, inverse problems.

Anechoic chambers for EMC testing in the frequency range from 30 MHz to 1 GHz have to fulfill stringent requirements before they are accepted for standardized emission and immunity measurements. In Europe, these requirements will lead to an exclusion of about 90% of the existing and 50% of the new chambers from an accreditation due to the formerly lower standards. This paper presents for the first time a complete, accurate and validated numerical analysis method of EMC anechoic chambers, which focuses especially on the problems below 200 MHz where the efficiency of most absorbers is quite poor. Examples of computed field configurations, normalized site attenuation (NSA) deviations, and field homogeneity zones are presented. They show the complexity of the frequency dependent field propagation mechanisms in anechoic chambers. Computational results coincide excellently with measurements obtained by scanning the chamber with a small field probe. Unlike trial and error or ray optical methods, often used for this purpose, the developed code can accurately predict the performance of new chambers during their design, before the realization. Measures are proposed for the functional improvement of existing as well as new chambers toward an accreditation. Index Terms - Anechoic chamber, absorber, propagation, EM theory.

An angle invariance property based on Hertz's principle of particle dynamics is employed to facilitate the surface-ray tracing on nondevelopable hybrid quadric surfaces of revolution (h-QUASOR's). This property, when used in conjunction with a Geodesic Constant Method, yields analytical expressions for all the ray-parameters required in the UTD formulation. Differential geometrical considerations require that some of the ray-parameters (defined heuristically in the UTD for the canonical convex surfaces) be modified before the UTD can be applied to such hybrid surfaces. Mutual coupling results for finite-dimensional slots have been presented as an example on a satellite launch vehicle modeled by general paraboloid of revolution and right circular cylinder.

A polarimetric wave propagation model for field strength forecasting and coverage prediction in the VHF/UHF frequency range is presented. The model uses a digital terrain data bank and considers multipath propagation (1). Based on the uniform geometrical theory of diffraction (UTD) and physical optics an approach is described for calculating the propagation effects in natural 3-D terrain, given by topographical and morphographical data. The method for field strength forecasting is described and methods for the analysis of the predicted multipath signal are discussed. It is shown how the complex probability density function (PDF) for the receiver field strength and the field strength delay spectrum (FDS) can be derived. Methods for further evaluation of the transmitting channel characteristics are discussed.

This paper describes the application of the finite-difference method for the determination of scattering parameters of passive, arbitrary three-dimensional, lossy structures. Maxwell's equations are solved in the frequency domain by solution of a boundary value problem. The generalized S-parameters can be computed for any one port or two port structure, while, for the first time, dielectric and conductor losses are taken into account. Higher order mode coupling can be considered and different geometries are allowed at the input and output ports. Verification calculations are given and results are presented for typical structures.

Various taper functions are well known to suppress side lobes in antenna array factors. These tapers are limited to either transmit or receive arrays and are mostly restricted to arrays with a high number of elements. This paper presents an approach for applying taper functions also to digital beamforming (DBF) arrays with transmitter multiplexing. For such DBF configurations, taper functions have not been known yet. An example for a multiple transmit DBF taper will be given which is dedicated for low number of elements. Index Terms--Digital beamforming (DBF), multitransmit digital beamforming, taper function.

Using a single offset cylindrical parabolic reflector and a parallel-plate Luneburg lens, a novel high-gain and high-efficiency antenna is developed for automotive radar at mm-wave frequencies. Particularly, a pair of small corrugated extensions is added to the lens for a cylindrical wave. Thus, by feeding a cylindrical wave to the reflector, especially the performance in elevation is enhanced, keeping the complete antenna compact. For verification, a prototype antenna for an adaptive cruise control radar at 76.5 GHz is designed and measured. The theoretical results are verified with good agreement. Index Terms--Adaptive cruise control (ACC) radar, mm-wave antennas, multiple beam antenna (MBA), parallel-plate Luneburg lens (PPLL).

A full-wave treatment of lossy three-dimensional structures using the finite-difference method in frequency domain is presented. This accounts for both, dielectric and conductor losses. By introduction of a surface resistance the effect of conductor losses and surface roughness can be modeled very efficiently. The modifications of the finite-difference frequency domain (FDFD) algorithm are presented. Comparisons between the conventional approach using elementary cells with finite conductivity and this new discretization method with surface current cells are given, and the advantages and limitations of the surface current model are shown.

This work is concerned with the calculation of natural electromagnetic (EM) wave propagation and the determination of the propagation channel characteristics in highway or railway tunnels in the ultrahigh-frequency (UHF) range and above ([is greater than] 300 MHz). A novel ray-tracing technique based on geometrical optics (GO) is presented. Contrary to classical ray tracing, where the one ray representing a locally plane wave front is searched, the new method requires multiple representatives of each physical EM wave at a time. The contribution of each ray to the total field at the receiver is determined by the proposed ray-density normalization (RDN). This technique has the further advantage of overcoming one of the major disadvantages of GO, the failure at caustics. In contrast to existing techniques, the new approach does not use ray tubes or adaptive reception spheres. Consequently, it does not suffer their restrictions to planar geometries. Therefore, it allows to predict the propagation of high-frequency EM waves in confined spaces with curved boundaries, like tunnels, with an adequate precision. The approach is verified theoretically with canonical examples and by various measurements at 120 GHz in scaled tunnel models. Index Terms--Geometrical optics (GO), measurements, millimeter waves, mobile communications, ray launching, ray tracing, tunnels, wave propagation.

An efficient method to solve electromagnetic scattering problems involving several metallic scatterers or bodies composed of dielectric and metallic regions is proposed. For this, the block Jacobi preconditioner is combined with the iterative transpose-free quasi-minimum residual (TFQMR) algorithm. For best performance, the blocks for the preconditioner are chosen according to individual scatterers or groups of scatterers, or according to the electric and magnetic current basis functions for dielectric/metallic scatterers. Index Terms--Boundary integral equations, iterative methods, linear algebra, moment methods.

A measurement campaign has been carried out in the Berlin subway to characterize electromagnetic wave propagation in underground railroad tunnels. The received power levels at 945 and 1853.4 MHz are used to evaluate the attenuation and the fading characteristics in a curved arched-shaped tunnel. The measurements are compared to ray-optical modeling results, which are based on ray density normalization. It is shown that the geometry of a tunnel, especially the cross-sectional shape and the course, is of major impact on the propagation behavior and thus on the accuracy of the modeling, while the material parameters of the building materials have less impact. Index Terms--Geometrical optics, measurements, mobile communications, ray launching, ray tracing, subway, tunnels, wave propagation.

Periodic and regular metal posts, a photonic bandgap (PBG) structure for guiding surface waves in a parallel-plate waveguide is proposed. The isotropic PBG structure is applied to the design of an asymmetric parallel-plate waveguide Luneburg lens (APWLL). The relation between the dimensions of the metal posts and the required refraction index in the lens is derived with the transmission-line theory and the transverse resonance method. Different lattices for the entire lens are also investigated. For verification, an antenna for a 76.5 GHz adaptive-cruise-control radar is fabricated, consisting of an APWLL, a primary feed, and symmetric corrugated flares to improve the property of the antenna in elevation. Measured results verify the PBG structure design in the APWLL. Index Terms--Adaptive cruise control radar, electromagnetic bandgap, Luneburg lens antennas, millimeter-wave antennas, photonic bandgap.