Surface Impedance Model Research Articles

FDTD surface impedance models for electrically thick dispersive material coatings

A new accurate finite‐difference time‐domain model of dielectric and conductive layers on metal surfaces is developed. The model is based on higher‐order surface impedance boundary conditions (SIBC). Several improvements to the existing numerical SIBC models are presented. The most important features of the proposed model are the inclusion of the tangential variations of the fields into the model and a considerably more accurate approximation of the impedance function than in the earlier works. In addition to the usual dielectric and conductive layers, the model can also handle layers of Lorentz, Debye, or Drude media. It is shown that the range of applicability of the proposed SIBC model is greatly increased as compared with the earlier models. Verification of the model is performed with one‐ and two‐dimensional simulations. The results are compared with the exact results in both time and frequency domains. Also, a comparison with an earlier model is provided.

Extracting surface impedance model parameters of a high-temperature superconductor from experimental characteristics of a microstrip resonator

We describe a method for extracting the model parameters of a high-temperature superconductor (HTSC) film from experimental resonance characteristics of a microstrip resonator. The method is based on a correct phenomenological model of the surface impedance of an HTSC film and the exact calculation of the microstrip resonator characteristics. The procedure of extracting the model parameters from experimental data is realized by using a genetic algorithm. Based on the models described, a special program implementing this mathematical algorithm was written, which allows the HTSC model parameters to be determined from experimental data.

Numerical Study of Mode Competition in Coaxial Cavity Gyrotrons with Corrugated Insert

A time-dependent self-consistent multimode code which allows to model the interaction between the modes of a coaxial cavity gyrotron operating at the fundamental and/or at higher harmonics of the cyclotron frequency is presented. The Method of Moments (MoM) is used to obtain a rigorous solution of the eigenvalue problem for a coaxial waveguide with corrugated insert. It is shown that the use of the Surface Impedance Model for the calculation of modes operating at higher harmonics leads to incorrect results in high-power coaxial cavity gyrotrons. However, the numerical simulations based on the MoM agree well with the available experimental results.

Conductivity and resistivity tensor rotation for surface impedance modeling of an anisotropic half‐space

The electromagnetic surface impedance of a half‐space with inclined conductivity anisotropy can be derived from the isotropic half‐space solution provided the conductivity term used in the expressions is the effective horizontal conductivity. For a TM‐mode plane wave incidence, the effective horizontal conductivity must be derived from the tensor rotation of the resistivity tensor and not from the tensor rotation of the conductivity tensor. For a TE‐mode incident with the same geometry as the TM‐mode, the surface impedance is independent upon the inclined anisotropy. This same formulation can then be extended to a multiple layered half‐space where each layer has an inclined anisotropy. For an anisotropic half‐space with coefficient of anisotropy of 4, with a horizontal conductivity of 0.001 S/m inclined at 45° with respect to the horizontal plane, the magnitude of the surface impedance calculated using the resistivity tensor rotation is approximately 53% larger than the magnitude of the surface impedance calculated using the conductivity tensor rotation.

Correction to “A self‐consistent impedance method for electromagnetic surface impedance modeling” by David V. Thiel and Raj Mittra

Correction to “A self‐consistent impedance method for electromagnetic surface impedance modeling” by David V. Thiel and Raj Mittra

2D‐FDTD modeling of wire antennas near artificial impedance surfaces

AbstractThis Letter deals with wire antennas radiating in the vicinity of an impedance surface. The use of impedance surfaces has been suggested, for instance, in the context of antenna reflectors. In this Letter a numerical surface impedance model is introduced. Some radiation patterns are calculated to validate the model. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 38–40, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10366

Phenomenological model of the microwave surface impedance of high-T/sub c/ superconducting films

The model of the microwave surface impedance of thin HTS films is based on a modified two-fluid approach and includes the phenomenological description of the normal and superconducting carrier density and the relaxation time of the normal carriers. A correlation between the London penetration depth at T=0 and the model parameter, which is responsible for the temperature dependence of the superconducting carrier density, was found from numerous experimental data. This correlation allows developing the phenomenological model of the HTS film surface impedance with minimal number of the fitting parameters: the transition temperature T/sub c/, the normal conductivity at T=T/sub c/, the residual resistance parameter, and the exponent describing the temperature dependence of the superconducting carrier density. The validity of the model was verified by a comparison with experimental data.

A self‐consistent impedance method for electromagnetic surface impedance modeling

A two‐dimensional, self‐consistent impedance method has been derived and used to calculate the electromagnetic surface impedance above buried objects at very low frequencies. The earth half space is discretized using an array of impedance elements. Inhomogeneities in the complex permittivity of the earth are reflected in variations in these impedance elements. The magnetic field is calculated for each cell in the solution space using a difference equation derived from Faraday's and Ampere's laws. It is necessary to include an air layer above the earth's surface to allow the scattered magnetic field to be calculated at the surface. The source field is applied above the earth's surface as a Dirichlet boundary condition, whereas the Neumann condition is employed at all other boundaries in the solution space. This, in turn, enables users to use both finite and infinite magnetic field sources as excitations. The technique is shown to be computationally efficient and yields reasonably accurate results when applied to a number of one‐ and two‐dimensional earth structures with a known surface impedance distribution.

Effective impedance of rough porous ground surfaces

Laboratory measurements at grazing angles of the impedance of sand surfaces containing artificial two-dimensional (2D) roughness are compared with predictions of an extended boss theory, which allows for incoherent scatter, randomness, and finite impedance effects. The data and predictions are found to be in reasonable agreement. The rough surface impedance model may be used, as an alternative to layering, to explain measurements where the reactance exceeds the resistance at low frequencies. The model also explains instances where the measured ground impedance tends to zero at higher frequencies. In addition, the model is found to predict features of measurements made with an elevated source and vertically separated receivers at a range of 30 m over cultivated ground surfaces.

Integral equation method with surface impedance model for 3D eddy current analysis in transformers

To estimate the stray loss distribution in power transmission systems like transformers, a new 3D eddy current analysis technique has been developed. This technique is based on the Integral Equation Method with Surface Impedance Model and is suitable for calculating eddy currents in a magnetized substance that has a complicated structure like transformers do. This technique was applied to an actual transformer and a new shield concept was proposed.

Revisiting the waves on a coated cylinder by using surface-impedance model

The results of analytical and numerical study of the modes propagating on an impedance-surface circular cylinder are communicated. Besides the well-known axially symmetric Sommerfeld's mode, another hybrid mode, having a single azimuthal variation, can propagate at any frequency. For a realistic cylindrical conformal antenna, the both wavenumbers are quite comparable. This may cause additional coupling between array antenna elements.

Microwave power dependent surface resistance of YBaCuO superconductors: material quality linkage

Three types of dependences of microwave (36 GHz) surface resistance on microwave magnetic field amplitude have been found for YBaCuO samples of various qualities. For the poor-quality samples is proportional to at low fields; then it changes to linear versus and, finally, saturates at large . The intermediate-quality samples demonstrate a threshold character of the nonlinearity with a jump-like growth of at field amplitudes higher than some characteristic value. Finally, the best samples exhibit an dependence which is initially quadratic with and then grows faster with the microwave field. The results are analysed in the framework of various models for the nonlinear surface impedance of type II superconductors.

A new FDTD surface impedance formalism to study PBG structures

This paper presents an analytical surface impedance model able to copy the behavior of photonic bandgap materials (PBG). The formalism introduced below is developed in the frequency domain and transposed into the time domain before being implemented in an FDTD algorithm. This new model is finally validated by considering a PBG plane reflector. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 18: 203–206, 1998.

A new FDTD surface impedance formalism to study PBG structures

This paper presents an analytical surface impedance model able to copy the behavior of photonic bandgap materials (PBG). The formalism introduced below is developed in the frequency domain and transposed into the time domain before being implemented in an FDTD algorithm. This new model is finally validated by considering a PBG plane reflector. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 18: 203–206, 1998.

Gyrotron coaxial cylindrical resonators with corrugated inner conductor: Theory and experiment

Gyrotron coaxial resonators with a longitudinally slotted inner cylinder are examined analytically using a surface impedance model, from which expressions for the electromagnetic field, ohmic quality (Q) factor, and characteristic equation of the transverse eigenvalues /spl chi//sub m,p/ are obtained. The major attributes of such resonators are expressed by the dependence of /spl chi//sub m,p/ on the parameter C-defined as the ratio of the outer to inner radii of the coaxial structure. In that connection, the effect of the corrugation parameters on /spl chi//sub m,p/ is particularly investigated on the basis of an expression derived for the slope function d/spl chi//sub m,p,p//dC. It is shown that the /spl chi//sub m,p/(C) curve may either exhibit oscillatory behavior or present a flat portion over a wide range of C depending on the corrugation parameters chosen. The theory is checked against experiment in which resonant frequencies and total Q factors were measured for TE modes operating in the range of 8-16 GHz in a coaxial cavity with 40 slots. Good agreement is found in that the magnitude of the relative error in frequency is less than 0.5%. Corrugated coaxial resonators prove to be relevant to megawatt gyrotrons where highly selective cavities are required to ensure high conversion efficiency.

The phenomenological model of the microwave surface impedance of high‐temperature superconducting films with minimized number of fitting parameters

A correct phenomenological model of the microwave surface impedance of a high-temperature superconducting (HTS) film with reduced number of fitting parameters is presented. Simple empirical expressions are proposed for London penetration depth at T=0 K and for the residual resistance parameter α versus the fitting parameter γ considered as a figure of merit of the HTS film at microwaves. The model is verified by a comparison of results of modeling and measurement data in a wide temperature range. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 16: 14–17, 1997.

The phenomenological model of the microwave surface impedance of high-temperature superconducting films with minimized number of fitting parameters

The phenomenological model of the microwave surface impedance of high-temperature superconducting films with minimized number of fitting parameters

Study of different realizations and calculation models for soft surfaces by using a vertical monopole on a soft disk as a test bed

We study a vertical monopole on a circular metallic disk which is loaded in different ways to provide a soft boundary condition, e.g., by transverse corrugations or by dielectric or lossy material coatings. The different realizations are calculated by the uniform geometrical theory of diffraction (UTD), the incremental theory of diffraction, the moment method (MM) with an impedance boundary model, and MM using equivalent currents on all material interfaces. The calculated results are compared with measured results for specific geometries. Some general characteristics of the different realizations of the soft surfaces are extracted from the results. It is verified that the artificially soft boundary condition can be realized by different surface loadings. In particular, a nearly soft boundary condition can be realized over a large bandwidth by coating a conductor with a thin layer of lossy magnetic material. It is found that the surface impedance model works very well for modeling corrugations. It may also work for material coated surfaces provided the surface wave is prohibited from radiating. The bandwidth of the different soft surfaces are given, including corrugations with different cross-sectional shapes.

Modeling of cascaded coplanar waveguide discontinuities by the mode-matching approach

Cascaded coplanar waveguide discontinuities with transverse dimensions of the order of some micrometers are analyzed by the mode-matching and generalized scattering matrix method. This approach permits a full-wave analysis of the electromagnetic field also in the metallic regions. Compared with full-wave analysis assuming perfect conductors and a subsequent loss computation based on the surface impedance model the accuracy is considerably enhanced. The mode-matching method is revisited in the context of a network representation of discontinuities. The implications of Tellegen's general network theorem on the mode-matching method, particularly on the testing functions, are investigated. Furthermore, the mode-matching method is related to the equivalence principle applied to discontinuities and to the continuity condition of voltage and current of quasi-TEM waves in the static approximation.

Surface impedance method using boundary elements for exterior regions

A surface impedance method using boundary element techniques for three-dimensional problems is proposed. In this method, a three-dimensional boundary element method in which unknown variables are normal and tangential components of magnetic flux density on the surfaces of the bodies is applied to the exterior region, and the body is approximated by the surface impedance modeling. Unknowns on the boundary surface for the exterior region and the body are combined by using the boundary conditions. As a result, the unknowns are reduced to the normal component of the magnetic flux density and an electric vector potential which expresses the electric current. The proposed method is validated by comparison of calculated results and theoretical values for a sphere model. >