P.S. Kooi

Electromagnetic dyadic Green's function in cylindrically multilayered media

A spectral-domain dyadic Greens function for electromagnetic fields in cylindrically multilayered media with circular cross section is derived in terms of matrices of the cylindrical vector wave functions. Some useful concepts, such as the effective plane wave reflection and transmission coefficients, are extended in the present spectral domain eigenfunction expansion. The coupling coefficient matrices of the scattering dyadic Greens functions are given by applying the principle of scattering superposition. The general solution has been applied to the case of axial symmetry (n=0, n is eigenvalue parameter in /spl phi/ direction) where the scattering coefficients are decoupled between TM and TE waves. Two specific geometries, i.e., two- and three-layered media that are frequently employed to model the practical problems are considered in detail, and the coupling coefficient matrices of their dyadic Greens functions are given, respectively.

A method for designing broad-band microstrip antennas in multilayered planar structures

The narrow bandwidth of a microstrip antenna is one of the important features that restrict its wide usage. A simple and practical method for the design of broad-band microstrip antennas is presented in this paper. Utilizing this design technique, several two-layer microstrip antennas have been proposed. To confirm the applicability of the method for the designs of antennas at L-band, experiments have been carried out. The measured results show that the proposed antennas have a bandwidth of up to 25.7%. Also, the method proposed in this paper is applicable to the design of other types of multilayered planar antennas.

Radio wave propagation along mixed paths through a four-layered model of rain forest: an analytic approach

This paper presents a novel full-wave analysis of the radio waves that are excited from a dipole antenna located in the trunk layer and propagate inside a four-layered forest medium. The dyadic Greens functions for the four-layered geometry are applied first to derive the integral expression of the electric fields. The closed form of the electric fields is then obtained by using the quasi-static approximation, saddle-point technique, and branch-cut integrations in the complex plane and, hence, expressed in terms of direct waves, multiple reflected waves, and lateral waves. Two kinds of images, i.e., the quasi-dynamic and complex images, are considered in the integration in the complex plane. Among those waves excited by a dipole antenna in the four-layered medium, it is shown theoretically and numerically that the lateral wave along the upper-side air-canopy interface plays a role of dominant modes. The propagation mechanism of other lateral waves due to the air-canopy, canopy-trunk, and trunk-ground interfaces is also discussed and analyzed so as to gain an insight into the wave characteristics. Transmission losses of the lateral waves are calculated numerically.

Microwave attenuation by realistically distorted raindrops: Part I. Theory

By using a new raindrop model developed earlier and applying the first-order perturbation-expansion technique, the effects of realistic and nonaxisymmetric distortion of varying raindrop shapes on microwave attenuation are re-investigated in this paper. Under the assumption of nonaxisymmetric raindrop scatterers, a system of general equations satisfied by the scattering and transmission coefficients of the electromagnetic (EM) fields are derived under the zeroth-order and the first order approximations. Both the horizontal and the vertical polarizations are discussed. The scattered and transmitted EM fields are obtained by means of expansion of spherical vector wave functions. Under the first-order perturbation approximation, the total cross section is calculated and discussed. Compared with the previous formulas presented for the axisymmetric raindrop scattering, the new results of total cross section obtained here consist of two additional terms resulting from the nonaxisymmetric distortion. These additional terms due to the raindrops nonaxisymmetry are represented analytically and discussed numerically. Part II of this paper will present an application of the analysis to the specific microwave attenuation due to rain in the moderate climatic region and in Singapores tropical region. >

A general expression of dyadic Green's functions in radially multilayered chiral media

A general expression of spectral-domain dyadic Greens function (DGF) is presented for defining the electromagnetic radiation fields in spherically arbitrary multilayered and chiral media. Without any loss of the generality, each of the radial multilayers could be the chiral layer with different permittivity, permeability, and chirality admittance, while both distribution and location of current sources are assumed to be arbitrary. The DGF is composed of the unbounded DGF and the scattering DGF, based on the method of scattering superposition. The scattering DGF in each layer is constructed in terms of the modified and normalized spherical vector wave functions. The coefficients of the scattering DGFs are derived and expressed in terms of the equivalent reflection and transmission coefficients, by applying boundary conditions satisfied by the coefficient matrices. >

An improved prediction of series resistance in spiral inductor modeling with eddy-current effect

Based on an earlier study by Kuhn and Ibrahim (see IEEE J. Trans. Microwave Theory Tech., vol. 49, no. 1, p. 31-38, 2001) on current crowding, an improved expression incorporating the skin effect for the prediction of series resistance in spiral inductor modeling has been derived. A modified model for the spiral inductor, which accounts for the eddy-current effect, is thus proposed. Relatively good agreements between the measured data and the results generated from the model are obtained.

On the eigenfunction expansion of dyadic Green's function in planarly stratified media

Using the eigenfunction expansion, a terse expression of the scattering dyadic Greens function is presented for defining electromagnetic fields in a planarly stratified medium with a current source arbitrarily located in one of the layers. The scattering dyadic Greens function for each layer is constructed in terms of the cylindrical vector wave functions by applying the method of scattering superposition. The general but succinct expression of the coefficients of the scattering dyadic Greens function for each layer of the multilayers is derived directly from the boundary conditions of the dyadic Greens function. To demonstrate how to apply this method, the coefficients of the scattering Green dyads for a three-layered medium, as an example, are reduced from the general expression for the cases where the current source is located in the first, the intermediate, and the last layers. The complete agreement of such coefficients derived in this article and obtained in the previous published papers asserts...

On the eigenfunction expansion of electromagnetic dyadic Green's functions in rectangular cavities and waveguides

The electric dyadic Greens functions of both the first and the second kinds due to the presence of electric and equivalent magnetic sources in rectangular cavities are obtained. A method for directly reducing the dyadic Greens functions for a rectangular cavity to those for a semi-infinite and an infinite rectangular waveguides is presented. The Green dyads of the second kind for an infinite and a semi-infinite rectangular waveguides, and a rectangular cavity are obtained. >

A novel approach for determining the GaAs MESFET small-signal equivalent-circuit elements

A simple way of extracting the small-signal equivalent circuit of a MESFET is proposed. The intrinsic elements and one of the extrinsic parameters are described as functions of the remaining extrinsic parameters. This drastically reduces the search space and the number of unknowns for optimization. It provides an important new insight into the correlation between the various extrinsic and intrinsic parameters. The method also reveals that there are two sets of solution for the parameters R/sub s/ and L/sub s/ which can fully satisfy the global solution. A numerical comparison in terms of both accuracy and speed between the proposed method and some conventional methods on a 400-/spl mu/m gatewidth GaAs MESFET is presented.

A two-year measurement of rainfall attenuation of CW microwaves in Singapore

The authors present two-year statistics on the rainfall attenuation of 21.225 GHz vertically polarized CW microwave signals in Singapore. The CCIR recommendation is found to underestimate the microwave specific attenuation (dB/km) per path average rain rate (mm/h) as well as the accumulated time of attenuation exceeding preset levels. A negative exponential raindrop size distribution based on spherical droplets is constructed from the measured data. >