## 1. Introduction

[2] Analysis of electromagnetic (EM) scattering by dielectric-coated curved objects is of significant practical interest, such as the radar cross section (RCS) of a missile structure or the installed radiation pattern of patch antennas conformal to rocket bodies. The underlying canonical configuration is a perfect electrically conducting (PEC) cylinder coated with one or more thin dielectric layers, the cylinder radius and length being much larger than the operating wavelength. In this paper, an asymptotic model is developed for the back-scattered field from a dielectric-coated cylinder, focusing on rigorous analytical characterization and validation of creeping waves. Detailed computations on a coated cylinder reveal stark contrast in propagation characteristics between the surface-guided field on a dielectric-coated cylinder and that on an uncoated metallic cylinder. The asymptotic approximations are facilitated by well-established representation of the source-excited field for a PEC cylinder in terms of an integral over the angular wave number spectrum using the theory of characteristic Green's functions [*Felsen and Marcuvitz*, 1973, pp. 685–697].

[3] The theoretical formulation describing the scattering by dielectric or dielectric-coated cylinders in terms of azimuthally propagating waves has existed for many years [*Elliott*, 1955; *Helstrom*, 1963; *Streifer*, 1964; *Tang*, 1957; *Wait*, 1960]. More recently, the advancement of numerical methods has enabled the computation of surface wave propagation constants and spectral integrals in the complex plane, resulting in the possibility of extending the asymptotic solution for a cylinder to more complex geometries. Several researchers have addressed the problem of asymptotic approximation of the EM field radiated by a line source or an infinitesimal dipole in the presence of an infinitely long coated cylinder, primarily focusing on numerical evaluation of azimuthally propagating waves derived from so-called creeping wave poles and their residues [*Albertsen*, 1989; *Erturk and Rojas*, 2000; *Felsen and Naishadham*, 1991; *Logan and Yee*, 1962; *Kim and Wang*, 1989; *Krasnojen*, 1998; *Naishadham and Felsen*, 1993; *Paknys and Wang*, 1986; *Paknys and Jackson*, 2005; *Pearson*, 1986, 1987; *Wang*, 1985]. Computation of the EM field itself, its verification with other methods, and its validation with either measured or computed data have received considerably less attention.

[4] In this paper, we evaluate asymptotically the monostatic RCS for a coated cylinder at normal incidence in terms of its geometrical optics (GO) field in the illuminated region and the creeping waves. Although the GO formulation is based on a standard approach [*Felsen and Marcuvitz*, 1973, pp. 693–697], the focus of this paper is on analytical treatment of creeping waves and evaluation of their contribution to the back-scattered field. We do not dwell on creeping wave propagation constants, as these are covered elsewhere [cf. *Paknys and Wang*, 1986]. *Kim and Wang* [1989] obtained the uniform theory of diffraction solution of the field exterior to a 2-D cylinder with a thin lossy coating by utilizing line source excitation at a fixed frequency. As the frequency increases, the coating becomes thicker relative to a wavelength in the dielectric, and the properties of wave propagation and the supported modes change significantly. To our best knowledge, the frequency behavior of the creeping wave modal field and its transition to leaky and trapped modes have not been investigated. We present new results on wideband characterization of creeping waves utilizing practical plane wave excitation, which show that creeping waves cannot be neglected for the coated cylinder. As the frequency is increased, the creeping wave exhibits a sharp modal cutoff, transitioning from an azimuthally propagating mode to a trapped mode. This should be contrasted with the relatively large monotonic decay of creeping waves with respect to frequency on a conducting cylinder.

[5] A second contribution of this paper is the rigorous validation of creeping wave scattering using the state space method (SSM) [*Piou*, 2005; *Naishadham and Piou*, 2005; *Naishadham and Piou*, 2008]. SSM is applied to extract spectral content and synthesize the wideband frequency response of creeping waves from a method of moments (MoM) solution for the scattering by the coated cylinder. It is shown that the extracted creeping wave contribution compares quite favorably with the analytical result for the creeping waves, on the basis of a 2-D formulation over a wide frequency band for a thick lossy dielectric coating.

[6] Mathematical details on the formulation of the creeping wave field, starting from line source excitation of an infinite coated cylinder and developing plane wave synthesis as a subsequent limiting case, are presented in section 2. SSM is summarized in section 3 for completeness. Representative computed results parameterized in terms of dielectric constant, polarization, and frequency are presented in section 4, emphasizing corroboration with wave phenomena extracted using the MoM/SSM spectral estimation approach. Important conclusions are summarized in section 5.