# Surface Waves on Stealth Aircraft

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# Full title

## The Characterization of Surface Waves on Low-Observable Structures

being a Thesis submitted for the Degree of

Master of Science

in the University of Hull

by

August 1997

# Abstract

Edge diffracted waves resulting from surface discontinuities contribute significantly to the radar cross section of an object. Although this problem could be alleviated by altering the shape of the edge discontinuity, this is not always possible due to other mission requirements.

The backscatter from edge diffracted waves may also be reduced by converting the incoming radar waves into surface waves whose intensity is significantly reduced before reaching the surface discontinuity. This can be achieved by employing isotropic surface wave absorbing materials backed by a metal surface. However, for plane surface waves, the effectiveness of these materials is shown to be strongly polarization dependent.

This work suggests a new strategy which involves replacing the scattering surface by an electromagnetic soft surface. This would result in a complete elimination of the edge diffracted waves in the radar direction, independently of radar polarization.

Furthermore, a new measuring apparatus based on a partially filled rectangular waveguide has been developed for determining the attenuation constant and phase constant of plane surface waves propagating along metal-backed surface wave absorbing materials. Measurements are presented which validate this new measuring method.

### Keywords

Radar Cross-Section (RCS) Management, Surface Waves, Radar Absorbing Materials, Electromagnetic Measurements

# Contents

 Abstract II Acknowledgements V 1. Introduction 1 1.1 Stealth Design 1 1.2 Reducing the RCS Contribution of Edge Diffracted Waves 10 1.3 Outline of this Text 13 1.4 Conclusions 14 1.5 References 14 2. Hertz Potentials 15 2.1 Introduction 15 2.2 Hertz’s Wave Equation for Source Free Homogeneous Linear Isotropic Media 17 2.3 Hertz’s Wave Equation in Orthogonal Curvilinear Coordinate Systems with Two Arbitrary Scale Factors 18 2.4 Hertz’s Wave Equation in a Cartesian Coordinate System 19 2.5 Hertz’s Wave Equation for a 2D-Uniform Guiding Structure 20 2.6 Hertz’s Wave Equation in a Circular Cylindrical Coordinate System 22 2.7 Conclusions 25 2.8 References 25 3. Plane Surface Waves Along Plane Layers of Isotropic Media 26 3.1 Definition 26 3.2 Plane Surface Waves and the Brewster Angle Phenomenon 27 3.3 Plane Surface Waves, Total Reflection and Leaky Waves 28 3.4 Plane Surface Waves along a Coated, Electric Perfectly Conducting Plane 30 3.5 Plane Surface Waves along a Planar Three-Layer Structure 79 3.6 Plane Surface Waves along the Plane Interface of Two Half Spaces 91 3.7 Appendix A: The Phase Velocity of an Inhomogeneous Wave in a Loss Free Medium 95 3.8 Appendix B: Proof of $$-\text{j}\sqrt{x} = \sqrt{-x}$$ 96 3.9 Conclusions 97 3.10 References 98 4. Axial Surface Waves in Isotropic Media 99 4.1 Definition 99 4.2 Axial Surface Waves along a Coated, Electric Perfectly Conducting Cylinder 100 4.3 Field Distribution of Axial Surface Waves along a Coated, Electric Perfectly Conducting Cylinder 105 4.4 Conclusions 107 4.5 References 107 5. RCS Management of Edge Diffracted Waves 108 5.1 Introduction 108 5.2 Converting the Incident Space Wave into Attenuated Surface Waves 109 5.3 Soft Surfaces 111 5.4 The Practical Realization of a Soft Surface 113 5.5 Conclusions 119 5.6 References 120 6. Surface Wave Absorber Measurements 121 6.1 Introduction 121 6.2 A Historical Overview of Surface Wave Measurement Techniques 122 6.3 A Plane Surface Wave Simulator Cell Based on a Partially Filled Rectangular Waveguide 126 6.4 Conclusions 158 6.5 References 158 7. Conclusions 159

Download the whole masterthesis in one file (10 MB PDF).

# Publications

5

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