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  • Topic: Radar, Permittivity, Polymers
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  • Published : April 4, 2013
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4/4/13

Resear ch Pr og r ess on Radar Absor bing M ater ials

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Research Progress on Radar Absorbing
Materials
Yanmin Wang, Shandong University of Science and Technology
August 18, 2008
This article briefly outlines the research and development activities in radar absorbing materials. First, the structure and working mechanism of traditional radar absorbing architectures such as the Salisbury screen and the adaptive radar absorber are introduced. Second, the major breakthroughs, including the successful fabrication of dynamically radar absorbing materials, are described. Finally, the applications of conducting polymers to radar absorbing materials and of conducting smart materials for microwave systems are summarized.

Since the advent of radar in 1940, there has been significant interest from military defense scientists to the possibility of using coating materials to render aircraft or other military vehicles less visible to radar and, preferably, to control such visibility. The highly conducting surface of a metal vehicle is an excellent reflector of radar, but an absorbing layer would suppress the radar signal at the receiver station. As well as the physical and chemical natures of the coating materials, the ‘shape’ or ‘architecture’ of the object is of importance. For more than 20 years, much work has been devoted all over the world to ‘radar absorbing materials’ (RAM) and their optimized shape,1 especially in the US and in the former USSR. Radar Abs orbing Archite cture s

The ideal radar absorber should be thin, light, durable, easily applied and inexpensive, and have a broadband frequency coverage. Simple, narrow- band, single layer absorbers were designed first and then formed the components for multi- layer broadband absorbers. Two of the oldest and simplest types of absorbers are represented by the Salisbury screen and the Dallenbach layers. The Salisbury screen is a resonant absorber created by placing a resistive sheet on a low dielectric constant spacer in front of a metal plate. The Dallenbach layer consists of a homogenous layer backed up by a metal plate and is the simplest type of RAM architecture since the reflection properties are only dependent on the thickness and radioelectric properties of one layer. Here, the Salisbury screen and two types of improved Salisbury screen, including the adaptive radar absorber and the dynamically adaptive radar absorber, are introduced. Salis bury Scre e n

The structure of the Salisbury screen is a thin sheet of resistive material separated from a perfectly electrical www.micr owavejour nal.com/ar ticles/pr int/6680- r esear ch- pr og r ess- on- r adar - absor bing - mater ials

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4/4/13

Resear ch Pr og r ess on Radar Absor bing M ater ials

conducting (PEC) back- plane by a low- loss dielectric material with a thickness d and relative permittivity εr, as shown in Fi gure 1. Assuming a plane- wave incidence, the reflectivity characteristics of the structure may be analyzed using the transmission line equivalent circuit shown. The free- space input impedance of the absorber for normal incidence is given by

where Zs is the sheet impedance, Z is the characteristic impedance of the dielectric spacer and

is the propagation constant in the spacer material. The corresponding free- space reflection coefficient of the absorber at normal incidence is given by

and the reflectivity by

where Z0represents the impedance of free space and is approximately equal to 377Ω. If the sheet impedance is purely resistive and equal to Z0, the absorber shows a zero reflectivity when βd =π/2, and this occurs when d=λ/4 (and at harmonically related distances of 3λ/4, 5λ/4, etc.).2 Adaptive Radar Abs orbe r

The starting point of the tuning configuration is the Salisbury screen, incorporating a layer containing a series combination of a fixed capacitance and a variable resistance, as shown in Fi gure 2,3 From transmission line theory, the input admittance...
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