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 Absorbing Architectures
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.
Salisbury Screen
The structure of the Salisbury screen is a thin sheet of resistive material separated from a perfectly electrical conducting (PEC) back-plane by a low-loss dielectric material with a thickness d and relative permittivity εr, as shown in Figure 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 Zsis 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 Absorber
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 Figure 2,3
From transmission line theory, the input admittance of this arrangement is given by
(i) If X is capacitive
(ii) If X is inductive
The absorber null frequency is determined by the susceptance term in the equation and hence can be tuned by varying R. The absorber may be transformed into a reflector by changing R from 377ω/sq either to 0 or to ∞. The reflectivity characteristics of the absorber configurations were examined using a computer program which sought to establish the optimum values of C giving the maximum tuneable null range for a minimum null depth of -20 dB. Confining the discussion to the RC case for the present, since there are only two variables in the equation, the computer program was based on an exhaustive search rather than an optimization technique and simply varied R and C over the ranges 0 to 1000 ω/sq and 0 to 300 fF/sq, respectively, so as to determine those values which resulted in the maximum fractional bandwidth at the -20 dB reflectivity level for a nominal center frequency of 10 GHz. The fractional bandwidth (FB) is defined as
The results of the computer investigation show that the incorporation of a controllable R into the series RC layer in the Salisbury screen increases the –20 dB fractional bandwidth by a factor of 2.52 as compared with that of the passive series RC case and by a factor of 3.45 as compared with that for the classical form of the Salisbury screen.
Dynamically Adaptive Radar Absorbing Materials (DARAM)
Strategies to realize a re-configurable electromagnetic surface (RES) have been widely investigated. With a Dallenbach RAM, the thin-layer electrical parameters ε or mu; might be varied by using a controllable E or H field,4but this was difficult to apply over a large-area surface. The ε of a layer can be modified by controlling the quantity of a high-permittivity liquid that permeates into a low-permittivity, porous matrix,5 but its response time was very slow. More recent papers have described small-area RES using photodielectric effects in AgCl,6silicon7,8 and semiconductor N-I-P-I structures.9 Other techniques rely on the incorporation of active devices such as negative-resistance10 and varactor diodes.11,12 However, not a single proposal could be realized for various reasons; the major drawbacks to these proposals are the cost and need for biasing networks.
Although conventional radar absorbing materials (RAM) have been used for over 50 years,13 dynamically adaptive radar absorbing materials (DARAM) have attracted more attention after recent advances in conducting polymer composites14-16 suggested the possibility of practical RES.17-19It is not envisioned that DARAM would necessarily replace conventional wideband RAM, but rather that it might be deployed in particular camouflage and deception roles. For these to be successful, the DARAM must have a sufficiently fast response time and it must be capable of being integrated effectively into an electronic countermeasures (ECM) system.
In general, the conducting polymer composite DARAM stems from the basic and conventional RAM structure such as the Salisbury screen. The conventional Salisbury screen can be modified to the simplest DARAM structure20 by applying tunable R and C, as shown in Figure 3, because R can control the null depth and, at the same time, the dynamic component of C results in the reflectivity frequency range. To optimize the null depth and frequency by independently controlling R and C, a skinned Salisbury screen DARAM is created by the addition of a spaced, high permittivity glass reinforced plastic (GRP) screen to protect against the environment.
Such a device can act as a wideband absorber with a reflectivity of -20 dB over the frequency range 8 to18 GHz. As the dynamic C component is increased, the upper frequency null diminishes and the lower frequency null moves down in frequency and also begins to diminish. The null depth of the latter can then be restored by adjusting the dynamic R component value to give a deep narrowband reflectivity ‘notch’, as shown in Figure 4, where curve A is for R=215Ω and C=0 fF, curve B for R=215Ω and C=100 fF, and curve C is for R=355&Omega and C=100 fF. To integrate the DARAM and its associated control circuitry into a vehicular ECM system for a successful operational deployment, the DARAM needs to self-monitor its electromagnetic state and pass this information to the ECM system. To realize this aim, it has been proposed that the PEC backplane could be replaced by a ‘leaky’ resistive surface, R2, with a sheet resistance of a few tens of ohms. The reflectivity state of the RAM at a particular frequency can then be inferred from a measurement of the level of the incident radar signal, which leaks through it. Just like the previous Salisbury screen DARAM structure, null tuning can be achieved by using an active C, but here it must be composed of two active layers arranged in a π-line configuration19for the transmission-loss maximum and reflectivity minimum characteristics of the leaky Salisbury screen to 'track'; closely with frequency (see Figure 5, where the line stretcher section is shown in bold).
The work at Sheffield, on RES and particularly DARAM, stemmed from an investigation of the microwave characteristics of textile substrates loaded with a conducting polymer (CP) such as polypyrrole (PPy) and polyaniline (PAni). The former research found that both PPy and PAni loaded substrates exhibit sheet resistance and sheet capacitance, and the magnitude of the latter depends on the substrate morphology, the chemical route and the percolation effects (the loading range of CP). The percolation effect suggests that, for a critical value of loading, it might be possible to ‘tune’ the sheet resistance and capacitance of the material using ‘molecular switches’ to make controlled connections between CP particles, the ‘switches’ being controlled by an external stimulus.
Applications of Conductive Polymers to RAM
It has been well observed and established by research that the physical, chemical and structural specific properties of conducting polymers influence the evolution of their microwave properties.21,22 However, much work needs to be done to achieve the ideal absorbing materials based on conducting polymers. First, taking into account the specific conductivity of conducting polymer (1 to 50 S/cm), it is necessary to dilute the conductive phase to obtain an absorbing material with conductivity in the range of σ-3to 10-1S/cm, which allows an appropriate surface impedance. Moreover, it is well known that conducting polymers are very difficult to process and their mechanical properties are very poor. As a conclusion, a good host material needs to be chosen for making an absorbing material based on conducting polymers. Well designed absorbing materials, based on conducting polymers, should include the following properties:
• The requested macroscopic conductivity, by controlling the quantity of the conducting phase. It is therefore possible to design very different materials: insulating-conducting blends or a laminate in the case of deposition on a substrate.
• The ability to be integrated in polymer processing techniques (extrusion, injection) or composite technique.
• Good mechanical properties when the conducting polymer is deposited on the surface of a textile or a honeycomb structure.
The microwave behavior of numerous conductive polymers has been published, such as polyaniline (1 MHz to 20 GHz), polypyrrole, poly(p-phenylenevinylene), polyparaphenylene, poly(p-phenylene-benzobis-thiazole) and polythiophene.
The effects of structural parameters such as the dopant name, molecular weight, defect rate and substituent size have been widely studied on PAno and polyalkyl thiophenes.22,23 Some physical effects, such as moisture absorption, temperature, protonation level, electron location, crystallinity and elongation, on the microwave properties of PAni have been found through characterization by the perturbation cavity method.24 Other methods have been employed and compared with the former one, such as a microwave impedance bridge at X-band (8.2 to 14.2 GHz)25 or the APC7 standard (130 MHz to 18 GHz),23with good agreement. The heterogeneous nature of PAni ("metallic islands") has been used in order to explain the experimental results26 and the tri-dimensional nature of metallic states has been explained.27 PAni has also been characterized in the low-frequency range by impedance measurement.28 Potential applications of the electromagnetic properties of PAni have been discussed.29
The variation of transmission, reflection and dielectric properties with doping level and temperature have been investigated in the case of PPy.30 New polymers such as poly(p-phenylene-benzobis-thiazole)31 have also been investigated. Tuning at 9 and 10 GHz has been obtained with this class of material. Double- and single-layer Salisbury screens using PBT (polybithiophene) were fabricated.31 An absorption of 90 percent was achieved with single-layer screens. The bandwidth of the absorber was found to increase from 2 to 7 GHz by constructing a multiple screen absorber. Fabrication of large area conducting polymer composites has also been reported.32 Broadband absorbers in the range 2 to 18 GHz have been presented.14 The composite of PPy grown on a cellulose paper substrate33 and PPy (or PAni) dispersed in silicon rubber (vinyl ester)34 were fabricated and their applications in the microwave range were described. Composites of polybithiophene and insulation polymers such as polycarbonate or polyacrylatestyrene copolymers have been studied.35 Another example of multilayer architecture is given by Rupich,36 who has built an electrochemical cell transparent to microwave (X-band) in which the transmission can be modulated by an applied potential.
Moreover, PPy or PAni loaded fabrics have been studied as microwave absorbers, either in the form of pure fabric or composites. The application involves their use, either under the form of multilayer absorbers, camouflage nets, or absorber with continuous variation of conductivity inside the plane (edge cards for use in low observable technologies).
Other examples of microwave property applications have been given in the field of thermoplastic welding by using conductive polymers at the interface37 or in microwave equipment.38
Importantly, since 1990, several patents had been delivered on applications of conductive polymers in the field of microwave absorption. Absorbers containing polyaniline have been patented.39-42 Some types of polypyrrole-based absorbers have also been patented.43,44
Conductive Polymer Smart Materials for Microwave Systems
The radar cross section (RCS) of the target is a very important coefficient in radar systems. It is determined by how effective the electromagnetic energy illuminated by the radar and reflected by the target can be detected. For instance, in some situations, traffic control or maritime tracking radar, the RSC of the target needs to be increased by using such devices as corner reflectors. On the other hand, a radar absorbing material (RAM) is used on the platform of the target to decrease its RSC so that it can avoid detection by the enemy radar, although geometrical shaping is the primary mechanism for controlling the RSC of the features. Much work by scientists was dedicated to research on radar absorbing materials during the past few years. Several types of RAM, for electromagnetic wave absorption, include ferrite-impregnated fiberglass composite, two-phase composites of spinel ferrites and lithium aluminosilicates, polychloroprene composites containing carbonyl-ion (CI) and doped ferrite powders, soft magnetic polymer with thin amorphous metal particles, etc. However, it is more advantageous to have the materials able to modify the radar signature of a target in response to the given operational environment. For example, the RCS of a military vehicle could be changed from a high value during peaceful time to a low value in combat situations. A microwave system with smart material, if it can be realized, should have significant practical value in this area. This aim can be reached by using a smart material whose microwave reflectivity can be controlled by a change in its resistance when an outer electric or optical field is applied. The resistance of the conductive polymer can be changed from high resistance to low resistance with the application of a voltage (electric field) and the original situation can be restored when the field is removed.
Some conductive polymer materials are found to undergo a redox reaction under the influence of an applied stimulus, so that their conductivities can change from low to high. The materials, in the previously published works45-47 in this field, were composites of PAni, PPy or PEDOT with + or Cu/Cu2+ as a reducing couple in a PEO matrix. Large, rapid and reversible changes in the microwave transmission/reflection coefficients were observed, when small (approximately 10 Vcm-1) DC/AC fields were applied across the edges of the composite samples in either waveguide or coaxial line test lines.
Conclusion
Radar absorbing materials have attracted many researchers’ attention. Based on the simple single-layer Salisbury screen, adaptive radar absorbers and dynamically adaptive radar absorbing materials were developed and their working mechanisms were studied. Lately, a variety of conducting polymers have been applied to radar absorbing materials and excellent results were obtained. Furthermore, conductive smart materials have been invented for microwave systems, opening a new page for research on radar absorbing materials.
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