Electromagnetic Behavior of Radar Absorbing

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Abstract The efficiency of recently developed RAM (Radar. Absorbing ... the experimental methodology used to characterize RAM based on conducting .... passing through them. .... (the RAM thickness was 3 mm in all cases) [11]. On RAM ...
Electromagnetic Behavior of Radar Absorbing Materials Based on Conducting Polymers M. Franchitto, R. Faez, A. J. F. Orlando, M. C. Rezende and I. M. Martin

Abstract  The efficiency of recently developed RAM (Radar Absorbing Materials) based on conducting polymers in sheet texture has been investigated at X-band (8 to 12 GHz) scattering measurements and, also by determination of their complex dielectric constant by waveguide method. This paper presents the experimental methodology used to characterize RAM based on conducting polymer called polyaniline. The correlation between the considerable loss tangent of the material and its reflectivity (return loss) suggests its application for aeronautical purposes, for example, to decrease the radar cross section of aircraft. Index Terms  Radar Absorbing Material, dielectric constant, scattering.

I. INTRODUCTION The first reported use of Radar Absorbing Materials was made during World War II, when the Germans applied a mixture of polymeric foam and carbon black on the submarines periscopes to avoid radar detection. Since then, many researches have been conducted with the aim of developing light and practical materials [1]. The purpose of the widely broadcasted “Stealth” technology is the reduction of the aircraft RCS (Radar Cross Section) that is a measure of power scattered in a given direction when a target is illuminated by an incident wave and is an exclusive characteristic of the target [2]. When the question of reducing radar detection is raised one talks about Radar Cross Section Reduction (RCSR). The “Stealth” technology makes use of geometrical means and material engineering in order to develop low reflection and high absorption structures – the so called RAM [3]. The most suitable way of reducing the reflection is by covering the aircraft with a RAM as of paint or polymeric sheet. This work is concerned exclusively with RAM in form of polymeric blend sheets that have been recently developed by CTA/ IAE-AMR. An important parameter for the characterization of a RAM is its “reflectivity” that has the same meaning as the return loss in microwave engineering.

M. Franchitto, R. Faez, and M. C. Rezende, Instituto de Aeronáutica e Espaço, S.J.Campos-SP, Tel +55-12-3947-4700, Fax: +55-12-3947-4797, email [email protected], [email protected], [email protected]; A. J. F. Orlando, Instituto Tecnológico de Aeronáutica, S.J.Campos-SP, e-mail [email protected]; I. M. Martin, Instituto de Física, UNICAMP – IFGW- P.O. Box 6165, 13083-970, Campinas-SP, [email protected].

Absorbing materials can be equally used in telecommunications and data transmission systems, besides in technological installations and other applications [4]. II. RAM PREPARATION Radar Absorbing Material preparation involves appropriate mixing of materials so that the final product should yield minimal scattering of waves when illuminated by a power source. Satisfactory results were achieved when conducting polymers, type polyaniline (PAni), were used. Detailed experimental procedures for the preparation of the RAM are described elsewhere [5,6]. The used PAni was synthesized in a pre-pilot scale using (NH4)2S2O8 as oxidant in HCl medium. PAni base emeraldine was obtained by dedoping in NH4OH medium. The conductive PAni was prepared by doping with dodecylbenzeno sulfonic acid (DBSA), PAni-DBSA. Binary polymer blends containing EPDM (ethylenepropylene-diene rubber) and the PAni-DBSA were prepared in an internal mixer coupled to a torque rheometer for different processing time. Flat sheets of 3 mm thick and 15,0 cm x 15,0 cm dimension were obtained by compressionmolding at 100ºC. Table I lists parameters of the prepared RAM. TABLE I RAM 1 2 3 4

PARAMETERS USED IN THE RAM PREPARATION Composition (EPDM rubber/polyaniline) Preparing Time (min) 70/30 35 70/30 10 50/50 44 50/50 15

III. SCATTERING MEASUREMENTS As any kind of product, radar absorbers are primarily specified by their intended application. The application with the properties of materials available to fabricate the product, lead to evolution of one or more concepts or structure. Measuring the absorber reflection coefficient is important in order to have an idea on how efficient the RAM is. The electromagnetic wave, traveling in a free space environment, can be partially reflected since there is an impedance difference between two media. Consider an interface between two semi-infinite media with impedances Z1 and Z2, respectively, and a transversal electromagnetic (TEM) wave traveling toward +x direction normally incident to the interface, as shown in Fig. 1.

Fig. 1. TEM electromagnetic wave on the interface of two semi-infinite media.

The tangential components of electric and magnetic fields must be continuous at the interface. Defining the reflection coefficient as ρ = E r , we have: Ei

Z 2 − Z1 Z 2 + Z1

ρ=

(1)

In a lossless medium, the impedance is expressed by the relative permittivity, ε r , or dielectric constant, and permeability, µ r . Then, µ r µ0 ε rε 0

Z=

(2)

=

2

Reflectivity (dBm)

-2

-4

Reference RAM # 1

ε0

In other words, the perfect absorber would have the relative parameter ε r 2 equal to µ r 2 . However, at microwave frequencies, ε r 2 generally does not approach the magnitude of µ r 2 [7]. This is one reason for working with loss absorbers. Furthermore, RAM are based on the fact that some materials absorb energy from the electromagnetic fields passing through them. By considering the medium conductivity σ , a frequency-dependent impedance is: Z=

On the NRL arch, an antenna is connected to a microwave transmitter and the other to a microwave receiver. Microwave energy is sent by the transmitting horn, reaches the material, is partially absorbed, and the rest is scattered towards the receiving horn. The four samples of RAM prepared for this work were submitted to NRL arch measurements and the upper curves are references from total reflection (metallic plates). Figures 3-6 show the RAM scattering measurements [9].

0

If medium 1 is the free-space, the perfect absorber with zero reflection coefficient must have: µ 2 µ0 (3) ε2

Fig. 2. The NRL arch used for Radar Absorbing Materials tests

jωµ r µ 0 σ + jωε r ε 0

(4)

The NRL (Naval Research Laboratory) arch free-space measurement method was chosen to validate the absorbing efficiency of the RAM. The NRL arch was widely used initially by the U.S. Navy for research testing purposes, and is a microwave measurement system that can measure the free space radar reflection coefficient. Basically, the NRL arch is a vertical semicircular framework, made of wood, as shown in Fig. 2, that allows a pair of horns to be positioned at a constant distance from the material under test [8].

-6

-8

-10 8

9

10

11

12

Frequency (GHz)

Fig. 3. RAM # 1 reflectivity on X-band.

R eflectivity 2 (dBm) 0 -2 -4

R eference R AM # 2

-6 -8 -10 -12 -14 -16 8

9

10

11

12

Frequency (GHz)

Fig. 4. RAM # 2 reflectivity on X-band.

length lε with the probe located at a new voltage minimum D. The RAM sample is adjacent to the short circuit. The impedance boundary conditions give:

Reflectivity (dBm)2 0 -2

tanh γ ( DR − D + lε ) tanh γ ε lε = γ lε γ ε lε

Reference RAM # 3

-4 -6

The dielectric constant is determined by solving the transcendental equation (5) as being:

-8 -10 -12

2

a  2 π  [γ ε ] + 1 εr = 2  2a    +1  λ g 

-14 8

9

10

11

12

Frequency (GHz)

Fig. 5. RAM # 3 reflectivity on X-band. Reflectivity (dBm) Reference RAM # 4

0

-1

-2

-3

(8) ε r = ε '− jε ' ' It must be emphasized that ε ' ' also includes the conductivity σ and the ratio ε ' ' / ε ' is called the loss tangent. The real and imaginary parts of the dielectric constant can be found by substituting (7) and (8) in (6): 2

a 2 2 π  α ε − β ε + 1 ε '= 2  2a    +1  λ g 

-5 9

10

11

12

Frequency (GHz)

[

]

a 2α ε β ε   π  ε '' = − 2  2a    +1  λ g 

Fig. 6. RAM # 4 reflectivity on X-band.

IV. DIELECTRIC CONSTANT The electromagnetic aspects of RAM design rest mainly on arrangements of dielectric or magnetic materials that provide appropriate impedance and reduce the scattered wave. RAM dielectric constant is determined by the two-point method involving the solution of a transcendental equation. An X-band rectangular waveguide with a short circuit termination, as shown in Fig 7, was used to measure the voltage minimum in two situations: with and without the RAM sample [10].

(6)

Both the propagation constant and the dielectric constant are complex quantities that can be written: (7) γ ε = α ε + jβ ε

-4

8

(5)

(9)

2

(10)

Point to point measurement at X-band frequencies are conducted by taking the voltage minimum position, repeating the process and solving the transcendental equations; one then obtains the frequency variation of the complex dielectric constant of the RAM sample. Figures 8 to 11 show the frequency behaviors of the four samples of RAM that were synthesized. Relative Permittivity 5

4

3

Real Imaginary

2

1

0

Fig. 7. Dielectric Constant Measurement with Short-Circuited Waveguide.

-1 8,0

Fig. 7 shows above, an empty short-circuited waveguide with a probe located at a voltage minimum DR, and below, the same waveguide, now containing a RAM sample of

8,5

9,0

9,5

10,0

10,5

11,0

11,5

12,0

Frequency (GHz)

Fig. 8. RAM # 1 Dielectric Constant in X-band frequencies.

Relative Permittivity 8

6

4

Real Imaginary

2

0

-2

-4 8,0

8,5

9,0

9,5

10,0

10,5

11,0

11,5

12,0

Frequency (GHz)

Fig. 9. RAM # 2 Dielectric Constant in X-band frequencies.

Relative Permittivity

an approximate thickness of λ / 4 matching occurs. Because of the considerable loss tangent ( ε ' ' / ε ' ), one can expect high absorption. (the RAM thickness was 3 mm in all cases) [11] On RAM # 3 and RAM # 4, Fig. 5 and 6 show a –8 dB and –4 dB, respectively, broadband average reflectivity at X-band frequencies. As RAM # 1 has a relative real permittivity about 4.2 (See Fig. 8) and RAM # 2 has about 5 (See Fig. 9), matching frequencies of 12.2 and 11.2 GHz, respectively, can be expected in agreement with the measured values. One can expect matching behavior for RAM # 3 and RAM # 4 at higher frequencies because of the lower real parts of the dielectric constant. The reflectivity depends on material structure, shape, thickness and the preparing time of RAM, that affects the material electromagnetic properties. The single layer structures encourages further development of multi-layered absorbers for broadband operation that is already in progress.

Real Imaginary

3

REFERENCES

2

1

0

-1

-2

-3 8,0

8,5

9,0

9,5

10,0

10,5

11,0

11,5

12,0

Frequency (GHz)

Fig. 10. RAM # 3 Dielectric Constant in X-band frequencies.

Relative Permittivity 2

Real Imaginary

1

0

-1

-2

-3 8,0

8,5

9,0

9,5

10,0

10,5

11,0

11,5

12,0

Frequency (GHz)

Fig. 11. RAM # 4 Dielectric Constant in X-band frequencies.

V. CONCLUSION Fig. 3-6 and thereto corresponding Fig. 8-11 indicate correlation between RAM efficiency (determined by the NRL arch measurement) and dielectric constant. Fig. 3 and 4 indicate best performance of their samples at 12 and 11.2 GHz, respectively. RAM # 2 shows a –15 dB of reflectivity at 11.2 GHz. This can be explained by the fact that the sheet works as a lossy impedance transformer and at

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