Proceedings of the 1st Iberic Conference on Theoretical and Experimental Mechanics and Materials /

11th National Congress on Experimental Mechanics. Porto/Portugal 4-7 November 2018.

Ed. J.F. Silva Gomes. INEGI/FEUP (2018); ISBN: 978-989-20-8771-9; pp. 183-192.

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PAPER REF: 7437

MEASUREMENT AND SIMULATION OF THE SHIELDING

EFFECTIVENESS OF MATERIALS USING THE ASTM D4935

FLANGED COAXIAL TRANSMISSION LINE

Hugo Tavares1(*)

, Nelson Matos2, Margarida Pinto

2, Guadalupe Gutiérrez Gutiérrez

3

1LEEQUE, ISQ, Avenida Professor Doutor Cavaco Silva 33, 2740-120 Porto Salvo, Oeiras, Portugal

2ID, ISQ, Avenida Professor Doutor Cavaco Silva 33, 2740-120 Porto Salvo, Oeiras, Portugal

3Airbus Defence and Space - EME & ASE, Paseo de John Lennon, 2, 28906 Getafe, Madrid, Spain

(*) Email: hvtavares@isq.pt

ABSTRACT

This paper presents an introduction to the application of the shielding effectiveness

measurement method, according to the ASTM D4935 standard [1]. Measurements of the

shielding effectiveness of a reference material, whose electromagnetic properties are well

known, are presented in this paper. In addition, a 3D electromagnetic model of the

measurement rig is created and validated. Finally, a finite-integration-technique simulation is

performed applying a commercial software package, CST Microwave Studio (CST MWS), on

the reference material sample. Despite the upper frequency limitations (due to excitation of

higher order modes) and the limitations due to sample thickness and re-reflected signals on

the air-material interfaces of the sample, it is shown that a good correlation between

simulation, laboratory measurements and expected theoretical values can be obtained. This

validation work will serve as the basis for future measurement programs of aerospace type of

carbon fibre based composite materials, in the RF frequency range, in which the double

flanged coaxial rig defined in ASTM D4935 is used.

Keywords: Shielding effectiveness (SE), reflection loss, 3D EM simulation, finite-

integration-technique (FIT), CST Microwave Studio (CST MWS).

INTRODUCTION

The shielding effectiveness is a characteristic of the attenuation of the electric field, , and/or

magnetic field, , due to the introduction of a material in the propagating medium of an

electromagnetic wave. It is defined for both and fields as:

= 20 ∗ , [Eq. 1]

= 20 ∗ , [Eq. 2]

where and are the electric and magnetic field magnitudes without the influence of the

shielding material, and and are the magnitudes of the electric and magnetic fields in the

presence of the shielding material. It is a macroscopic material property that is dependent on

the constitutive parameters of the material: permittivity (ε), permeability () and electrical

conductivity ().

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Given the definition of SE as a relative measurement, a two-step measurement process is

required: 1) a reference normalization of magnitude and phase characteristics of the

propagation medium are normalized without the sample being present and 2) with the sample

present in the propagating medium of the electromagnetic wave. The propagating medium in

which the shielding effectiveness of the material is measured defines the conditions in which

its application is valid: near/far field conditions, polarization of the electromagnetic wave,

angle of incidence, etc. It is therefore of uttermost importance to characterize the

electromagnetic environment in which the shielding effectiveness measurements are

performed in order to provide the most reliable data for the scope of shielding application of

concern. One of the widest used methods to characterize the shielding effectiveness is the

double flanged coaxial transmission line shown in Figures 1a-b.

Fig. 1(a) - ASTM D4935 flanged

coaxial transmission line.

Fig. 1(b) - Flanged interfaces of the coaxial transmission line used

to fit the material under test.

Fig. 1(c) - Relevant mechanical dimensions for the TE and TM

cut-off frequencies.

It is comprised of two identical sections of a tapered coaxial transmission line. Each section is

tapered from the typical dimensions of coaxial cable to an external diameter of 7.62 cm, to

allow the insertion of the sample under test between the flanges of both tapered sections. The

mechanical dimensions of both the tapper and flanged sections can be consulted on ASTM D-

4935 [1], and the relevant electromagnetic dimensions illustrated of Figure 1c.

This coaxial transmission guide excites a Transverse Electro-Magnetic, TEM wave, between

the inner and outer conductors of the coaxial line. The TEM propagation mode has no cut-off

frequency and the electric field and magnetic field components are perpendicular to the

propagation vector, , as shown in Figure 2. The cut-off frequencies of the higher order TE

and TM propagation modes that can be excited in this waveguide are [2]:

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, =

!"#!$

[Eq. 3]

,% =

!"&!$

[Eq. 4]

where ' is the speed of light in the vacuum, ( and () are the inner and outer conductor

diameters of the flanged section, and * and + are different integer values that relate to higher

order TE and TM propagation modes on the transmission line. Since the TEM mode has no

cut-off frequency, free space conditions are dominant up to the first frequency of the first TE

mode. Therefore, up to the first TE mode, it is possible to extract in near free-space conditions

the scattering parameters that relate to the shielding effectiveness of a material. By

appropriately selecting the values of ( and (), it is possible to design a measurement jig to

operate in a broadband spectrum, assuming the feasibility of the mechanical dimensions when

compared to the required sample diameter, avoiding the higher order propagation modes

where the relationship between and is more complex. In free-space, the relationship

between E and H is:

= ,

[Eq. 5]

where - is the free space (vacuum) impedance,120/Ω12 377Ω5, and the , and

vectors are orthogonal, as shown in Figure 2 ( normal to both and ).

Scattering parameters (S-parameters) are measured in vector network analysers (VNA). These

multi-port magnitude and phase measurement instruments are generally fitted with RF ports

whose impedance is 50 Ohms. In order to improve the impedance match between the VNA

and the measurement rig, the latter is additionally designed with the restriction to have a

characteristic impedance, - , of 50 Ω. On an air dielectric coaxial transmission line, the

relationship between (, () and - is defined by:

- = 138 7!"!$8 [Eq. 6]

Therefore, for a 50 Ω transmission line we have approximately ()~2,3(. The measurement

rig defined in ASTM 4935D uses !"!$= :,;)

<,< which closely matches this condition.

Fig. 2 - and field lines inside the coaxial

transmission guide.

Fig. 3 - Reflection, absorption and multiple reflection

on an infinite sheet of material.

As derived in [3] the shielding effectiveness properties of a material are due to three different

physical phenomena: 1) reflection loss, =,2) absorption loss, >,and 3) multiple

reflections, ?=. Therefore, for an isotropic, linear and homogeneous material the shielding effectiveness, of a material are a combination of all these losses mechanisms [3]:

= = @ > @?=. [Eq. 7]

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Figure 3 illustrates the behaviour of an electromagnetic wave incident on a material of finite

thickness and infinite transversal dimensions. The reflection loss mechanism on the material

is due to the transition of the electromagnetic wave propagating of free-space conditions, in

which - = - , to a material in which the propagating constant is a function of the

permittivity, permeability, and electrical conductivity. Absorption losses occur due to the

finite conductivity of the material and are a function of the skin depth, δ, and its thickness -

these losses are reflected in the material through heat exchange. Multiple reflection losses

occur due to the multiple reflection of the travelling wave between the air-material interface

on either side of the material. The reflection loss is generally the dominant mechanism for

good conductors. Further discussions on the modelling of the internal loss mechanisms can be

found on [4] and are outside the scope of this paper. In the case of a thin and good conductor,

non-magnetic material, the shielding effectiveness can be approximated by:

2 = 2 168 @ 10 7BC8 [Eq. 8]

or alternatively by:

2 = 2 20 71 @ DD,EF8 [Eq. 9]

where the surface resistivity =G is defined by:

= = B [Eq. 10]

in which H is the thickness of the material.

Applying equations [9-10], a theoretical shielding effectiveness value of IJ,!C = 33HK is

obtained for a gold sample reference material with the properties as in Table 1.

Table 1 - Gold reference sample parameters.

Electrical conductivity, σ 4,42 × 10:/*

Thickness, H ~5,2P6,6+*

Surface resistance, = ~4P5,6Ω/QRST(U

MEASUREMENT METHOD

The measurement procedure of the shielding effectiveness is thoroughly described in [1] and

the reader is directed to the standard for explicit instructions. Only a brief description of the

most relevant points will be presented on this paper. In order to extract the S-parameters, the

measurement is performed with the aid of a VNA and one 10 dB attenuator at the coaxial port

of each flange, in order to improve the mismatch between the reflection caused by the

insertion of the sample in the coaxial guide and the respective port of the VNA, as depicted in

Figure 4a. First, a set of calibration samples, shown in Figure 5a, with appropriate dimensions

are inserted in the flange and inner conductor to establish a reference path. The calibration

samples are an annular disk and a solid disk as shown in Figure 4a. The annular disk has

inner/outer diameters of the inner/outer diameter of the flange. The smaller disk has a

diameter identical to the diameter of the inner conductor of the coaxial rig.

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Fig. 4(a) - Through calibration setup

Fig. 4/b) - Sample measurement setup.

Second, the measurement sample, shown in Figure 5b, is inserted between the flanges of both

sections of the measurement rig and the measurement is performed, refer to Figure 4b. The

measurement sample is a solid disk with the diameter of the flange. All of these samples are

made of the same material to be evaluated.

Fig. 5(a) - Set of samples to perform the reference

measurement - annular disk (outer) and solid inner

conductor disk (inner).

Fig. 5(b) - Sample to perform the shielding

effectiveness measurement.

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This procedure was applied to a Mylar coated gold specimen with the dimensions shown in

Figure 5. The measurement was performed between 200?V and 1,5WVafter performing

the calibration steps previously described. Refer to Figure 6 for the measurement setup and to

Figure 7 for the measured data.

Fig. 6 - Measurement setup.

Fig. 7 - Shielding effectiveness measurement results.

3D FULL FIELD SIMULATION

A 3D Electromagnetic Model of the ASTM D4935 rig was created with the aid of CST

Microwave Studio®. Refer to Figures 8-a to 8-d to the final assembled parts of the

measurement rig. As excitations, two waveguide ports were defined on both sides of the

measurement rig.

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-33

-32

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200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Sh

ield

ing

eff

ect

ive

ne

ss,

dB

Frequency, MHz

SE measurement, [dB]

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Fig. 8(a) - Perspective view of the flanged coaxial CAD model for simulation.

Fig. 8(b) - Lateral view of the flanged coaxial CAD model for simulation.

Fig. 8(c) - Cutting plane view of the inner conductor of the measurement

rig (excitation port planes in red).

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Fig. 8(d) - Cutting plane view of the complete measurement rig.

A simulation of the propagation modes excited on the waveguide has revealed as expected

that the TEM mode was correctly excited and both ports provide an expected input impedance

of approximately 50 Ω. Following this verification, and )) parameters were extracted

without any sample fitted in the flanges. CST MWS TLM mesh uses conformal PBA cell

warping and lumping of cells (octree mesh) away from object to reduce overall mesh count

and improve accuracy. With modest parameters for the discretization of the mesh, it was

shown that reflection values better than X25 to X35HK can obtained, therefore validating the

model and the standard requirements for the input impedance of the measurement rig. For

these simulations, a time domain technique based on a FIT implementation was used. To

obtain broadband results, this technique applies a Gaussian pulse, with spectral content up to

1,5WV, and performs a Fourier transform on the scattering parameters to obtain frequency

domain data. A truncation of the simulation is performed when a level down to X40HK of the

initial energy excited into the port has left the system. The 3D model space is truncated with

open space boundaries, and no symmetries were applied, even though the S parameters should

be symmetrical and field symmetries exist in the structure.

The reference material thickness is extremely small. As so, one possible material model to use

is the Ohmic sheet material model [5], with varying surface resistivity in the range of

4 X 5,6Ω/QRST(U, as shown in Figure 9.

Fig. 9 - Cutting plane view of the complete measurement rig with the sample fitted.

The model mesh parameters are shown in Table 2 and the simulation results in Figure 10.

Table 2 - EM model mesh data.

Smallest cell 2,0032** Largest cell 17,9236** Number of cells 139 104

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Fig. 10 - Shielding effectiveness simulation results X=~4P5,6Ω/QRST(U.

RESULTS AND COMPARISONS

As can be seen on Figure 7 for the measured shielding effectiveness and Figure 10 for the

simulated shielding effectiveness, the results compare very well, with differences in the order

of 1 X 2,5HK. The expected theoretical values for thin samples with surface resistivity are

shown in Table 3.

Table 3 - Theoretical results for low frequency approximation.

=, Ω/QRST(U , HK =, Ω/QRST(U , HK =, Ω/QRST(U , HK

4 X33,6 4,6 X32,4 5,2 X31,4

4,2 X33,2 4,8 X32,1 5,4 X31,1

4,4 X32,8 5 X31,7 5,6 X29,8

These results illustrate the accuracy between simulated, theoretical and measured values for a

sample in which the reflection loss mechanism is the main loss mechanism. Electromagnetic

simulation has allowed to validate via simulations that the measurement rig design effectively

complies with the standard requirements but also show a very good correlation between

measured and simulated values.

FUTURE WORKS

Future works on the theme of electromagnetic shielding shall now be undertaken for more

complex samples. For instance, evaluations of incrusted conducting polymers and metallic

dopants on dielectric substrates, specialty shielding textiles for industrial application and

carbon fibre composite material used in the aerospace industry are materials that should be

characterized using the method herein described.

ACKNOWLEDGMENTS

This project has received funding from the Clean Sky 2 Joint Undertaking under the European

Union’s Horizon 2020 research and innovation program under grant agreement No 807083.

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REFERENCES

[1]-ASTM D4935-10, Standard Test Method for Measuring the Electromagnetic Shielding

Effectiveness of Planar Materials, American Society for Testing and Materials, 2010.

[2]-E. Hund, Microwave Communications - Components and Circuits, McGraw Hill, 1989.

[3]-A. López, L. Vojtěch, Comparative Among Models to Estimate the Shielding

Effectiveness Applied o Conductive Textiles, Information and Communication Technologies

and Services, 2013.

[4]-L. Vojtěch, M. Neruda, J. Hájek, Planar Materials Electromagnetic Shielding Efficiency

Derivation, International Journal on Communications Antenna and Propagation. 2011.

[5]-Modelling Thin Materials in CST STUDIO SUITE, Computer Simulation Technology,

2012.