Dielectric Pro of Foams_Kamlesh Patel IJ

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Dielectric Properties of Foams by Traceable Free-Space Transmission Method in X-band Range

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Dielectric Properties of Foams by Traceable Free-Space

Transmission Method in X-band Range

KAMLESH PATEL

Electrical & Electronics Standards, National Physical Laboratory

Dr. K. S. Krishnan Road, New Delhi-110012

E-mail: [email protected]

Abstract

Complex permittivity of flexible polyurethane foam and rigid polyethylene foam are determined by the free-

space transmission method in 8.2-12.4 GHz frequency range. The calculated dielectric constant were found in

the range of 1.011 to 1.075 and 1.004 to 1.032 for the polyurethane foam and polyethylene foam respectively,whereas the loss tangents are in the range of 2.33 x 10-3 to 0.04 x 10-2 and 3.65 x 10-4 to 2.77 x 10-2. The

results along with their combined uncertainty may be treated as a standard data for the applications of these

easily available foams in the microwave frequency range. To establish the reliability, a traceability link of these

dielectric property measurements is also presented through the S-parameters measurements, frequency and

dimensional metrology.

Key words : Dielectric property, material sheet, measurement uncertainty, calibration, free-space

method, traceability, Vector network analyzer.

1. Introduction

Dielectric properties of materials aredetermined by the measurement of the reflection

and transmission coefficients and its modeling using

free space method [1-5]. Free-space measurements

based on VNA have been well established to

characterize various materials in the form of sheets.

The microwave nondestructive testing of concrete

structures, coatings and paints is reported for the

detection of cracks, defects, inhomogeneities, and

moisture contents etc. [6-8]. These papers briefly

explained the implementation of Thru-Reflect-Line

calibration technique by employing Free-space

standards on VNA for dielectric and magnetic

properties. The accuracy consideration is discussed

on the basis of two kinds of errors, one associated

with the residual post calibration and the other due

to small changes in the references planes. The

maximum errors evaluated for dielectric constant

(2.08) is less than 3.0% and error in loss tangent

(3.7 x 10 -4) is 0.025 at 10 GHz. Dielectric

properties of cereal Grain and oil seed at microwave

frequency ranges have been reported with bulkdensity and moisture content [9-11]. The

expressions of relative errors for dielectric

properties were also derived to estimate the

uncertainty in bulk density and moisture content.

Based on these derived expressions, the

uncertainties are reported in the ranges of 13.3 -

18.9 kg m-3 for bulk density and 0.49-0.67% for

moisture content.

This paper presents the evaluation of the

dielectric properties of foams made of the

polyethylene (rigid white) and polyurethane (flexible

yellow) at microwave frequencies. The work is the

extension of the previous study on the performance

of established free-space method and its validation

through Teflon and Plaster of Paris sheets

characterization [12]. These foams are used as

weatherproof RF transparent randomes, blankets

Invertis Journal of Science & Technology Vol. 4, No.3, 2011 ; pp. 133-139


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Kamlesh Patel

134

and covering where radar transparency is required.

The other applications are antenna spacer,

instrument housing and thermal barriers in electric

and electronics applications. The details of the

electrical and mechanical properties for these foamsare readily available [13-14]. In the present work,

theoretical expressions for the uncertainty

estimation have been derived and discussed in details

in terms of measured attenuation and phase,

operating wavelength and thickness of the material

sheets. A traceability path of dielectric property

measurements to the national standards of physical

and electrical parameters is also proposed for

reliability and effectiveness of these results.

2. Material details and Measurement Set-up

The size of material sheet has been chosen to

cover the complete focusing area of antennas. The

width, height and thickness dimensions of

polyethylene and polyurethane foams are 55.30 x

42.70 x 7.00 cm (white) and 64.00 x 40.80 x 4.880

cm (yellow) respectively. The given thickness here

is for uncompressed foams shown in Fig.1.

The setup, measurement conditions and

operating procedure are given elsewhere [12].

Figure 2 shows the free-space transmission

measurement set up used in the present work.For the analysis purpose, the relative errors

given by equations (22) and (23) in [11] are re-

produced here as equations (1) and (2),

(1)

= + +

'

'"

"

2

A d

A d(2)

The theoretical expressions of uncertainty are re-

written from [12] as follows.

(3)

(a)

(b)

Fig.1. Experimental foams in the form of sheets (a)white polyethylene 55.30 x 42.70 x 7.00 cm3

(white) and (b) yellow polyurethane 64.00 x

40.80 x 4.880 cm3

(4)

(5)

where, the attenuation A and phase shift are

determined from the measurement of S21

and the

phase as follows in case of low reflection,

A(dB) = 20 log /S21

and () = - 2.n, n

=1,2,3

= + +

+ +

0

'0 0

2' 2 720. .

360. 360.d

d d d

00"

0 0

" 2

(360. . )

d dd

d d d

= + +

+

'0

" '0

" 1

8.686 2

A d

A d

= + + +

2

tan ' . " ". '

tan ( ')

+ =

/


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135

0

(cm) is the free space wavelength and d (cm) is

the sheet thickness.

The equations (3) to (5) are modified version

of the earlier reported (1) and (2) with the addition

of uncertainty in 0

and some of the constant factors

also. In this work for uncertainty estimation of

dielectric constant, loss factor and loss tangent, the

derived equations (3)-(5) are used. Figure 3 displays

the major contributors of the uncertainties in the

calculated , and tan .

The uncertainty in realizing the calibration

standards is equally contributed to S-parameters and

phase measurements due to any error in antenna

alignment and positioning. The uncertainty in the

measured complex S-parameters on VNA has been

estimated as per EA-Guidelines [15] and verified

by the calibrated attenuator and airline prior to

applying the Free-space method. The uncertainty

contributions for attenuation and phase shift havebeen taken from the previous study [16] whereas

for0and d, the scale used is linked to Vernier caliper

calibration. Thus based on the uncertainty

contributors a self- explanatory traceability route

can be established for dielectric property

measurements, which is presented in Fig. 4.

3. Results and Discussion

After performing TRL recalibrations for free-

space from 8.2 to 12.4 GHz band, a foam sheet

was placed between two antennas using a stand

treated as a sample holder respectively for the

foams. Figure 5 represents the transmission

characteristics of white and yellow foam sheets, as

measured insertion loss, which in turn called theshielding efficiencies of the respective materials.

It was found that yellow foam is lossier than the

white foam due to higher density though the pattern

is similar over maximum frequency points.

Compositioning with other solvable carbon

products, these foams can provide a promising

shielding to electromagnetic energy with low

manufacturing cost, better structure and solidness.

From the transmission characteristics of white and

yellow foams, their dielectric properties have been

evaluated using equations given in [11-12]. The

variations of, and tan with uncertainty over

the X-band frequency range for white and yellow

foams are shown in Figs.6 (a-c).

Both the foams are having almost the same

range of dielectric constant as that of air, thus mixing

Fig.2. Free-space transmission measurement set up using Vector network analyzer


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Kamlesh Patel

136

Fig. 3. Major contributors of uncertainty in dielectric properties

Fig. 4. Traceability path for Dielectric property measurements by Free-space transmission method


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with tiny metallic compounds or solvable ferric or

copper solution, one can develop a series of negative

refractive index materials i.e. metamaterials. As

these foams are spongy in structure i.e. they return

to its normal thickness after getting compressed

and have low density, thus, mixing with metallic

atoms is comparatively easy for changing the design

of the unit cell in the developed material. The yellow

foam has a higher dielectric constant and is lossierthan the white foam because of higher density and

vulnerability to moisture. Except at few frequencies,

both have extremely low loss factor and low loss

tangent over the entire range, this suggests that

measurements with higher precision are again

required to claim their values accurately. These

foams are transparent to electromagnetic energy

due to the low dielectric constant with nominal

change against variation in frequency at the present

state, mixing with conducting polymers may prove

them as effective shielding materials. Table 1

summarizes the overall results with the associated

uncertainties.

4. Conclusion

Establishment of a reliable free-space

Fig. 5. Shielding properties of specimen foams

Fig. 6. Dielectric properties of white and yellow

foams (a) Dielectic constant, (b) Loss factorand (c) Loss tangent


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Kamlesh Patel

138

transmission method is comparatively difficult task

and the major challenges are:

During calibration, keeping the reflection

standard at right place

The accuracy and size of the reflection standard Movement of the antennas for desired distance

to obtain line standards

Keeping the measurement environment free

from other radiation sources.

A number of re-calibrations have been

performed to reduce the uncertainty contributors

as low as possible. Also the size of the reflection

standard have been kept larger than the size of

material under test to get better results, however

getting accurate reflection standard with such a large

size is quite problematic and future work will focus

on it. Getting high absorbing material for a chamber

is also costly affair. However the efforts are

implemented to reduce the external sources as low

as possible.

White rigid Polyethylene foam and yellow flexible

Polyurethane foam are characterized for their

dielectric properties and the results are found in

good agreement with the manufacturer's

specifications. The applications of these foams as

shielding materials and metamaterials will be

explored based on the results obtained. Thus the

established method is very useful to counter check

the dielectric properties of supplied material sheets

from the reference data in the X-band frequency

range. The efforts are going on to evaluate the

magnetic properties in the same range and

implementing the proper arrangements to enhance

the accuracy of the results. A traceability link of the

dielectric properties measurements is established

to the national standards of the other parameters

and presented to consider the various uncertainty

contributors while evaluating the combined standarduncertainty. This method will help in the primary

and quick nondestructive evaluation of the newly

developed materials for its dielectric properties at

NPL, India.

5. Acknowledgement

The author gratefully acknowledges the

contributions made by the authors in the field

previously and support from Director, National

Physical Laboratory and HOD for providing the

Laboratory facilities for the present work. The

author would also like to extend appreciations to

his colleagues Mr. P.S. Negi, Dr. Sreekumar C. and

Dr. R. P. Aloysius' of NPL for their useful technical

comments and suggestions on the manuscript.

References

[1] P.K. Kadaba, Simultaneous measurement of

complex permittivity and permeability in

the millimeter region by a frequency -

domain technique, IEEE Trans. on Instru. andMeas., 33 (1984) 336.

[2] A.L. Cullen, A new free-wave method for

ferrite measurements at millimeter

wavelengths, Radio Science, 22 (1987)

1168.

[3] J.C. Joseph, R.J. Jost and E.L. Utt, Multiple

Table 1

Dielectric properties of Foams in 8.2-12.4 GHz frequency range

Parameter Dielectric constant Dielectric Loss Loss tangent tan

/ Material

Range Uncertainty Range Uncertainty Range Uncertainty Polyethylene Max 1.032 0.014 0.028 3.84E-04 0.028 2.14E-05

Foam Min 1.004 0.014 3.70E-04 2.79E-05 3.65E-04 1.19E-08

Polyurethane Max 1.075 0.014 0.040 0.001 0.040 4.18E-07

Foam Min 1.011 0.014 0.003 6.25E-05 0.002 2.1E-07


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angle of incidence measurement technique

for the permittivity and permeability of

lossy materials at millimeter wavelengths,

IEEE AP-S Int. Symp. Diges t, (1987) 640.

[4] Jesse Lai, Dana Hughes, Eric Gallaher andReza Zoughi, Determination of the thickness

and dielectric constant of a dielectric slab

backed by Free-space or a conductor

through inversion of the reflection

coefficient of a rectangular waveguide probe

IMTC 2004-Instrumentation and Measure-

ment Technology Conference, Como Italy, 18-

20 May (2004).

[5] Yasushi Iijima, Takashi Tanaka, Masafumi

Kimura and Risaburo Sato, Measurement of

complex permittivity and permeability atmillimeter wavelength using a free space

method, Int. Symposium on Electromagnetic

Compatibility, 19P206 (1999) 412.

[6] D.K. Ghodgaonkar, V.V. Varadan and V.K.

Varadan, A free- space method for

measurement of dielectric constants and loss

tangents at microwave frequencies, IEEE

Trans. on Instru. and Meas.,38 (1989) 789.

[7] D.K. Ghodgaonkar, V.V. Varadan and V.K.

Varadan, Free- Space measurement of

complex permittivity and complexpermeability of magnetic materials at

microwave frequencies, IEEE Trans. on

Instru. and Meas., 19 (1990) 387.

[8] D.K. Ghodgaonkar, Wan Mahmood B.W. A.

Majid and Hilmi Husin, Accurate

measurement of dielectric constants and

loss tangents for nondestructive evaluation

of Malaysian timber at microwave

frequencies International conference on

Timber Engineering, PTEC99, Rotorua, New

Zealand, 15-20 March (1999) 87.

[9] Samir Trabelsi, Andrzej W. Kraszewski and

Stuart O. Nelson, Free-space measurement

of dielectric properties of cereal Grain and

oilseed at microwave frequencies,Meas. Sci

& Technology,14 (2003) 589.[10] Samir Trabelsi and Stuart O. Nelson, Free-

space measurement of dielectric properties

of Moist Granular materials at microwave

frequencies, IMTC 2003- Instrumentation

and Measurement Technology conference, Vail

CO USA 20-22 May (2003).

[11] Samir Trabelsi, Andrzej W. Kraszewski and

Stuart O. Nelson, Nondestructive microwave

characterization for determining the bulk

density and moisture content of shelled corn,

Meas. Sci. Technology, 9 (1998) 1548.

[12] Kamlesh Patel, Sandhya M. Patel and P.S.

Negi (submitted), Performance of VNA

based free-space method for the complete

dielectric property characterization, Invertis

Journal of Science and Technology.

[13] www.eccosorb.com, Emerson and Cuming

Microwave Products, Inc, USA

[14] www.cuming.com, Cuming Corporation,

USA

[15] EA Guidelines on the Evaluation of VectorNetwork Analyzers (VNA) 2000 Publication

reference EA-10/12, European cooperation

for Accreditation.

[16] Kamlesh Patel, P.S. Negi and P.C. Kothari,

Extension of Attenuation measurement

range in Intermediate frequency (IF)

substitution and Vector Network analyzer

(VNA) techniques. Proceedings of the First

international symposium-RMO, Dubronvik

Croatia, 12-15 Nov (2008) 125.

Dielectric Pro of Foams_Kamlesh Patel IJ

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