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CHAPTER 4

DESIGN AND DEVELOPMENT OF ELECTROMAGNETIC

SHIELDING TESTER FOR CONDUCTIVE TEXTILE

MATERIALS

4.1 INTRODUCTION

Shielding effectiveness is a key parameter which often determines

the scope for application of a given material. The shielding effectiveness for

metal shields can be determined by knowing the materials’ electrical and

magnetic parameters, whereas the materials containing inter-twined metallic

or graphite threads, plastic materials having metallised surfaces or composite

materials, the shielding effectiveness can be determined by actual measuring.

There are several methods available which allow the shielding effectiveness

to be measured. However, for flat shielding structures, there are currently no

standards defining the evaluation of small samples.

The shielding effectiveness measurement results obtained using

currently known methods depend not only on the properties/parameters of the

shielding material but also on the size of the test sample, the geometry of the

test setup, and the parameters of the source of electromagnetic radiation. At

the current state of research and development, it is not always possible to take

all of these additional factors into account. It should also be noted that there is

currently no effective method for comparing the results of shielding

effectiveness measurement obtained based on MIL-STD 285 and IEEE-STD-

299 for comparison with ASTM D4935. There is also a lack of generally

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accepted standardised method for measuring shielding effectiveness of

conductive textile materials.

Considering this, in this research, modified the exiting testing

equipment was developed, which is suitable for measuring the

electromagnetic shielding effectiveness of textile materials from 500MHz to

12GHz. Various fabric samples were tested for their electromagnetic

shielding effectiveness using modified electromagnetic shielding tester and

network analyzer tester, and the values were compared and correlated. The

effect of the size of the test sample, the geometry of the test setup, and the

parameters of the source of electromagnetic radiation were analyzed and

optimized by Taguchi design and ANOVA.

4.2 MODIFICATION OF ELECTROMAGNETIC SHIELDING

TESTER

In the present work, Electromagnetic shielding testing apparatus

was modified and fabricated to enable only to measure the Electromagnetic

shielding effectiveness of conductive textile materials in the frequency ranges

of 500 MHz to 12 GHz. The basic shielding mechanisms are reflection,

absorption and internal re-reflection of shielding materials. The fabric is

placed in between the source and the receiving antenna. The principle

involved in the shielding tester is that to capture the waves that get through

the fabric when it is subjected to frequency range. The shielding effectiveness

of materials is the ratio of the magnetic strength at the receiver antenna

without testing materials to that with the testing materials. In case of MIL-

STD 285 system, the output of electromagnetic shielding effectiveness is

taken by network analyzer / spectrum analyser which is used to measure the

shielding effectiveness in decibal.

A spectrum analyzer is a device that measures the power spectrum.

It is often used to examine the components of a waveform, whether it is

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electrical, acoustic or optical in form. There are basically two kinds of

spectrum analyzers, the analog and the digital. It also displays the received

signal and compares the bandwidth to the frequency. A comparison is often

done with an Oscilloscope, which compares the strength of the signal against

the time. Spectrum analyzers are also useful in analyzing amplitude against

the frequency. Amplitude is normally measured in power or in dBm instead of

volts, which is normally used in most spectrum analyzer. The reason behind

this is the fact that there are low signal strengths and frequency of movements

that may not be measured. Spectrum Analyzers can only measure the

frequency of the response at powers as low as –100 dBm. These are the levels

that are frequently seen in microwave but it is very costly when compared to

power meter.

The Vector Network Analyzer (VNA) operates from 200 kHz to

100 MHz, and connects to a personal computer using a USB 1.1 interface.

The VNA is one of the more useful pieces of test equipment for designers and

experimenters. It can measure the forward and reverse gain and phase

response of a circuit, and the input and output reflection properties (complex

impedance). The VNA is used to measure and adjust filters, coaxial cables,

amplifiers, antenna input impedance vs. frequency. Generally a VNA

measures and displays data in the frequency domain.

The Nonlinear Vector Network Analyzer (NVNA) performs a

variety of measurements for a number of different application areas. These

measurement applications are of importance for those designing active

components such as amplifiers and in general any component that exhibits

nonlinear behavior. The NVNA can also be used to analyze complex signal

behavior while applying VNA error correction providing extremely accurate

signal analysis capabilities. The NVNA provides vector corrected

measurements of the absolute amplitude and cross-frequency phase of

frequencies to/from the component with the highest level of accuracy.

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Traditional VNAs do not have the ability to accurately measure the

vector corrected absolute amplitude of the frequencies and the phase

relationship between frequencies (cross-frequency phase) of waves. The

NVNA, however, can make these measurements with the highest level of

accuracy. Once these absolute quantities are measured, they can then be used

to analyze component and complex signal behavior. But VNA it is very costly

when compare to power meter. But in case of Power meters, measuring radio

frequency power levels are an essential tool for any RF design, test or repair

laboratory. In fact the use of RF power meters increases as the frequency of

signals increases. RF power measurement is a key parameter to measure to

determine the operation of a circuit at RF or microwave frequencies. This is

because as the frequency rises, detecting voltage and current is less easy to

undertake. Additionally RF power is one of the key parameters of interest as

RF circuits need to deliver power of various levels into other circuits or loads.

Accordingly these test instruments are used in conjunction with work on radio

receivers, transmitters and RF circuits of all types from wi-fi to cellular and

broadcast transmitting to domestic radio and television receiver design.

In view of this fact, RF power is widely used in the specification of

components and interfaces. Typically input ports will have maximum power

levels specified, and outputs will be characterized in terms of the output RF

power they can deliver. Naturally other parameters including impedance and

the like are also provided, but power is one of the most universal RF

measurements. Hence in this study, the output of electromagnetic shielding

effectiveness is taken by power sensor and power meter and then computed in

computer to estimate the electromagnetic shielding effectiveness in dB using

C Programme. The measurement systems were carried out in open space,

thereby eliminating the potential reflections that would introduce errors. The

distance between the transmitter and receiver is calculated and used in the

system to avoid the ground reflection, where as the MIL-STD 285 standard

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systems, the measurement were carried out in anechoic chambers made of

absorbing materials. In case of MIL-STD 285 system, there is no separate

fabric clamp, which enables to measure the electromagnetic shielding but in

this present study we have designed a separate frame for fixing the test

sample in different sizes (0.3x0.3, 0.5x0.5 and 1.0x1.0 m) to measure the

electromagnetic shielding effectiveness. It is made up of stainless steel and

distance between transmitting and receiving antenna, and samples frame can

be adjusted according to the frequency as shown in Figures 4.1, 4.2 and 4.3.

The free space measurement system used in the present study consists of RF

signal source, an isolator, attenuator, a cavity wave meter, transmitting

antenna, receiving antenna, power sensor, along with a power meter to

measure the output power. The Basic test set up of electromagnetic shielding

tester is shown in the Figures 4.1, 4.2 and 4.3. The experimentations have

been carried out for low frequencies range of 500MHz to high frequencies

range of 12 GHz at open space.

4.2.1 RF Signal Source

This consists of both a Klystron power supply and a Klystron

oscillator which generates electro-magnetic radiations and a source capable of

generating a sinusoidal signal over the desired portion of the frequency range

as specified. A 50-Ω output impedance is needed to minimize reflections due

to mismatches. Precision step attenuators are useful in increasing the effective

dynamic range for SE measurements. It is shown in Figure 4.5.

4.2.2 Isolator

An optical isolator consists of a magnetic garnet crystal having a

Faraday effect, a permanent magnet for applying a designated magnetic field,

and polarizing elements that permit only forward waves to pass while

blocking backward waves. It is shown in Figure 4.5.

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4.2.3 Attenuator

It is used to decrease the power level if needed i.e. to propagate the

electromagnetic waves correctly and accurately without any disturbances.

These are devices used to isolate the specimen holder from the signal

generator and the receiver. The main purpose in this system is for impedance

matching. A 10-dB, 50-Ω attenuator should be used on each end of the

specimen holder. The material under test usually causes a large reflection of

energy back into the signal generator. This may also cause variations of the

incident power by changing the generator impedance loading. Use of a

bidirectional coupler allows monitoring and correcting any changes in

incident power due to this loading. Attenuators greater than 10-dB will

excessively decrease the dynamic range of the measurement system. It is

shown in Figure 4.5.

4.2.4 Cavity Wave Meter

The cavity wave meter is of the type commonly used for the

measurement of microwave frequencies. The device uses a resonant cavity.

The resonant frequency of the cavity is varied by means of a plunger, which is

mechanically connected to a micrometer mechanism. Movement of the

plunger into the cavity reduces the cavity size and increases the resonant

frequency. Conversely, an increase in the size of the cavity (made by

withdrawing the plunger) lowers the resonant frequency. The microwave

energy from the equipment being tested is fed into the wave meter through

one of two inputs, A or B. The crystal rectifier then detects (rectifies) the

signal. The rectified current is indicated on current meter. The cavity wave

meter is shown in Figure 4.5.

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4.2.5 Antenna

An antenna (bi-conical antenna and parabolic disc antenna) is a

transition device, or transducer, between a guided wave and a free-space

wave, or vice-versa. The antenna is a device which interfaces a circuit and

space. The parabolic disc antenna is shown in Figure 4.5.

4.2.5.1 Parabolic Antenna and Bi-Conical Antenna

A parabolic antenna is a high-gain reflector antenna used for radio,

television and data communications, and also for radio location (radar), on the

UHF and SHF parts of the electromagnetic spectrum. The relatively short

wavelength of electromagnetic (radio) energy at these frequencies allows

reasonably sized reflectors to exhibit the very desirable highly directional

response for both receiving and transmitting. A typical parabolic antenna

consists of a parabolic reflector with a small feed antenna at its focus. The

reflector is a metallic surface formed into a paraboloid of revolution and

(usually) truncated in a circular rim that forms the diameter of the antenna.

This parabolic possesses a distinct focal point by virtue of having the

reflective property of parabolas in that a point light source at this focus

produces a parallel light beam aligned with the axis of revolution. The feed

antenna at the reflector's focus is typically a low-gain type such as a half-

wave dipole or a small waveguide horn.

A bi-conical antenna consists of an arrangement of two conical

conductors, which is driven by potential, charge, or an alternating magnetic

field (and the associated alternating electric current) at the vertex. The

conductors have a common axis and vertex. The two cones face in opposite

directions. Bi-conical antennas are broadband dipole antennas, typically

exhibiting a bandwidth of 3 octaves or more. The transmitting antenna

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(bi-conical antenna and parabolic disc antenna) is a region of transition from a

guided wave on a transmission line to a free-space wave.

4.2.5.2 Receiving Antenna

The receiving antenna (bi-conical antenna and parabolic disc

antenna) is a region of transition from a space wave to a guided wave on a

transmission line. It is shown in Figure 4.7.

4.2.6 Power Sensor

The power sensor is shown in the Figure 4.6. The cable used to

connect the receiving antenna and the power meter to convert the readings in

terms of digital by thermal current. Zeroing a power sensor is performed to

reduce zero offset and noise impact to improve RF power measurement

accuracy. The USB power sensor has two types of zeroing namely: Internal

Zeroing (INT) and External Zeroing (EXT).

4.2.7 Power Meter

The power meter is shown in Figure 4.6. It is a digital device used

to measure the received power values in watts or dBm. It has a pointer to

indicate the accurate value. It is absolute low power measurement instruments

designed for use with temperature – compensated thermistor mount. High

accuracy over a wide temperature range is obtained by measuring the output

voltage of thermistor bridges and computing the corresponding power, even

higher accuracy + 0.2% microwall can be obtained. It has automatic zeroing

facility. The basic measuring circuit consists of a D.C. bridge circuit instead

of a 10KHz AC which has benefit of no signal emission from mount to

disturb circuit and instrument is not effected by capacitance change during the

movement of thermistor cable.

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Power Meter

Transmitting Antenna Receiving

antenna

T E X T I LE M A T E R I A L

RF Signal Source

Isolator Attenuator

Figure 4.1 Basic Test Setup of Electromagnetic Shielding Tester

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Power Meter

Transmitting Antenna Receiving

antenna

RF Signal Source

Isolator Attenuator

Bolts

Test Fixture

Gasket

Figure 4.2 Shielding Effectiveness in Open Reference Test Setup

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Power Meter

Transmitting Antenna

Receiving antenna RF Signal

Source

Isolator Attenuator

Test Fixture

Gasket

Fabric sample

Figure 4.3 Shielding Effectiveness in Closed Reference Test Setup

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4.3 WORKING PRINCIPLE

The methodology of designing and fabricating electromagnetic

shielding tester is that the electromagnetic waves from the radiofrequency

source are passed on to the textile material and finding out the shielding

effectiveness of the material. The electromagnetic wave generated by the RF

signal source which generates 500 MHz to 12 GHz, is actuated by the

antenna. Then, the Electromagnetic waves are passed on to the textile material

which is mounted between the transmitting antenna and receiving antenna on

the specimen holder, as shown in the Figure 4.3. The majority of the

Electromagnetic waves are absorbed and reflected by textile material and

remaining electro magnetic waves are received by the receiving antenna. The

power received by the receiver is sensed by the Power Sensor and actuated to

the Power Meter. The Power Meter is used to indicate the amount of

electromagnetic waves absorbed by the conductive textile material. The

shielding effectiveness is ratio of the magnetic field strength at the receiving

antenna without the testing material (HO) to that with the testing material (H1)

SE = 20log10 (HO)/ (H1) in dB (4.1)

The Figure 4.4 is a module diagram of electromagnetic shielding tester. If a

transmission line, propagating energy is left open at one end, there will be

radiation from this end. In case of a rectangular waveguide this antenna

presents a mismatch of about 2 m and it radiates in many directions. The

match will improve if the open waveguide is a horn shape. The radiation

pattern of an antenna is a plot of field strength of the power intensity as a

function of the aspect angle at a constant distance from the radiating antenna.

An antenna pattern is of course three dimensional but for practical reasons it

is normally presented as a two dimensional pattern in one or several planes.

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An antenna pattern consists of several lobes, the main lobe, side lobes and the

back lobe. The major power is concentrated in the main lobe and it is required

to keep the power in the side lobes and back lobe as low as possible. The

power intensity at the maximum in the main lobe compared to the power

intensity achieved from an imaginary omni-directional antenna (radiating

equally in all directions) with the same power fed to the antenna is defined as

gain of the antenna.

4.3.1 3dB Beam Width

This is the angle between the two points on a main lobe where the

power intensity is half the maximum power intensity. The antenna pattern

measurement is always done in far field region.

4.3.2 Radiation Field Beginning

Far field pattern is achieved at a minimum distance -2D2/λo,Where

D is size of the diameter of the antenna and λo is free space wavelength. D is

also very important to avoid reflection, Antenna measurement is done at

outdoor ranges or in anechoic chambers made of absorbing materials. The

electromagnetic waves perform two phenomenons: radiation and induction.

The radiation field begins only beyond the distance of 2D2/λo

103

TEST SETUP FOR MEASURING THE SHIELDING EFFECTIVENESS

KLYSTRON POWER

SUPPLY GS-610

KLYSTRON OSCILLATOR

XG-11

ISOLATOR XI-621

PIN MODULATOR

XM-55

VARIABLE ATTENUATOR XA-455

VSWR METER SW-

115

FREQUENCY METER XF-

455

DETECTOR MOUNT XD-

451

HORN HORN

Figure 4.4 Module Diagram for electromagnetic shielding tester

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Cavity Wave Meter

Isolator

Attenuator

Transmitting Antenna – Parabolic Dish Antenna

Figure 4.5 Microwave Test Bench – Individual Components

Power Meter

Klystron Power Supply

Figure 4.6 Klystron Power Supply and Power Meter

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Receiving Antenna – Parabolic Dish Antenna

Figure 4.7 Receiver

4.3.3 Distance between the Sample and Antennas

The distance between sample and transmitting antenna, influence

the shielding effectiveness of conductive textile materials. As the diameter of

the antenna and frequency increases, the distance between sample and

transmitting antenna also increases.

4.4 TEST PROCEDURE

Before tests, test specimens are conditioned for 48 hr at 27 ± 2° C

and 60 ± 5% relative humidity. Tests must be performed immediately upon

removal from conditioning environment.

i. Set up the equipment as shown in the Figure 4.2. Keeping the

axis of both antennas in same line.

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ii. Energize the Klystron Oscillator for maximum output at

desired frequency with square wave modulation by tuning

square wave amplitude and frequency of modulating signal of

Klystron Power Supply and by tuning the detector.

iii. Also tune the S S Tuner in the line for maximum output (if S S

Tuner is in the set-up).

iv. Obtain full scale deflection (0 dB) on normal dB scale (0-10

dB) at any convenient range switch position of the VSWR

(Voltage standing Wave Ratio) Meter by gain control knob of

VSWR meter or by variable attenuator.

v. Tune the receiving antenna to the left in 2º or 5º steps upto 4º -

5º and note the corresponding VSWR dB reading in normal

dB range. When necessary, change the range switch to next

higher range and add 10 dB to the observed value.

vi. Repeat the above step, but this time, turn the receiving antenna

to the right and note down the readings.

vii. Determine 3 dB - width (beam width) of the receiving

antenna.

viii. Measure the reading value for with and without material.

4.5 EXPERIMENTAL PROCEDURE

In this investigation, the various fabric parameters of copper core

conductive textile materials were taken to measure the electromagnetic

shielding effectiveness in the modified electromagnetic shielding tester,

which are shown in the Table 4.1. The measurement values of modified

electromagnetic shielding tester were compared with military standard system

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(MIL-STD 285). In order to optimize the size of the test sample, distance

between sample and transmitting antenna, distance between the sample and

receiving antenna and RH%, the Taguchi design method and ANOVA were

selected and applied for electromagnetic shielding effectiveness performance

characteristic of conductive textile materials.

Taguchi design method replicates each experiment with the aid of

an outer array that deliberately include the sources of variation that a product

would come across while in service. Such a design is called a minimum

sensitivity design or a robust design and the Robust Design method is called

Taguchi method. To achieve the optimum design factor setting, Taguchi

advocated a combination of two stage process in which the first step is related

to the selection of robustness seeking factors and the second step with the

selection of adjustment factors to achieve the desired target performance. The

various stages in the experimental design have been dealt by many authors in

the past. In other experimental designs, noise factors are kept under

observation during experimentation, whereas Taguchi methods include those

factors in the experimentation to make the design a robust one in the form of

S/ N ratios.

In robust design, one minimizes sensitivity to noise by seeking

combinations of DP settings. The most appropriate S/N ratio can be selected

depending upon the properties of interest (Table 4.3), for both scaling factors

and adjusting factors. The experiments using the listed variables at different

levels (Table 4.4 (a), 4.4 (b) and 4.5) were carried out randomly to avoid the

systematic errors. The selection of appropriate orthogonal array (OA) is a

critical step in Taguchi’s experimental design. Four design parameters at three

different levels were selected for optimization purpose in this study. The

control factors are sample size (m), distance between sample and transmitting

antenna (cm), distance between sample and receiving antenna (cm) and RH

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.Temperature and frequency were considered as the two different noise factors

because they influence the electro magnetic shielding effectiveness

individually to significant extent. which are shown in Table 4.4(a), 4.4(b)

and 4.5 The OA selected should satisfy the following criterion: Degrees of

freedom (DOF) of OA > Total DOF required .Therefore, L9 Taguchi

orthogonal array and 3 levels were selected to assign various columns. The

experiments were conducted according to the trial conditions specified in L9

OA. A total of 36 experiments (three repetitions at each trial condition) were

conducted.

Using Taguchi’s analysis and analysis of variance (ANOVA), the

optimal performance parameters were determined and the optimal values were

predicted. The average values of performance characteristics at each level and

against each parameter were calculated and are given in Table 4.5.

4.6 RESULTS AND DISCUSSION

The Table 4.1 shows that various fabric parameters of Copper Core

Conductive textile materials which were taken to measure Electromagnetic

Shielding Effectiveness. In order to analyse the electromagnetic shielding

effectiveness, the samples DCS1, DCS2, DCS3, DCS4, DCS5, DCS6, DCS7

and DCS8 were considered for measurement by the modified electromagnetic

shielding tester and network analyzer (MIL-STD 285). From Table 4.2 and

the Figure 4.8, 4.9, 4.10and 4.11, it is clear that there is no significant

difference of electromagnetic shielding effectiveness with modified

measurement system and network analyzer (MIL-STD 285) at 95%

confidence level in the frequency ranges of 500MHz to 8GHz.

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Table 4.1 Copper Core Conductive Fabric Parameters

Sam

ple

Thickness (mm) EP

I

PPI

Wea

ve

Cou

nt

(Tex

) Copper Diameter

(mm)

Cover Factor

DCS1 1.05 22 20 Plain 179 0.09 18.4

DCS2 0.9 22 24 2/2 Twill 179 0..09 19.6

DCS3 0.8 22 24 Plain 98 0.09 15.06

DCS4 0.45 22 24 2/2 Twill 98 0.09 15.06

DCS5 0.72 22 25 Plain 118 0.1 17.1

DCS6 1.38 22 24 2/2 Twill 118 0.1 16.79

DCS7 0.9 20 25 Plain 84 0.1 14.5

DCS8 0.55 22 28 2/2 Twill 84 0.1 15.8

It can be observed that the measured values are approximately

equal to the standard values of network analyzer (MIL-STD 285). The

readings were taken in open space, thereby eliminating the potential

reflections that would introduce errors. The distance between the transmitter

and receiver is calculated and used in the system to avoid the ground

reflection, whereas the standard values were carried out in chambers which

exhibited accurate results. At low frequency ranges (in terms of MHz), the

measured EMSE were approximately equal to the standard measurements

were done in anechoic chambers but the small variations are due to the

influence of the environmental condition like human influence, atmospheric

conditions, complicity and dynamics of scenarios.

The T-tests were carried out for modified electromagnetic shielding

tester measurement result with standard values of network analyzer

(MIL-STD 285). Modified electromagnetic shielding tester measurement

results did not show any significant difference at 95% confidences level in the

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frequency range of 500 MHz to 8GHz, all the measurement values were

closer to that of standard values of network analyzer (MIL-STD 285). But, the

modified measurement values of samples shows, a significant difference at

95% confidence level in the frequency range of 8GHz to 12GHz which is

shown in the Table 4.2. It was also observed that the measured values showed

good EMSE than the network analyzer (MIL-STD 285) standard values in the

high frequencies of 8-12 GHz, due to narrow beam width of the parabolic dish

antenna before the radiation field begins. The standard values which were

given from the anechoic could not be subjected to low frequency sources.

Thus, in the case of low frequency sources like that of a mobile the anechoic

chambers had to be modified accordingly and hence it required capital

investment.

It was observed that the developed EMS measurement system can

be used to measure the EMSE of conductive textile materials with respect to

high frequency ranges of 500MHz-12 GHz.

The Table 4.2 compares the Electromagnetic Shielding

Effectiveness values of Copper Core Conductive textile materials between the

developed and standard measurement systems.

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Table 4.2 Shielding Effectiveness of Developed and Standard Measurement Systems

Frequency

*DC

S1

^CS1

Significance at 95%

confidence *DC

S2

^CS2

Significance at 95%

confidence *DC

S3

^CS3

Significance at 95%

confidence *DC

S4

^CS4

Significance at 95% confidence

550 MHz 30 31 No 24 24 No 22 23 No 24 25 No 600 MHz 39 40 No 35 35 No 33 34 No 38 39 No 650 MHz 46 48 No 36 38 No 37 38 No 47 47 No 700 MHz 35 35 No 30 31 No 32 33 No 31 33 No 750 MHz 43 45 No 42 42 No 43 45 No 44 44 No 800 MHz 49 49 No 37 39 No 46 46 No 44 47 No 850 MHz 52 54 No 44 43 No 52 51 No 54 54 No

1GHz 59 59 No 52 54 No 47 48 No 57 57 No 3GHz 33 34 No 18 18 No 21 22 No 26 29 No 6GHz 34 34 No 17 17 No 20 21 No 23 23 No 8GHz 22 22 No 14 14 No 12 13 No 17 17 No

9.04 GHz 20 16 Yes 18 14 Yes 15 11 Yes 20 16 Yes 9.14 GHz 19 15 Yes 18 14 Yes 15 11 Yes 19 15 Yes 9.22 GHz 19 15 Yes 17 13 Yes 14 10 Yes 17 13 Yes 10 GHz 19 17 Yes 12 10 Yes 11 10 Yes 18 17 Yes 11 GHz 18 15 Yes 14 12 Yes 15 11 Yes 18 15 Yes 12 GHz 17 16 Yes 10 8 Yes 10 8 Yes 15 12 Yes

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Table 4.2 (Continued)

Frequency

*DC

S5

^CS5

Significance at 95%

confidence *DC

S6

^CS6

Significance at 95%

confidence *DC

S7

^CS7

Significance at 95%

confidence *DC

S8

^CS8

Significance at 95% confidence

550 MHz 26 26 No 24 24 No 23 24 No 28 28 No 600 MHz 31 33 No 35 37 No 33 33 No 39 39 No 650 MHz 38 39 No 39 39 No 39 39 No 41 43 No 700 MHz 30 32 No 30 31 No 29 31 No 34 35 No 750 MHz 42 43 No 31 31 No 39 41 No 45 45 No 800 MHz 46 46 No 44 45 No 43 45 No 42 44 No 850 MHz 57 57 No 49 49 No 52 52 No 53 53 No

1GHz 51 53 No 45 46 No 48 49 No 47 47 No 3GHz 27 27 No 36 36 No 26 26 No 28 28 No 6GHz 22 23 No 22 22 No 20 21 No 27 27 No 8GHz 21 23 No 16 16 No 19 20 No 20 20 No

9.04 GHz 22 18 Yes 17 13 Yes 21 17 Yes 22 18 Yes 9.14 GHz 21 17 Yes 16 12 Yes 20 16 Yes 21 17 Yes 9.22 GHz 19 15 Yes 14 10 Yes 16 12 Yes 18 14 Yes 10 GHz 20 19 Yes 14 11 Yes 21 18 Yes 20 17 Yes 11 GHz 20 18 Yes 14 12 Yes 18 15 Yes 19 17 Yes 12 GHz 21 19 Yes 11 9 Yes 15 12 Yes 21 19 Yes

*DCS – Modified Electromagnetic shielding tester – EMSE value, ^ CS – MIL –STD 285 Test method by network analyzer – EMSE value

113

Table 4.3 Signal-to-Noise ratio for EMSE of EMS tester and its

significance

S.No. Case S/N ratio 1 Target is the best S/N (θ) = 10 log10 (τ2 / s2)

2 Small-the-better S/N (θ) = -10 log10 (yi2 / n)

3 Larger-the-better-EMSE of EMS tester

S/N (θ) = 10 log10 [(1/yi2) / n]

4 Binary scale (GO/NO-GO) S/N (θ) = 10 log10 (p/1-p) p= proportion of good products

Table 4.4(a) Controlled Variables

S.No. Control Factors \ Levels Levels

1 2 3 1 Sample Size (m) 0.3x0.3 0.5x0.5 1.0x1.0

2 Distance between sample and transmitting antenna (cm)

100 150 200

3 Distance between sample to Receiving Antenna(cm)

10 15 20

4 RH% 40 50 60

Table 4.4(b) Noise Factor

S.No. Noise Factors \ Levels Noise Level 1 Noise Level 2 1 Temperature N1(HIGH)-270C N2(LOW)-210C

2 Frequency N3 (HIGH)-9 GHz N4(LOW)-3GHz

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0

10

20

30

40

50

60

70

550

MH

z

600

MH

z

650

MH

z

700

MH

z

750

MH

z

800

MH

z

850

MH

z

1GH

z

3GH

z

6GH

z

8GH

z

9.04

GH

z

9.14

GH

z

9.22

GH

z

10 G

Hz

11 G

Hz

12 G

Hz

Frequency

Shi

eldi

ng E

ffect

iven

ess

dB

DCS1

DCS2

CS1

CS2

Figure 4.8 Comparison of DCS1, DCS2, CS1 and CS2

0

10

20

30

40

50

60

550

MH

z

600

MH

z

650

MH

z

700

MH

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750

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850

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1GH

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3GH

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6GH

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8GH

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9.04

GH

z

9.14

GH

z

9.22

GH

z

10 G

Hz

11 G

Hz

12 G

Hz

Frequency

Shie

ldin

g Ef

fect

iven

ess

dB

DCS3

DCS4

CS3

CS4

Figure 4.9 Comparison of DCS3, DCS4, CS3 and CS4

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0

10

20

30

40

50

60

550

MH

z

600

MH

z

650

MH

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MH

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1GH

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3GH

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6GH

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GH

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9.14

GH

z

9.22

GH

z

10 G

Hz

11 G

Hz

12 G

Hz

Frequency

Shie

ldin

g Ef

fect

iven

ess

dB

DCS5

DCS6

CS5

CS6

Figure.4.10 Comparison of DCS5, DCS6, CS5 and CS6

0

10

20

30

40

50

60

550

MH

z

600

MH

z

650

MH

z

700

MH

z

750

MH

z

800

MH

z

850

MH

z

1GH

z

3GH

z

6GH

z

8GH

z

9.04

GH

z

9.14

GH

z

9.22

GH

z

10 G

Hz

11 G

Hz

12 G

Hz

Frequency

Shie

ldin

g E

ffect

iven

ess

dB

DCS7

DCS8

CS7

CS8

Figure.4.11 Comparison of DCS7, DCS8, CS7 and CS8

116

4.6.1 Effect of Sample Size on EMSE

The Shielding effectiveness measurement results obtained using

this methods depend not only on the properties / parameters of the shielding

materials but also on the size of the test samples, the geometry of the test set-

up and the parameters of the source of electro magnetic radiation. In order to

analyse the effect of sample size on electro magnetic shielding effectiveness

during the measurement, the three samples size (0.3 x 0.3, 0.5 x 0.5 and 1.0 x

1.0m) are considered. The Figure.4.12 shows the effect of various control

factors on signal to noise of sample size, distance between the transmitted

antenna to sample size, distance between the sample to receiving antenna and

RH%. Design factors used in the experiments appeared to exercise strong

influence over electromagnetic Shielding effectiveness of conductive textile

materials in all the measurements. Signal to noise ratio, larger-the-better,

showed the highest value for the sample measured with the highest sample

size and distance between sample and transmitting antenna.

It can be also observed that the sample size 1.0 x 1.0m have good

electro magnetic shielding result than 0.3 x 0.3 and 0.5 x 0.5m sample sizes.

Similarly the ANOVA carried out for electro magnetic shielding effectiveness

of fabric showed the dominant effect of sample size and distance between

sample to transmitted antenna compared to distance between the sample and

receiving antenna and RH% in determining the electro magnetic shielding

effectiveness of the fabric samples during measurements. However, all the

design parameters appeared to influence the Electromagnetic shielding

effectiveness shown by analysis of variance in terms of factor effects and F-

values (Table 4.6).

117

4.6.2 Effect of Distance between Sample and Transmitting Antenna

on EMSE

The distance between the samples and transmitting antenna, also

influences the electromagnetic shielding effectiveness of conductive

materials. In order to study the effect of distance between the sample and

transmitting antenna, three distances (100 cm, 150cm and 200cm) are

considered for optimization of distance between transmitted antenna and

samples at same frequency with same antenna diameters. The Figure.4.12

shows that the distance between transmitting antenna and sample has

significant effects on electromagnetic shielding effectiveness of conductive

material. It can be observed that maximum electromagnetic shielding

effectiveness can be obtained in the distance of 150 cm between transmitting

antenna and samples than other two distances.

The ANOVA carried out for electromagnetic shielding

effectiveness of fabric showed the significant effect on distance between

samples to transmitted antenna (Table 4.6). It is also observed that as the

diameter of the antenna and frequency increases, the distance between

samples and transmitting antenna also increase.

4.6.3 Effect of Distance between the Sample and Receiving Antenna

on EMSE

Distance between sample and receiving antenna also influences the

electromagnetic shielding effectiveness of the conducting materials. in order

to study effect of the distance between sample and receiving antenna, three

distances (10cm, 50cm and 20cm) are considered for optimization of distance

between the sample and receiving antenna, measured at same frequency with

same antenna diameters. The Figure.4.12 shows that the distance between the

118

sample and receiving antenna has little effects on electro magnetic shielding

effectiveness of conducting materials. It can be observed that the maximum

electromagnetic shielding effectiveness can be obtained in the distance of 20

cm between the sample and receiving antenna than the other two distance.

The ANOVA carried out for electromagnetic shielding effectiveness of fabric

showed the neutral/ negligible effect on distance between samples to

receiving antenna (Table 4.6).

4.6.4 Effect of RH% on EMSE

To study the effect of RH % on Electro Magnetic shielding

Effectiveness, copper core yarn fabrics are considered. It was observed that

RH% has a little influence on Electro Magnetic shielding Effectiveness of

copper core yarn fabrics. Figure 4.12 shows the Effect of RH% of copper core

yarn woven fabrics, with an increase in RH%, a general increase in shielding

effectiveness of sample. It may be due to the water vapour and oxygen in the

atmosphere. The specific attenuation is strongly dependent on frequency and

temperature of the atmosphere.

The RH% attenuation increases with frequency and becomes a

major contributor in the frequency band. The rayleigh scattering occurs when

particles are very small compared to the wave length. Electro Magnetic waves

are absorbed in the atmosphere according to wave length. The two compound

of oxygen (O2) and water vapor are responsible for the majority of signal

absorption. These could be particle such as small specks of dust (or) nitrogen

and oxygen molecules. Mie scattering occurs when the particle are just about

the same size as the wave length of the radiations. The water vapour is

common cause of mie scattering which tend to affect longer wave length than

those affected by rayleigh scattering. Absorption is the other main mechanism

at work due to which electro magnetic radiation interacts with the atmosphere.

119

In contrast to scattering, this phenomena causes molecules in the atmosphere

to absorb energy at various wave lengths. Water vapour in the atmosphere

absorbs much of the incoming long wave infrared and short wave variations

(between 22mm and 1mm). Water vapour has a permanent dipole moment,

and so a strong pure rotations spectrum beginning at about 25µm and

extending with greater and greater absorptions to long wavelengths. The

ANOVA carried out for electromagnetic shielding effectiveness of fabric

showed the neutral/ negligible effect on RH% (Table 4.6)

Table 4.5 Effect of process parameters on signal-to-noise ratio of

maximization of EMSE

Expt

. No.

Sample Size (m)

Distance between

sample and transmitting

antenna (cm)

Distance between sample

and from Receiving Antenna

(cm)

RH

%

N1 N3

N2 N3

N1 N4

N2 N4

SN R

atio

(L

arge

r-th

e Be

tter

)

1 0.3x0.3 100 10 40 20 24 35 39 28.45

2 0.3x0.3 150 15 50 23 25 37 40 29.17

3 0.3x0.3 200 20 60 21 24 36 40 28.70

4 0.5x0.5 100 15 60 27 29 40 42 30.28

5 0.5x0.5 150 20 40 29 29 40 43 30.53

6 0.5x0.5 200 10 50 25 27 38 41 29.73

7 1.0x1.0 100 20 50 31 31 33 36 31.11

8 1.0x1.0 150 10 60 33 31 48 50 31.56

9 1.0x1.0 200 15 40 29 30 41 44 30.69

120

Table 4.6 Factor effects and F-Value of response Variables

Design factor

Deg

ree

of

free

dom

Fact

or e

ffec

t (%

)

F –

Val

ue

(Bef

ore

pool

ing)

Empt

y or

po

oled

F=

<1.5

F af

ter

pool

ing

Dom

inan

t or

signi

fican

t or

neut

ral/

negl

igib

le

Opt

imum

lev

el

Sample size 2

90 45 No 88

Dom

inan

t

1.

0 x

1.0

Distance between sample and Transmitting antenna (cm)

2 8 4 No 8

sign

ifica

nt

150

Distance between sample and Receiving antenna (cm)

2 1 0 Pooled ---

neut

ral/

negl

igib

le

---

RH% 2 1

1 Pooled ---

neut

ral/

negl

igib

le

---

Table 4.7 Confirmation Test with scaling and adjustments factors

Parameter

Original results

Confor-mation results

Significance at 95%

confidential level (Yes/No)

Scaling factor

Adjustment factors

550 MHz 30 31 No A3,B2 C3,D3 750 MHz 43 43 No A3,B2 C3,D3

1GHz 59 58 No A3,B2 C3,D3 3GHz 33 33 No A3,B2 C3,D3 6GHz 34 34 No A3,B2 C3,D3

9.22 GHz 19 18 No A3,B2 C3,D3 10 GHz 19 19 No A3,B2 C3,D3 12 GHz 17 17 No A3,B2 C3,D3

121

Ov Mean

27.00

27.50

28.00

28.50

29.00

29.50

30.00

30.50

31.00

31.50

32.00

0.3x

O.3

0.5x

0.5

1.0x

1.0

100

150

200 10 15 20 40 50 60

Ov-

Mea

n

Control Factor Levels

Elec

trom

agne

tic s

hiel

ding

eff

ectiv

enes

s S/

N R

atio

(LA

RG

ER-t

he-

bett

er) i

n dB

Figure 4.12 Effect of process parameters on EMSE of the fabric

4.6.5 Confirmation Test

Methodology advocated by Taguchi for optimization problems

involve typically four stages namely, problem formulation, Data collection /

simulation, factor effects analysis and confirmation test. The confirmation test

in the Taguchi methods supplements, assures validity of the results obtained

in the experimental design and orthogonal array selected in the study and

various levels of the design parameters and the introductions. Confirmation

test carried out with every set of optimum parameters for all the response

variable showed the closer result to that of original results and did not show

any significant difference at 95% confidential level. Table 4.7 shows the

values obtained in confirmation tests along with the original values for all

response variables, considered in the study.

Sample size (m) Distance between sample and

transmitting antenna (cm) Distance between sample

and receiving antenna (cm) RH%

122

4.7 CONCLUSIONS

The modified electromagnetic shielding tester can be used for

characterization of the electromagnetic shielding effectiveness of conductive

textile materials in the low frequency range to high frequency range (500 MHz to 12 GHz)

i. The modified electromagnetic shielding tester showed good

EMSE results than the Standard measurement system (MIL – STD-285) in the High frequency range of 8-12 GHz.

ii. The technique is simple and reproducible, and is neither labor-intensive nor capital-intensive. It can be used to generate statistical data.

iii. The modified electromagnetic shielding tester can be used to

estimate less than 1mm thickness of thin conductive textile materials

iv. All the design factors selected in the experiments

demonstrated pronounced effects on various parameters used in the assessment of electromagnetic shielding effectiveness of

conductive textile materials. The electromagnetic shielding

effectiveness of conductive textile materials were found to be influenced by sample size, distance between sample and

transmitting antenna, distance between sample to receiving

antenna and RH% selected in the experiments. The electromagnetic shielding effectiveness of conductive textile

materials were dominantly influenced by sample size and

significantly influenced by distance between sample and transmitting antenna.

v. The possible extension would be the inclusion of Anechoic Chamber which would further improve the accuracy of the

result in case of low frequency.