- 1 INTRODUCTION

CHAPTER 1

INTRODUCTION

Microwave technology owes its origin to the design and development of radar and

gained a tremendous progress during the World War 11. In the earlier stages of

development, the invention of microwave generators like klystron, magnetron etc.

opened the gigahertz frequency region of electro-magnetic spectrum to

communication engineers. Hence the major development especially comes in the

field of satellite communication. It can be seen that microwaves constitute only a

small portion of electromagnetic spectrum, but their uses have become increasingly

important in the material characterization for industrial, scientific and medical

applications.

Material characterisation is essential for the proper selection and

implementation of a substance when used in industrial, scientific and medical

applications. The dielectric parameters over a wide range of temperature on low

loss dielectrics are needed to assess their suitability for use in telecommunications,

dielectric waveguides, lenses, radomes, dielectric resonators and microwave

integrated circuit (MIC) substrates; and on lossy materials for estimating their

heating response in microwave heating applications. The dielectric data would also

be required on lossy ceramics for their use as microwave absorbers, lossy pastes for

the design of new food packages, for heating in microwave ovens and on biological

materials for diathermy. The measurement of the dielectric parameters will serve as

a tool for investigating the intermolecular and intramolecular mechanisms of

compounds. The dielectric data have also been used to estimate the amount of

moisture in wcxd, sand and agricultural products. The dielectric properties are

needed for the calculation of internal electric fields resulting from the exposure to

non-ionizing electromagnetic (EM) fields and are thus important in the development

of diagnostic and therapeutic medical applications of this energy and studies of

possible hazards of EM fields. Electromagnetic waves in the radio and microwave

frequencies are effectively used in the treatment of hyperthermia of tumors and other

disorders.

In the industrial, scientific and medical (ISM) band which covers from few

kHz to several GHz, the range of relative permittivity (q) for the materials of

interest is very large. The real part of relative complex permittivity < can vary

from a value of about two to a value of a few thousand while the imaginary part E",

of complex permittivity from a small fraction to a large value of about hundreds for

some materials.

It is found that any single method of complex permittivity measurement is

not suitable over such a wide range of frequency and complex permittivity.

Besides, different materials may be available in different physical states such as gas,

Liquid, powder, paste, solid etc. Even in the case of solids, they are available in

different shapes such as flat sheet, grains and hard-to-grind arbitrary geometrical

shapes such as rocks. Due to the variations in the values of E' and E", frequency,

physical state and shape, different materials need different techniques for complex

permittivity measurements.

The methods for measuring the complex permittivities of materials can be

broadly classified into two categories:

I . Frequency domain methods

2. Time domain methods

The frequency domain methods are well established and have been in use for

over the past 50 years. But the time domain methods are recently developed.

However, time domain methods are growing in acceptance as these provide quick

measurement to estimate the dielectric response of a material over a wide frequency

range. Nowadays, this range covers beyond 10 GHz with the presently available

equipment. Figure 1.1 shows the various categories of methods for complex

permittivity measurements.

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Fiqure 1 . 1 . Classification of dielectric lneasurement metMs.

1.1 FREQUENCY DOMAIN METHODS

First of all, various categories of frequency domain techniques are

considered.

1.1.1 Free Space Methods

Free space methods are based on the optical type measurements. (Figure

1.2) Generally these methods were considered suitable for frequencies in the

millimeter wave region. Both reflection and transmission methods have been used.

A major drawback of these methods was the requirement of a large sample to avoid

diffraction effect around sample edges for performing measurements in the

centimeter wave region. However it has been shown that with precision horn lens

antennas whch have better far-field focussing ability, it is possible to make accurate

free space measurements in the low frequency bands (below X-band).

Compared to other methods, free space methods have the following

advantages.

(a) These provide contactless methods for the complex permittivity

measurement. Hence they are more suitable for high temperature

measurements.

(b) In the shorted waveguide method and cavity method, it is necessary to

machine the sample so as to fit exactly in the waveguide or cavity. During

the sample preparation, it is very difficult to machine the sample exactly to

the required dimensions. This requirement limits the accuracy of

measurement of hard and brittle materials. Free space methods do not suffer

from the above limitations, because no sample preparation may be required,

if the material is available as a sheet.

HP 8341 B HP 8510 B

SWEEP OSCILLATOR NETWORK ANALYSER

HP 9000/300 S-PARAMETER

INSTRUMENTATION COMPUTER

DIELECTRIC SHEET

TRANSMITTING A W E NNA

-

RECEIVING

-

Figure 1.2 Free space experimental set-up for complex permittivity measurement

Different free space methods are thoroughly discussed by Musil and Zacek

I l l . Ghodgaonkar et al. 121 have given the measurement set-up using network

analyser. They have reported results on some materials that agree very well with the

previous resulls obtained using more established methods.

1.1.2 Transmission Line Methods

In transmission line methods, a sample of the dielecvic material is placed

either between the outer and inner conductors of a coaxial line or inside a

waveguide. The sample when placed at the end of the waveguide may be terminated

with either a short or some other known impedance. The complex permittivity of

the material is then determined by the measurements of the line without the sample

and with the sample. Most important and widely used method was that developed by

Roberts and Von Hippel 13) (Figure 1.3). In this method, a waveguide is

terminated by the sample in physical contact with a short circuit. This niethod has

been extensively used in the past and is recommended as a standard method by the

American Society for Testing and Materials (ASTM). Nelson (41 has made the

measurement more convenient by writing a computer programme which exactly

computes 2 for high loss as well as low lass materials. This programme is

applicable to measurements made in rectangular and circular waveguides as well as

to coaxial lines Certain corrections are also included in the programme for the

influence of the slot in the slotted-waveguide section, and the difference in velocity

of propagation in air and vacuum. Even if these corrections are ignored, it is

claimed that the error in computed results is only 0.5% for t' and 2 - 3% for e".

Using the standard waveguide components, waveguide methtds have been used up

to 140 GHz. Beyond this frequency, waveguide dimensions become very small and

transmission losses very high 151.

PROBE

7

SBORT C I R C U I T

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DIELECTRIC GENERATOR

Figure 1.3 Transmission line method

Various transmission line methods with different designs for holding the

sample have been discussed in the literature and the modification is still going on.

For example, a two port circular cell which requires a sample of cylindrical shape

machined with a hole in the middle to fit hetween the imer and outer conductors of

the cell has been used by Nicolson and Ross [6] , and Weir 171. In another

configuration the sample is placed between a coaxial cable and either a short or open

[8]. Taherian et al. [9] have discussed the use of a two port cell, for measurements

at 1.1 GHz on brine solution, which has a symmetric cylindrical sample holder

comected to a coaxial line section at each end. This cell is different from those

used earlier such that the sample is in the shape of a solid circular disc without a

hole in its center. Scott [ lo] has proposed a new type of fixture in which very little

sample preparation is required for planar materials. In waveguide methods, at low

frequencies (<2GHz) a large sample and big temperature controlled oven are

required. Due to these demerits, the waveguide methods are not considered very

suitable for ISM applications.

1.1.3 Automatic Network Analyzer Methods

In certain aspects, automatic network analyser (ANA) methods can also be

considered as transmission line methods because the sample is held in a transmission

line and helecmc constant is determined by measuring the reflection andlor

transmission coefficient. The scattering parameters are measured with a vector

network analyser (HP 8510 type) consisting of a synthesized sweeper and an S

parameter test set. A PC (90001300 series instrumentation computer) can be used

for automation, data acquisition, printing of S-parameters etc. Even though these

methods have similarity with transmission line methods, they can be considered as

separate category because ANA methods have emerged as powerful modem methods

with which dielectric measurements can he canied out over a wide range of

frequencies in single measurement. A typical measurement set-up is given by

Ghodgaonkar ( 1 1 1 and details are given by Somlo 1121 (Figure 1.4).

HP 8341 B R P 8510 B

SWEEP OSCILLATOR NETWORK ANALYSER

HP 8514 B

S-PARAMETER T E S T S E T I NSTRUIIEUL'ATION

COMPUTER

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F i g u r e 1.4 A u t o m a t i c n e t w o r k analyeer m e t h o d

In ANA methods, various measurement errors are significantly reduced by a

calibration procedure. The most general procedure uses a through path connection,

matched termination, a short and an open (TMSO) 1131. With wide-band sweep

oscillators, and under the control of a computer, dielectric measurements have been

done accurately by ANA's over a wide range of frequencies and materials 114-161.

In the case of broad band measurements, the TMSO calibration procedure

suffers in accuracy. Therefore different researchers have suggested the use of other

methods of calibration. In one such method an open ended coaxial probe (sensor) is

immersed in a material of known permittivity [17]. A major development in this

area is the use of open ended waveguide or coaxial line. This is a non-destructive

method of dielectric measurement in which the problem of sample preparation is

greatly reduced. Gardiol et al. [18,19] was the first to propose this method using

rectangular waveguide. Athey et al. 1171 have suggested the use of open ended

elliptical probe instead of a circular coaxial probe. They have pointed out that (i)

fabrication of such a probe is easier and (ii) the sensitivity of measurements is

improved. The open ended transmission line method is quite suitable for lossy

dielectric materials, as the infinite half space condition assumed in the analysis can

he. achieved more readily than with medium or low loss dielectric materials.

However, the ANA methods suffer from the following disadvantages.

( i ) In the case of hard and brittle solids, it is impossible to perfectly fit the

sample hetween outer and inner conductor.

(ii) Unless the sample is filling the entire co-axial line, it is very difficult to

define a reference plane which is necessary for the calibration of the

measurement system.

(iii) The presence of the sample may generate higher order modes in the

transmission line.

(iv) The initial capital cost of Hewlett Packard type ANA set-up is very high.

Therefore, methods are more useful for applications where data is required

over a wide band of frequencies and others are not convenient.

1.1.4 Cavity Perturbation Techniques

It was Bethe and Schwinger (201 who proposed cavity perturbation technique

for the first time. A material sample, when introduced into a cavity, alters its

characteristic parameters namely the resonance frequency (fr) and the loaded quality

factor (Q,). The changes depend upon the real and imaginary parts of the complex

permittivity or permeability and on the geometry of the sample and cavity. (Figure

1.5). In the past, most researchers have preferred simple geometrical shapes like

cylindrical 121) and rectangular cavities 1221. In such cases, the perturbation

equations are easier to derive. However, cavities having strong fields in some

regions have also been used to increase the sensitivity of measurement. For example

Sen et al. 1231 devised a re-entrant cylindrical cavity for complex permittivity

measurement (Figure 1.6).

SAMPLE

COUPLING PROBE

Figure 1.5(a) Cross-sectional view of cylindrical transmission type cavity resonator

SAMPLE -

PROBE

Figure 1.5( b ) Cross-sectional view of cylindrical reflection type cavity resonator

SAMPLE

PROBE

Fiqure 1 .6 R e - e n t r a n t c a v i t y .

1.1.5 Resonant Cavity Methods

ln the past few decades, the closed cavity resonator operated in the low-

attenuation TEol mode, which consists of a hollow metallic cylinder of circular

cross-section terminated with two short-circuited plates, has been used to measure

the complex permittivity of materials (especially low-loss solids) 124). Reviews of

literature on dielectric measuring methods including cavity resonator techniques have

been given by Bussy 1251, Lynch (261, Chamberlain and Chanhy [27] and more

recently by Birch and Clarke (281.

Resonant cavity methods can be classified into two categories.

1 . Length Tuning Method (LTM)

2. Frequency Tuning Method (FTM)

in the case of length tuning method, the real and imaginary parts of complex

permittivity are obtained at a fixed frequency, from the measured changes of the

length and of the Q-factor of the cavity at resonance. In order to tune the

resonator, the short-circuit can be moved in the axial direction to vary the resonator

length by a bearing and positioning device (Figure 1.7).

In frequency tuned method (FTM), no movable mechanical parts are used.

Here the resonator is terminated only by the two fixed short-circuits. At constant

resonator Length, the frequency is altered to tune it to resonance. The real and

imaginary parts of complex permittivity are measured from the changes of

resonance frequency and Q-factor of the cavity 1291.

MICROMETER HEAD

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DETECTOR PROBE

EXCITER PROBE

DIELECTRIC UNDER TEST

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Figure 1.7 Length tuning resonant cavity methods

Figure 1 . 8 Cross - sec t iona l view of t h e c a v i t y

Boifot 1301 devised a coaxial dielectrometer for broad band complex

permittivity measurement (Figure 1.8). In the resonant cavity method suggested by

Seaman et al. [31], the plunger position is adjusted for resonance with and without

the sample. Then the change in position gives dielectric constant and loss tangent is

found from the change in resonant current or from the width of the resonance curve.

In another method, the position of the dielectric sample can be adjusted by a

micrometer plunger. Power is coupled into or out of the cavity through two

coupling loops. Difficulty due to the variation of the minimum position may be

eliminated by using two travelling probes instead of coupling loops. In some better

methods 132,331 the two loops are located at the short circuit and hence are always

at a node. Then the length of the transmission line is changed, but the distance from

the sample to one end remains constant.

1.2 TIME DOMAIN METHODS

The time domain methods are modem techniques for the material

characterization. They are becoming popular very fast as wide band dielectric

measurements are required in many cases such as characterization of components

and in determining dielectric relaxations, in materials like muscle tissue, which

range from 100 Hz to 100 GHz. Till late 1960's, dielectric measurements on

materials were mainly carried out by means of frequency domain methods.

However, since then, due to the availability of fast sampling oscilloscopes and

tunnel diode step generators with picoseconds rise time, it has become possible to

use time domain techniques in the microwave region (Figure 1.9). Time domain

methods offer three distinct advantages over frequency domain methods. First, time

domain techniques are inherently of wide band as compared to the traditional

frequency domain methods. In the case of waveguide techniques, at least four

waveguide systems are required to measure the dielectric properties over 1 GHz to

10 GHz range, along with samples of different shapes and sizes compatible with the

waveguide dimensions for different bands. Second, the time required for

performing a time domain measurement valid over a wide range of frequencies is

considerably less than that required for multiple frequency domain measurements.

Hence time domain methods are more suitable for wide band measurements on

materials whose characteristic may not remain constant over a long period of time

(for example, biological materials). Thud, the instrumentation required for time

domain methods requires less capital investment as compared to an automated vector

network analyzer (ANA).

Time domain methods can be broadly classified into two categories.

(a) Reflection methods

(b) Transmission methods

Initially, reflection methods were used in electrical engineering for locating

the faults in the cables. These techniques are called "Time Domain Reflectometry"

(TDR) 1341. The measurement of reflection coefficient and the time delay gives the

type and distance of the fault from the secondary end. These faults could be a

break in the cable conductor, cable shield, or perhaps water in the cable. Van

Gemert 1351 used transmission methods for dielectric measurements and he discussed

the different variations on the time domain reflection and transmission methods.

Most researchers have preferred reflection methods to transmission methods. Of the

few who have used the transmission method, only Gestblom et al. 1361 have given

quantitative experimental results.

GENERATOR

SAMPLE HOLDER

Figure 1 . 9 Schematic representation of the experimental set-up for time domain tranamiasion measurement method

The reflection and transmission time domain techniques are collectively

called "Time Domain Spectroscopy" (TDS). Time domain measurement methods

have also been used for performing broad hand measurements on filters, directional

couplers, amplifiers and studying the electronic system response to lightning and

EMP [37].

Although TDS is hasically a time domain technique, the data is usually

converted to the frequency domain using a Fast Fourier Transform (FFT) analysis.

Fellner-Feldegg [38] suggested time domain data directly to obtain G , r, and the

dielectric relaxation time. Whittingham [39] and Van Gemert [40] pointed out

certain conceptual difficulties with that approach and indicated that only % and E,

can be accurately determined directly from the time domain data. Later Cole 1411

gave an analysis to compute complex permittivity directly from the time domain

waveforms. But this method can give unacceptable errors for polar materials.

Cormack et al. 1421 have suggested the use of Extended Function Fast Fourier

Transform (EF-FFT) for computation of the spectral components of an infinite

duration step-like waveform. Cormack has shown that the EF-FFT technique is

more accurate than the conventional techniques. Nozaki et al. 1431 have reported

that with the recent improvement it is possible to conduct precise measurements

from about 100 kHz to 25 GHz using this technique.

1.3 MISCELLANEOUS METHODS

For complex permittivity measurements, certain new techniques are

discussed in the literature (44-461. Scott et al. [44] suggested a method based on the

measurement of the input impedance of an antenna over a range of frequencies in a

standard medium of known permittivity like air. The antenna is then immersed into

the medium whose permittivity is to be determined and the impedance of the antenna

is measured over a range of frequencies. The measured impedance is then used to

calculate the complex permittivity of the material. This method has been used over

a frequency range of 50 MHz to 10 GHz and looks more suitable for liquids.

In the method described by Shimin 1451, a rectangular microsbip antenna has

been used for measuring the dielectric constant of thin slab substrates. Neelkanta et

al. 1461 suggested another method in which the sample is put on a planar conducting

surface in the form of a thin film. This structure is excited by a monopole and the

change in directivity due to the inclusion of the dielectric film on the conducting

sheet gives the dielectric constant of the film material. Bernard et al. 1471 suggested

a microstrip ring resonator technique for dielectric constant measurement. This

method is based on the fact that the effective permittivity will change if the alumina

or air boundary is modified by placing a dielectric material above the alumina

substrate, thereby changing the resonant frequency of the ring. The variational

calculation of the line capacitance is used to compute the effective permittivity of the

multilayer microstrip like ring resonator and hence the resonant frequency for

various test materials.

1.4 MOTIVATION OF THE PRESENT STUDY

Among the various methods discussed in the previous sections, the cavity

perturbation techniques are more accurate and precise than other methods. For I to

10 GHz range, rectangular or circular waveguide cavities are generally employed.

Earlier, the measurements were carried out at single frequency due to non-

availability of microwave sources covering wide frequency bands. With the

advancement of microwave technology, the synthesized sweep generators and vector

network analysers are available. Also these instruments can be interfaced with series

insbumentation computers. Although different waveguide cavities are required for

different bands, it is possible to measure the dielectric parameters at different

frequencies in single band with the help of modem equipments mentioned above.

So the major aims of the present work are development of modified cavity

perturbation techniques and their applications to material characterization.

1.5 BRIEF SKETCH OF PRESENT STUDY

The scheme of the work presented in this thesis is given below:

A review of important research work done in the field of cavity perturbation

techniques and its applications is presented in Chapter 2. Special emphasis is given

on the complex permittivity measurements.

In Chapter 3, the details of the design and fabrication of various resonators

employed for measurements are discussed.

In Chapter 4, the first part describes the theoretical analysis for the

measurements of complex permittivity and complex permeability of materials. The

second part deals with the measurement procedures.

The experimental results are discussed in detail in Chapter 5

The conclusions drawn from the sadies and further scope of the present

work are discussed in Chapter 6.

The experimental results of a few investigations in the related fields done by

the author are included in the thesis as four appendices. Appendix A deals with the

application of a cross-iris coupling for enhancing the loaded Q-value of rectangular

waveguide cavities. in Appendix B, the design and development of a Triple Comer

Reflector antenna are discussed. Appendix C describes the investigation of an

asymmetric hollow sectoral dielectric horn antenna. Appendix D deals with the

synthesis and characterisation of a ceramic dielectric resonator.