NAT'L INST. OF STAND & TECH

AlllDb M1S122

NI8T PUBLICATIONS NIST Special Publication 260-159

Relative Permittivity and Loss

Tangent Measurement Using the

NIST 60 mm Cylindrical Cavity

Michael D. Janezic Jolene D. Splett

Kevin J. Coakley

Raian F. Kaiser

John H. Grosvenor

Qc

\oo

VLSI

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\At Boulder, CO 80305 "Some elements at Boulder, CO
NIST Special Publication 260-159

Relative Permittivity and Loss

Tangent Measurement Using the

NIST 60 mm Cylindrical Cavity

Michael D. Janezic Raian F. Kaiser John H. Grosvenor

Electromagnetics Division Electromagnetics and Electrical Engineering Laboratory

Jolene D. Splett Kevin J. Coakley

Statistical Engineering Division Information Technology Laboratory

National Institute ofStandards and Technology

Boulder, Colorado 80305

September 2005

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration

Michelle O 'Neill, Acting Under Secretaryfor Technology

National Institute of Standards and Technology

William A. Jeffrey, Director
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such

identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 260-159

Natl. Inst. Stand. Technol. Spec. Publ. 260-159, 65 pages (September 2005)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2004

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 —Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
Contents

1 Introduction 2

2 Cylindrical Cavity Specifications 4

3 Circular-Cylindrical Cavity Theory 7

3.1 Circular-Cylindrical Waveguide 7

3.2 Empty Circular-Cylindrical Cavity 1

3.2.1 Electric and Magnetic Fields 11 3.2.2 T E0lp Mode Resonant Frequency 12

3.2.3 TEolp Mode Quality Factor 13

3.3 Sample-Loaded Circular-Cylindrical Cavity 15

3.3.1 Electric and Magnetic Fields in the Air-Filled Region 15

3.3.2 Electric and Magnetic Fields in the Sample-Filled Region 16

3.3.3 TE Mode Resonant Frequency 16

3.3.4 TE0lp ModeQualityFactor 17

4 Sample Specifications 19

4.1 SRM Sample Selection 19

4.2 Sample Dimensions 19

4.2.1 Sample Diameter 19

4.2.2 Sample Thickness 20

4.3 Sample Cleaning 20

5 Measurement System Characterization 21

5.1 Sample Thickness Estimation 21

5.2 Environmental Variables 25

5.2.1 Speed of Light 25

5.2.2 Temperature 25

5.2.3 Relative Humidity 25

5.2.4 Barometric Pressure 26

5.3 Cylindrical Cavity Length and Radius 27

5.4 Resonant Frequency and Quality Factor 29

5.5 Cylindrical Cavity Conductor Losses 30

iii
6 Permittivity and Loss Tangent Measurements 32

6.1 Measurement Procedure 32

6.1.1 Speed of Light in Laboratory 32

6.1.2 Sample Thickness 32

6.1.3 Cylindrical Cavity Length and Radius 32

6.1.4 Empty Cavity Measurement 33

6.1.5 Sample-Loaded Cavity Measurement 33

6.2 Relative Permittivity and Loss Tangent Results 34

6.3 Measurement Intercomparison 35

6.4 Broadband Measurements 36

7 Uncertainty Analysis 40

7.1 Monte Carlo Simulation Model 40

7.1.1 Random Errors 40

7.1.2 Systematic Errors 42

7.2 Observational Repeatability Study 45

7.3 Relative Permittivity and Loss Tangent Uncertainty 52

8 Measurement Quality Assurance 56

9 Conclusion 59

10 References 60

iv
Relative Permittivity and Loss Tangent Measurement

using the NIST 60 mm Cylindrical Cavity

Michael D. Janezic*, Jolene D. Splett*, Kevin J. Coakley*, Raian F. Kaiser"1", and John H. Grosvenor^"

^Electromagnetics Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Boulder, CO 80305 USA

* Statistical Engineering Division

Information Technology Laboratory

National Institute of Standards and Technology

Boulder, CO 80305 USA

In order to develop a dielectric Standard Reference Material (SRM), a measurement

system for measuring the relative permittivity and loss tangent of dielectric materials

is presented. To achieve the necessary level of measurement accuracy, we selected the

circular-cylindrical cavity method. Expressions for determining the relative permittivity and loss tangent are derived from theoretical models of an empty and sample-loaded

circular-cylindrical cavity. We describe the circular-cylindrical cavity's specifications and the detailed measurement procedure we employed. We present a comprehensive

uncertainty analysis for both the relative permittivity and loss tangent as well as a measurement assurance plan for ensuring the integrity of the measurement system.

Key words: circular-cylindrical cavity; dielectric constant; loss tangent; relative permittivity; resonator.

1
1. Introduction

For over fifty years, researchers have measured the relative permittivity, e'r , and loss tangent, tan S, of low-loss dielectric solids at microwave frequencies with circular-cylindrical

cavities [1-4]. The circular-cylindrical cavity method remains popular today because the measurement accuracy is high over a wide range of relative permittivities and loss tangents, the cavity is simple to construct, and the dielectric samples are easily machined. In addition, the measurement theory used to calculate the sample's permittivity and loss tangent is derived from a relatively simple boundary-value problem. Because of these advantages, the NIST Electromagnetic Properties of Materials program chose this method to certify the rela- tive permittivity and loss tangent of low-loss dielectric materials for distribution as Standard

Reference Materials (SRMs).

In this paper we outline the steps taken to characterize the dielectric properties of a

low-loss dielectric material. In particular, using the circular-cylindrical cavity method, we

characterize the permittivity and loss tangent of cross-linked polystyrene samples at 10 GHz. We begin by describing the cylindrical cavity fixture, including details about its construction,

in the Cylindrical Cavity Specifications section. We outline the necessary measurement theory in the Cylindrical Cavity Theory section. We first derive the electromagnetic fields for a cylindrical waveguide and then extend this theory to the cases of the empty and sample-loaded cylindrical cavity. Solving the boundaryvalue problems for these two cases, we find expressions for the resonant frequency and quality

factor of the cylindrical cavity with and without sample present, plus equations for calculat- ing the dielectric sample's relative permittivity and loss tangent. The Sample Specifications section discusses the criteria used for selecting cross-linked polystyrene as the first SRM material and includes details regarding the specific specimen

sheet we ordered. In order to reduce the overall measurement uncertainty, we also provide

guidance for selecting the sample diameter and thickness. Finally, we outline the cleaning

procedure used for the samples.

In the Measurement System Characterization section, we specify how we determined the

variables necessary to determine the sample's permittivity and loss tangent. The variables include sample thickness, environmental variables used to calculate the speed of light in the

laboratory, length and radius of the cylindrical cavity, resonant frequency and quality factor, and the conductive metal losses of the cylindrical-cavity endplates and wall. The Permittivity and Loss Tangent Measurements section outlines the step-by-step measurement procedure used to calculate the sample's permittivity and loss tangent. We present measurement results at 10 GHz for the 18 cross-linked polystyrene samples we considered. In order to check the consistency of the cylindrical-cavity measurements, we provide permittivity and loss tangent results for cross-linked polystyrene made with four other measurement

methods. Finally, we provide additional permittivity and loss tangent data from 8 to 11 GHz

to show how the dielectric properties of cross-linked polystyrene vary at frequencies near 10

2
GHz.The Uncertainty Analysis section addresses how we estimated the uncertainty of both

relative permittivity and loss tangent measurements. We list the possible random and sys- tematic measurement errors and describe the Monte Carlo model we used to simulate the

effects of these errors on the sample's relative permittivity and loss tangent. In order to quantify the stability of the measurement process, we outline our long-term repeatability

study of cross-linked polystyrene as well as a similar study involving a single-crystal quartz check standard.

Finally, the Measurement Quality Assurance section summarizes the procedures we will employ to ensure the integrity of future cylindrical-cavity measurements. Specifically, these

include the development of measurement control charts for two of the cross-linked polystyrene

samples. Measurements of these two samples prior to every measurement will enable us to detect problems related to the cylindrical-cavity measurement procedure or measurement

software.

3
2. Cylindrical Cavity Specifications

The cylindrical cavity resonator shown in Figure 1 is nominally 450 mm long and 60 mm

in diameter. The cavity is primarily composed of a helically wound cylindrical waveguide

terminated by two gold-plated, metallic endplates. As in References [2] and [3], we employ helical waveguide to attenuate some of the undesired resonant modes while leaving the TEoin resonant modes unperturbed. Our particular helical waveguide consists of two cop- per wires embedded in epoxy and surrounded by a fiberglass cylinder epoxied into a steel pipe. Although we must take into account the increased surface resistance of the cylindrical

waveguide walls due to the epoxy's dielectric losses in order to accurately measure the loss tangent, the benefit of eliminating many of the unwanted resonant modes outweighs this small inconvenience. A layer of heat shrink was added to the steel pipe to seal it from the water jacket.

The two gold-plated endplates that terminate the cylindrical cavity are optically polished.

One endplate is supported by an upper flange, shown in Figure 2. The bottom endplate, with

a diameter slightly smaller than that of the cylindrical waveguide, is supported by the tuning

endplate assembly shown in Figure 3. Using a precision dc motor connected to a micrometer

positioned below the endplate, the cylindrical cavity length can be varied by approximately one inch. The bottom endplate is connected to a yoke contained within the tuner assembly. As the motor drive moves the bottom endplate up and down within the cylindrical cavity, an electronic probe measures the change in the bottom endplate's position.

Figure 1. NIST 60 mm cylindrical cavity resonator.

4
Figure 2. Upper flange of cylindrical cavity.

Figure 3. Tuning endplate assembly of cylindrical cavity.

To allow placement of a dielectric sample in the cylindrical cavity, the tuning endplate

assembly is lowered so that the bottom endplate is positioned below the cylindrical waveguide

section, as shown in Figure 4. The sample is then placed on the bottom endplate and the tuner assembly is raised back into position so that both the sample and bottom endplate

are positioned inside the cylindrical waveguide section. In order to ensure that the bottom

endplate remains parallel to the top endplate, the tuner assembly travels on three hardenedsteel guide shafts.

5
Figure 4. Dielectric sample on bottom endplate of opened, lower tuner assembly.

Located near the upper flange are two coupling loops that extend into the cavity from two circular holes located on opposite sides near the top of the cylindrical cavity. In order to

excite a resonance in the cylindrical cavity, each coupling loop is connected to an automatic network analyzer via a coaxial transmission line. The extent to which the coupling loops

protrude inside the cylindrical cavity is varied in order to change the degree of cavity coupling and the amplitude of the resonance. This is an improvement upon the design reported in Reference [4], where the cavity was excited by rectangular irises fed by X-band waveguide

feeds. In that design, the coupling level was approximately -30 dB, resulting in significant coupling losses. With the new design, we are able to lower the coupling level to a point where the coupling losses can be neglected.

The dimensions of the cylindrical cavity must be well characterized in order to achieve high-accuracy measurements of permittivity and loss tangent. To ensure the stability of the

cavity dimensions despite temperature variations in the laboratory, the cylindrical cavity is surrounded by a water jacket connected to a water bath that controls its temperature to within ±0.1 °C.

G
3. Circular- Cylindrical Cavity Theory

Before outlining the procedure for calculating the sample's relative permittivity and loss tangent, we examine the theory of an empty and a sample-loaded circular-cylindrical cavity.

Specifically, we first derive the electric and magnetic fields of TEmn modes in a circular- cylindrical waveguide. We then extend this theory to the resonant frequency and quality

factor of the TEq\p mode for an empty circular-cylindrical cavity. These expressions are important for calculating the length and diameter of the cavity, as well as measuring the

surface resistance of the cavity walls. Next, we consider the case of a sample-loaded cylindrical cavity and derive equations for resonant frequency and quality factor of the TEq\v mode

used to solve for the sample's relative permittivity and loss tangent in Section 6.

3.1 Circular-Cylindrical Waveguide

For a forward-traveling wave in a circular-cylindrical waveguide the magnetic and electric

fields take the form H(p, cj>, z) = [hr(p, 0) + hz (p, 0)] exp(-72) (1) and

E{p, 0, z) = [eT(p, 0) + ez {p, 4>)} exp(-72), (2) where hr and &r are the transverse magnetic and electric field components, hz and ez are the

longitudinal magnetic and electric field components, 7 = a+jP is the propagation constant, a is the attenuation constant, and (5 is the phase constant. For a lossless waveguide, eqs.

(1) and (2) reduce to H(p, 0, z) = [hr(p, 0) + hz (p, 0)] exp(-^z) (3) and

E(p, 0, z) = [eT (p, 0) + e 2 (p, 0)] exp(-j^). (4) Since the longitudinal electric field component e z is zero for TEmn modes, we can determine

the transverse electric and magnetic field components from the longitudinal magnetic field hz

.

Longitudinal Magnetic Field Component hz

If we assume that the electromagnetic fields have exp(jcot) time dependence, then the wave

equation for a TEmn mode is [V2 + u

2 p0 eoea -P2}hz (p^)=0, (5) where u is the frequency in radians, po is the permeability of free space, eo is the permittivity

of free space, and eais the relative permittivity of the air inside the waveguide. Expressed

7
in circular-cylindrical coordinates, eq. (5) becomes

where

k

2 = uj2voe0 ea - (3 2

. (7) Solutions for hz are found using the method of separations of variables. We let hz (pr cf)) = R(p)$(<j>) (8) and substitute this into eq. (6) to obtain

1 d ( dR\ 2;2 1 d

2 $

Since p and z are independent variables, both sides of eq. (9) must equal an arbitrary

constant: R%{plTp)

+pk^-*W2 =^ (10) where is the separation constant.

Solution for <£(</>) The solution to the differential equation + k

2 <P = 0 (11)

$(0) = A0 cos(/c0 0) + B0 sin(A^), (12) where Aq and Bq are constants. Since the fields must be periodic in 0, we can apply two

boundary conditions. First, enforcing

$(tt) = #(-7r), (13) we find B0 = 0. (14) With the second boundary condition—I -—I (15) we find

kcj) = m, where m = 1, 2, 3, . . . . (16) Thus, the solution to eq. (11) is = Ao cos(m0). (17)

8
Solution for R(p)

The solution to the differential equation

1_ _d / dR

R

P dp V dp + P

2 k2

c = 4 (18)

R(p) = AyJm {kcp) + B.Ymihp), (19) where and B\ are constants, JTO is the Bessel function of the first kind of order m, and Ym is the Bessel function of the second kind of order m. The fields must be finite inside the

circular waveguide, so from the condition that \R{p-*0)\ is finite, (20) we find = 0. (21) We also assume that the metallic walls of the circular waveguide are made up of perfect conductors, so that ^[*(P)1U = 0, (22) which yields

fcc = ^, (23) a where a is the radius of the waveguide and jmn is the nth zero of J'm {kcp). The solution to

eq. (19) is therefore R(p) = A1 Jm(kcP). (24) Substituting eqs. (17) and (24) into eq. (8) we have hz (p, (f)) = AJm (kc p) cos(ra0). (25) Transverse Electric and Magnetic Field Components

With the expression for hz in eq. (25), we solve for the tranverse electric field components

using (u2 p0 e0 €a - P

2 )eT {p, 0) = jup0 az x Vhz (p, (p). (26) Substituting eq. (25) into eq. (26) we get

&pM) = A^-smim^Jmihp), (27)

A jup0 , A—^-cos{m(p)

TTL —Jm{kc p) - kc Jm+i{kcp) P

(28)
In the same way, we find the transverse magnetic field components using

(u;

2 /wl - P2 )hT {pA) = -jPVhz {p,4>).

Substituting eq. (25) into eq. (29) we get jB

hp (p,4>) = -A—^cos(m(p)

j 0 vrt h${p,4>) = A——sm(m(f))JTn (kcp). k p

Jm{kc p) ~ kc Jm+i(kcp)

(29)

(30)

(31)

Electric and Magnetic Fields Knowing the electric and magnetic field components, we can write expressions for the magnetic and electric fields of a TEmn mode in a circular-cylindrical waveguide:

where

Ep {p, 0, z)

E,{p,<l>,z),

Hp (p, <j>, z)

kc P

A—r£-cos(m(f)) Jm{kcp) — kc Jm+\(kcp)

-Jm{kc p) ~ kc Jm+1 (kcp)

j(3 -A— cos(m0)

kc j (3 Tfl A—— sin(m0) Jm (/cc p) exp(-j/3z), kc P

exp(-jpz),

exp{-j(3z),

H^pA.z)

Hz {p,<j>,z) = AJm (kcp)cos(m(f))exp{-j/3z),

k] = u

2 p0 ^'a ~ ft ^ _ Jmn

(32)

(33)

(34)

(35)

(36)

(37)

(38)

(39)

10
coupling loops

L

1 * (P) 2a

*

Figure 5. Empty cylindrical cavity.

3.2 Empty Circular-Cylindrical Cavity

3.2.1 Electric and Magnetic Fields

In a circular-cylindrical cavity, shown in Figure 5, the two ends of a circular waveguide are terminated with conductive plates. The boundary conditions along the waveguide walls are the same as those of the circular waveguide, and the radial and circumferential variations in the electric and magnetic fields remain the same. However, since there are standing waves in the cavity instead of a traveling wave propagating along the axis of the circular waveguide, we must modify the z-dependence of the electric and magnetic fields. If we assume a standing wave inside the cavity, the electric field components are Ep (p^z) = A

J^-sm{m<f) )Jrn (kc p)[Bsm(Pz) + Ccos{pz)}, (40) kc P E^p, 0, z) = A^y^- cos(ra0)

771 —Jm(kcp) - kc Jm+1 (kcp) p [Bsm(pz) + Ccos{(3z)}, (41)

where B and C are constants to be determined. Assuming that the two endplates are

perfectly conducting, the tangential electric fields at z — 0 and z = L are zero. Applying

these two boundary conditions on Ep and E^, we find that C = 0, (42) and

(3 = ^ P=l,2,3,... . (43)

11
Therefore, eqs. (40) and (41) reduce to Ep (p,<f>,z) = A^^—sm(m<l>)Jm(kc p)sm(Pz) :

Using Faraday's law,

k

2 P A-p-cos(m0) Jm{kc p) — kc Jm+i(kc p) sm(pz).

V x E = -jcjp0 H, we can find the magnetic field components as follows: Hp (p,4>,z) = A—cos(m(f))

P Jm{kcp) — kc Jmjr i(kc p) cos(/?z), H^p^.z) = -A^—sm(m(f))Jm(kc p)cos(pz),

kc p Hz (p,(f),z) = AJm (kcp)cos(m(f))sm(Pz).

In particular, the electric and magnetic fields for a TEqip mode are Ep {p,(j),z

E<p(p,(f),z Hp

(p,(f),z

H<j,(p,<t>,z Hz {p,4>,z

0,A^J^kap) gmtfz)

A—Ji(ka p) cos(/3z),

= AJ0 (kc p) sm(Pz).

3.2.2 TEoip Mode Resonant Frequency

For the TEmnp resonant mode, the electric- and magnetic-field components are

where

Ep (p, <j>, z E^p^z

Hp (p, 0, z

Ht{p,4>,z

Hz {p,(j>,z

A

JUJfiom gin(m^) Jm ( fcc/9 ) Si n(^)

kl p = A^cos(r^)

A— cos(m0)

Jm{kc p) — kc Jm+i(kc p) sin(/3z),

-Jm{kc p) - kc Jm+i(kc p) cos(/3z), = -Aj£— sm(m(f))Jm(kc p)cos(pz),

kc P = AJm (kc p)cos(m(f))sm(Pz),

k

2

c = u}2

ii0 e0 ea - p

2

,

12
and

(62) Substituting eqs. (61) and (62) into eq. (60), one can determine the resonant frequency,

fmnp, of the TEmnp mode as

Jmnp ~

mere

For the case of the TEq\p mode, eq. (63) reduces to

(63)

(64)

/dp

2tt (65)

3.2.3 TEoip Mode Quality Factor One of the more important parameters of a cavity is its quality factor, Q. In particular we

are interested in the quality factor of the TE0 i p mode. The quality factor for the empty

cylindrical cavity is defined asstored energy in cavity = u) , dissipated power in cavity Pw + P\, + Pt (66)

where to is the resonant frequency of the TE0 i p mode in radians, Wo is the time-ave energy stored within the cavity, Pw is the power dissipated on the cavity wall, Pb is the power

dissipated on the bottom cavity endplate, and Pt is the power dissipated on the top cavity

endplate. If there were no conductive losses within the cavity, the denominator of eq. (66) would be zero and the quality factor would approach infinity. However, for real conductors,

there is power dissipated in the cavity walls and endplates, and the quality factor is some

finite value. Using the electric and magnetic fields for the TEqip mode (50-54), we derive expressions

for the both the stored energy and dissipated power in the cavity: ^o=^/ \E\ 2 dv= e-^f

L r r \E^pdpdct>dz=

2 Jv 2 Jz=0 Jp=0 Js=q 4k2 Pw = % / l^l

2 ^ = ^t I*" \HZ \ 2 P dpdz I = A

2 ^J*(ka)Rsw , (68)

13
and

2k? Jl(ka)Rsh , (69)

= A^-^Jl(ka)R,u (70) where Rsw , Rsb, and Rst are the surface resistivities of the cavity wall, bottom endplate, and top endplate. Substituting eqs. (67-70) into eq. (66) we obtain the expression for the

quality factor of the TEq\p mode,

Q

1 / Mo P7TV L)

3/2 i(f)W*,] + ifr

(71)

14
3.3 Sample-Loaded Circular-Cylindrical Cavity

Sample

Figure 6. Sample-loaded circular-cylindrical cavity.

3.3.1 Electric and Magnetic Fields in the Air-Filled Region

For the TEq\p mode, only the azimuthal ^-component of the electric field exists: E0<t> {p,z) = -AQ ^Jx {kc p) [B0 sm(P0 z) + C0 cos{P0 z)} , (72) where

(51 = u;

2 /wl - k2

c , (73) and e'a

is the relative permittivity of the air. The tangential electric field must be zero on

the upper cavity endplate. Therefore, applying the boundary condition E0(t) (p, z = L) = 0, (74) we find that C0 = -50 tan(A>L). ( 75) Therefore, the electric field in the air-filled cavity region is EQ<t> {p, z) = -AQ 3-^Jx {kc p) [sin(A)z) - tan(A)L) cos(A^)] . (76) Using Faraday's law, V x E = -jujplqH, (77)

15
we can find the remaining magnetic field components:

H0p {p,z) = -A0 ^Ji(M[cos(^) + tan(/50 L)sin(/50 2)] (78) H0z {p, z) = A0 J0 (kc p) [sm((3oz) - tan(/30 L) cos{p0 z)] . (79)

3.3.2 Electric and Magnetic Fields in the Sample-Filled Region

The electric and magnetic fields in the sample-filled region are derived in a manner similar to that for the air-filled region. As in that case, only the 0-component of the electric field exists: E14> (p, z) = -Ar^p-J^kcp) [B, sm(Ps z) + d cos(&z)] , (80) where P

2

s =u2 p0 e0 e's -kl (81) and e's is the relative permittivity of the sample. The tangential electric field must be zero on the bottom cavity endplate. Therefore, applying the boundary condition E1 +{pt z = Q) = Q, (82) we find that Ci = 0. (83) Therefore, the electric field in the sample-filled region reduces to EH{p,z) = -Al ^Jl {kcp)sin{(3s z). (84)

Using eq. (77), we find the remaining magnetic field components:

Hlp (p,z) = -Al ^J1 (kc p)cos(ps z), (85) Hlz (p,z) = AiJ0 (A^)sin(ft*). ( 86)

3.3.3 TEoip Mode Resonant Frequency

In order to determine the TEq\v resonant frequencies for the sample-loaded cavity, we must match the tangential electric and magnetic fields at the sample-air boundary z = b. From

the boundary condition on the tangential electric field, Eo^p, 4>,z = b) = EH{p, (j>,z = b), (87) we get Ai sm{ps b) = A0 [sin(A)&) - tan(/30 L) cos(/30 6)] . (88)

16
From the boundary condition on the tangential magnetic field, H0p (p, 4>,z = b) = Hlp {p, <f>, z = 6), (89) we get AiPa cos(&6) = AoPo [cos(Pob) + tan(/?0 L) sm(/30b)] . (90) Combining eqs. (88) and (90) we get tan(&6) +

tan [P0 (L - b)} = Q #s Po

In the case of the empty cavity, we can calculate the resonant frequencies by use of eq. (63). This is not the case for the sample-loaded cavity. We must use an iterative technique to find the zeroes of eq. (91). The p-th zero of this function is the resonant frequency of the TEq\p mode. Note that this equation can also be used to obtain the relative permittivity of the sample es when the resonant frequency is measured.

3.3.4 TE01p Mode Quality Factor As in the case of the air-filled cylindrical cavity, we can derive an expression for the quality

factor of the sample-loaded cavity. In particular, we derive an expression for the quality

factor of the TEqip mode. The quality factor is defined as _ stored energy in cavity Ws + Wa ,_ oN

LJ - to — uj dissipated power in cavity Pws + Pwa + P(, + Pt + Ps where u is the resonant frequency of the TEq\v mode in radians, Ws is the time-averaged

energy stored within the air-filled portion of the cavity, Wa is the time-averaged energy stored within the sample-filled portion of the cavity, Pws is the power dissipated on the cavity wall

in the sample-filled region, Pwa is the power dissipated along the cavity wall in the air-filled region, Pb is the power dissipated on the bottom endplate, Pt is the power dissipated on the top endplate, and Ps is the power dissipated in the sample.

Using the electric and magnetic field for the TEq\p mode, we derive expressions for both

the stored-energy and dissipated-power terms in the sample-loaded cylindrical cavity: W- = — J f J

\Eu\2pdpd^dz = Af

eoe^

a

Jl{ka)

z=0 p=0 4>=0 2b Sin2A6

(93)

z=bp=04>=o ^u 1 2(L - b)

Ps

sin [2p0 (L-b)}

(94)

17
p

P-sw

b 2tt ! f \Hlz \ 2 pd(f>dz\ =A\^J$(ka)

J J <P=a 4

z=0 0=0

2b

sin 2/3s b

P-sw )

f 7 \u |2 a±a I a2TM t2fu \ 1 fo/r t\ sin [2/?0 (L - 6)]

/ / |#02 p = A0 —J0 {ka) 2{L-b)

J J 'p=a 4 cosz pqL po

z=b 0=0

ft = 4f / / V | s=0 = A?-^fjt(ka)Rsi ,

(95)

Psw 1

(96)

(97) p=0 0=0

a 2tt

p=0 0=0

a 6 27Ttt o on

t f f f t 7TUJ LL CL = - tan Sscu€0 € s J J J lEi^p d(p dp dz = Al tan

6

s e0 e'

s —^|—Jo(£:a) p=0 2=0 0=0

Solving eq. (92) for the sample loss tangent tan<5s , we get

2b

(98)

sin(2&6)'

(99)

tan 5, 'ujjWg + WS ) Q

Puis Pwa Pb Pt

, 7TU3 p

20 a2 2

s —

Jo<Aa< 26 sin(2&6)

(100) Note that several variables must be measured or calculated before we can employ eq. (100) to calculate the loss tangent of the sample. In the case of the sample, we must know

its dimensions and permittivity. For the cylindrical cavity, we must also know the cavity dimensions and the microwave surface resistivities of the cavity wall and endplates. In

addition to these quantities, we must measure the resonant frequency and quality factor when the sample is present in the cylindrical cavity. As for the unknown constants Aq and

Ai, we arbitrarily set A\ to unity and determine Aq in term of A\ from eq. (90).

18
4. Sample Specifications

4.1 SRM Sample Selection

Our criteria for selecting a Standard Reference Material (SRM) for permittivity and loss tangent measurements included linearity, homogeneity, isotropy and stability with changes in temperature and frequency [5] . Based on these criteria, we selected cross-linked polystyrene

as the first low-loss, low-permittivity material to be characterized as an SRM. Specifically, we purchased a 61 cm x 61 cm x 1.9 cm sheet of Rexolite 14221 a thermoset cross-linked styrene copolymer. To prepare for the machining of the SRM samples, we divided the sheet into 56 sections

as shown Figure 7. The first row was reserved for making preliminary SRM samples in order

to refine the machining process as well as to make samples for measuring the permittivity and loss tangent using other measurement methods. Eighteen samples, whose locations are shown in bold in Figure 7, were fabricated into cylindrical samples that fit inside the 60 mm

cylindrical cavity. We discuss the SRM sample dimensions and tolerances in the following

sections.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

17 18 19 20 21 22 23 24

25 26 27 28 29 30 31 32

33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

49 50 51 52 53 54 55 56

Figure 7. Subdivision of cross-linked polystyrene sheet. Bold sites indicate the location of the first 18 cavity samples.

4.2 Sample Dimensions

4.2.1 Sample Diameter

The first quantity we considered was the diameter of the sample. To allow the sample to

fit within the cavity, the diameter of the sample must be slightly less than the diameter

of the cylindrical cavity. Therefore, we specified the diameter of the sample to be 59.7 Certain commercial equipment, instruments, or materials are identified in this paper only to provide complete technical description. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

19
mm, 0.3 mm less than the nominal cavity diameter of 60 mm. The theory for determining

the sample permittivity assumes an ideal sample with no airgap between the sample and

cavity. However, given that the gap is small, and that the permittivity of the cross-linked polystyrene sample is low («2.54), and that the electric field near the cavity wall approaches

zero, we expect no systematic error on the permittivity due to this small air gap [6] [7].

4.2.2 Sample Thickness

The next quantity we specified was the thickness of the sample. Since the bottom cavity

endplate and the faces of the dielectric sample are not perfectly flat, there exists a residual gap between the sample and the endplate. Because the size of this gap depends on the machining quality of both the sample and endplate, the gap will vary for each sample and is difficult to characterize [8] . In order to minimize the effect of this gap and the imperfections

of the machining process on the sample's top and bottom surface, we specified the sample

thickness to be an integer multiple of half-wavelengths in the sample, thus forcing the electric

field to be nearly zero on both the top and bottom surfaces of the sample. References [3] and [4] show that the uncertainty in the sample permittivity due to uncertainty in the sample

thickness is minimized when the sample thickness is an integer multiple of a half-wavelength

in the sample. For a TEoip mode, the expression for a half-guided wavelength A/2 in the sample located

inside the cylindrical cavity is

2 ^w.-(£)

J

where u is the resonant frequency in radians, /x0

is the permeability of free space, e0 is the

permittivity of free space, e'

s is the relative permittivity of the sample, j'

Q1 is the first zero of J0 (kcp), and a is the radius of the cylindrical cavity. For measurements of the SRM samples at 10 GHz in a cylindrical cavity with a radius

of 30 mm, and a relative permittivity of 2.535 for cross-linked polystyrene, we obtained an

optimal sample thickness value of approximately 10.2 mm using eq. (101).

4.3 Sample Cleaning

Prior to any measurements, contaminants on the surfaces of the samples should be removed

as they can be a source of measurement error, especially for the measurement of loss tangent. These contaminants include particles and lubricants used in the fabrication process as well

as oils from handling. To remove the contaminants, we cleaned each sample with 99 % pure

isopropyl alcohol and a lint-free cloth. In addition, we used lint-free gloves and avoided

contact with the top and bottom sample faces.

20
5. Measurement System Characterization

5.1 Sample Thickness Estimation

We used a Sylvac electronic probe, shown in Figure 8, to measure the sample thickness b at 21 different locations on the sample. A circular template with 21 small holes defines the

(x,y) measurement locations (see Figure 9). In addition, the sample radius was measured

eight times using a caliper.

Figure 8. Electronic probe station for measuring sample thickness. We modeled the spatially varying measured thickness of a cross-linked polystyrene sample

with the polynomial model

N

= °° + ©+ «

(f) + a3 (I) g) + a, (ff + a5 (ff + e, (102)

Using the method of ordinary least squares, we estimated ao, ai, a2, ^3, <^4, and as. The (ar, y) coordinates are scaled by

#o = -E^, ( 103) n i=i where R{ is the nth measurement of the radius of the sample. The estimated mean thickness

of the sample is

21
Figure 9. Thickness measurement locations.

where S)

,+

-(ft)

!

Based on eqs. (104) and (105), we estimated the mean thickness as

o = a0 + -04 + -a5

.

(105)

(106)

Assuming that our model is valid, we modeled the random variability of the b measurements

as VAR(6) - s\ + 4 + 4, (107)

22
where VAR(ao) + —[VAR(a4 ) + VAR(a5 )] + i[COV(d0 ,a4 ) + COV(a0 ,a5 )] + icOV(a4 ,a5 ), (108) and

( (109)

Whereas s\ represents random error of the estimate (assuming that the model is valid) given perfect knowledge of the sample radius and template coordinates, the quantity s%

represents uncertainty due to inexact knowledge of R. The quantity sf. represents uncertainty due to inexact knowledge of template placement. To compute St, we assume that all the (x,y) locations in the model have the same

translation error (ex ,ey

). We generate new (x,y) coordinates, (x',y'), using

where ex and e

y are normally distributed random variables each with a mean of zero and

variance <r 2

, and a = pRq. A plausible value of p is 0.02. One hundred sets of 21 (x',y') coordinates were generated, and b is estimated based on the polynomial model for each set of coordinates using the 21 observed thickness measurements. The standard deviation of these 100 thickness estimates is st- The estimated polynomial model parameters depend on the angular orientation of the

template. This angular variation cannot be explained by random measurement error. To

quantify the orientation error, measurements are made for 30 different angular rotations of the template for one side of sample #36. The 30 rotation angles are marked on the template

at 12° increments (12 x k, where k = 0, ...,29). Data were taken at randomly selected

angles. A plot of the mean thickness estimates and their associated "one-sigma" error bars (based on eq. (108)) at each angle of rotation is shown in Figure 10. The data indicate that

there is no systematic variation of the estimated sample thickness as a function of angle. The error in b due to the angular orientation of the template was estimated using a vari- ance components analysis based on the 30 estimates of mean thickness from the polynomial

model. The error in angular orientation is assumed to be the same for all samples.

X^ — X{ -f" (z x (110) and

Vi = V% + e

: (111)

23
I

t

I t h

—r~

100

Orientation Angle (degrees)

Figure 10. Estimated mean thickness of one side of sample #36 with associated uncertaintyintervals (± standard error (6), (107)) for 30 different template orientations. The uncertainties were computed based on the assumption that the model (eq. (102)) is valid.

24
5.2 Environmental Variables

5.2.1 Speed of Light

Determining the speed of light cair in the air-filled portion of the cylindrical cavity is important not only for the calculation of the sample permittivity and loss tangent, but also for the calculation of the cylindrical cavity diameter and length. Due to the presence of water

vapor and oxygen in the air, the speed of light is slightly less in air than in vacuum. From

measurements of three environmental variables, namely temperature T, barometric pressure P and relative humidity H, we calculate cai r [9] as:

(112)

1 + iVo where N0 = l(T6 (2.588Pi + 41.QE6 + 2.39£)fl, (113) with

Pi = P-E, (114) H95 41.51 x 109 - 834* and

(115) *=2t5W < 1W> We measured temperature in units of Celsius, barometric pressure in units of kilopascals, and relative humidity in percent. The computed value of ca{ r is an estimate of the speed of

light in the. air-filled portion of the cylindrical cavity. Uncertainties in measurements of the environmental variables P, H, and T lead to uncertainty in our estimate of cai r .

5.2.2 Temperature

The temperature of the cavity was controlled by a constant-temperature circulating water

bath. A thermometer in the water bath in conjunction with a remote thermometer in the water jacket surrounding the cylindrical cavity provide feedback to the temperature

controller. After setting the water bath temperature controller to 23 C, we allowed 30 minutes for the cylindrical cavity to reach thermal equilibrium. At that point, the two

thermometers are in agreement.

5.2.3 Relative Humidity

We measured the relative humidity with a sling psychrometer that meets a recommendation

of the U.S. National Weather Service. It is made up of two mercury-filled thermometers mounted on a stainless steel platform. One bulb has a wick attached that is moistened with

deionized water, while the other bulb remains dry. Spinning the two thermometers until the

25
temperatures reach equilibrium gives the wet-bulb and dry-bulb temperatures. From these two temperatures, we determine the relative humidity from high-altitude (3901 to 6100 feet) relative humidity charts provided by the U.S National Weather Service.

5.2.4 Barometric Pressure We measured the barometric pressure using a Fortin-type mercurial barometer that is trace- able back to the National Institute of Standards and Technology. We corrected for both

gravity and the temperature of the barometer mercury [10-12]. First we calculated the temperature-corrected barometric pressure Pt using

Pt = Pu + Ct , (117) Ct = pX + l—l -ll (US)

1 + 0.0001818T and Pu is the measured, uncorrected barometric pressure reading. Then, we corrected the barometric pressure for gravity using

P = Pt~ Cg , (119) where

r 980 616 1 C9 = pt\ nonccE (l - 0.0026373 cos(2L) + 0.0000059 cos

2 (2L)) - 1 (120) L980.665 ^ 'J

and L is the latitude in degrees north or south. In the case of our laboratory, the latitude is 40 degrees north.

26
5.3 Cylindrical Cavity Length and Radius

Direct measurement of the length and radius of the cylindrical cavity is difficult because the bottom endplate is not fixed and the cylindrical waveguide is composed of a helical waveguide. Therefore, we determine the cavity length and radius from observed resonant TEq\p modes [13] when the cavity is empty. In section 3.2.2, we derived the measured resonance

frequency for the pth TE01p resonant mode as

fa(P) Cair Cf)

2+

27 (?f e(p), (121)

where p takes on the values 22, 23, and 24. The quantity fa {p) is estimated from a resonance

curve for the pth mode by use of the same algorithm developed for estimating Q and /o (Section 5.4). Because the cylindrical-cavity walls and endplates have a finite conductivity, the resonant frequency is shifted slightly downwards. We can correct the resonant frequency due to this skin-depth effect [14] as

/*(P>=/a(p) + 4^, (122)

where Q is the measured quality factor of the pth TEq\p resonant mode. In our case, the frequency correction was rather small, a mere 70 kHz correction for a resonant frequency fa of 10 GHz. One way to estimate a and L is to linearize the above equation so that it is of the form

y(p) = Po + x(p)p1 + e*(p), where

y{p)

(P) A)

2*7; (p)

x{p) = p , 2

2

.hi

and A

The method of linear least squares is used to determine Pq and j3i . By minimizing

Y\v(p)-^-x{p)(5l ]\ (123)

p 27
we obtain

Joi a = (124)

and

(125)

Since fa (p) is measured with error, squaring the measured value introduces a possible sys- tematic estimation error (E(/ + e)

2 = E(/2

) + E(e2

), assuming E(e) = 0). Hence, this method

may not be reliable in general. More generally reliable estimates of a and L are obtained by mimimizing

by use of a nonlinear fitting technique, which minimizes a general, unconstrained objective function using the analytic gradient and Hessian of the function. The parameter estimates from a linear least squares fit are used as the starting values in the nonlinear algorithm.

We minimize the objective function over many sets of randomly selected initial parameter

values to avoid local minima. The random initial conditions are simulated by adding random

noise to the least-squares estimates. The standard deviation of the random noise is based on the asymptotic standard errors of a and L from a least-squares fit. The procedure used

to compute asymptotic standard errors (ASEs) is similar to the one used for Q and f0 [15].

£[/» - ra (p)}

2 (126)

v

28
5.4 Resonant Frequency and Quality Factor

We estimate the resonant frequency fo of the microwave cavity and the corresponding Q

factor using a nonlinear parameter estimation algorithm [15]. At the A;th frequency, the observed resonance curve is modeled as TM= i+Si-fj + bg + (i27)

where BG is background and e{fk) is additive noise. To estimate the parameters in an

optimal way, the frequency-dependent noise in the measurement system must first be characterized. The variance of the additive noise is modeled as VAR(e( A)) = a\h) =

1 + Q2^_£)2 + l\ (128)

The variance function parameters 71 and 72 are estimated from the residuals computed from a

least-squares fit of the resonance curve model to the observed data. Based on the empirical estimates of the variance function parameters, estimates of Q and fo are determined by

weighted nonlinear least squares. Given the parameters that characterize the resonance curve (127), and the variance of the additive noise (128), asymptotic statistical theory [16] predicts the covariance of the pa- rameter estimates. For details regarding the estimation of the parameters and the covariance

of the parameter estimates, see Reference [15]. The nonlinear fitting routine used to estimate the model parameters minimizes a general, unconstrained objective function using the analytic gradient and Hessian of the objective function [17]. Some difficulty with convergence was encountered since the function would

often converge to a local rather than a global minimum. To prevent this problem, the

function was minimized for a large number of randomly generated initial conditions. The

final parameter estimates are those that yield the smallest value of the objective function.

29
5.5 Cylindrical Cavity Conductor Losses

In order to accurately measure the sample loss tangent, we characterized the conductive losses

in the walls and endplates of the cylindrical cavity. For the cylindrical-cavity endplates, we

measured the surface resistance of the two endplates using the Courtney method [18] at 10 GHz. With the Courtney method, a dielectric sample is sandwiched between two metallic

plates and a TE0 i p resonant mode is excited with a pair of coupling loops. This method is normally used to measure the relative permittivity and loss tangent of the dielectric samples. However, if the relative permittivity and loss tangent of the dielectric resonator are known, we can determine the surface resistance of the metallic endplates that sandwich the dielectric resonator. In our case, we machined an oriented, single-crystal sapphire resonator, placed it between two silver plates, and measured its relative permittivity and loss tangent with the Courtney method. Then, substituting one of the silver endplates with one of the cylindrical- cavity endplates, we measured the surface resistance of the top and bottom cylindrical-cavity

endplates at 10 GHz as Rst = 39.7 ± 6.5 mft (129) and Rsb = 80.7 ± 5.9 mH. (130) The bottom endplate suffers from wear caused by the samples being located directly on

the bottom endplate surface, resulting in small scratches. Therefore, we were not surprised to

find that the surface resistance of the bottom endplate was significantly higher than that of the top endplate, which has no contact with any samples. We plan to occasionally remeasure

the bottom endplate in order to monitor the increase in surface resistance over time. At some point, the surface resistance of this endplate will reach a value where remachining and

polishing of the surface will become necessary in order to reduce the metal losses. Once the surface resistance of the two cylindrical-cavity endplates was measured, we

determined the surface resistance of the cavity walls from a measurement of the quality

factor for the resonant mode. In section 3.2.3, we found an expression for the quality factor of a TEqip mode:

Q

3/2

CqtrMO

2 (131)

In this expression, Rst and Rsb represent the surface resistance of the top and bottom endplates respectively, while Rsw is the surface resistance of the cavity walls. Solving for Rsw we obtain

Rsw —

3/2 W+ (¥)

2

i(

(132)

30
Given the measured quality factor Q of the TEq\p mode and the measured surface resistance

of the two cavity endplates, we can calculate Rsw from eq. (132).

31
6. Permittivity and Loss Tangent Measurements

In this section we outline the step-by-step procedure used to measure the permittivity and

loss tangent of the cross-linked polystyrene SRM samples in the NIST 60 mm cylindrical

cavity at 10 GHz. We show individual measurements for the 18 samples characterized. In order to verify the cross-linked polystyrene measurements made with the cylindrical cavity, we also show measurements near 10 GHz using four other measurement methods. Although we certify the cross-linked polystyrene properties at 10 GHz, we also made measurements

from 8 to 11 GHz in the cylindrical cavity to show how the relative permittivity and loss tangent vary as a function of frequency.

6.1 Measurement Procedure

6.1.1 Speed of Light in Laboratory

An important first step is to determine the speed of light cair in the air-filled portion of the cylindrical cavity, since we need this quantity not only for the calculation of the sample

permittivity and loss tangent, but also for the calculation of the length and diameter of the

cylindrical cavity. First, we allowed the water bath that circulates water around the cylin- drical cavity at 23 °C to reach equilibrium for approximately 30 minutes. Next, we measured

the relative humidity with a sling psychrometer. Then we measured the barometric pres- sure with a Fortin-type mercurial barometer. With these three measured quantities, namely

temperature, relative humidity, and barometric pressure, we calculated cai r as outlined in Section 5.2.1.

6.1.2 Sample Thickness

The next step in the process was to measure the sample thickness b. We placed a circular template, shown in Figure 9, on top of the sample, and using a Sylvac probe, we measured

the thickness of the sample at the 21 template hole locations. In order to calculate the sample thickness, we also measured the diameter of the sample with a dial caliper. Then,

using the algorithm outlined in Section 5.1, we calculated the sample thickness.

6.1.3 Cylindrical Cavity Length and Radius

Because the bottom endplate of the cylindrical cavity is not fixed and the cavity walls are composed of helical waveguide, the cavity length L and radius a were difficult to measure

directly. In Section 3.2.2, we derived an equation for the resonant frequency /oip of an TEq\p mode in an empty cavity:

(133)

32
Note that the resonant frequency depends on both the cavity length and radius, so by

measuring several resonant frequencies, we can calculate L and a. First, with the motorized

drive below the bottom endplate, we adjusted the length of the empty cavity until the TEq\23 mode resonated at 10 GHz. We then measured the resonant frequencies of the TE0122, TE0123, and TE0124 modes using the method described in Section 5.4. The measured

resonant frequency was lowered slightly by the penetration of the electromagnetic fields into the conductive wall and endplates of the cylindrical cavity. To minimize this potential

systematic error in L and a, we corrected each of the measured resonant frequencies fmoip [14] using

/()lp = /moip + 9 /T°

1P

' ( 134) where Qmoip is the measured quality factor of the /oip mode. In the case of the T.E0123 mode,

the resonant frequency was approximately 10 GHz with a quality factor near 70 000, so the resonant-frequency correction, due to the skin depth, was under 100 kHz or 0.001 %. With the three corrected resonant frequencies and eq. (133), we use the method of least squares to determine the cavity length and diameter, as outlined in Section 5.3. With the

cavity dimensions determined, we zeroed the digital probe that monitors the travel of the bottom endplate.

6.1.4 Empty Cavity Measurement

Once the cavity dimensions had been calculated, the next step was to characterize the wall and endplate losses of the cylindrical cavity at 10 GHz. This is a very important step as the conductive losses in the cavity are significant and must be properly accounted for in order to accurately measure the loss tangent. The conductive losses vary as a function of frequency,

so characterizing these losses at the frequency of interest is also important. As summarized in Section 5.5, we had previously measured the surface resistances of the top and bottom cavity endplates Rst and Rsb at 10 GHz using the Courtney method.

Therefore, the remaining conductive loss left to characterize was the surface resistance of the cylindrical cavity wall Rsw . We measured the resonant frequency and quality factor of the TE0123 at 10 GHz. From the measured resonant frequency and quality factor, as well as the dimensions of the cylindrical cavity and surface resistances of the two endplates, we can

calculate Rsw using the method described in Section 5.5.

6.1.5 Sample-Loaded Cavity Measurement

In previous steps, we had already calculated the speed of light, cavity dimensions, and

the cavity losses due to the conductive cavity wall and endplates. Left remaining was the measurement of the sample's relative permittivity and loss tangent. We began by lowering

the lower endplate assembly, shown in Figure 4. We placed the sample on the bottom

33
endplate and raised the endplate assembly back into its original position as shown in Figure

3. The presence of the sample inside the cavity shifts the resonant frequency of the TE0123 mode significantly. Since we characterized the conductive losses at 10 GHz, the cavity must

be retuned such that the T£oi23 mode resonates again at 10 GHz for the sample-loaded

cavity. Using the motorized drive below the bottom endplate, we retuned the resonant frequency by shortening the cavity by a length AL. Since we zeroed the electronic probe

monitoring the bottom endplate movement after we measured the cavity length L and radius

a, the electronic probe can measure AL, the change in the cavity length, directly. Once the TE0123 mode was tuned to 10 GHz, we measured the resonant frequency and quality factor using the method described in Section 5.4. At this point, we had completed all the measurements necessary to calculate the sample

relative permittivity and loss tangent. First, we calculated the sample's relative permittivity

using the transcendental equation we derived in Section 3.3.3: tan(&6) | tan[A)(L-fr)] _ n Ps +

Po ~°' (35)

where Ps is a function of the sample's relative permittivity e

s

. In order to calculate the

relative permittivity from this equation we used the Newton-Raphson iterative method.

Since the method is iterative, an initial guess for the relative permittivity was necessary. Once the sample's relative permittivity was determined, we explicitly calculate the loss tangent using the equation derived in section 3.3.4:

tan <L u(Wa + Ws) _ *ws *wa -*o m

Q

(136)

6.2 Relative Permittivity and Loss Tangent Results

In order to verify the long-term stability of our relative permittivity measurements and

optimize the measurement procedure, we performed a repeatability study, described in detail

in Section 7.2. In this study, we measured three samples: a single-crystal quartz check

standard, and two (#9 and #36) cross-linked polystyrene samples (see Figure 7). Once the

repeatability study was completed we machined 16 additional cavity samples taken from

various parts of the cross-linked polystyrene sheet, as shown in Figure 7. The 16 samples form the first set of SRM samples.

Using the procedure described in the last section, we measured the relative permittivity and loss tangent of all eighteen cross-linked polystyrene samples at 10 GHz using the cylin- drical cavity. We collected three sets of data for relative permittivity, shown in Table 1, and

loss tangent, shown in Table 2. Each set of eighteen cross-linked polystyrene measurements,

as well as two single-crystal quartz check-standard measurements, were collected on a sin- gle day, allowing for a week between each set of measurements. As shown in Table 1, the

34
Table 1. Measured relative permittivity for cross-linked polystyrene SRM samples.

Sample Measurement Average

number 1 2 3 9 2.5353 2.5340 2.5351 2.535

10 2.5363 2.5357 2.5345 2.536

12 2.5338 2.5346 2.5341 2.534

15 2.5357 2.5338 2.5334 2.534

16 2.5349 2.5349 2.5324 2.534 17 2.5357 2.5345 2.5341 2.535 24 2.5341 2.5348 2.5329 2.534 28 2.5343 2.5345 2.5332 2.534 29 2.5350 2.5341 2.5343 2.534 36 2.5352 2.5354 2.5359 2.536 37 2.5351 2.5343 2.5347 2.535 41 2.5348 2.5342 2.5368 2.535 48 2.5338 2.5342 2.5356 2.535 49 2.5354 2.5328 2.5338 2.534 50 2.5356 2.5349 2.5353 2.535 53 2.5358 2.5350 2.5333 2.535 55 2.5372 2.5353 2.5343 2.536 56 2.5363 2.5340 2.5333 2.535

range of values for the average relative permittivity varied between 2.534 and 2.536 for all 18 samples while the average loss tangent ranged from 4.6xl0-4 to 4.7xl0-4

. This variation

in both the relative permittivity and loss tangent is less than or equal to the measurement

uncertainty calculated for relative permittivity and loss tangent as shown in Section 7.3. Consequently, within measurement precision, we were not able to detect any variation in the dielectric properties as a function of the sample position on the cross-linked polystyrene

sheet, thus verifying our original assumption of the material's homogeneity.

6.3 Measurement Intercomparison

As a consistency check, we compared the relative permittivity and loss tangent data of the

cross-linked polystyrene with data from four other measurement methods operating near 10 GHz. These methods included the parallel plate resonator, split-cylinder resonator, dielectric resonator, and another cylindrical cavity [19]. We machined samples for these other methods

from the first row of the sheet where the SRM materials were taken. Figures 11 and 12

35
Table 2. Measured loss tangent for cross-linked polystyrene SRM samples.

Sample Measurement Average

number 1 2 3 tan<5s (xl04

) 9 4.59 4.60 4.61 4.6 10 4.69 4.66 4.74 4.7 12 4.59 4.59 4.61 4.6 15 4.64 4.61 4.57 4.6 16 4.59 4.60 4.58 4.6 17 4.71 4.68 4.66 4.7 24 4.61 4.61 4.65 4.6 28 4.64 4.65 4.60 4.6 29 4.68 4.61 4.65 4.6 36 4.62 4.62 4.59 4.6 37 4.69 4.68 4.70 4.7 41 4.74 4.67 4.69 4.7 48 4.63 4.62 4.65 4.6 49 4.70 4.65 4.61 4.7 50 4.74 4.71 4.67 4.7 53 4.66 4.63 4.63 4.6 55 4.64 4.57 4.59 4.6 56 4.66 4.66 4.66 4.7

compare the permittivity and loss tangent measurements using the four methods. Also shown in this plot are data for one of the cross-linked polystyrene measurements made with

the 60 mm cylindrical cavity. As seen in Figures 11 and 12, there is very good agreement between the 60 mm cylindrical cavity results and measurement made using the four other

techniques.

6.4 Broadband Measurements

Although we certified the permittivity and loss tangent of the cross-linked polystyrene material at 10 GHz, we made additional measurements using the cylindrical cavity resonator

over a wider frequency range. Using eleven TE0 i n modes between n = 16 and n = 26 we

were able to cover the frequency range between 8 to 11 GHz. Above 11 GHz, we encountered

difficulty in making measurements, due to the presence of interfering, higher-order resonant modes. Although higher-order modes do not propagate below 8 GHz, the quality factors of the TEoin modes are relatively small, thereby decreasing the measurement sensitivity.

36
Shown in Figures 13 and 14 are the measured permittivity and loss tangent of the cross- linked polystyrene as a function of frequency. Note that the permittivity is relatively flat, which is expected for a low-loss material, while the loss tangent increases with frequency.

2.56

2.52

O 50 mm Cylindrical Cavity

Parallel Plate Resonator A Split-Cylinder Resonator V Dielectric Resonator • 60 mm Cylindrical Cavity

2.50 10 Frequency (GHz)

Figure 11. Comparison of cross-linked polystyrene relative permittivity measurements.

37
5.5x10^

5.0

4.5 -

4.0

O 50 mm Cylindrical Cavity

Parallel Plate Resonator A Split-Cylinder Resonator V Dielectric Resonator • 60 mm Cyli ndri cal Cavi ty

10 Frequency (GHz)

Figure 12. Comparison of cross-linked polystyrene loss tangent measurements.

2.55

2.54

2.53

2.52

9 10 Frequency (GHz)

Figure 13. Relative permittivity of cross-linked polystyrene measured in cylindrical cavity

resonator.

38
5.0xlO"

4

4.8

'-

4.6

4.4

4.2 r

9 10 Frequency (GHz)

Figure 14. Loss tangent of cross-linked polystyrene measured in cylindrical cavity resonator.

39
7. Uncertainty Analysis

Estimates of permittivity and loss tangent are uncertain due to both random and systematic

errors. In a repeatability study, we quantify uncertainties due to random variability in some, but not all, measured quantities. By a Monte Carlo method, we quantify the effects of two random errors that are not captured in the repeatability study. Using the same Monte Carlo method, the effects of systematic errors on uncertainty are also quantified. Assuming validity

of the model, the Monte Carlo approach allows us to estimate the effect of random errors in measured quantities, one at a time or jointly, on the estimates. Based on the repeatability

study, we demonstrate the stability of the measurement process. A measurement assurance program based on the repeatability study data is described in Section 8.

7.1 Monte Carlo Simulation Model

The simulation studies utilize a computer program for simulating the effects of random and

systematic errors on the measurement process.

7.1.1 Random Errors

Theoretical Study

Based on nominal values and their associated standard deviations for an actual sample

(Table 3), we quantify the effect of random errors on estimates of permittivity and loss tangent (Table 4). Standard deviations for Tc , Tg, H, and Pu are based on stated bounds

for measurement error provided by the instrument manufacturers. To compute a standard

deviation from the upper and lower bounds, we assume that error is a uniformly distributed random variable. Table 4 lists uncertainties in intermediate quantities (ca; r , a, L) as well

as in the permittivity and loss tangent. The individual and joint effects of uncertainties are shown in Table 4. Based on results in Table 4, the random variability of estimates of cair , a, L, and permittivity is due mostly to random variation in the humidity measurement. However, random

errors in estimates of the quality factors Qa and Qs are the biggest contributors to uncertainty

in the estimate of loss tangent. The random errors for Q and f0 correspond to variability expected in a single measure- ment at a particular time. Repeatability studies reveal additional random variability due

to long-term variability in the measurement system. The approximate standard deviations

associated with the total (long-term and short-term) random variability are 200 and 400 for Qa and Qs , and 11 000 Hz and 14 000 Hz for fa and fs

. Tables 3 and 4 do not account for long-term measurement variability. Although the measured values of endplate resistances, Rsb,ioGHZ and Rst,ioGHz, are uncertain, they were not remeasured during the repeatability study; they were measured once and

40
Table 3. Nominal values of measured and estimated quantities and their standard deviations

for cross-linked polystyrene sample #36.

Variable Nominal Value Standard Deviation

Bj„t (degrees) 39.995 0.00 AL (m) 0.0086994 0.00 Tc (°C) 22.9 0.06 T» (°C) 20.7 0.06

if (%) 44% 1.2%

Pu (mm of Hg) 632.1 0.3 Rsb,10GHz (f2/m2

) 0.0807 5.9E-03

Rst,10GHz {0,/m2

) 0.0397 6.5E-03

b (m) 0.01018566 Measurement 1.5E-07 Template Angular Orientation 1.2E-06

Is (Hz) 10.00002407E09 145

/a (Hz) 10.00000897E09 49 Qs 45223.0 121 Qa 73945.0 99 faiTEons) (Hz) 9.728749568E09 61 fa(TE012,) (Hz) 10.000004096E09 40 fa(TE0125 ) (Hz) 10.275665920E09 44 Q(T£oi23 ) 69493.461 125 Q(T£0 124) 73822.742 84 Q(rE0 i 25 ) 75711.453 85

treated as constants throughout the study. Similarly, the angular template orientation for the sample thickness measurement was fixed for each sample throughout the repeatability

study. Thus, the random errors due to endplate resistance and template angular orientation

are not quantified by the repeatability study. The uncertainty associated with these errors was added to the overall uncertainty separately from the random measurement error.

41
Table 4. Uncertainty of permittivity and loss tangent due to random effects when sources of uncertainty are perturbed individually and jointly (see last line).

Source Std. Dev. UCaiT (m/s) Ua (m) Ul (in) Ue>. Tc 0.06 °C 36.2 3.63E-09 5.25E-08 1.06E-06 6.25E-11 TB 0.06 °C 0.6 6.33E-11 9.17E-10 1.85E-08 1.09E-12 H 1.2% 411.6 4.12E-08 5.97E-07 1.20E-05 7.10E-10 Pu 0.3 mm of Hg 30.3 3.03E-09 4.39E-08 8.85E-07 5.22E-11

Rsb,\QGHz 5.9E-03 Q/m2 4.67&07

Rst,\QGHz 6.5E-03 Q/m2 5.13E-07

b Measurement 1.5E-07 m 1.30E-10 7.11E-11

b Angular 1.2E-06 m 1.13E-09 5.76E-10

/. 144.6 Hz 4.22E-06 5.93E-10

fa 48.9 Hz 1.17E-12 Qs 121.5 3.31E-06 Qa 99.4 1.05E-06

fa(P) 60.5, 39.9, 43.9 Hz 3.56E-09 2.97E-08 8.32E-07 1.23E-10 Q(p) 124.9, 83.8, 85.0 7.06E-09 5.87E-08 1.49E-06 2.29E-10

All 412.7 4.18E-08 6.05E-07 1.29E-05 3.52E-06

7.1.2 Systematic ErrorsCavity Radius a and Length L

We studied the systematic error associated with the position of the cavity endplate for the estimates of a and L. Thirty pairs of (a, L) measurements were taken at 30 different elevations of the cavity endplate (Figure 15). Holding all other parameters and measured

quantities fixed to values used in Table 3, permittivity and loss tangent were estimated

for perturbed values of a and L (from the Table 3 measurement) based on the deviations

of the 30 (a, L) pairs from their associated mean values. We also estimated loss tangent and permittivity for each of the 30 deviations of a or L individually (the other parameter was equated to its mean value.) To quantify systematic error due to uncertainty in a and

L, the standard deviations of the 30 estimates of permittivity and loss tangent (Table 5) were computed. Parenthetically, the propagation-of-errors approximation for the standard

deviation of the estimates of permittivity or loss tangent is very poor unless the covariance

structure is accounted for.

42
E

=L O -

0 5 10 15 20 25 30

Endplate Position Index

10 15 20

Endplate Position Index

25 30

Figure 15. a — a versus endplate position index (top), and L — L versus endplate position index (bottom).

43
Table 5. Uncertainties of e

s and tan 5S due to systematic errors in a and L. Type of Estimate U

u tan ds Monte Carlo Simulation Vary a 3.7E-03 5.4E-07 Vary L 4.1E-03 5.5E-07 Vary a and L 1.9E-03 2.7E-07 Propagation of Errors Without Covariance 5.5E-03 7.7E-07 With Covariance 1.9E-03 2.7E-07

Sample Thickness, b We studied the uncertainty in permittivity and loss tangent due to systematic errors

in sample thickness b using the simulation model. The two sources of systematic error are inexact knowledge of both the sample radius and the probe error. For each source of error, the appropriate standard deviation was used to perturb the

value of b in the simulation model while all other parameter values were held constant. The

standard deviation of the resulting permittivity and loss tangent represents the uncertainty

for the source of error being quantified.

The value of standard deviation (sb) used in the simulation model for estimating the uncertainty of permittivity and loss tangent due to the sample radius was based on the worstcase (or maximum) error observed in the template angular orientation study. (See Section 5.1

for details regarding estimation of sample thickness.) Although s# is dependent in part on the estimated model parameters (see eq.(109)), sb is considered systematic because the sample

radius was measured on one occasion and was constant for all subsequent measurements of permittivity and loss tangent. Probe error is the manufacturer's specified error for measurements of sample thickness provided by the probe. The simulation model utilizes a standard error of b based on a uniform distribution bounded by the manufacturer's specifications (see section 7.1.1). Change in Cavity Length AL

We determined the uncertainty in estimates of permittivity and loss tangent due to systematic errors in AL using the simulation model. (See Section 6.1.5 for details regarding

AL.) Systematic errors in AL are caused by inexact knowledge of the distance traveled by

the cavity endplate after the sample is loaded. The standard error of AL was based on a uniform distribution bounded by the manufacturer's specifications of the probe accuracy.

44
Frequency Drift

In the measurement process, a resonance curve T was sampled at 201 frequencies. The

kth. frequency is

where fref = 10 GHz and df is the frequency spacing. For each frequency fk, we simulated 500 values of T. Over the course of these 500 measurements, we assumed that the resonance

frequency was drifting upward in a linear fashion. For each fk, we averaged the 500 simulated

values of T. At the first measurement of T(/i), fo = fref. During the last measurement of T(f201), fo = fref + <5, where S is frequency drift. Finally, the simulated resonance curve was

normalized so that its maximum value is 1. A four-parameter model (with flat background) was fitted to the noise-free simulated

resonance curve using the method of least squares. The difference between Q and Q is the estimated bias in Q. The bias of the resonance frequency estimator, fo was estimated in a similar manner. Figure 16 displays the bias of Q and fo versus frequency spacing df for Q = 50 000 and four levels of resonance frequency drift. The bias of Q was approximated as follows:

where fref = 1 GHz, dx = -1.000004, d2 = 396455.6, and d3 = -4.05579 when Q = 50 000. We estimated the bias of Q, for a specific frequency drift using (138). Based on repeated

observations of resonance curves, the frequency drift was estimated to be at most 500 Hz. For frequency spacings of 3500 Hz (empty cavity) and 5600 Hz (sample-loaded cavity), 1000 random values of drift between zero and 500 Hz were simulated. For each realization of the frequency drift, we estimated the bias in the Q estimate using eq. (138), and esti- mated both permittivity and loss tangent (holding all other variables fixed). We quantified the uncertainty in the estimates of permittivity and loss tangent due to frequency drift by

computing the standard errors of these 1000 estimates of permittivity and loss tangent.

7.2 Observational Repeatability Study

We quantified both short-term and long-term variation to determine the stability of the measurement process. In general, a repeatability study captures much more variability than does a theoretical study because actual measurements typically depend on sources of

variability not accounted for in theoretical simulation models. Based on the repeatability

study data, we determine the statistical significance of the effects of four factors (sample

orientation, operator, sample number, and measurement day) on the measured values of permittivity and loss tangent.

fk = fref + (k ~ 101)d/, (137)

(138)

45
CO

CD ^

10A-7 5*10A-7 5*10A-6 Frequency Spacing (GHz)

5*1(^-5

Figure 16. Estimated bias of Q and f0 for Q = 50 000.

46
The measurement plan requires two operators to measure each of two cross-linked polystyrene

samples (#9 and #36) for two orientations (up and down) on eight different occasions. The

template specifying coordinates for sample thickness measurements is placed in a fixed loca- tion on each sample for all thickness measurements. A sample made of single quartz crystal served as a "check standard" and is measured at the beginning of each operator's measure- ment session. The measurement sequence for the first day of the study is shown in Table

6.

47
Table 6. Measurement plan for the first day. Operator number Sample Sample orientation

1 Quartz Down

Dnwri l—SKJ VV 11 #36 Ud

#9 Ud

#36 Down

2 Quartz Up

#36 Down

#9 Up

#9 Down

#36 Up

Measured values of permittivity and loss tangent on the cross-linked polystyrene samples

over time are displayed in Figure 17. Figure 18 shows measurements of the single quartz

crystal over time. Since Figures 17 and 18 indicate that there are no time trends in the data, we conclude that the measurement process is stable over the duration of the study.

An analysis of variance [20] was performed using the repeat measurement data to: (1) determine whether estimated permittivity and loss tangent are statistically different de- pending on the orientation of the sample in the cavity; (2) determine whether differences in permittivity and loss tangent among samples are statistically significant; and (3) estimate

the contributions of operators and measurement time to the overall variance of the estimates. (See Reference [21] regarding experiment design and analysis.) The four factors and their respective levels used in the analysis of variance are displayed in Table 7.

Table 7. Analysis of variance factors.

Factor name Symbol Factor levels Sample orientation Ti Up, Down

Operator Oj 1,2 Sample Number sk #9, #36

Day Di 1, 8

48
Samples #9 and #36

•

i : • •

•

• • •

• # •

8

• i

•

• • •: •

i •

•

•

•

••

•

•

•

• i

•

•

•

• • •

•

•20APR01 30APR01 10MAY01 20MAY01 30MAY01

Date

20APR01 30APR01 10MAY01

Date

20MAY01 30MAY01

Figure 17. Permittivity and loss tangent of cross-linked polystyrene samples over time.

49
Single Quartz Crystal

30MAY01

20APR01 30APR01 10MAY01 20MAY01 30MAY01

Date

Figure 18. Permittivity and loss tangent of single quartz crystal over time.

50
The analysis of variance for the cross-linked polystyrene samples is based on the model

Vijkim = v + Ti + Oj + Sk + Dl + (139) (TO)y

- + (TS)ik + (TD)u +

(OS)jk + {OD)3l +

(SD)ki + eijklm

for a single measurement yijkim, where \i represents the overall mean and €ijkim is a random

error component. The terms containing symbols for two factors, such as (TO)ij, represent

interactions (or dependencies) among the factors. Although the model could include inter- actions comprised of more than two factors, the general model selected for the analysis is adequate for identifying possible dependencies among the factors. The analysis of variance reveals that only one factor, measurement day, is significant at the 0.1 level for both permittivity (p-value = 0.10) and loss tangent (p-value = 0.06). Since measurement day is the only significant factor, the total random measurement variation is the sum of the component of variance due to measurement day (called between-day or long- term variation) and the component of variance due to random error (called within-day or short-term variation). Table 8 displays the variance component estimates for permittivity and loss tangent.

Table 8. Estimated variance components for permittivity and loss tangent. The number in parentheses represents the percentage of total variance. 4 tan 5S Source Variance Stan. dev. Variance Stan. dev. Between-Day 0.41E-06 (51 %) 6.4E-04 0.011E-08 (97 %) 1.04E-05 Within-Day 0.40E-06 (49 %) 6.3E-04 0.037E-10 (3 %) 0.19E-05

Total 0.81E-06 9.0E-04 0.011E-08 1.06E-05

The estimated standard deviation of a single future measurement is the total standard

deviation listed in Table 8. This repeat measurement error is one component of the overall uncertainty. The individual components of variance shown in Table 8 can also be used

to develop uncertainties for averages of measurements. The percent errors, based on the

repeat measurement error divided by the average of all 64 measurements, are 0.035 % for permittivity and 2.2 % for loss tangent. We also performed an analysis of variance for the single quartz crystal. No factors are

statistically significant. The repeat measurement error for permittivity and loss tangent are 0.0033 (0.075 %) and 2.0E-06 (14.7 %), respectively.

51
After completing the analysis of the repeatability study data, additional measurements

were taken for three samples to verify the stability of the measurement system. Plots of the

old and new observations reveal a shift in permittivity for the quartz sample (Figure 19). Further investigation suggests that the mean level shift in the quartz data could be related

to backlash error in the motorized micrometer. Additional data were collected for the three samples after altering the measurement procedure to minimize backlash error. The new

data indicate that the measurement system is now stable. Interestingly, the measurements

of cross-linked polystyrene samples do not appear to be effected by the backlash error (Figure

19).The next step in the analysis is to determine whether the new data can be combined with

the old data to produce a better estimate of the repeat measurement error. A mixed-model

analysis of variance reveals that the mean levels corresponding to old and new permittivity and loss tangent data associated with cross-linked polystyrene samples are not statistically

different. In addition, there is insufficient evidence to suggest that the short-term and long- term variation of the estimates has changed. Thus, we conclude that the old and new cross- linked polystyrene data can be combined. However, the mixed model analysis for the quartz data indicated that the old and new data should not be combined. The repeat measurement

errors based on the combined data for cross-linked polystyrene are shown in Table 9.

Table 9. Estimated variance components for permittivity and loss tangent based on combined

data. The numbers in parentheses represents the percentage of total variance. tan 8S Source Variance Stan. dev. Variance Stan. dev. Between-Day 0.40E-06 (49 %) 6.3E-04 0.0086E-08 (96 %) 9.3E-06 Within-Day 0.41E-06 (51 %) 6.4E-04 0.0032E-09 (4 %) 1.8E-06

Total 0.82E-06 9.0E-04 0.0089E-08 9.4E-06

7.3 Relative Permittivity and Loss Tangent Uncertainty

We list all sources of error studied and their effects on the uncertainty of the permittivity and

loss tangent estimates in Table 10. The table displays the combined standard uncertainty,

which is the square root of the sum of the individual squared uncertainties.

52
Single Quartz Crystal

•

4.435

•

• •

• •

•

•

•

•

•

t

•

• •

• •

• o

CO

*fr

- • •

•

•

•

•

• •

LOCM ••

•

•

•

• o

CM •

• 20APR01 20MAY01 19JUN01 19JUL01 18AUG01 17SEP01

Date Cross-Linked Polystyrene

• CO • • CO

LO - •

• • •

c\i •: • •

•

•

t

•

• LO

CO \

•

•

•

• •

•

•

• 1 LO -

c\j •

I •t •

•

t

9

•

•

••

•

•

•

0 •

•

• •

•

•

•

• t

* CO • • LO -

• •

•

C\j •

•

•

•

•

• • CO • 0 CO

LO -

Cxi •

•

0

20APR01 20MAY01 19JUN01 19JUL01 18AUG01 17SEP01

Date

Figure 19. Permittivity single quartz crystal and cross-linked polystyrene over time.

53
Table 10. Combined standard uncertainty for e's and tan£s . Source of uncertainty lype 01 eriect j r Measurement Random (A) 9.0E-04 9.4E-06

Rsb,\QGHz Random (B) 4.7E-07

Rst,\0GHz Random (B) 5.1E-07

b Template angular orientation Random (B) 1.1E-09 5.7E-10

b Probe Systematic (B) 7.7E-10 4.1E-10 Sample radius Systematic (B) 1.6E-12 9.0E-13

i)Keg Ri£3Q JJldo rlnp LiLlt. tr\ \j\J frpniipnr'Af 11 LLwi.ll> nrift

V

Oy oLcIildLIL. pm afip [Ri ^J_>

J

1 IF D7 O.UILrUO a and L Systematic (B) 1.9E-03 2.7E-07 AL Probe Systematic (B) 3.7E-04 5.2E-08 Combined standard uncertainty, Uc 2.1E-03 9.4E-06 Coverage factor, k 2 2 Expanded uncertainty, kUc 4.4E-03 1.9E-05

The overall uncertainty for a single measurement (made on any sample, by any operator, and any sample orientation) is the combined standard uncertainty, Uc- The uncertainty due

to systematic effects on tan Ss is negligible compared to the combined standard uncertainty.

The certified value that we shall report for each SRM sample is based on three repeated measurements taken over a span of several weeks. Thus, the uncertainties for certified values

are slightly different from those reported in Table 10. To determine the uncertainty for a certified value, we assume that the observed data for the i th sample can be modeled by

Vijk = fa + Dj + 6ijk + ebprobe + ^radius + ^Qbias + &aL + 6 ALprobe-, (140) where Dj is the j

th measurement day, and eijk is the error of the k

th measurement on the

j

th day (here k = 1). The quantities ebprobe , eradlus , eQbias , eaL , and eALprobe represent the systematic errors listed in Table 10. The best estimate of the certified value for a single sample is the simple average of the three measurements. We assume that the correction factors for the systematic errors are zero. However, the correction factors are not known perfectly, and thus have some

uncertainty associated with the correction factors. The combined standard uncertainty associated with the certified value is ^c = sj {u2D + ul)/m + u

2bprohe + u2radlus + u2Qbias + u\L + u

2ALprobe , (141) where m = 3 (the number of measurement days), uD and ue represent the long-term and

short-term components of variance (see Table 9 for example), and unrobe, uradius, UQUas-, uaL,

54
and u&Lprobe represent uncertainties associated with systematic errors. The effective degrees

of freedom were computed by use of the Satterthwaite approximation.

55
8. Measurement Quality Assurance

To ensure the integrity of the measurement system, measurements are performed periodically on three check standards: cross-linked polystyrene samples #9 and #36 and a single quartz

crystal sample. New measurements are compared to past measurements using control charts

for individual measurements and moving ranges. (The moving range chart contains the

differences between adjacent values over time.) The "individuals" control chart monitors

the nominal value of the measurements, while the moving range chart is a tool for detecting

changes in the variation of the measured values; see Reference [22] for details regarding the

construction of control charts. Examples of control charts for permittivity measurements of cross-linked polystyrene

sample #9 are shown in Figure 20. Repeat measurement error and the associated degrees of freedom (determined by the Satterthwaite approximation) from the repeatability study (see Section 7.2) were used to define boundaries for acceptable measurements. The top graph

displays a 99.6 % (three-sigma) control chart for single future measurements of e's

, and the bottom chart is the associated moving range chart. In addition to traditional control charts, we generated a control region for new measurements of the two cross-linked polystyrene check standards completed on the same day. Same-day measurements are related since they will share similar environmental conditions. An elliptical region that contains 99.6% (roughly a ± three sigma interval) of the bivariate distribution of the two sample measurements, y\ and y2 , is defined by

(yi - iifjal + a

2

) - 2{ Vl - fi){y2 - fi)a

2D + (y2 - fi) 2 {a2D + a2

) 2

where /i is the mean permittivity (assumed to be the same for both samples), and o

2D and a

2 are the between-day and within-day variances, respectively [23]. In the equation, fi, o

2D

and a

2 denote true, but unknown, parameters. The parameters were estimated using the repeat measurement data (see Table 9, Section 7.2, so that the resulting control region is approximate.) Figure 21 displays an approximate 99.6 % elliptical control region for a single pair of future measurements of samples #9 and #36 taken on the same day. The region was

generated using p, = 2.53, a

2D = 0.401E-06, and a2 = 0.414E-06.

56
Sample #9

20APR01 30MAY01 09JUL01 18AUG01 27SEP01 06NOV01

Date

(a)

20APR01 30MAY01 09JUL01 18AUG01 27SEP01 06NOV01

Date

(b)

Figure 20. (a) Control chart with 99.6 % probability limits for individual measurements of

e's for sample #9. (b) Moving range control chart. Circles represent historical data used to generate control limits, while the dot represents a new observation. 57
2.532 2.533 2.534 2.535 2.536 2.537 Sample #9 Permittivity

Figure 21. Approximate 99.6 % (three-sigma) bivariate control region for pairs of es measurements. Circles represent historical data, while the dot represents a new observation.

58
9. Conclusion

We have demonstrated that the circular-cylindrical cavity method has the necessary measurement accuracy to characterize dielectric standard reference materials. Using this method, we were able to measure the relative permittivity of cross-linked polystyrene samples with

less than 0.5 percent uncertainty, while the loss tangent uncertainty was less than 2 xlCT5. This low level of uncertainty was achieved because we identified and characterized many dif- ferent aspects of the measurement method. Highlights include a new method for measuring

a cavity's resonant frequency and quality factor, the inclusion of the surface resistance of the circular-cylindrical cavity's wall and endplates in the measurement theory, and a study

of the effects of coupling losses. In our uncertainty analysis for both the relative permittivity and loss tangent, we identified and characterized the random sources of uncertainty through a comprehensive repeatability study, and also included the effects of several systematic sources of error. To verify the performance of this method, we compared the relative permittivity and loss tangent, measured with the circular-cylindrical cavity, to measurement made in several other resonant

techniques and found good agreement. Finally, to ensure the integrity of the measurement

system over time, a measurement assurance plan is outlined. Acknowledgments

The authors acknowledge Dom Vecchia and Hari Iyer for many helpful discussions, and in particular for the development of the uncertainty of the certified value of SRMS. Thanks

also to Jack Wang for providing information regarding bivariate control ellipses.

59
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60
[14] R.A. Waldron, Theory of Guided Electromagnetic Waves, New York : Van Nostrand

Reinhold, 1970.

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