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National Institute of Standards and Technology

NIST Technical Note 7386

Intercomparison of Permeability and

Permittivity Measurements Using the

Transmission/Reflection IVIethod in

7 and 14 mm Coaxial Air Lines

Claude M. Weil

Michael D. Janezic

Eric J. Vanzura

tie

00

NIST Technical Note 1386

Intercomparison of Permeability and

Permittivity Measurements Using the

Transmission/Reflection Method in

7 and 14 mm Coaxial Air Lines

Claude M. Weil

Michael D. Janezic

Eric J. Vanzura

Electromagnetic Fields Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303-3328

March 1997

^^^TES O*

U.S. DEPARTMENT OF COMMERCE, William M Daley, Secretary

TECHNOLOGY ADMINISTRATION, Mary L. Good, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arati Prabhakar. Director
National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Technol., Tech. Note 1386, 44 pages (March 1997)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1997

For sale by the Superintendent of Docunnents, U.S. Government Printing Office, Washington, DC 20402-9325
Intercomparison of Permeability and Permittivity

Measurements Using the Transmission/Reflection Method

in 7 and 14 mm Coaxial Air Lines

Claude M. Weil, Michael D. Janezic, and Eric J. Vanzura

Electromagnetic Fields Division,

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303

We discuss a measurement intercomparison, designed as a follow-up to that reported

by Vanzura et al. In this effort, 13 participants performed broadband (3 MHz to 10

GHz) measurements of the magnetic and dielectric properties of five different ferrite

samples using the transmission/reflection (T/R) method in 7 and 14 mm diameter

coaxial air lines. Agreement within ±5 percent was obtained for the measured

permeability data for fi-equencies between 50 and 100 MHz. However, consistent

with the findings of the earlier study, significant variability (±15 percent) was found

to exist in the permittivity data, due to air-gap effects.

Key Words: coaxial air line; dielectric properties, ferrites; intercomparison; magnetic

properties; materials; measurements; microwaves

1. INTRODUCTION

As part of a major effort to assess the national quality of RF electromagnetic properties of materials

measurements, the National Institute of Standards and Technology (NIST) has organized a number

ofnational measurement intercomparisons involving industry and government laboratories. For these,

it is obviously necessary to select a well-known and standardized measurement technique that

potential participants are familiar with and for which they have ready access to needed fixtures,

instrumentation, and operating soft:ware. The T/R method in coaxial air lines is a well-known

technique that has been implemented in many laboratories for the broadband characterization of

medium-to-high loss materials in the RF-microwave spectrum. It has recently been documented as a standardized measurement method by ASTM [1].

In this method, a toroidal sample of the material under test is precisely machined to the air line's

dimensions and positioned inside the line. Transmission line scattering parameters, both reflected and

transmitted, are then measured over a broad fi^equency range, usually by means of an automatic

network analyzer (ANA). Data on the complex relative permittivity, e' - e/ - je/' and permeability,

^/= ^r" Jl^r" ^6 derived from measured scattering parameter data using various available reduction

algorithms [3-6] (see section 3, below).
The coaxial air line method was selected by NIST for use in two separate measurement

intercomparisons that have been conducted over the past six years. In the first study, 1 1 participants,

including NIST, measured the complex permittivity of five different bulk low-loss dielectrics with e/

ranging from approximately 6.8 to 50. This study, which was completed in 1993 and has been

reported on by Vanzura et al. [2], was primarily intended to compare measurements of e^', the real

part of e/. However, participants were also asked to measure the imaginary part, e" . The ready

availability of very accurate reference data obtained using the NIST 60 mm diameter cylindrical

cavity [6] at X-Band fi-equencies (7 to 11 GHz) was the primary reason that NIST selected low-loss

dielectrics for use in this study. The cavity data provided a very accurate reference against which the

coaxial line data could be compared.

The dielectric study did not provide a very satisfactory comparison of dielectric loss data, e" [2, §

IV B] nor did it include any measurements of complex permeability. It is well known that

characterization methods based on transmission line measurements cannot satisfactorily measure the

dielectric loss of low-loss materials ( e/' < 0.05) due to the low-Q characteristics of transmission line structures and resulting loss measurement insensitivity. Consequently, measured e." data for the lowloss dielectric samples were not very meaningful. NIST did include two medium-loss glass materials,

with €/' t 0.05, in this study. Measurements of these did, therefore, provide some more meaningful

comparisons of e/' data. However, one of these materials (No. 2, lead-oxide glass) proved to be

unreliable owing to problems of inhomogeneity and sample-to-sample variability. Such variability

is a problem in any measurement intercomparison that involves multiple samples of the same material.

Because ofthe inadequacies in the first study, detailed above, NIST sought to organize a follow-up

study involving both complex permeability and permittivity measurements of some bulk

polycrystalline ferrites, using exactly the same measurement methodology. These materials, in their

demagnetized state, exhibit medium-to-high magnetic loss at frequencies below gyromagnetic

resonance, which for most ferrites is at nonmicrowave frequencies less than 1 GHz. In this region,

the coaxial air-line technique is well suited to the characterization of ferrites, so such a measurement

intercomparison should produce more meaningful results. At microwave frequencies above 1 GHz,

near or above gyromagnetic resonance, most demagnetized ferrites exhibit only weak intrinsic

permeability properties (|ir < 1) ^th correspondingly low magnetic losses. Because of these factors,

NIST deliberately tried to emphasize measurements at low frequencies. This is the reason that

participants who possessed a low-frequency ANA were requested to perform their measurements

over the fi^equency range 3 MHz to3 GHz. Those who did not have this type of ANA available were

requested instead to perform measurements over the range 50 MHz to 10 GHz, using the more

commonly available high-frequency ANA.

The materials used in this study are summarized in Table 1 . As in the earlier study, the composition

of the samples was not disclosed to the participants until after the study was completed. For this

study, there were 10 participants who had also taken part in the dielectric study, plus 4 new

participants. NIST experience with all of these materials has shown that significant variability can

exist from sample to sample, so a decision was made at the start of this study that only one kit,

containing 7 mm diameter samples, would be circulated amongst participants, rather than the three

used in the dielectric study. Thirteen participants, who were anonymously coded by the letters "A"
Table 1 . Compositions of intercomparison ferrites with nominal permeability and permittivity

properties as specified by their suppliers.

Material Composition ^r' Hr"

Yttrium iron garnet,

Ms = 142 kA/m

Nickel-zinc ferrite

(Manufacturer A)

Ferrite-loaded polymer

Lithium ferrite,

Ms =135 kA/m

Nickel-zinc ferrite

(Manufacturer B)

134 at 15 ±5% 0.003 IkHz

800 at 77 at IMHz IMHz

3.0 at 0.39 at 17 at 1.2 at IGHz IGHz IGHz IGHz

94 at 17 ± 5% 0.0085

IkHz

1.2 at 12 at 19 at 8.2 at IGHz IGHz IGHz IGHz

through "P", took part in measuring the 7 mm kit; usable data were obtained from 12 participants,

including NIST, and are intercompared in Section 4.

As the study progressed, it soon became apparent that having only one kit in circulation was

unnecessarily impeding the progress ofthe study. NIST also happened to have available some 14 mm

diameter coaxial samples in materials 1, 2, 3, and 5, which had been prepared for other investigations.

In an attempt to speed up the progress of the study, we decided to circulate an additional kit containing these 14 mm samples to a few select laboratories who have the capabilities to perform

measurements in a 14 mm coaxial transmission line. A secondary objective for using 14 mm diameter

lines was to determine whether this improved measurement accuracies at all. Five participants (A,

C, I, M, P) including NIST, took part in measuring the 14 mm kit, of which three (A, C, I) also

measured the 7 mm kit. However, some of these were unable to perform measurements because the

samples did not fit inside their 14 mm coaxial air line; this was true of all samples for Participant I and

of samples 3 and 5 for Participant C. Details regarding the circulation of the two sample kits among

the participants are given in Section 2.

Together with the sample kit, each participant received a cover letter of invitation plus measurement

guidelines giving recommended procedures for performing measurements and reducing and reporting
data, along with a data sheet that participants were asked to fill out. The data sheet was used to

record information on the condition of the samples received, ANA configuration, details of ANA

calibration used, air-line dimensions, laboratory environment, file names, and details of algorithm used

for data reduction, as well as sample dimensions and method of gap correction used, if any (see

Section 3B).

One of the key findings of the dielectric intercomparison [2] is that much of the variability seen in

permittivity measurements can be attributed to whether or not participants corrected for the presence

of air gaps between sample and air-line conductors. Hence, during this study, we tried to make

participants more aware ofthis important issue by bringing it to their attention in the invitation letter and by including updated NIST data on sample dimensions in the participant data sheet (see Section

2). Participants were urged to measure the dimensions of their air-line fixtures and to repeat those

for the samples, if possible, in order to develop their own air-gap estimates and correct for these.

Many investigators, who use the coaxial air-line method, have avoided the need for air gap correction

by using conductive fillers in the air gap. This works well, particularly for high-permittivity materials.

However, because the intercomparison samples are porous and because the fillers contain emulsifiers

that will migrate into the pores, participants were requested to refi^ain fi-om using such fillers during

this study.

2. INTERCOMPARISON SAMPLES

The five ferrite materials selected for use in this study are well known to NIST and have already been

extensively characterized using various techniques. They are of possible interest to NIST as fijture

RF magnetic reference materials. These materials are supplied in the form of tiles, bar stock, or

cylindrical slugs that have been pressed and fired by the ferrite supplier. The 7 mm and 14 mm

diameter toroidal samples were machined fi^om these by a NIST subcontractor.

At the beginning ofthe study and after each round ofmeasurements by a participating laboratory, the

samples were carefully inspected by NIST personnel, replaced or refaced if necessary, dimensionally

remeasured and then recharacterized. Sample replacement or refacing was necessitated by breakage

and cumulative damage that occurred in these very brittle materials during the course of

measurements by both participants and NIST. Following the fifth round of 7 mm kit measurements,

both Samples 1 and 4 were found to be broken. NIST was able to replace Sample 1, but had no

material on hand fi"om which to prepare a replacement for Sample 4. When we were informed by the

ferrite supplier that several weeks lead time would be required to obtain a replacement material, we

decided to withdraw Sample 4 fi^om circulation. Similarly, following the fourth round of 14 mm kit

measurements, Sample 1 was found to be broken and the kit withdrawn fi-om circulation. All samples

in the 7 mm kit were refaced after the sixth and eleventh rounds. Samples 2 and 5 in the 14 mm kit had to be refaced after the first round and Sample 3 was refaced after the third round.

NIST measurements of sample length and diameter were conducted at the NIST-Boulder laboratory

using a coordinate measuring machine (CMM) of specified uncertainty ±1.5 ^m. Measurements of
inner and outer diameter were conducted at positions spaced every millimeter along the sample axis

and an average and standard deviation were computed. These data, were subsequently recorded on

the participant data sheets and furnished to participants.

Tables 2a and 2b provide details of changes made in the samples of both the 7 and 14 mm diameter

kits, including dimensional changes, as well as which particular samples were measured by the various

participants. As in the earlier study, participants are only identified by a letter code (A through P)

in order to preserve the anonymity of measured data. In column 2 of Table 2, a code is used for

sample identification; the first number denotes the original or replaced ferrite material, while the final letter denotes the original (IXXa) or refaced sample (IXXb, c, . .

. , etc.).

Table 2a. NIST dimensional data for 7 mm diameter samples.

MaterialSample

Code

Measured by

Participant:

Length

(mm)

Inner Diameter (mm) Outer Diameter (mm)

1 lYG

2YGa

b

c

A,D,E,F,K

JA,B,C,G,H,I

A,L,N

7.406

10.360

8.900

8.170

3.0505 ± 0.003

3.0524 ±0.002

3.0559 ±0.004

3.0548 ±0.005

6.9930

6.9890 ± 0.002

6.9897 ±0.001

6.9904 ±0.001

2 IFRa

b

c

A,D,E,F,J,K

A,B,C,G,H,I

A,L,N

7.722

6.746

6.607

3.0725 ±0.012

3.0756 ±0.010

3.0657 ±0.010

6.991

6.9859 ± 0.003

6.9884 ± 0.002

3 IMFa

b

A,B,C,D,E,F,

G,H,I,J,K

A,L.N

12.710

12.640

3.0754 ±0.015

3.0754 ±0.015

6.9688 ± 0.002

6.9862 ±0.012

4 ILF A,D,E,F 12.71 3.0780 ±0.009 6.991

5 IZNa

b

c

A,D,E,F,J,K

A,B,C,G,H,I

A,L,N

6.281

5.621

5.614

3.0540 ±0.004

3.0593 ±0.005

3.0562 ±0.004

6.992

6.9874 ± 0.002

6.9888 ±0.001
TABLE 2b. NIST dimensional data for 14 mm diameter samples.

MaterialSample

Code

Measured by

Participant:

Length

(mm)

Inner Diameter (mm) Outer Diameter (mm)

1 lYG A,C,I,M,P 6.931 6.2172 ±0.0076 14.269 ±0.0035

2 IFRa

b

A,M

A,C,I,P

7.052

6.571

6.2152 ±0.0064

6.2144 ±0.0056

14.282 ±0.0021

14.278 ± 0.0030

3 IMFa

b

A,I,M

A,P

12.800

11.020

6.2063 ± 0.0034

6.2081 ±0.0040

14.282 ±0.0071

14.281 ±0.0041

5 IZNa

b

A,M

A,I,P

6.638

3.647

6.1999 ±0.0049

6.2100 ±0.0014

14.273 ±0.0018

14.266 ±0.0035

3. MEASUREMENT METHODOLOGY

3.1 Data-Reduction Algorithms

The algorithm often used for deriving complex permittivity and permeability parameters from the

measured S-parameters is that based on the explicit solution of Nicolson and Ross [3] and Weir [4].

It has been incorporated in the software of a major commercial instrument supplier and is widely

used. As pointed out in the supplier's descriptive literature [5], this algorithm generally performs

satisfactorily for medium- and high-loss materials, particularly ferrites.

One ofthe findings ofthe earlier dielectric intercomparison [2, § IIIA] is that this algorithm does not

work well for low-loss dielectrics. This is because the Nicolson-Ross-Weir (NRW) solution is very

sensitive to the inevitable phase errors in S-parameter measurements that arise whenever resonances of the sample-under-test are approached. These instabilities are manifested as periodic off-scale

departures or "drop-outs" in the permittivity versus frequency plots. The lower the dielectric loss of

the material under test is, the greater the Q of the resonant system is and the greater the magnitude

of the instabilities are. Iterative algorithms, such as the NIST-developed code EPSMU3 [6], are

better able to handle low-loss dielectrics. Data from the dielectric intercomparison [2] showed that

those who had used an iteratively based code (all but two participants) obtained more stable and more

accurate results. For high-loss materials, such as ferrites in the low-frequency regime, the resonances

are largely damped out, so that the instabilities are no longer as apparent.
3.2 Air-Gap Corrections

As already discussed, air gaps between the inner conductor and sample inner diameter (inner gap) and

between sample outer diameter and outer conductor (outer gap) must be dimensionally estimated and

corrected for. This requires dimensional measurements of the sample's inner and outer diameter, as well as the inner and outer conductor diameter ofthe coaxial air line. The required correction, which

increases significantly as material permittivity increases, is usually determined using a static coaxial

capacitor model; this correction is included as an optional feature of the NIST software. In this

model, the gaps are assumed to be of uniform thickness and are represented by two concentric

capacitors in series [6, p. 111]. Because the electric field intensity is greatest near the center

conductor, the correction for the inner gap is much greater than that for the outer gap.

4. DISCUSSION OF MEASUREMENT RESULTS

Intercomparison measurement results are shown in Figures 1 through 5, corresponding to each ferrite

material measured. Table 3 relates the plot symbols used in Figures 1 through 5 with participant

codes; the figure in parentheses denotes measurements of the 7 or 14 mm sample kit. Participants

N and P are not included in Table 3 because, although they took part in the measurements, they did

not submit usable data. Details ofthe reduction algorithm and method of air-gap correction used by

the participants are also given in column 3 of Table 3 where EPSMU3 denotes use of the NISTdeveloped iterative code [5]. The numbers following, where provided, denote the version of the

NIST software used. In most cases, these were early versions, such as Version 1.1, which have

exhibited some problems of numerical implementation. Later versions of the NIST software (the

latest is Version 3.4) contain significant improvements that have overcome the early problems. In column 3, "internal" denotes use of an internally developed code that is usually proprietary to the

participant. In some cases, a few details of these codes were provided to NIST; they are generally

based on the explicit NRW solution [3,4].

Column 4 gives details of whether or not participants applied an air-gap correction to their data.

Where participants used the NIST code, they applied the gap correction feature available in the

software. Corrections based on the same model were also available in some, but not all of the

internal codes. Gap widths were computed by participants using either the sample dimensions

supplied by NIST as well as their own coaxial line dimensions or sample and line measurements

performed by participants. The latter case is denoted in Table 3 by "internal data." With some

exceptions, most participants provided no details of how they performed dimensional measurements.

4.1 Reference Data

It is essential to provide the reader with some accurate reference points for these materials, against

which measured data can be compared. This is particularly important where significant variation in the measured data is apparent. Such data need to have been generated using a technique that is more
accurate than the coaxial air line method used in this study. In contrast to the earlier permittivity-only

intercomparison study [2], where accurate X-band comparison data obtained using the NIST 60 mm

diameter cylindrical cavity [7] were available for the low-loss dielectrics used in that study, it has not

been straightforward to compile needed reference data for these materials. This is because a variety

Table 3. Key relating data symbol used to participant codes, plus details of data-reduction algorithms

and gap correction used (if any).

Participant

Code

Symbol Algorithm Type Air-Gap Estimation and Correction

A (7)

A (14)

B(7)

C(7)

C(14)

D(7)

E(7)

F(7)

G(7)

H(7)

1(7)

J (7)

K(7)

L(7)

M(14)

Reference

Data

-OX

EPSMU3

EPSMU3

EPSMU3A

EPSMU3, 1.1 EPSMU3, 1.1

Int. (NRW)

Internal

Int. (NRW)

EPSMU3

EPSMU3, 1.1

Int. (NRW)

Int. (NRW)

& EPSMU3

Internal

CMM measurements, gap corrected

CMM measurements, gap corrected

Internal data, gap corrected

Used NIST data, gap corrected

Used NIST data, gap corrected

Not gap corrected

Not gap corrected

CMM measurements, gap corrected

Internal data, inside gap

corrected only

Used NIST data, gap corrected

Internal data, gap corrected

Internal data, gap corrected

Inner dia. estimated with gauge pins,

inside gap corrected only

Int. (NRW) Not gap corrected

Internal No gap correction, used silver

paste

Permeameter

Other

(see 4.1)
of alternate techniques are needed to derive the required reference permeability and permittivity data over the very broad 1 MHz to 10 GHz frequency range used in this study. These are reviewed below.

At the lowest frequencies, 0.3 to 100 MHz, |i/ and e' measurements are usually made using some

type of impedance measuring technique involving an inductance-resistance measurement from which

the complex permeability of magnetic materials is derived or a capacitance-conductance measurement

from which the complex permittivity of dielectric materials is derived [8]. Though modem impedance

analyzers work satisfactorily up to frequencies greater than 1 GHz, their application in accurate

material measurements appears limited to less than 300 MHz. In two very similar methods, termed

the permeameter [9] and permittimeter [10], a toroidal sample of the material under test is mounted

inside the identical 7 or 14 mm diameter coaxial air lines used in this study. This means that the

actual intercomparison samples can be conveniently measured in this way. NIST obtained some

useful low frequency \i* reference data on Materials 2, 3, and 5 using the permeameter method. We

did not attempt to use the permittimeter method for deriving low frequency e* reference data,

because it is very prone to air gap-errors.

Accurate measurements of both complex permeability and permittivity can be realized in the midfrequency region, 50 to 1000 MHz, using the coaxial re-entrant cavity method [11]. For permeability

measurements, a toroidal sample offerrite is placed at the end of the cavity [12]. However, we were

unable to perform any permeability measurements using this method, because insufficient material was

available to fabricate the large toroid needed for such measurements. Using the re-entrant cavity

method, in which a disc sample ofthe ferrite under test is placed in the center conductor gap, we were

able to obtain a single-frequency e* reference data point for Materials 2, 3, and 5, which is accurate

to v^thin ±2 percent. For these measurements, the upper and lower faces of the sample were coated

with silver paste in order to reduce the air gap errors.

Another usefiil technique for mid-frequency permeability measurements is the air-filled stripline

resonator method [13]. Because the method relies on small-perturbation theory for computation of

n/ and e' [14], it is not considered as accurate as the re-entrant cavity method and is not

recommended for permittivity measurements. During the past five years, NIST has evaluated this

technique and has used it for characterizing ferrites, including Materials 3 and 5 [13].

For the microwave region, 1 to 10 GHz, the most accurate and most sensitive method for

characterizing demagnetized ferrites is the dielectric-post resonator (or "Courtney") method [15], in

which a rod of the ferrite under test is resonated in the Hon niode between two parallel conducting

plates. NIST has obtained extensive |i/ data on a variety of ferrites, including Material 1, over the

range 2 to 20 GHz, using a variation of this technique [16]. Accurate characterization of ferrite

permittivity in this frequency range requires that measurements be performed in the presence of a

large (minimum 1200 kA/m) magnetic biasing field, as discussed by Courtney [15]. Data on the

complex permittivity properties of a number of garnets are given in Courtney's paper [15, Tables IV,
V] and have been included in Figure le for Material 1. NIST is currently fabricating a fixture

specifically designed to measure the microwave dielectric properties of ferrites.

4.2 Complex Permeability Data

Measured data on relative permeability, ^r' ^re presented in Figures la through 5a for all five

materials using linear-log scales and in Figures lb, 2b, and 5b for Materials 1, 2, and 5 using log-log

scales. Logarithmic scales allow for permeability data to be displayed over the full amplitude range

of 0.01 <, [i^ <. 1000 that the transmission line technique is capable of resolving. However, because

of logarithmic scale compression, these measurement data may appear to agree better than they really

do. Similarly, measured data on magnetic loss, li" are presented in Figures Ic through 5c using

linear-log scales and in Figures Id, 2d, and 5d using log-log scales. For Materials 2, 3, 4, and 5 the

measured \x* data generally agree within ±5% of mean for fi-equencies between 50 and 1000 MHz,

but much increased variability (up to ±20% or more of mean) is seen in both the (i/ and \i" data as

fi-equency is reduced below 50 MHz, particularly for Material 2. An uncertainty analysis performed

by NIST for the transmission line method [6, §2.5], shows that the measurement accuracy for both

\i' and e/ degrades rapidly when the normalized sample length, L/A„ < 0.2, due to the inability to

resolve small phase differences. This is therefore consistent with the increased measurement

variability seen below 50 MHz. For Material 1 (YIG), significantly greater variability (±25% of

mean) is seen in the \i* data (see Figures la through Id) throughout the 0.003 to 1 GHz spectrum.

The reasons for this are not clear, but may be caused by a well-known temperature sensitivity in

YIGs.

The low-fi-equency |i/ comparison data, obtained using the permeameter method [9] are included in

Figures 2a through 2d, 3a,c, and 5a through 5d; the uncertainty of these measurements is estimated

to be ±1 .5%. With the exception ofFigure 2c, Participant A agreed very closely with these data. For

Participants D and M, who were the only others to also attempt measurements below 50 MHz, the

agreement with the comparison data was generally not as good.

Selected mid-fi^equency \i* comparison data, obtained using the stripline resonator method [13], are

also included with error bounds in Figures 2a through 2d, 3a,c and 5a through 5d. The agreement

for Material 5 is very close for all participants, whereas that for Material 3 is not as good. The only

applicable microwave comparison data available [15] are for Material 1 (YIG) and are shown in

Figures lb and Id.

In general, it is seen that most measured \i* data lie within the error bounds of the comparison data,

thereby providing confidence in the validity of the participants' measured permeability data using the

coaxial air line method.

10
4.3 Complex Permittivity Data

Measured data on relative permittivity e/ are presented in Figures le through 5e for all five materials

using linear-log scales. These data show that there is generally more variability in the e/ data (±15

% about the mean) than in the [ij data. Comparison e/ data, obtained using the coaxial re-entrant

cavity method [1 1] at about 500 MHz, are shown with error bounds (±2%) in Figures 2e, 3e, and 5e

and it is seen that most of the measured data lie well below these reference levels. However, some

ofthe participants obtained comparable results; the data for Participant A consistently lay within the error bounds of these comparison measurements for Materials 2, 3, and 5. Similarly, the data of

Participant M agreed within these limits for Materials 3 and 5 and was only slightly low for Material

2. The data of Participants C, F, H, and I also agreed within the error bounds of the comparison

data, but for only one material.

The only comparison e/ data available for Material 1 are shown on Figure le at about 6 GHz. It is obviously impossible to draw any meaningfiil comparisons with measured data at this frequency.

However, because of its very low dielectric loss properties. Material 1 (YIG) exhibits an almost

constant e,' value with frequency. Hence, it can be reasonably assumed that the comparison value

for this Material lies in the range e/ = 1 5.7 to 1 5.9 throughout the frequency range measured. Figure

le shows that the measured data of Participants A and H approach the closest to this level.

Reference to Table 3 shows that Participants A, C, F, H, and I, discussed above, included air-gap

corrections, using either their own internally generated dimensional data or the NIST-provided data.

Participant M was unaware of NIST instructions not to use any conductive fillers and used a silver

paste as a gap-correction technique. This further confirms one of the principal findings of the earlier

dielectrics-only study [2], that air-gap correction is essential for accurate complex permittivity

measurements using the coaxial air line method.

Participants, A, C, and M who used the 14 mm diameter coaxial air line appeared to obtain e/ data

that were consistently more accurate than those obtained using 7 mm diameter lines. This therefore

suggests that use of the larger diameter 14 mm coaxial line yields somewhat more accurate e/ data.

This is consistent with earlier findings that the influence of the air gap is proportionally reduced in

larger-diameter air lines [6].

Measured data on dielectric loss e/' are similarly presented in Figures If through 5f For Materials

2 and 3, which possess measurable dielectric loss, the variability in measured dielectric loss, e" is seen

in Figures 2f and 3f to be large, approaching ±50% or more about the mean at 100 MHz. For the

remaining Materials 1, 4, and 5, which are all low-loss dielectrics, the e" data are seen in Figures If,

4f, and 5f to be meaningless. This again confirms what was learned in the earlier permittivity-only

study [2], that the transmission line technique does not have sufficient measurement sensitivity to

characterize low-loss dielectrics accurately.

Analysis ofboth the \i* and e* data shows that either type of reduction algorithm (see Section 3.1)

appeared to work equally well when measuring these materials at nonmicrowave frequencies, less

11
than 1 GHz. The sudden data drop-outs seen in Figures 2b, 2d, 3a, 3c, 3d, 5a, and 5c show evidence

that the data-reduction algorithms used by Participants D, L, and M lost the correct root during the

reduction process. All three used some type of internally developed algorithm for which few details are known. In most ofthe plots, there is also much evidence of algorithm instabilities around 10 GHz

(for Material 1, these begin at about 6 GHz). These instabilities are most apparent for Materials 1 and 5, which possess relatively low dielectric loss. Such instabilities are attributed to TEM mode and

higher-order mode resonances, that are no longer damped out by the high magnetic losses which

exist at the lower frequencies, and are apparent for both type of algorithms.

5. CONCLUSIONS

This study has demonstrated that the coaxial air line technique very accurately measures the complex

permeability properties of ferrites at frequencies above 50 MHz, but below the frequency of

gyromagnetic resonance for the ferrite. For this specific application, the study demonstrated that all participants were able to obtain very accurate results using it, so it remains the preferred measurement

method and is considered superior to any other. For the critical low-frequency region, below 50

MHz, where ferrites exhibit very significant magnetic properties with |i/ > 300, the method rapidly

loses accuracy due to its inability to resolve small phase differences. In this region, the permeameter

technique [9] provides more accurate results and is preferred. For the microwave region, at frequencies equal to or above gyromagnetic resonance, the method is still remarkably capable of

resolving values of jir and \i" down to approximately 0.01. However, due to its obviously inadequate

measurement sensitivity, other measurement techniques such as the dielectric post resonator [16] are preferred for this frequency region.

The study also further confirmed the findings of the earlier NIST study [2] that use of the coaxial air line technique for performing broadband complex permittivity measurements of dielectrics remains

problematic. The air gap remains a major source of error for €/ measurements and most participants were unable to adequately correct for it, despite our best efforts to alert them to the importance of

this issue. Furthermore, the technique cannot satisfactorily resolve the dielectric loss of low-loss

materials, including many ferrites, due to its inadequate measurement sensitivity. For measurements

above about 300 MHz, other resonator-type methods, such as the coaxial re-entrant cavity [1 1] or

the dielectric rod resonator [15-17], provide much greater accuracy and sensitivity and are much

preferred. While these methods are not broadband, they are nonetheless capable of providing multiple

fi^equency data using a variety of different methods, such as cavity tuning and operation in different

resonator modes.

For low frequencies in the range 3 to 300 MHz, the permittimeter technique [10] provides the best

accuracy and sensitivity for low-loss dielectrics, provided that air-gap corrections are properly

applied. When correctly used, the coaxial air line method works satisfactorily for medium-to-high

loss materials at fi-equencies above 3 MHz. This study provided limited evidence that larger diameter

(14, 77.5, and 155 mm) coaxial structures at these frequencies result in somewhat more accurate

measurements of e

12
Some of the internally developed reduction algorithms appeared to exhibit difficulties in staying on

the correct root. The reasons for this are unclear. Both types of reduction algorithms used by

participants, which are based either on the explicit NRW solution or the implicit iterative technique,

generally appeared to work equally well for these materials.

6. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the significant contributions of the following NIST personnel:

James Baker-Jarvis and Bill Riddle for re-entrant cavity measurements, Richard Geyer for dielectric

post-resonator data, John Grosvenor, Jr., for some of the sample characterization measurements,

Chriss Jones for stripline resonator data, and Glen Sherwood for dimensional metrology of samples.

The assistance of Bud O'Connor of Colorado Precision Optics, who prepared the intercomparison

samples, is also acknowledged. Last, but not least, the authors recognize the significant support

provided by the following organizations who voluntarily participated in this measurement

intercomparison: Boeing Defense and Space Group, Seattle, WA; Eaton/AIL Systems, Deer Park,

NY; Georgia Tech Research Institute, Atlanta, GA; Hewlett-Packard Co., Santa Rosa, CA; Hughes

Electro-Optical Systems, El Segundo, CA; Lockheed Martin Corp., Sunnyvale, CA; McDonnell

Douglas Aerospace, St. Louis, MO; Naval Research Laboratory, Washington, DC; NorthropGrumman Corp., Pico Rivera, CA and Bethpage, NY; SRI International, Menlo Park, CA; Texas

Instruments Inc., McKinney, TX; 3M Corp., St. Paul, MN; Wright Laboratory, WPAFB, OH.

7. REFERENCES

[1] ASTM Standard D 5568-95, Standard test method for measuring relative complex

permittivity and relative magnetic permeability for solid materials at microwave frequencies.

In 1996 Annual Book of ASTM Standards, Vol. 10.02, Electrical Insulation (II). ASTM,

West Conshohocken, PA 19428.

[2] Vanzura, E.J.; Baker-Jarvis, J.R.; Grosvenor, J.H., Jr.; Janezic, M.D. Intercomparison of

permittivity measurements using the transmission/reflection method in 7 mm coaxial

transmission lines, IEEE Trans. Microwave Theory Tech. 42: 2063-2970; Nov. 1994.

[3] Nicolson, A.M.; Ross, G.F. Measurement of the intrinsic properties of materials by time

domain techniques, IEEE Trans. Instrum. Meas. IM-19: 377-382; Nov. 1970.

[4] Weir, W.B. Automatic measurement of complex dielectric constant and permeability at microwave frequencies, Proc, IEEE 62: 33-36; Jan. 1974.

[5] HP 85071 A Materials Measurement Software, Hewlett-Packard Co. (1990).

13
[6] Baker-Jarvis, J.R.; Janezic, M.D.; Grosvenor, J.H., Jr.; Geyer, R.G. Transmission/

reflection and short-circuit line methods for measuring permittivity and permeability, Natl.

Inst. Stand. Technol. Tech. Note 1355 (revised); Dec. 1993.

[7] Vanzura, E.J.; Geyer, R.G.; Janezic, M.D. The NIST 60-mm diameter cylindrical cavity resonator: Performance evaluation for permittivity measurements, Natl. Inst. Stand. Technol.

Tech. Note 1354, Aug. 1993.

[8] New technologies for wide impedance range measurements to 1.8 GHz, Product Note No.

4291-1, Hewlett-Packard Co. (1994).

[9] Goldfarb, R.B.; Bussey, H.E. Method for measuring complex permeability at radio

frequencies. Rev. Sci. Instrum. 58: 624-627; April 1987.

[10] Powell, R.C.; Rasmussen, A.L. A radio-frequency permittimeter, IRE Trans. Instrum. 8: 179-184; Sept. 1960.

[1 1] Works, C.N. ; Dakin, T. W. ; Boggs, F. W. A resonant cavity method for measuring dielectric

properties at ultrahigh frequencies, ALEE Trans. 63: 1092-1098; 1944.

[12] Harrington, R.D.; Powell, R.C.; Haas, P.H. A re-entrant cavity for measurement of

complex permeability in the very-high-frequency region, Nat. Bur. Stand. (U.S) J. Res. 56:

129-133; March 1956.

[13] Jones, C.A.; Kantor, Y.; Grosvenor, J.H., Jr.; Janezic, M.D. Stripline resonator for

electromagnetic measurements of materials, to be published in Natl. Inst. Stand. Technol.

J. Res.; 1997.

[14] Waldron, R.A. Theory of strip-line cavity for measurement of dielectric constants and

gyromagnetic-resonance line-widths, IEEE Trans. Microwave Theory Tech. MTT-12: 123-

131; Jan. 1964.

[15] Courtney, W.E. Analysis and evaluation of a method of measuring the complex permittivity

and permeability of microwave insulators, IEEE Trans. Microwave Theory Tech. MTT-18:

476-485; Aug. 1970.

[16] Krupka, J.; Geyer, R.G. Complex permeability of demagnetized microwave ferrites near and above gyromagnetic resonance, IEEE Trans. Magn. 32: 1924-1933; May 1996.

[17] Kobayashi, Y.; Katoh, M. Microwave measurement of dielectric properties of low-loss

materials by the dielectric rod resonator method, IEEE Trans. Microwave Theory Tech.

MTT-33: 586-592; July 1985.

14
0.01 0.1

Frequency (GHz)

Figure la. Measured relative permeability, |i/ for Material 1 (linear)

15
100 '^

3

<D

10

0.1

0.01

0.001 0.01 0.1 1

Frequency (GHz)

Figure lb. Measured relative permeability, n/ for Material 1 (log)

16
70

60

50

I

40

| 30

a

^ 20

10

0.01 0.1 1

Frequency (GHz)

10

Figure Ic. Measured magnetic loss, n/' for Material 1 (linear)

17
100

10

in

O

•1—

(

OX)

0.1

0.01

I I I J I

I I

I I I I M I I II I I I I 0.001 0.01 0.1 1

Frequency (GHz)

10

Figure Id. Measured magnetic loss, n/' for Material 1 (log)

18
18

17

C^ 16

I 15

S

fc 14

13 -

12

0.01 0.1 1

Frequency (GHz)

10

Table le. Measured relative permittivity, e,' for Material 1

19
0.5

0.4

o

O

o

<D

1—1

Q

0.3

0.2

0.1

-0.1

0.01 0.1 1

Frequency (GHz)

10

Figure If. Measured dielectric loss, e," for Material 1

20
450

400

350

=1 300

•1—

1

250

•1—

<

Xi

200

B

l-H (U 150

100

50 -

0.01 0.1

Frequency (GHz)

Figure 2a. Measured relative permeability, (i,' for Material 2 (linear)

21
1000 F

=3.

a

Ph

0.01 0.1 1

Frequency (GHz)

Figure 2b. Measured relative permeability, \i,' for Material 2 (log)

22
250

200

1/5 HI

iJU

O

hJ

o•i-H S-J

PI 100

dO

c^

50

0.01 0.1 1

Frequency (GHz)

10

Figure 2c. Measured magnetic loss, n," for Material 2 (linear)

23
100

t/1 O

10

0.1 J l__J I I I I I I I

I I A I I L-_J I

I I " I I 0.001 0.01 0.1

Frequency (GHz)

10

Figure 2d. Measured magnetic loss, ^i/' for Material 2 (log)

24
18

17

16

^

15

•S 14

S 13

12

11

10

0.01

J \ \ I I'll

0.1 1

Frequency (GHz)

10

Figure 2e. Measured relative permittivity, e/ for Material 2

25
O

o

« 2

Q

0.01 0.1 1

Frequency (GHz)

10

Figure 2f. Measured dielectric loss, e" for Material 2

26
2

i:1h

0.01

J I I I

I I I I I

. I I I I I'll

0.1 1

Frequency (GHz)

10

Figure 3 a. Measured relative permeability, ji/ for Material 3

27
2.5 -

2 -

III O

03

1.5

0.5

0.01 0.1 1

Frequency (GHz)

10

Figure 3 c. Measured magnetic loss, ^i," for Material 3

28
18

16

vu

>

14

g 12

<u

10

8

0.01

I I I I I I I J I I I I'll

0.1 1

Frequency (GHz)

10

Figure 3e. Measured relative permittivity, e/ for Material 3

29
J/J O

o

o

Q

-1

0.01

J I I I I'll

0.1 1

Frequency (GHz)

10

Figure 3f. Measured dielectric loss, e/ for Material 3

30
60

50

40

3 30

g

^ 20

10

0.01

J I I

I I I L 0.1

Frequency (GHz)

Figure 4a. Measured relative permeability, ^r' ^^ Material 4 (linear)

31
100

(U

10

1

0.1

0.01

J I I I I I J I I I I I KEnaa^ '

' I

0.1 1

Frequency (GHz)

10

Figure 4b. Measured relative permeability, \i,' for Material 4 (log)

32
25

20

15

O

10

i^ 5

-5 J I I I

1 1

1

0.01

J I I I I'll

0.1 1

Frequency (GHz)

10

Figure 4c. Measured magnetic loss, n/' for Material 4

33
20

19

18

V*J 17

>-l

-M

•»—

1

>•1-H 16 'M

-«-^ •1^

a

15

Ph

14

13

12

^^

0.01

TM

J I I \ I I I I I

\ I I I L-L J I L-JU I nil 1

1

0.1 1

Frequency (GHz)

10

Figure 4e. Measured relative permittivity, e/ for Material 4

34
0.5

vu

1/2 O

J

o• f—

I

in

Q

-0.5 -

-1

0.01 0.1 1

Frequency (GHz)

10

Figure 4f. Measured dielectric loss, e/' for Material 4

35
350

300

^^ 250

3

200 •<—

1

05

B

150

l-H <u Oh

100

50

0.01 0.1

Frequency (GHz)

1

Figure 5 a. Measured relative permeability, n/ for Material 5 (linear)

36
1000

100

2;.

a

10

0.1

0.01

0.001 0.01 0.1 1

Frequency (GHz)

10

Figure 5b. Measured relative permeability, ^; for Material 5 (log)

37
180

160

140

120

ifl o 100

hJ

u• 1-H 80

<L> a

<^

cd 60

40

20

J I

I I I I I

I I I I I'll

0.01 0.1 1

Frequency (GHz)

10

Figure 5c. Measured magnetic loss, n/' for Material 5 (linear)

38
100

10

O

a

0.1

0.01

0.001 0.01 0.1

Frequency (GHz)

10

Figure 5d. Measured magnetic loss, |i/' for Material 5 (log)

39
14

13

I 12

Oh

11

10

0.01 0.1 1

Frequency (GHz)

10

Figure 5e. Measured relative permittivity, e/ for Material 5

40
1.5

vum

O

J

in

Q

0.5

-0.5

0.01

J I

0.1 1

Frequency (GHz)

H

10

Figure 5f. Measured dielectric loss, e/' for Material 5

41

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