Magneto-dielectric Characterization and Antenna Design

Kyu Han

1,2, Madhavan Swaminathan

1,2, P. Markondeya Raj

3, Himani Sharma

3, Rao Tummala

3 and Vijay Nair

4

1Interconnect and Packaging Center (IPC), SRC Center of Excellence @ GT

2School of Electrical and Computer Engineering, Georgia Institute of Technology

266 Ferst Drive, Atlanta, GA 30332 USA

Email: khan7@gatech.edu 3Packaging Research Center, Georgia Institute of Technology

4Intel Corporation, Chandler, Arizona

Abstract

Antenna size has fundamental limits based on the

frequency of operation and performance required. In the past,

various methods have been developed to miniaturize antennas

with limited success. Magneto-dielectric materials, however,

have been reported as providing new opportunities for

effective antenna size reduction in many recent studies. In this

paper, a novel material characterization method which is a

cavity perturbation technique (CPT) with substrate integrated

waveguide (SIW) cavity resonator is presented for measuring

electric and magnetic properties of magneto-dielectric

material. CPT formulas for extracting complex permittivity

and complex permeability are explained and modification

process using 3D EM simulation tool is discussed. Design and

fabrication of SIW cavity resonators is presented. The

frequency dependent properties of permittivity and

permeability for synthesized magneto-dielectric material are

extracted in the frequency range of 1-4 GHz. Planar inverted

F antenna (PIFA) working at 1GHz on magneto-dielectric

substrate has been designed and simulated in this paper.

Introduction

In wireless communication systems, size of mobile device

is a key specification. Since the size of the antenna is

determined by its electrical length, miniaturization of the

antenna can be very challenging. One method for decreasing

the antenna size is by using high permittivity materials [1].

However, using the high dielectric constant material as the

antenna substrate leads to narrow bandwidth and low

efficiency [2]. Recently, magneto-dielectric materials, which

have both permittivity and permeability greater than 1, have

stirred the interest of antenna designers since the material can

reduce antenna size without deteriorating antenna

performance [3].

Magneto-dielectric materials are not available readily in

nature and have to be realized using material synthesis where

magnetic metal particles are mixed with low loss dielectric

materials. Since the antenna response is affected by the

frequency dependent permeability and permittivity of the

material, an accurate method is required to extract the

frequency dependent properties (ε’, ε”, μ’ and μ”) of the

magneto-dielectric material. This can be challenging since the

electric and magnetic properties need to be separated through

measurements. This has been achieved by using two different

structures, as described in [4] and [5]. One structure is

sensitive to change in permittivity and electric loss tangent

while the other structure is sensitive to change in permeability

and magnetic loss tangent. In this paper, a cavity perturbation

technique (CPT) is used to measure both electric and

magnetic properties using a single substrate integrated

waveguide (SIW) structure. Simulation and measurements

have been used to design the SIW and demonstrate the

characterization process on a magneto-dielectric material

synthesized using nano-cobalt magnetic particles in a polymer

dielectric material. The frequency dependent permittivity and

permeability have been extracted for this material in the

frequency range 1-4 GHz. The second part of the paper uses

the magneto-dielectric material for PIFA design and presents

improvement in size reduction, bandwidth and radiation

efficiency. The sensitivity of the material parameters for fine

tuning the antenna is discussed, to determine the parameters

that have the largest effect on antenna performance.

CPT for SIW

CPT is a well-known method for extracting

electromagnetic properties of dielectrics, semiconductors,

magnetic materials, and composite materials [6]. Permittivity

and permeability of the magneto-dielectric sample can be

calculated from changes in the resonant frequency and quality

factor by introducing the sample at positions where the

electric field and magnetic field are maximum in the cavity,

respectively. For complex permittivity measurements,

modified CPT formulae from [7] can be used which are:

(

)

. (1)

(

) (

)

. (2)

where ε’s and ε”s correspond to real and imaginary

permittivity of the sample, respectively, ε’r and ε”r are real

and imaginary part of relative permittivity of cavity substrate.

Qo and Qs are the quality factors of the empty and loaded with

sample cavity. fo and fs are the resonant frequencies before

and after the sample perturbation, respectively. Vc is a volume

of the cavity and Vs is a volume of the sample. In the above

equations constants A and B are obtained experimentally by

using standard samples with known dielectric properties.

Similarly, for complex permeability measurement, equation (3)

and (4) can be used [6].

(

) . (3)

(

). (4)

978-1-4799-2407-3/14/$31.00 ©2014 IEEE 782 2014 Electronic Components & Technology Conference

where μ’s and μ”s are real and imaginary part of permeability

of the sample where constants C and D in equations (3) and (4)

are also obtained from the measurement of standard sample.

Design of SIW Cavities and CPT Analysis

SIW technology has been used to implement cavities for

CPT in this paper, as shown in Fig. 1, since they have high Q,

are highly sensitive to material properties and have minimum

radiation effect. This technology has been used to measure

complex permittivity of dielectric materials in [7] and [8]. The

resonant frequency of these resonators for TEm0k mode is

related to the width W and the length L of the cavity as

follows [7]:

√

√(

) (

) (5)

where c is the speed of light in free space, ε’r and μ’r are the

relative permittivity and permeability of SIW substrate

respectively and m, k are mode numbers.

Figure 1. TE103 mode SIW cavity resonator with GSG probe

excitation.

The SIW cavities are designed for the dominant TE102 or

TE103 mode. TE102 mode can be used to measure material

properties in the low frequency range since it can reduce the

SIW cavity dimension. Fig. 2 shows the electric and magnetic

field distribution in the SIW with TE103 mode. The cavity is

excited at one of the maximum positions of the electric field

where a GSG probe is used for the excitation, with the signal

probe on the center patch and ground probes on the SIW on

either side (corner-to-corner probing). In the CPT, placing the

sample at either the E or H-field maximum position as shown

in Fig. 2 with dashed circles, changes the resonance frequency

of the SIW cavity based on the sample’s permittivity and

permeability, respectively. During measurements it is required

that, the permeability of the sample does not perturb the

permittivity measurement and also that the permittivity of the

sample does not affect the permeability measurement.

Separating the permittivity and permeability parameters can

be a very challenging process during characterization of

magneto-dielectric materials. 3D EM simulations with CST

microwave studio have been used in this paper to design the

SIW cavity with appropriate locations for excitation and

sample placement such that the permittivity and permeability

measurements have minimum effect on each other. The

results of simulations for the SIW cavity resonating at ~2GHz

is shown in Fig. 3 and 4 where the complex permittivity and

permeability parameters are varied to demonstrate that these

parameters are isolated from each other. In these figures, εr’

and μr’ of the sample are varied between 6-8 and 1-3

respectively while both electric and magnetic loss tangent of

the sample are varied from 0.01 to 0.03. As shown in Fig. 3,

magnetic properties are not sensitive to the resonant frequency

and quality factor when the sample is located at the E-field

maximum position. Similarly electric properties of the sample

are not sensitive to changes in the cavity response when the

sample is located at the H-field maximum position as shown

in Fig. 4.

(a) (b)

Figure 2. Field distribution of TE103 mode SIW cavity (a) E-

Field, (b) H-Field (Circle shows E and H-Field maximum).

(a)

(b)

(c)

(d)

Figure 3. Parameter sweep when the sample is at E-field

maximum position: (a) εr, (b) µr, (c) electric loss tangent, (d)

magnetic loss tangent.

Re

turn

lo

ss,

S1

1 (

dB

)

Frequency (GHz)

Frequency (GHz)

Re

turn

lo

ss,

S1

1 (

dB

)

Frequency (GHz)

Re

turn

lo

ss,

S1

1 (

dB

)

Frequency (GHz)

Re

turn

lo

ss,

S1

1 (

dB

)

783

(a)

(b)

(c)

(d)

Figure 4. Parameter sweep when the sample is at H-field

maximum position: (a) εr, (b) µr, (c) electric loss tangent, (d)

magnetic loss tangent.

Fabrication of SIW Cavities

In order to validate the simulation, FR4 material was used

to fabricate SIW cavities in [9], Rogers 3003 material which

has parameters εr’=3.0, tanδ=0.0013 (electric loss tangent) has

been used instead in this study. Thickness of the cavity is

1.524mm. Since Rogers 3003 has less dielectric loss than FR4

material, cavity with Rogers 3003 has higher Q factor than

cavity with FR4. This advantage can give better accuracy for

loss tangent extraction. Seven SIW cavities were designed and

fabricated to characterize the magneto-dielectric sample in the

frequency range 1-4 GHz, as shown in Fig. 5. Table 1 shows

the SIW cavity operating mode, resonant frequency and

dimension. Fig. 6 shows the fabricated SIW cavity which is

resonating at 3 GHz with drilled hole for sample insertion.

Dimension of the hole is 6 x 6mm2. SIW cavity in Fig. 6 (a)

has a hole at the E-field maximum position and this cavity

was used for permittivity measurement. Another SIW cavity

in Fig. 6 (b) has a hole at the H-field maximum position and it

was used for permeability measurement.

Figure 5. Fabricated SIW cavities with different resonant

frequency

Figure 6. SIW cavity with different holes location for (a)

permittivity (b) permeability measurements.

Table 1. SIW cavity specification

Resonance

(GHz)

Operation

Mode

W (mm) L (mm)

1 TE102 242 126

1.5 TE102 162 86

2 TE103 190 66

2.5 TE102 117 45

3 TE103 123 41

3.5 TE103 109 35

4 TE103 93 31

Magneto-dielectric Material Synthesis

The magneto-dielectric material used in this study was

synthesized using high volume loading (50-70 vol %) of

cobalt nano-particles in a dielectric polymer matrix. [10],

where it shown in Fig 7 (a). Higher permeability at high

frequencies can be achieved by reducing the metal particle

size and the separation between adjacent metal particles down

to the nano-scale [11]. The partially-oxide-passivated cobalt

nanoparticles were commercially obtained from US

Nanomaterial as powders while the polymer was obtained

from Asahi Inc. The hard metal aggregates of cobalt were

broken down to their primary particle sizes of ~20-30 nm

using ball-milling process. As-received metal powders were

suspended in anhydrous toluene solvent and milled for 10-15

hours with zirconia balls to break the aggregates. The

dielectric polymer was then added to the suspension and

Re

turn

lo

ss,

S1

1(d

B)

Frequency (GHz)

Frequency (GHz)

Re

turn

lo

ss,

S1

1(d

B)

Re

turn

lo

ss,

S1

1 (

dB

)

Frequency (GHz)

Re

turn

lo

ss,

S1

1 (

dB

)

Frequency (GHz)

1.5GHz

1.5GHz

3.5GHz

1GHz

4GHz 3GHz

2GHz Hole for

sample Insertion

(a)

(b)

784

milled again for 4-6 hours to ensure complete homogenization

of the polymer and the metal particles. The final polymer-

metal slurry was dried into a powder at 80oC for 30 minutes in

a nitrogen atmosphere.

The dried metal-polymer composite powder was

compacted using a mechanical hydraulic press in varied

shapes and sizes. For the high-frequency measurements, the

compacts were made with 6 x 6 mm2 stainless steel mold

(shown in Fig. 7(b)), while the thicknesses were maintained

close to 1.5 mm to match the SIW cavity substrate height. A

high mechanical load of 2 Tons/cm2 was applied on the 6x6

mm2 mold, to ensure good packing density is achieved in the

pressed compacts. The compacts were then thermally treated

in inert atmosphere at 250oC for 90 minutes to cure the

fluoropolymer matrix. A picture of the compacted pellets is

shown in Fig. 7(a).

Figure 7. (a) Magneto-dielectric composite samples, (b)

Schematic of the compaction set-up.

Magneto-dielectric Material Characterization

The magneto-dielectric composite samples were

characterized in the range 1-4 GHz. Rogers dielectric material

RO4360, TMM 6 and TMM 10 were chosen as the standard

sample to obtain the constants A and B in equations (1) and

(2) for each frequency. Similarly, Cuming Microwave FLX-

10 magnetic absorber material was used as the standard

sample for obtaining the constants C and D in equations (3) –

(4) for each frequency. The samples were prepared in a

hexahedron form with dimension of 6x6x1.524mm3 and were

inserted into the hole machined in the cavity. The cavity

response was measured with a VNA using SOLT calibration.

Corner-to-corner probing method is shown in Fig. 8. GSG 500

probe was used to excite the SIW cavity. As shown in Fig. 8,

samples were inserted in the hole and copper tape was used to

cover the top and bottom of the hole. Fig. 9 shows the

measured response of the 2GHz SIW cavity, with the various

samples inserted in the cavity. From Fig. 9, the samples with

higher permittivity or permeability values have lower resonant

frequencies and samples with higher loss tangent show wider

3dB bandwidth corresponding to a lower Q factor.

Figure 8. Corner-to-corner probing method.

(a)

(b)

Figure 9. SIW cavity measurement with various sample

materials (a) @ E-field maximum and (b) @ H-field

maximum.

Fig. 10 shows the extracted properties of the magneto-

dielectric composite material using the seven SIW cavities

provided in Table I. In Fig. 10 (a) and (b), the relative

permeability μr’ gradually decreases as frequency increases

Frequency (GHz)

Re

turn

Loss (

dB

)

Before Perturbation

RO4360

TMM10

Magneto-dielectric Composite

Frequency (GHz)

Re

turn

Loss (

dB

)

Before Perturbation

Magneto-dielectric Composite

FLX10

Uniaxial Pressure

Upper punch

Lower punch

Die

Magneto-dielectric

composite

(a) (b)

Sample

785

while εr’ is fairly constant. The value of the extracted relative

permittivity (εr’) is 12±0.5 and relative permeability (μr

’) was

1.9±0.2 in the frequency range 1-4 GHz. Electric loss tangent

is 0.0035±0.001. Magnetic loss tangent increases with

frequency increases. It is 0.0614 at 1GHz and increases to

0.48 at 4GHz.

Figure 10. Extracted Cobalt nano particle composite material

properties (a) Permittivity, (b) Electric loss tangent,

(c) Permeability and (d) Magnetic loss tangent.

PIFA on magneto-dielectric substrate

A 1 GHz Planar Inverted-F Antenna (PIFA) on magneto-

dielectric substrate was designed using CST and the material

properties from previous section was used for the design, as

shown in Fig. 11. The relative permittivity and permeability

of the magneto-dielectric used was 11.9 and 2.153 at 1GHz

respectively. Also electric loss tangent and magnetic loss

tangent used was 0.0032 and 0.0614 at 1GHz respectively.

These values are extracted material properties at 1 GHz. As

shown in Fig. 11, ground plane of the antenna is supported by

FR4 material and magneto-dielectric material was used as

substrate for PIFA. Size of the magneto-dielectric substrate is

20 x 20mm2. Height of FR4 and magneto-dielectric substrate

is 1 mm and 1.5 mm respectively. Size of the FR4 material is

60 x 120mm2. Actual dimension of two patterned conductors

is shown in Fig. 12. U shaped top plane is connected to the

ground plane with 2mm width shorting pin at top left corner

of the antenna. Distance between the shorting pin and port is

4.8mm and it is optimized for good matching. This antenna

showed 9.73% bandwidth and 72.11% efficiency as shown in

Fig. 13.

Figure 11. Planar inverted-F antenna with magneto-dielectric

material (a) Perspective view, (b) top view, (c) side view.

Figure 12. Dimension of patterned conductor

(a) U-shaped patch, (b) Ground plane.

Dielectric substrate with εr=21, tan δ=0.0646 and the same

thickness h=1.5 mm was also designed and simulated to

FR4

MD

(a)

(b)

(c)

Port

Short

GND FR4

U Patch

MD

120mm

60mm

5 5 10

14.8

6

22

60

120

Units=mm

(b) (a)

Frequency (GHz)

(a)

1 1.5 2 2.5 3 3.5 410

11

12

13

14

Relative Permittivity

Mag

nitu

de

Frequency (GHz)

(b)

Mag

nitu

de

1 1.5 2 2.5 3 3.5 41.7

1.8

1.9

2

2.1

2.2

Relative Permeability

Frequency (GHz)

(c)

Mag

nitu

de

1 1.5 2 2.5 3 3.5 40

0.1

0.2

0.3

0.4

0.5

Magnetic Loss Tangent

Mag

nitu

de

(d)

1 1.5 2 2.5 3 3.5 40

0.002

0.004

0.006

0.008

0.01

Electric Loss Tangent

786

compare antenna performance between the magneto-dielectric

substrate and the high dielectric constant substrate. Dielectric

constant of 21 was chosen to maintain the same antenna size

and resonant frequency. For comparison, electric loss tangent

of 0.0646 was chosen for the high dielectric constant material

to equal the total loss of magneto-dielectric material. The high

dielectric constant substrate antenna showed 8.62%

bandwidth and lower efficiency of 51.6% as shown in Fig. 13.

PIFA with the magneto-dielectric material substrate shows

better performance for both bandwidth and efficiency than the

antenna with high dielectric constant material substrate. It

shows that the magneto-dielectric material is an effective

material for antenna miniaturization. Losses of magneto-

dielectric substrate effect on antenna performance have also

been analyzed using EM simulation. Without changing the

values of real permittivity and real permeability, if the

magnetic loss tangent was reduced to 0.0307, the antenna

efficiency increased to 82.5% and the bandwidth decreased a

little to 8.82% as shown in Fig. 13. This narrow bandwidth

can be compensated using patch design optimization.

Therefore, if the loss of the magneto-dielectric material is

reduced and patch design optimized, antenna performance can

be improved further.

Figure 13. Antenna performance of PIFA with different

substrates (a) Return loss, (b) Efficiency.

Conclusions

In this paper, CPT with SIW cavity resonators for

magneto-dielectric material is discussed and demonstrated

with simulation and measurement. The novelty arises in the

use of a single SIW cavity structure to extract both properties,

which otherwise would require two separate structures.

Simulation and experiment shows that magnetic properties of

the sample do not affect electric properties extraction and vice

versa. Magneto-dielectric composite material which is

synthesized with cobalt metal particles has been measured

with this method. PIFA on magneto-dielectric material with

extracted properties has been designed using CST. The

magneto-dielectric substrate antenna shows better bandwidth

and efficiency as compared to using the high dielectric

constant material substrate. If the total loss of magneto-

dielectric material is reduced using enhanced material

synthesis technique, antenna performance can be improved

further. In conclusion, effective antenna miniaturization can

be achieved by using the magneto-dielectric material.

References

1. C. A. Balanis, Antenna Theory: Analysis and Design,

Wiley, New York, 1997, pp. 812-813.

2. J. S. Colburn and Y. Rahmat-Samii, “Patch antennas on

externally perforated high dielectric constant substrates,”

IEEE Trans. Antennas Propag., vol. 47, no. 12, pp.1785-

1784, 1999.

3. P. M. T. Ikonen and S. A. Tretyakov, “On the advantages

of magnetic materials in microstrip antenna

miniaturization,” Microwave Opt. Technol. Lett., vol. 52,

pp. 3131-3134, 2008.

4. N. Altunyult, M. Swaminathan, P. M. Raj, V. Nair,

“Antenna miniaturization using magneto-dielectric

substrates,” in Proc. IEEE Electronic Components and

Technol. Conf. (ECTC), May 2009, pp. 801-808.

5. K. Han, M. Swaminathna, P. M. Raj, H. Sharma, K. P.

Murali, R. Tummala, V. Nair, “Extraction of Electricl

Properties of Nanomagnetic Materials through Meander-

Shaped Inductor and Inverted-F Antenna Structures,” in

Proc. IEEE Electronic components and Technol. Comf.

(ECTC), 2012, pp.1808-1813.

6. L. F. Chen, C. K. Ong, C. P. Neo, V. V. Varadan, V. K.

Varada, Microwave Electronics: Measurement and

Materials Characterization, Wiley, New York, 2004, pp.

250-286.

7. H. Lobato-Morales, A. Corona-Chavex, D. V. B. Murthy,

J. L. Olvera-Cervantes, "Complex permittivity

measurements using cavity perturbation technique with

substrate integrated waveguide cavities," Review of

Scientific Instruments, 81.6, 2010.

8. K. Saeed, R. D. Pollard, I. C. Hunter, “Substrate integrated

waveguide cavity resonators for complex permittivity

characterization of materials,” IEEE Trans. Microwave

Theory and Techniques, vol. 56, no. 10, pp.2340-2347

2008.

9. K. Han, M. Swaminathan, P. M. Raj, H. Sharma, R.

Tummala, V. Nair, “Magneto-dielectric material

characterization and antenna design for RF applications,”

European Conference on Antenna Propagation (EuCAP),

2014, paper accepted.

Retu

rn L

oss (

dB

)

Frequency (GHz) (a)

90

80

70

60

50

40

30

20

10

0

Effic

iency (

%)

Frequency (GHz) (b)

Magneto-dielectric High dielectric constant Magneto-dielectric w/ low loss

Magneto-dielectric High dielectric constant Magneto-dielectric w/ low loss

787

10. P. M. Raj, H. Sharma, D. Mishra, K. P. Murali, K. Han, M.

Swaminathan, R. Tummala, “Nanomagnetics for high-

performance, miniaturized power, and RF components,”

IEEE Nanotechnology magazine, vol. 6, no. 3, pp.18-23

2012.

11. N. Tang, W. Zhong, X. Wu, H. Jiang, W. Liu, and Y. Du,

“Synthesis and complex permeability of Co/SiO2

nanocomposites,” Matter. Lett., vol. 59, no. 14-15,

pp.1723-1726, 2005.

788