Magneto-dielectric Characterization and Antenna Design

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1 Magneto-dielectric Characterization and Antenna Design Kyu Han,, Madhavan Swaminathan,, P. Markondeya Raj, Himani Sharma, Rao Tummala and Vijay Nair Interconnect and Packaging Center IPC), SRC Center of GT School of Electrical and Computer Engineering, Georgia Institute of Technology 66 Ferst Drive, Atlanta, GA 0 USA khan7@gatech.edu Packaging Research Center, Georgia Institute of Technology Intel 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 D 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 - GHz. Planar inverted F antenna PIFA) working at GHz 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 []. However, using the high dielectric constant material as the antenna substrate leads to narrow bandwidth and low efficiency []. Recently, magneto-dielectric materials, which have both permittivity and permeability greater than, have stirred the interest of antenna designers since the material can reduce antenna size without deteriorating antenna performance []. 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 [] 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 //$.00 0 IEEE 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 - 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: ) ). ) ). ) 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 ) and ) can be used [6]. 78 ). ). ) ) 0 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 ) and ) are also obtained from the measurement of standard sample.

, since they have high Q, are highly sensitive to material properties and have minimum radiation effect.

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]: ) ) permeability parameters are varied to demonstrate that these

In these figures, ε r and μr of the sample are varied between 6-8 and - respectively while both electric and magnetic loss tangent of the sample are varied from 0.0 to 0.0. As shown in Fig.

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.. 5) Figure.

2 where μ s and μ s are real and imaginary part of permeability of the sample where constants C and D in equations ) and ) 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., 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]: ) ) permeability parameters are varied to demonstrate that these parameters are isolated from each other. In these figures, ε r and μr of the sample are varied between 6-8 and - respectively while both electric and magnetic loss tangent of the sample are varied from 0.0 to 0.0. As shown in Fig., 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.. 5) Figure. Field distribution of TE0 mode SIW cavity EField, H-Field Circle shows E and H-Field maximum). 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. 78 The SIW cavities are designed for the dominant TE0 or TE0 mode. TE0 mode can be used to measure material properties in the low frequency range since it can reduce the SIW cavity dimension. Fig. shows the electric and magnetic field distribution in the SIW with TE0 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. 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. D 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 ~GHz is shown in Fig. and where the complex permittivity and Figure. TE0 mode SIW cavity resonator with GSG probe excitation. d) Figure. Parameter sweep when the sample is at E-field maximum position: εr, µr, electric loss tangent, d) magnetic loss tangent.

.5GHz Return loss, SdB) GHz GHz Hole for sample Insertion GHz Return loss, SdB) GHz.5GHz.5GHz Figure 5. Fabricated SIW cavities with different resonant frequency Figure 6.

Parameter sweep when the sample is at H-field maximum position: εr, µr, electric loss tangent, d) magnetic loss tangent.

00 electric loss tangent) has been used instead in this study. Thickness of the cavity is.5mm.

Seven SIW cavities were designed and fabricated to characterize the magneto-dielectric sample in the frequency range - GHz, as shown in Fig. 5.

Dimension of the hole is 6 x 6mm. SIW cavity in Fig. 6 has a hole at the E-field maximum position and this cavity was used for permittivity measurement. Another SIW cavity in Fig.

5 TE0 09 TE0 9 L mm) 6 86 66 5 5 Magneto-dielectric Material Synthesis The magneto-dielectric material used in this study was synthesized using high volume loading 50-70 vol %) of cobalt

3 .5GHz Return loss, SdB) GHz GHz Hole for sample Insertion GHz Return loss, SdB) GHz.5GHz.5GHz Figure 5. Fabricated SIW cavities with different resonant frequency Figure 6. SIW cavity with different holes location for permittivity permeability measurements. d) Figure. Parameter sweep when the sample is at H-field maximum position: εr, µr, electric loss tangent, d) magnetic loss tangent. Fabrication of SIW Cavities In order to validate the simulation, FR material was used to fabricate SIW cavities in [9], Rogers 00 material which has parameters εr =.0, tanδ=0.00 electric loss tangent) has been used instead in this study. Thickness of the cavity is.5mm. Since Rogers 00 has less dielectric loss than FR material, cavity with Rogers 00 has higher Q factor than cavity with FR. 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 - GHz, as shown in Fig. 5. Table shows the SIW cavity operating mode, resonant frequency and dimension. Fig. 6 shows the fabricated SIW cavity which is resonating at GHz with drilled hole for sample insertion. Dimension of the hole is 6 x 6mm. SIW cavity in Fig. 6 has a hole at the E-field maximum position and this cavity was used for permittivity measurement. Another SIW cavity in Fig. 6 has a hole at the H-field maximum position and it was used for permeability measurement. Table. SIW cavity specification Resonance Operation W mm) GHz) Mode TE0.5 TE0 6 TE TE0 7 TE0.5 TE0 09 TE0 9 L mm) Magneto-dielectric Material Synthesis The magneto-dielectric material used in this study was synthesized using high volume loading vol %) of cobalt nano-particles in a dielectric polymer matrix. [0], where it shown in Fig 7. 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 []. 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 ~0-0 nm using ball-milling process. As-received metal powders were suspended in anhydrous toluene solvent and milled for 0-5 hours with zirconia balls to break the aggregates. The dielectric polymer was then added to the suspension and 78

milled again for -6 hours to ensure complete homogenization of the polymer and the metal particles.

The dried metal-polymer composite powder was compacted using a mechanical hydraulic press in varied shapes and sizes.

5 mm to match the SIW cavity substrate height. A high mechanical load of Tons/cm was applied on the 6x6 mm mold, to ensure good packing density is achieved in the pressed compacts.

4 milled again for -6 hours to ensure complete homogenization of the polymer and the metal particles. The final polymermetal slurry was dried into a powder at 80oC for 0 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 mm stainless steel mold shown in Fig. 7), while the thicknesses were maintained close to.5 mm to match the SIW cavity substrate height. A high mechanical load of Tons/cm was applied on the 6x6 mm mold, to ensure good packing density is achieved in the pressed compacts. The compacts were then thermally treated in inert atmosphere at 50oC for 90 minutes to cure the fluoropolymer matrix. A picture of the compacted pellets is shown in Fig. 7. Sample Figure 8. Corner-to-corner probing method. Uniaxial Pressure Return Loss db) Upper punch Magnetodielectric composite Die Lower punch Before Perturbation RO60 Magnetodielectric Composite TMM0 Magneto-dielectric Material Characterization The magneto-dielectric composite samples were characterized in the range - GHz. Rogers dielectric material RO60, TMM 6 and TMM 0 were chosen as the standard sample to obtain the constants A and B in equations ) and ) for each frequency. Similarly, Cuming Microwave FLX0 magnetic absorber material was used as the standard sample for obtaining the constants C and D in equations ) ) for each frequency. The samples were prepared in a hexahedron form with dimension of 6x6x.5mm 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 GHz 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 db bandwidth corresponding to a lower Q factor. Return Loss db) Figure 7. Magneto-dielectric composite samples, Schematic of the compaction set-up. Magnetodielectric Composite Before Perturbation FLX0 Figure 9. SIW cavity measurement with various sample E-field maximum H-field maximum. Fig. 0 shows the extracted properties of the magnetodielectric composite material using the seven SIW cavities provided in Table I. In Fig. 0 and, the relative permeability μr gradually decreases as frequency increases 785

while εr is fairly constant. The value of the extracted relative permittivity εr ) is ±0.5 and relative permeability μr ) was.9±0. in the frequency range - GHz. Electric loss tangent is 0.005

008 PIFA on magneto-dielectric substrate A GHz Planar Inverted-F Antenna PIFA) on magnetodielectric substrate was designed using CST and the material properties from previous section was used for the

5 while εr is fairly constant. The value of the extracted relative permittivity εr ) is ±0.5 and relative permeability μr ) was.9±0. in the frequency range - GHz. Electric loss tangent is 0.005±0.00. Magnetic loss tangent increases with frequency increases. It is 0.06 at GHz and increases to 0.8 at GHz. Relative Permittivity Electric Loss Tangent PIFA on magneto-dielectric substrate A GHz Planar Inverted-F Antenna PIFA) on magnetodielectric substrate was designed using CST and the material properties from previous section was used for the design, as shown in Fig.. The relative permittivity and permeability of the magneto-dielectric used was.9 and.5 at GHz respectively. Also electric loss tangent and magnetic loss tangent used was 0.00 and 0.06 at GHz respectively. These values are extracted material properties at GHz. As shown in Fig., ground plane of the antenna is supported by FR material and magneto-dielectric material was used as substrate for PIFA. Size of the magneto-dielectric substrate is 0 x 0mm. Height of FR and magneto-dielectric substrate is mm and.5 mm respectively. Size of the FR material is 60 x 0mm. Actual dimension of two patterned conductors is shown in Fig.. U shaped top plane is connected to the ground plane with mm width shorting pin at top left corner of the antenna. Distance between the shorting pin and port is.8mm and it is optimized for good matching. This antenna showed 9.7% bandwidth and 7.% efficiency as shown in Fig.. 0mm FR GND U Patch 60mm MD Port. MD Short Relative Permeability. FR Figure. Planar inverted-f antenna with magneto-dielectric material Perspective view, top view, side view Units=mm Magnetic Loss Tangent d) Figure. Dimension of patterned conductor U-shaped patch, Ground plane. Figure 0. Extracted Cobalt nano particle composite material properties Permittivity, Electric loss tangent, Permeability and d) Magnetic loss tangent. Dielectric substrate with εr=, tan δ=0.066 and the same thickness h=.5 mm was also designed and simulated to 786

066 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.

6 Return Loss db) compare antenna performance between the magneto-dielectric substrate and the high dielectric constant substrate. Dielectric constant of was chosen to maintain the same antenna size and resonant frequency. For comparison, electric loss tangent of was chosen for the high dielectric constant material to equal the total loss of magneto-dielectric material. The high dielectric constant substrate antenna showed 8.6% bandwidth and lower efficiency of 5.6% as shown in Fig.. 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 magnetodielectric 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 0.007, the antenna efficiency increased to 8.5% and the bandwidth decreased a little to 8.8% as shown in Fig.. 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. Magneto-dielectric High dielectric constant Magneto-dielectric w/ low loss Efficiency %) Magneto-dielectric High dielectric constant Magneto-dielectric w/ low loss Figure. Antenna performance of PIFA with different substrates Return loss, 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 magnetodielectric 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. C. A. Balanis, Antenna Theory: Analysis and Design, Wiley, New York, 997, pp J. S. Colburn and Y. Rahmat-Samii, Patch antennas on externally perforated high dielectric constant substrates, IEEE Trans. Antennas Propag., vol. 7, no., pp.78578, P. M. T. Ikonen and S. A. Tretyakov, On the advantages of magnetic materials in microstrip antenna miniaturization, Microwave Opt. Technol. Lett., vol. 5, pp. -, 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 009, pp K. Han, M. Swaminathna, P. M. Raj, H. Sharma, K. P. Murali, R. Tummala, V. Nair, Extraction of Electricl Properties of Nanomagnetic Materials through MeanderShaped Inductor and Inverted-F Antenna Structures, in Proc. IEEE Electronic components and Technol. Comf. ECTC), 0, pp L. F. Chen, C. K. Ong, C. P. Neo, V. V. Varadan, V. K. Varada, Microwave Electronics: Measurement and Materials Characterization, Wiley, New York, 00, pp 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, 8.6, 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. 0, pp 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), 0, paper accepted. 787

7 0. P. M. Raj, H. Sharma, D. Mishra, K. P. Murali, K. Han, M. Swaminathan, R. Tummala, Nanomagnetics for highperformance, miniaturized power, and RF components, IEEE Nanotechnology magazine, vol. 6, no., pp N. Tang, W. Zhong, X. Wu, H. Jiang, W. Liu, and Y. Du, Synthesis and complex permeability of Co/SiO nanocomposites, Matter. Lett., vol. 59, no. -5, pp.7-76,

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TODAY S global trends in mobile industry is smaller, 1850 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 5, NO. 12, DECEMBER 2015 RF Characterization of Magnetodielectric Material Using Cavity Perturbation Technique Kyu Han,