improved planar resonant rf sensor for retrieval of...

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Abstract— In this work, the improved planar resonant sensors

based on the generalized SRR configuration are proposed for the

permittivity and the permeability testing of magneto-dielectric

samples. It is observed that the proposed inter-digital capacitor

based split ring resonator (IDC-SRR) and the meandered line

based split ring resonator (ML-SRR) provide better sensitivity

for dielectric and magnetic measurement, respectively, when

compared with the standard SRR based microwave sensors. Both

the sensors are first modeled under unloaded condition, using the

full wave electromagnetic solver, the CST Microwave Studio, in

order to ensure their operating frequency range near 2.45 GHz of

ISM band. In the next step, the numerical simulation of these

sensors is carried out by loading them with a number of

reference materials in order to develop empirical model for the

determination of permittivity and permeability of test samples.

Additionally, the equivalent circuit models of these sensors are

obtained using the ADS circuit simulator and results are

compared with the numerical simulation. All the sensors are

designed and fabricated on 1.27 mm thick RT/Duroid 6006

substrate, and testing is carried out using the network analyzer.

A number of standard dielectric and magneto-dielectric samples

are tested using the proposed scheme in order to retrieve their

permittivity and permeability values. The measured data of each

sample are in good match with the corresponding reference

values available in literature having a typical error of less than

6%.

Index Terms— Characterization, Inter digital capacitor,

Permeability, Permittivity, Split ring resonator, Magneto-

dielectric.

Md. Shafi K T was with the Department of Electrical Engineering, Indian

Institute of Technology Kanpur, Uttar Pradesh - 208016, India. He is now

with the Department of Electrical and Computer Engineering, Khalifa

University of Science and Technology-The Petroleum Institute, Abu Dhabi, UAE (e-mail: [email protected]).

A. K. Jha was with the Department of Electrical Engineering, Indian

Institute of Technology Kanpur, Uttar Pradesh-208016, India. He is now with the Institute of Photonics and Electronics, Czech Academy of Sciences,

Prague - 18251, Czech Republic ([email protected]).

M. J. Akhtar is with the Department of Electrical Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh - 208016, India (e-mail:

[email protected]).

This work was partially supported by the DST under Grant DST/TSG/ME/2015/97.

I. INTRODUCTION

HERE are various microwave material characterization

techniques available for determination of permittivity and

permeability. Among different characterization techniques, the

resonant methods usually provide quite accurate parameter

extraction, especially for low loss samples, provided that the

material parameters are required only over a very narrow

frequency band [1]. However, since most of the resonators,

antennas, couplers and filters are specifically designed to work

in narrow band of frequencies, the resonant methods appear to

be adequate to accurately estimate the electromagnetic

properties of materials in the desired frequency band. The

cavity perturbation technique is one of the most preferred

characterization methods for narrow band application [2]-[5].

This technique requires sample placement at the maximum

magnetic or electric field position for the magnetic or

dielectric characterization. However, it requires bulky and

costly metal cavities for performing the characterization. In

recent years, the planar resonant RF sensors have been used to

replace the bulky cavities, since they have advantages such as

low cost, ease of fabrication and easy integration with other

monolithic integrated devices [6]-[8].

The planar microwave sensors for material characterization

are currently being explored by several groups. Recently, a

number of planar RF resonant sensors using various

configurations such as the substrate integrated waveguide

(SIW) [9], the complimentary split ring resonator (CSRR)

[10]-[13], the split ring resonator (SRR) [14]-[17], and the

inter digital capacitor (IDC) [6],[18],[19] have been proposed.

Among various configurations listed above, the CSRR and

SRR based resonant sensors are used more often due to their

simple fabrication process and more compactness in design.

From the early works done in this domain, it is also observed

that the metamaterial inspired structures based on SRR and

CSRR can provide better sensitivity as compared to other

planar resonant sensors.

Conventionally, the CSRR based RF sensors have been

used in the past for testing of dielectric materials, while the

SRR based RF sensors appear to be good choice for testing of

magnetic materials due to the field configuration of the

microstrip geometry [20]. However, the CSRR based sensor

may not be compatible with the multilayer technology where

Improved Planar Resonant RF Sensor for

Retrieval of Permittivity and Permeability of

Materials

Muhammed Shafi K T, Abhishek Kumar Jha, Member, IEEE and M Jaleel Akhtar, Senior Member,

IEEE

T




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the access of the ground plane is restricted to minimize the

possible fringing effect. In order to integrate the microwave

sensors with other compact systems having multi-layer

structures, the SRR based sensors would provide more degree

of freedom in system design as compared to the CSRR based

sensor.

Now, the recent trend in RF design is to use the magneto-

dielectric substrates to achieve miniaturization of microwave

circuits and systems [21]-[27]. The magneto-dielectric

substrates introduce the magnetic properties to the substrate

material, which provide an extra parameter in terms of the

relative permeability for optimizing several parameters of

microwave devices. Since the magneto-dielectric substrates

possess the permeability and the permittivity values greater

than unity, the devices fabricated using these substrates would

be smaller in size compared to the devices made on pure

dielectric substrates. However, the dielectric and magnetic

properties of these magneto-dielectric substrates need to be

accurately measured before using them to design a RF device.

From above discussion, it can be inferred that the accurate

electromagnetic characterization of magneto-dielectric

substrates is quite important in today’s scenario and it should

preferably be carried out using the SRR based approach in

order to conform to the modern microstrip multilayer

technology. It is mainly due to this reason that the generalized

SRR based approach is proposed in this work for the RF

characterization of magneto-dielectric samples at 2.45 GHz of

ISM band. To this end, a novel IDC based SRR (IDC-SRR)

RF sensor for dielectric testing, and a new type of meandered

line based split ring resonator (ML-SRR) RF sensor for

magnetic testing, of magneto-dielectric samples are proposed.

The newly designed sensors are compared with other

equivalent configurations in terms of sensitivity, which shows

the potential advantage of the proposed sensors. The proposed

sensors are modeled using the standard equivalent circuit

approach, and the scattering parameters of the equivalent

circuit model are compared with those of the numerical

simulation results. In the next step, the numerical method

based on the finite integration technique is used to develop an

empirical formula for calculating the relative permittivity and

permeability of the magneto-dielectric samples in terms of the

measured resonant frequency and the transmission coefficient

of the loaded sensor. The prototype is fabricated on the 1.27

mm thick RT/Duroid 6006 substrate (εr = 6.15). Lastly, the

measurements of various standard materials are performed

using the fabricated prototype of the proposed sensors with the

help of Keysight’s vector network analyzer N5230C.

The overall paper is organized as follows. First of all, the

design procedure of the planar RF sensor for dielectric

material testing is explained in section II. Section III contains

the detailed information of the microwave sensor design for

calculating relative permeability of material under test (MUT).

The numerical modeling of each proposed sensor using the

electromagnetic simulation tool, the CST Microwave Studio is

included in section IV. Section V presents the measurement

results for different magneto-dielectric materials using

proposed sensors. The values of relative permittivity and

permeability of different materials are tabulated with their

reference values available in literature. Section VI deals with

the details of error analysis and finally, the conclusion is

provided in section VII.

II. DESIGN OF DIELECTRIC MATERIAL SENSOR

Measurement of dielectric and magnetic properties of

material requires different RF sensing spots since the

permittivity measurement is primarily carried out by placing

the sample in the maximum electric field region while the

permeability determination usually requires the maximum

magnetic field region. Here, first of all, the design of RF

sensor for dielectric material characterization is considered.

All the RF sensor structures presented in this paper are

simulated using the full wave electromagnetic solver, the CST

Microwave Studio [28]

A. Theory

Microwave resonant method for characterization of

materials using planar technology is based on the field

perturbation theory. When the resonance occurs, the total

electric field will be confined to a smaller region of split ring

resonator, where the sample is usually placed. This confined

electric field is capable of sensing even a smaller change in the

dielectric constant of the test sample. The response of the

microwave sensor to the change in the effective dielectric

constant of the surrounding can be noticed in terms of the

change in resonant frequency and the quality factor of the

loaded structure.

Fig. 1. (a) Structure of SRR based dielectric material sensor with enlarged

view of coupled SRR structure. The dimensions of the design are, L = 50 mm,

W = 30 mm, L1 = 6.45 mm, L2 = 10 mm, g = 0.3 mm, wd = 0.3 mm. (b)

Electric field configuration near SRR at 2.45 GHz.

Fig. 2. (a) Structure of capacitive loaded SRR based dielectric material sensor

with dimensions, L1 = 5 mm, L2 = 9 mm, g = 0.3 mm, wd = 0.3 mm. b = 0.9

mm, La = 1.8 mm. (b) Electric field configuration near capacitive loaded SRR

at 2.45 GHz.




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Fig. 3. (a) Structure of IDC - SRR based dielectric sensor with dimensions, L1

= 4 mm, L2 = 7 mm, g = 0.3 mm, wd = 0.3 mm. b = 1.5 mm. (b) Electric field

configuration near IDC based SRR at 2.45 GHz.

For the planar microwave sensor based on the rectangular

shaped SRR, the maximum electric field is limited to the

dielectric gap of the SRR at resonance, thereby giving a

sensing spot for dielectric material characterization. Fig. 1 (a)

shows the structure of a rectangular shaped SRR based planar

microwave sensor. Fig. 1(b) shows the electric field confined

to dielectric gap of the SRR at resonance condition. The SRR

structure is excited by the magnetic field of the microstrip line

[29]. Since it is magnetically coupled, the spacing between the

microstrip line and the SRR should be minimum to get

maximum energy transfer from the microstrip to the SRR

structure. Now, as one of the major goals of the sensor design

is to improve the sensitivity, the SRR based dielectric sensor

should accordingly be modified. The confinement of electric

field in the gap of the SRR can be improved by increasing the

effective capacitance of this gap, which can be achieved by

maximizing the local area of the ring on both sides of the gap

resulting into a capacitive loaded SRR structure as shown in

Fig. 2(a). Fig. 2(b) shows the electric field of the proposed

capacitive loaded SRR structure magnetically coupled to the

microstrip line. After comparing the electric field of the

capacitive loaded SRR with that of the simple SRR shown in

Fig. 1(b) at the resonant frequency, it can be observed that the

modified SRR structure substantially improves the electric

field in the gap region which is akin to increasing the

sensitivity. The field in the gap region of the SRR is further

improved here to obtain higher sensitivity by introducing an

IDC in the gap of SRR as shown in Fig. 3(a). The IDC based

structure shown in Fig. 3(a) basically provides higher effective

capacitance as compared to the simple parallel plate like

structure shown in Fig. 2 (a), which leads to higher electric

field in the small sensing region leading to higher sensitivity

as shown in Fig. 3(b).

B. Comparison of sensitivity

For comparing the sensitivity of various RF sensors, each

sensor is modeled using the CST microwave studio with a test

material placed on corresponding sensing area of each sensor.

The substrate material for all the sensor design is chosen as

1.27 mm thick RT/Duroid 6006 (εr = 6.15) with a rectangular

cube shaped test sample having dimension of 5 mm × 5 mm ×5

mm. Since the electromagnetic properties of materials are

usually dispersive in nature, the proposed sensors are modeled

to resonate at the ISM frequency of 2.45 GHz for getting the

dielectric and magnetic properties at a common frequency.

The resonant frequency of each sensor model is optimized to

get same operating frequency by adjusting the dimensions of

each structure in the CST simulation environment. Later the

sensor is loaded with test specimens having different

electromagnetic properties, and the change in resonant

frequency (f) values are noted down. Since the sensor is

designed for determination of dielectric permittivity, the value

of relative permeability is taken as unity (relative permeability

of free space) during the simulation. The parameter f,

representing the difference in resonant frequency (fr) of the

sensor loaded with a particular material from the resonant

frequency of the sensor without MUT (f0), is then plotted

versus the relative permittivity value for different sensor

configurations as shown in Fig. 4. From this figure, it can be

concluded that the sensitivity of the proposed IDC-SRR based

sensor is much higher as compared to that of the simple SRR

based sensor for extraction of relative dielectric permittivity of

unknown samples. Hence, the proposed IDC-SRR based RF

sensor, which appears to be suitable for integrating with multi-

layer planar devices; is selected in this work for further study.

Fig. 4. Sensitivity comparison of different dielectric sensor designs.

C. Equivalent circuit for IDC – SRR based sensor

For carrying out a detailed analysis of the proposed sensor

structure, an equivalent circuit with lumped elements is used

as shown in Fig. 5(a). In Fig. 5(a), since it is difficult to

calculate the mutual coupling value, M for the microstrip

based circuits, the proposed equivalent circuit model is

transformed as shown in Fig. 5(b). The parameter values of

the equivalent circuit are extracted using the standard

procedure [30]. Table I gives the values of the lumped

elements of the proposed equivalent circuit corresponding to

simple SRR and IDC based SRR dielectric sensor designs. It

can be observed from Table I that the capacitance value of the

proposed resonator (C1a), is larger as compared to the simple

SRR design, which clearly validates the higher sensitivity of

IDC based sensor over the simple sensor design. The proposed

equivalent circuit is simulated in ADS software, and the S21

plot from the circuit simulation is compared with the

electromagnetic simulation data obtained using the CST as

shown in Fig. 6. A good match between the circuit simulation




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and the EM simulation is obtained as shown in Fig. 6.

Fig. 5. Proposed equivalent circuit for IDC-SRR based dielectric sensor

TABLE I

LUMPED ELEMENT VALUES OF PROPOSED EQUIVALENT CIRCUIT

Sensor

design C (pF) Lc (nH) C

1a (pF) L

1a (nH) R ( )

Simple

SRR 0.71 4.03 13.52 0.31 279.7

IDC based SRR

0.24 3.77 32.66 0.128 282.7

Fig. 6. Comparison of S21 plot of CST Simulation and ADS Simulation

III. DESIGN OF MAGNETIC MATERIAL SENSOR

After designing the modified SRR based structure for the

dielectric material characterization, the next step is to enhance

the sensitivity of the SRR based RF sensors in order to

characterize magnetic materials. The magnetic permeability

estimation using the SRR employs the conductive loop of the

SRR which confines maximum magnetic field around the

conductor when the SRR is at resonance because of the

inductance offered by the conductive loop. Fig. 7(a) shows the

structure of the rectangular shaped SRR used for the

estimation of relative permeability. From Fig. 7(b), it can be

observed that the magnetic field is maximum at the far edge of

the SRR from microstrip line. The orientation of the SRR

structure with respect to the microstrip line is chosen in order

to place the MUT on the conductive part of the SRR without

effecting the coupling between the microstrip line and the

SRR structure. Since the sensitivity of the magnetic material

characterization depends on the amplitude of the magnetic

field around the sensing region of conductive loop of the SRR,

the conductive loop of the SRR is meandered here in order to

confine maximum magnetic field in the test region. Fig. 8(a)

shows the proposed meandered line magnetic material sensor.

As it can be seen from the Fig. 8(b), the magnetic field is more

densely concentrated in the meandered line region as

compared to that of the simple SRR sensor shown in Fig. 7(b).

Fig. 7. (a) Structure of SRR based magnetic sensor with the enlarged view of

coupled SRR structure, The dimensions of the design are, L = 80 mm, W = 40 mm, L1 = 4.8 mm, L2 = 10 mm, wd = 0.3 mm, g =0.3 mm. (b) Magnetic field

configuration at resonant frequency.

Fig. 8. (a) Structure of meandered line SRR based magnetic sensor with

dimensions, wd = 0.3 mm, g = 0.3 mm, L1 = 5.855 mm, L2 = 10 mm, La = 2

mm. (b) Magnetic field at the resonant frequency

Fig. 9. Sensitivity comparison of different magnetic sensor designs

The dimensions of both simple SRR and the meandered line

based SRR sensor are optimized in order to achieve the

operating frequency of 2.45 GHz using the CST Microwave

Studio. These sensors are then loaded with various magnetic

materials having same dimension as previously used for the

study of dielectric sensors, and the resonant frequencies of

both the magnetic material sensors are recorded for each




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relative permeability value while keeping the relative

permittivity value equal to one. The difference in the resonant

frequency value for each test specimen is plotted versus the

relative permeability value for both configurations as shown in

Fig. 9. It can be observed from this figure that the meandered

line SRR magnetic sensor provides more sensitivity with

respect to the change in the relative permeability values as

compared to that of the simple rectangular SRR based sensor.

From Fig. 9, it can also be observed that the proposed

meandered line sensor is having quite good sensitivity for

materials having lower values of the relative permeability,

which is quite important from practical point of view as the

relative permeability of most of the magnetic composite

substrates lies between 1 and 2.

A. Equivalent circuit for Meandered Line SRR based sensor

The meandered line based SRR, similar to the simple SRR,

is magnetically coupled to the microstrip line. Hence, the

equivalent circuit model for the meandered line SRR magnetic

sensor can also be represented using the topology shown in

Fig. 5. The values of the lumped elements of equivalent circuit

are extracted using the same procedure described earlier for

the case of dielectric sensor, and the extracted values for the

simple SRR and meandered SRR based magnetic sensors are

tabulated in Table II. It can be concluded from Table II that

the inductance value of the proposed resonator (L1a) is higher

than that of the simple SRR sensor, which clearly validates the

higher sensitivity of the meandered SRR magnetic sensor. The

proposed equivalent circuit model is simulated using the ADS

software, and S21 plots of the circuit simulation and that of the

electromagnetic simulation using CST are compared in Fig.

10. As observed from this graph, a good match between the

two plots is obtained which authenticates the validity of the

proposed equivalent circuit model of the meandered line SRR

based magnetic sensor.

TABLE II

LUMPED ELEMENT VALUES OF PROPOSED EQUIVALENT CIRCUIT

Sensor

Design C (pF) Lc (nH) C

1a(pF) L

1a (nH) R ( )

Simple

SRR 2.67 1.72 234.5 0.018 97.7

Meandered

SRR 2.14 1.94 70.43 0.06 77.7

Fig. 10. Comparison of S21 plot of CST Simulation and ADS Simulation.

IV. NUMERICAL MODEL OF PROPOSED SENSORS

A. IDC-SRR based dielectric sensor

In order to obtain the mathematical relationship between the

resonant frequency of the IDC based SRR dielectric material

sensor with that of the relative permittivity of the MUT, the

response of the IDC based SRR dielectric sensor to the various

dielectric values of the test sample is used. The variation in

resonant frequency difference plotted against relative

permittivity corresponding to IDC based SRR dielectric sensor

is already shown in Fig. 4. A curve fitting tool of Origin is

utilized to develop the relation based on numerical simulation

data and the fitting function is chosen based on the minimum

chi-square value. The following expression is finally obtained

using this procedure for estimation of real part of the

permittivity of the MUT.

0 0.91781ln

0.1341 1.04966r

rf f

(1)

where f0 represents the resonant frequency of the sensor

without MUT, which helps in normalizing the results after

fabrication.

B. Meandered Line SRR based magnetic sensor

The dependence of resonant frequency of the meandered

line based SRR magnetic material sensor on the real value of

complex permeability is analyzed by keeping a MUT on top of

the meandered line region of the proposed sensor. The

dimensions of rectangular cube shaped MUT are kept same as

in the case of dielectric sensor. The relative permeability of

the MUT is varied and the corresponding resonant frequency

shift of the meandered line SRR based magnetic material

sensor is already shown in Fig. 9. The same numerical

simulation data are used to determine the relationship between

the change in resonant frequency and the relative permeability

value of the MUT. Since the relative permeability value of

materials in microwave frequency is very small, more number

of frequency points is selected between the relative

permeability value of 1 and 2 in order to have accurate

modeling of the proposed sensor. Finally, curve fitting tool of

the Origin is used as discussed above to obtain a mathematical

formula for calculation of the relative permeability of the

MUT as shown below.

00.225931

ln0.66825 0.43511

r

rf f

(2)

The f0 in this case represents the resonant frequency of the

proposed meandered line SRR sensor without MUT.

V. MEASUREMENT AND RESULTS

The prototype of each sensor designs is fabricated using

standard photolithography, and the measurement is carried out

by connecting each sensor to the vector network analyzer

(VNA) through a set of coaxial cables and SMA adapters. The

VNA input power level is kept at 0 dBm, and the IF

bandwidth is set to 50 Hz for getting better resolution. The 2




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port OSLT calibration is carried out using the modified

settings of the VNA and all the measurements are carried out

under same settings. The design of each sensor is validated by

measuring permittivity and permeability of some standard

samples.

A. IDC-SRR based Dielectric Sensor

The proposed single band dielectric material sensor is

validated by measuring a number of standard dielectric and

magneto-dielectric samples. Fig. 11 shows the fabricated

prototype of proposed sensor and Fig. 12 shows the measured

S21 parameter for different dielectric and magneto-dielectric

samples.

The fabricated prototype is having unloaded resonant

frequency of 2.26 GHz. A shift in resonant frequency from its

design value of 2.45 GHz is observed here due to the

tolerances in standard fabrication process. The values of the

permittivity for each sample are estimated using (1) and are

compared with the reference values available in literature.

Table III gives the measured values of the relative permittivity

of various dielectric and magneto-dielectric materials

measured using the IDC-SRR based dielectric sensor along

with the percentage error.

Fig. 11. Fabricated prototype of IDC-SRR dielectric sensor with an enlarged

view of the resonant structure. The dotted curve represents the position of the

sample.

Fig. 12. Measured S21 plot for different dielectric and magneto-dielectric

materials.

B. Meandered Line SRR based Magnetic Sensor

Finally, the meandered line SRR based magnetic material

sensor is fabricated and tested using the same measurement

setup described earlier. The relative permeability of various

magneto-dielectric materials are measured using the proposed

sensor and the VNA. Fig. 13 shows the fabricated prototype,

and Fig. 14 shows the S21 plot measured using the VNA for

different magneto-dielectric samples loaded on the designed

sensor. The relative permeability values of various magneto-

dielectric samples measured using the mathematical relation

given in (2) are tabulated in Table IV, where the extracted data

of each sample is compared with its reference value available

in literature.

TABLE III MEASURED PERMITTIVITY OF DIFFERENT DIELECTRIC AND MAGNETO-

DIELECTRIC MATERIALS.

MUT εr-Measured

εr

% Error

Carbonyl Iron 5.84 5.5 [31] 6.18

Ni0.6Co0.4Fe2O4 3.23 - -

30% Cobalt-Polystyrene 2.74 - -

20% Cobalt-Polystyrene 2.34 - -

Teflon 2.09 2.1 [8] 0.48

PVC 2.62 2.65 [8] 1.13

Polyethylene 2.23 2.26 [8] 1.33

Plexiglas 2.65 2.6 [8] 1.92

Natural rubber 2.57 - -

Epoxy resin 3.29 3.5 [33] 6.00

Fig. 13. Fabricated prototype of meandered-SRR magnetic sensor with an enlarged view of resonant structure. The dotted curve represents the position

of the sample.

Fig. 14. Measured S21 data for different magneto-dielectric and dielectric

materials.




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TABLE IV MEASURED RELATIVE PERMEABILITY OF DIFFERENT MAGNETO-DIELECTRIC

AND DIELECTRIC MATERIALS.

MUT µr Measured µr % Error

Carbonyl Iron 1.37 1.4 [31] 2.14

Ni0.6Co0.4Fe2O4 1.02 - -

30% Cobalt-Polystyrene 1.34 1.36 [32] 1.47

20% Cobalt-Polystyrene 1.11 1.15 [32] 3.47

Teflon 1 1 0

PVC 1 1 0

VI. ERROR ANALYSIS

Fig. 15. Shift in the resonant frequency corresponding to change in the

permittivity (permeability) at a constant permeability (permittivity) using

magnetic (dielectric) sensor.

One of the main objectives of this work is to characterize

magneto-dielectric materials, which have non-unity value for

both relative permittivity and relative permeability. However,

for simplifying the analysis, the dielectric (magnetic) material

testing using the proposed IDC (MLSRR) based senor is

performed after considering the value of relative permeability

(permittivity) to be one as explained in Fig 4 (Fig 9). Here, a

detailed analysis is carried out to observe the change in

resonant frequency, while changing the relative permeability

(permittivity) of the sample from unity in case of the dielectric

(magnetic) material testing as shown in Fig. 15. From this

figure, it can be observed that the change in resonant

frequency corresponding to the value of relative permeability

(permittivity) other than unity in case of the dielectric

(magnetic) material testing is nearly less than 1/10th of the

change shown in Figs. 4 and 9 under similar conditions.

Hence, it can be concluded that accuracy of the measurement

of dielectric (magnetic) MUT would not be significantly

affected if the relative permeability (relative permittivity) of

the actual test sample is other than unity.

VII. CONCLUSION

The modified SRR based RF planar sensors have been

designed fabricated and tested for the characterization of

dielectric and magneto-dielectric materials at 2.45 GHz of the

ISM band. The IDC based SRR structure shows enhanced

sensitivity as compared to that of the simple SRR

configuration. The proposed IDC based SRR sensor appears to

be better suited for integration with the multilayer planar

configuration as compared to the conventional CSRR based

sensors due to the fact that the ground is not disturbed in this

case. The inclusion of IDC structure on the SRR significantly

increases the sensitivity of the sensor for dielectric testing

which is not possible otherwise. Similarly, the meandered line

based SRR has higher sensitivity as compared to the simple

rectangular SRR for the characterization of magnetic

materials. A basic equivalent circuit model is used to explain

the microwave sensor theoretically, and a close match between

the circuit model and the numerical model results has been

achieved in each case. The empirical models of the proposed

sensors obtained using the numerical approach and the curve

fitting tools have been experimentally verified. The prototypes

of both the sensors have been fabricated, and a number of

standard dielectric and magneto-dielectric samples have been

tested using these sensors. The measured relative permittivity

and relative permeability values are in good match with

reference values available in literature having a typical

accuracy of more than 94%.

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Muhammed Shafi K. T received the B.Tech. degree in electronics and communication

engineering from Calicut University, Calicut,

India, in 2013, and the M.Tech degree in electrical engineering from the Indian Institute of

Technology (IIT) Kanpur, India, in 2016.

Currently, he is a research assistant in Robotics and Advanced Sensing Laboratory of Department

of Electrical and Computer Engineering, Khalifa

University of Science and technology-The Petroleum Institute, Abu Dhabi, UAE. His research interests include design

and development of microwave sensors for characterization of dielectric and

magneto-dielectric materials, Non-destructive testing of composite materials and near field microwave imaging of multi-layer dielectric materials.

He was associated with the Microwave Imaging and Material Testing

Laboratory, Department of Electrical Engineering, IIT Kanpur. There he was involved in design and development of compact microwave sensors for

various applications.

Abhishek Kumar Jha (S’14–M’17) received the

B.E (Hons.) and M.Tech. (Hons.) degrees in

electronics and communication engineering from the University of Burdwan, West Bengal, India,

and the Ph.D. degree in Electrical Engineering

from the Indian Institute of Technology Kanpur (IITK), India.

He is currently a postdoctoral research fellow with Bioelectrodynamics research group at the

Institute of Photonics and Electronics, The Czech Academy of Sciences,

Prague, Czech Republic. His research interests encompass the design and development of high-frequency planar sensors and biochips for

electromagnetic analysis and label-free sensing of biomolecules and cells,

numerical analysis of microwave circuits and waveguide components, advanced RF sensors, frequency selective surfaces and techniques for

nondestructive testing of materials, printed RF electronics etc. He has

authored/coauthored more than 30 scientific articles published in peer-reviewed international journals and conference proceedings.

He founded and chaired the IEEE MTT-S SBC IITK, Uttar Pradesh Section, India, and served the IEEE APS SBC IITK, Uttar Pradesh Section, India, as

the secretary. He was associated with the MTRDC, DRDO Bangalore, India,

where he was involved in medium power injection locked magnetron. He was an Assistant Professor with the Electronics & Communication Engineering

Department, Seacom Engineering College, Howrah, India.

Dr. Jha is a winner of various competitive and prestigious awards including the BIRAC-GYTI award under the technological edge category, the

Microwave Graduate Fellowship in recognition of his academic achievement

and excellence, the IEEE SIGHT design competition award etc. He is also the recipient of the University Gold Medals for being first in the first class of B.E

and M.Tech degrees in 2009 and 2011, respectively. He is also a member of

International Association of Engineers, Hong Kong. He is serving as one of

the potential reviewers of IEEE Sensor Journal, IEEE Transactions on

Instrumentation and Measurement and IEEE Transactions on Microwave

Theory and Techniques.

Mohammad Jaleel Akhtar (S’99–M’03–SM’09) received the Ph.D. and Dr.

Ing. degrees in electrical engineering from the Otto-von-Guericke University of Magdeburg, Magdeburg, Germany, in 2003.

He was a Scientist with the Central Electronics Engineering Research Institute, Pilani, India, from

1994 to 1997, where he was involved in the design

and development of high power microwave tubes. From 2003 to 2009, he was a Post-Doctoral

Research Scientist and a Project Leader with the

Institute for Pulsed Power and Microwave Technology, Karlsruhe Institute of Technology,

Karlsruhe, Germany, where he was involved in a

number of projects in the field of microwave material processing. In 2009, he joined the Department of Electrical Engineering, IIT Kanpur, Kanpur, India,

where he is currently an Associate Professor. He has authored two books, two

book chapters, and has authored or co-authored over 150 papers in various peer-reviewed international journals and conference proceedings. He holds

two patent on RF sensors for material testing. His current research interests

include RF, microwave and THz imaging, microwave nondestructive testing, RF sensors, functional materials, wideband electromagnetic absorbers, UWB

antennas for imaging, RF energy harvesting and design of RF filters and

components using the electromagnetic inverse scattering.

Dr. Akhtar is a fellow of the Institution of Electronics and Telecommunication

Engineers, New Delhi, India, and a Life Member of the Indian Physics Association and the Indo-French Technical Association. He is a recipient of

the CST University Publication Award in 2009 from the CST AG, Darmstadt,

Germany. He served as a Chair of the IEEE Microwave Theory and Techniques Society-S Uttar Pradesh Chapter from 2013 to 2015, and the Vice-

Chair of the IEEE Uttar Pradesh Section in 2015.

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