Artificial Circular Dielectric Resonator with Resonant Mode Selectability

Rizqa Hanif Basuki, Hepi Ludiyati, Achmad Munir* Radio Telecommunication and Microwave Laboratory

School of Electrical Engineering and Informatics, Institut Teknologi Bandung Bandung Indonesia *munir@ieee.org

Abstract—Resonant frequencies of natural circular dielectric resonator between its resonant modes has a close range each other, so some desired mode has to be chosen selectively from other modes. Therefore, a circular dielectric resonator with a capability to select the resonant mode is required. This can be overcome by artificially modifying some property of natural circular dielectric resonator, e.g. resonant mode, so that it has a unique characteristic. In this paper, some attempt to select the resonant mode of circular dielectric resonator is proposed by use of artificial circular dielectric resonator. The proposed artificial circular dielectric resonator is constructed of some open ring metal strips etched on a circular shape of dielectric substrate. The use of open ring metal strip aims to select the desired resonant mode of resonator. The effect of number of metal strips and its gaps as well as the effect of number of stacking dielectric substrate are used to evaluate the performance of proposed resonator in producing resonant frequency and inter-mode frequency separation. The investigation result shows that the resonator configuration consists of 1 strip with 4 gaps and of 2 strips with 4 gaps in different gap position has the lowest resonant frequency of TE01δ mode.

Keywords-artificial circular dielectric resonator; resonant frequency; resonant mode; selectability

I. INTRODUCTION In dielectric materials, the constituent atoms will

experience polarization under the influence of an applied external electric field. The ability to yield polarization is determined by dielectric constant or permittivity. The greater permittivity of dielectric, the easier to have dielectric polarization [1]. There are many kinds of materials which have such properties of dielectric in the form of solid, liquid, and gas. For electromagnetics field application, solid dielectrics such as porcelain, glass, and plastic are commonly used and can be found in various devices including capacitor, isolator, and dielectric resonator. The latter one is an electronic component that exhibits resonance for a certain range of frequencies. It is usually implemented in the filtering process for wireless communication systems. This type of filters is widely used due to the low attenuation and suitable for miniaturization [2].

The performance of a dielectric resonator depends on its characteristics. As microwave filter applications, dielectric resonators with anisotropic permittivity can be applied for improving the filter response. Hence for specific application, it

is required to implement dielectric resonators with mode selectivity [3]. Those kinds of characteristics are rarely obtainable through simple chemical treatment such as natural dielectric. Therefore, other treatment is required to yield dielectric resonators with anisotropic permittivity.

In principle, the technology of artificial materials which are also known as metamaterials was developed to overcome such kind of problem. The notion metamaterials basically includes several artificial materials such as chiral materials, artificial magnetic, Veselago medium, and artificial dielectric [4]. Artificial material that was invented more than fifty years ago [5] offers the artificial dielectric as a solution to these matters. In this paper, an artificial circular dielectric resonator with resonant mode selectability is proposed for the investigation. The proposed resonator is designed with a specific pattern of metal strip etched on dielectric substrate to excite a TE01δ mode with the resonant frequency in S band frequency range.

II. BRIEF OVERVIEW OF ARTIFICIAL DIELECTRIC AND CIRCULAR WAVEGUIDE

A. Artificial Dielectric Artificial dielectric is a kind of metamaterials in which its

particles are arranged in regular or irregular, homogeneous or inhomogeneous, isotropic or anisotropic structure. The arrangement of particles in artificial dielectric determines the characteristics of dielectric material from macroscopic viewpoint. The orientation of each particle will determine isotropic or anisotropic characteristics. Hence, homogeneous or inhomogeneous characteristics will depend on the distribution of each particle [4].

Basically, all kind of materials are composed of unit particles such as atoms, ions, and electrons. Similarly, an artificial dielectric is also composed of unit particles. The unit particles can be made of thin wire, rectangular strip, strip circular, spiral, and cylinders. The unit particles, orientation and their distribution are the main factors that determine the macroscopic characteristics of an artificial dielectric material. For instance, an artificial dielectric material made of unevenly distributed thin metal with certain orientation direction will have anisotropic characteristic. This characteristic can be applied to affect the eigen frequency of each resonance mode. As a result, the artificial dielectric resonator will have the capability to select in producing resonant mode

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B. Circular Waveguide Circular waveguide is a kind of transmission line consists

of single conductor. As shown in Fig. 1 [6], cylindrical shaped of circular waveguide has uniform circular section of a finite radius a. It has lowest attenuation rather than other shape of waveguide which makes its application in long distance low-loss communication link. The cutoff frequency of circular waveguide can be expressed in (1).

Figure 1. Circular waveguide

r

mnmn

a

pcf

επ2

′⋅= , (1)

where p’nm is a result from Bessel differential function 2nd order, c is light velocity, a is radius of circular waveguide, and εr is dielectric constant inside the waveguide. The index of m and n denotes the wave number of resonant mode. The dominant mode with lowest cutoff frequency in a circular waveguide is TE11 mode. Out of the multitude of possible modes in circular waveguides, the modes of highest practical interests are TE11, TE01, and TM01 [7]. Based on the distribution field in inside of circular waveguide, the TE01 mode has a simple configuration since no variation of electric field in angular direction as illustrated in Fig. 2.

Figure 2. Distribution field inside of circular waveguide for TE01δ mode

III. SIMULATION, EXPERIMENTATION AND DISCUSSION

A. Basic Configuration and Number of Metal Strip A basic configuration of artificial circular dielectric

resonator consists of thin layer copper metal strip printed on a 0.8mm thick circular shape of FR4 Epoxy dielectric substrate with the radius of 49.7mm as shown in Fig. 3. The width of closed ring metal strip is 2mm and the distance from edge of substrate to the edge of outer ring is 1mm.

Figure 3. Basic configuration of closed ring metal strip

The ring-shape of metal strip is chosen to control the polarization effect which is useable to control the anisotropic effect. For obtaining a TE01δ mode resonator, metal strip made longer in angular direction (φ) since the relative permittivity in any direction can be attained by controlling the polarization effect to that direction [8]. To investigate the effect of number of metal strip, the other concentric metal strips in smaller radius is added up to 5 with the spacing between metal strips is 2mm. The resonant frequency of simulated result for basic resonator configuration in various number of full ring metal strip is plotted in Fig. 4 with resonant frequencies of simulated result for the hollow waveguide and the waveguide with dielectric substrate plotted together as comparison.

1 2 3 4 53.7749

3.775

3.7751

Number of metal strip

Freq

uenc

y (G

Hz)

hollow waveguide only waveguide with dielectric substrate only basic configuration with full metal strip

Figure 4. Simulated results for basic configuration in various number of closed ring metal strips

From Fig. 4, it is shown that the addition of metal strip affects on increasing resonant frequency of dielectric resonator. Since the width of metal strip is too wide, the shield effect of metal strip will be greater and will reflect the incoming power of incident wave. The greater power of reflected wave will affect to the smaller power absorbed by dielectric. As a result, the transmitted power will be greater yields the increase of resonant frequency.

B. Open Ring Resonator and Final Configuration Next, the resonator configuration that is modified from

basic configuration, as illustrated in Fig. 5, i.e. open ring resonator, is investigated to observe the effect of gap on the metal strip to the resonant frequency of TE01δ mode. To discover the effect of additional gap to the resonant frequency,

y

z x

a

φ

p

electric fields

magnetic

diameter 99.4mm

distance from edge to strip = 1mm

strip width = 2mm

metal strip

FR4 Epoxy dielectric substrate

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the number of gaps on metal strip is varied from 8 gaps to 16 gaps for each metal strip with certain spacing between adjacent gaps. It should be noted that the gap width is 1mm. From the investigation result, it shows that the trend of resonant frequency of each design gives different result. For a certain number of gaps, the resonant frequency decreases, but in other number of gaps, the resonant frequency increases. Here, the resonator configuration with gaps that gives the lowest resonant frequency is chosen for the next investigation.

Figure 5. Open ring resonator modified from basic configuration

Based on the result on previous investigation, the resonator configurations consist of 1 strip 4 gaps and 2 strips with 2 different positions of gaps in each strip have given the lowest resonant frequency. Then, each design will be investigated by stacking it with similar resonators in 3 layers. The resonant frequency of both designs as shown in Figs. 6 and 7 is investigated in cylindrical section of waveguide converter [9]. To analyze the shifting resonant frequency of both designs, the TE01δ mode operates at the resonant frequency 3.78GHz will be used as frequency reference.

(a) 1st layer (b) 2nd layer (c) 3rd layer

Figure 6. Design of resonator configuration with 1 strip 4 gaps in different position of gap

(a) 1st layer (b) 2nd layer (c) 3rd layer

Figure 7. Design of resonator configuration with 2 strips and 2 different positions of gaps in each strip

The observation is conducted around the frequency 3.7GHz in which the reflection coefficient value is below 10dB. Hence, the simulated results for the resonator configurations consist of 1 strip 4 gaps and 2 strips with 2 different positions of gaps in each strip are depicted in Figs. 8 and 9, respectively. It shows in the figures that the hollow waveguide operates at resonant frequency of 3.8GHz, while the frequencies obtained after addition of the dielectric substrate and the resonator configuration are tabulated in Tables 1 and 2 for the designs of resonator configuration shown in Figs. 6 and 7, respectively. From the tables, it notes that inaccurate frequency shifting occurs due to the thickness of resonator configuration which is too thin rather than the volume of waveguide converter. From the reflection coefficient viewpoint, the addition of different layer of dielectric substrate with and without metal strips arrangement impacts to the increasing of return loss. The greater number of stacked layer the greater return loss produced which means the more positive value of return loss. As the return loss is represent the comparison of reflected power to incident power of wave, the more positive return loss the more power of incoming wave is reflected. It means the transmitted power will be smaller which impacts to frequency decreasing. While the frequency decreasing means that the relative permittivity of resonators configuration increases.

2.5 3 3.5 4 4.5-60

-40

-20

0

Frequency (GHz)

Ref

lect

ion

Coe

ffici

ent (

dB)

1 layer waveguide only 2 layers substrate only 3 layers

Figure 8. Simulated results of resonator configurations consist of 1 strip 4

gaps for different number of layer

2.5 3 3.5 4 4.5-60

-40

-20

0

Frequency (GHz)

Ref

lect

ion

Coe

ffici

ent (

dB)

1 layer waveguide only 2 layers substrate only 3 layers

Figure 9. Simulation results of resonator configurations consist of 2 strips with 2 different positions of gaps in each strip for different number of layer

diameter 99.4mm

distance from edge to strip = 1mm

strip width = 2mm

metal strip

FR4 Epoxy dielectric substrate

gap width = 1mm

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TABLE I. SIMULATED FREQUENCY OF RESONATOR CONFIGURATIONS CONSIST OF 1 STRIP 4 GAPS FOR DIFFERENT NUMBER OF LAYER

Substrate 1 strip 1 layer

1 strip 2 layer

1 strip 3 layer

Frequency (GHz) 3.76 3.74 3.74 3.74

TABLE II. SIMULATED FREQUENCY OF RESONATOR CONFIGURATIONS CONSIST OF 2 STRIPS WITH 2 DIFFERENT POSITIONS OF GAPS IN EACH STRIP FOR

DIFFERENT NUMBER OF LAYER

Substrate 1 strip 1 layer

1 strip 2 layer

1 strip 3 layer

Frequency (GHz) 3.76 3.74 3.74 3.72

To verify the simulated results, the experimental

measurement is carried out using prototypes of realized resonator configurations. The measurement results for the resonator configurations consist of 1 strip 4 gaps and 2 strips with 2 different positions of gaps in each strip are depicted in Figs. 10 and 11, respectively. Hence, the measured frequencies which are obtained after addition of the dielectric substrate and the resonator configuration are tabulated in Tables 3 and 4. As shown in Tables 3 and 4, frequency shifting is not too significant since the thickness of dielectric substrate too thin rather than the waveguide length.

2.5 3 3.5 4 4.5-40

-20

0

Frequency (GHz)

Ref

lect

ion

Coe

ffici

ent (

dB)

1 layer waveguide only 2 layers substrate only 3 layers

Figure 10. Measured results of resonator configurations consist of 1 strip 4

gaps for different number of layer

2.5 3 3.5 4 4.5-40

-30

-20

-10

0

Frequency (GHz)

Ref

lect

ion

Coe

ffici

ent (

dB)

1 layer waveguide only 2 layers substrate only 3 layers

Figure 11. Measured results of resonator configurations consist of 2 strips with 2 different positions of gaps in each strip for different number of layer

TABLE III. MEASURED FREQUENCY OF RESONATOR CONFIGURATIONS CONSIST OF 1 STRIP 4 GAPS FOR DIFFERENT NUMBER OF LAYER

Substrate 1 strip 1 layer

1 strip 2 layer

1 strip 3 layer

Frequency (GHz) 3.77 3.74 3.74 3.74

TABLE IV. MEASURED FREQUENCY OF RESONATOR CONFIGURATIONS CONSIST OF 2 STRIPS WITH 2 DIFFERENT POSITIONS OF GAPS IN EACH STRIP FOR

DIFFERENT NUMBER OF LAYER

Substrate 2 strip 1 layer

2 strip 2 layer

2 strip 3 layer

Frequency (GHz) 3.77 3.77 3.74 3.725

IV. CONCLUSIONS The characteristic of artificial circular dielectric resonator

with resonant mode selectability made from simple structures such as a stack of dielectric substrate which metal strip printed on each layer has been demonstrated numerically and experimentally. It has been shown that the increase of number of metal strips has given significant impact to the increase of resonant frequency. In other hand, the increase of number of gaps on metal strip has affected to the decrease of resonant frequency with the frequency shifting varies from 15MHz to 45MHz. From the characterization result, it can be concluded that the frequency shifting to the lower frequency is produced by the resonator configuration with high relative permittivity. This frequency shifting will affect to the resonant mode selectability of the resonator configuration. With this behavior, the range of resonant frequency of the adjacent mode will be wider, so the unwanted mode can be suppressed.

REFERENCES [1] M.F. Iskander, Electromagnetic fields and waves, Illinois: Waveland

Press Inc., 2000. [2] R. Zhang and R.R. Mansour, “Low cost dielectric resonator filters with

improved spurious performance”, IEEE Transactions on Microwave Theory and Techniques, vol. 55, no.10, Oct. 2007.

[3] I. Awai, H. Kubo, H. Kohno, T. Iribe, A. Sanada, “Dielectric resonator based on artificial dielectrics and its application to a microwave BPF”, Proceeding of 32nd European Microwave Conference, Milan, Italy, 2002.

[4] A. Munir, “Study of artificial dielectric resonators and its microwave application”, Doctoral Dissertation, Yamaguchi University, Japan, 2005.

[5] W.E. Kock “Metallic delay lenses”, Bell System Technical Journal, Vol.27, pp. 58-82, May 1948.

[6] D.M. Pozar, Microwave engineering, 2nd Edition, New York: John Wiley & Sons Inc., 1998.

[7] A. Das and S. K. Das, Microwave engineering, New Delhi: Tata McGraw Hill, 2009.

[8] A. Munir and H. Kubo, “Study of artificial dielectric resonator with anisotropic permittivity encapsulated in a circular waveguide and its filter application”, Proceeding of Asia-Pacific Microwave Conference, Suzhou, China, 2005.

[9] A. Munir and M.F.Y. Musthofa, “Rectangular to circular waveguide converter for microwave devices characterization,” International Journal of Electrical Engineering and Informatics, vol. 3, no. 3, pp. 350-359, Oct. 2011.

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