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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 5, MAY 2014 321

A Novel Resonant Ground Structure

Based on a Cavity-Backed DGSImseob Shin, Tae-Hak Lee, Juseop Lee , Member, IEEE , and Young-Sik Kim , Member, IEEE

Abstract— In this letter, a new resonant ground structure (RGS)

based on a cavity-backed DGS is proposed. The proposed RGSis similar to a DGS and is modeled by a parallel RLC resonance

circuit. Since the proposed RGS utilizes a substrate-filled metalliccavity (SFMC) underneath the DGS, it leads to no back-radiationand high quality factor. The frequency responses and structuralparameters are analyzed and verified via simulation and measure-

ment. The quality factor of the proposed RGS with a spiral-shapeddefect is larger than that of the DGS with the same defect by afactor of 7.4.

Index Terms— Defected ground structure (DGS), electromag-

netic bandgap (EBG), photonic bandgap (PBG), resonant groundstructure (RGS).

I. I NTRODUCTION

T HE bandgap structures such as a photonic bandgap (PBG)

and a DGS have been widely investigated [1]–[7]. In gen-

eral, most of DGSs are constructed with many artificial defects

on the ground plane to provide the bandgap effect, and have a

low quality (Q) factor and high back-radiation. The DGSs with

one single defect, which are especially modeled to a parallel

RLC resonance circuit, have been widely utilized to resonators

[3], [4], filters [5], [6], dividers [7], and so on.

In this letter, a new RGS based on a cavity-backed DGS

is presented. As the proposed RGS features a substrate-filledmetallic cavity (SFMC) which is placed under the DGS, this

proposed RGS improves the Q factor and reduces back-radi-

ation. The proposed RGS can be modeled by a parallel RLC

resonance circuit. The structural and extracted parameters are

analyzed via the EM simulator. The experimented results are

compared with the results of the EM simulation and the circuit

model analysis.

II. MODELING AND STRUCTURE PARAMETERS

A. Modeling and Equivalent Circuit

Fig. 1(a) and (b) show the configuration of the proposed RGS

on a microstrip line. The proposed RGS consists of a dumbbell-shaped DGS and the SFMC which is slightly larger than or

Manuscript received September 16, 2013; revised December 04, 2013 andJanuary 14, 2014; accepted January 27, 2014. Date of publication March 21,

2014; date of current version May 06, 2014. This work was supported by the

Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology

(2012R1A111004665).The authors are with the Department of Computer and Radio Commu-

nications Engineering, Korea University, Seoul 136-713, Korea (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LMWC.2014.2309079

Fig. 1. (a) Three-dimensional view of the proposed RGS with a dumbbell-shaped defect. (b) XY cross-section.

equal to the defect size of a DGS. The bottom and four sides

of the SFMC are wrapped up with copper. Because the DGS

is clothed with a metal, it is expected that there is no back-ra-

diation loss. This SFMC plays two important roles. One is to

increase the effective capacitance. The dielectric material with

high permitivity in the SFMC is placed on the etched defect of

the DGS and, thus the effective capacitance increases with the

increase of the permitivity value. The other is to change the cur-

rent distribution which is formed by the DGS. In other words,

the SFMC and the etched defect together make a new current

distribution.

For illustration, we designed both DGS and RGS. The dimen-

sions of the conventional DGS are mm,

mm, and mm and the substrate with a thickness of

0.787 mm, a relative dielectric constant of 2.2, and a loss tan-

gent of 0.0009 is used. The same design parameters are set for

the proposed RGS and the SFMC has a width of 5.0 mm, alength of 11.4 mm, a height of 1.27 mm, a dielectric con-

stant of 10.2, and a loss tangent of 0.0023. The simulation has

been carried out by ANSYS HFSS v12.

Fig. 2(a) shows the S-parameters of theproposed RGS and the

conventional DGS. The proposed RGS yields a narrower band-

width and higher rejection at the resonant frequency than those

of the DGS because the RGS scheme may capture the field on

the DGS inside of the SFMC and then has a high Q character-

istic. The equivalent circuit of the proposed RGS can be mod-

eled by a parallel RLC resonance circuit because its frequency

responses are similar to those of the DGS. The equivalent cir-

cuit of both RGS and DGS is shown in the inset of Fig. 2(a). The

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322 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 5, MAY 2014

Fig. 2. (a) Comparison of the EM simulation results of the proposed RGS withthose of the DGS. (b) Current distributions of the DGS (left) and the proposed

RGS (right) at each resonant frequency.

TABLE I

EXTRACTED PARAMETERS OF THE EQUIVALENT CIRCUITS

current distributions at each resonant frequency (8.4 GHz in the

DGS and 7.15 GHz in the RGS) are plotted in the Fig. 2(b).

It is shown that the current distribution in the proposed RGS

is formed not along the etched defect but along the SFMC. In

Table I, the equivalent circuit parameters extracted from the EM

simulation results in Fig. 2(a) are compared for two configura-

tions. These parameters can be extracted using the following

equations [3]:

(1)

(2)

(3)

where is a characteristic impedance of the transmission line.

is the resonant frequency and is a 10 dB bandwidth. The

effective capacitance increases approximately 4.7 times, but the

effective inductance decreases almost 3.4 times. Therefore, the

Q factor of the proposed RGS increases about

4.2 times than that of the DGS.

B. Structure Parameters

In order to examine the structural characteristics of the

SFMC, the proposed RGSs are simulated by changing three

physical parameters, the dielectric constant, thickness, and size

of the SFMC.

Fig. 3. Simulated transmission responses of the proposed RGS for different

dielectric constants .

TABLE II

EXTRACTED CIRCUIT PARAMETERS BY CHANGING DIELECTRIC CONSTANT

Fig. 4. Simulated transmission responses of the proposed RGS for differentthicknesses of the SFMC (h).

TABLE IIIEXTRACTED CIRCUIT PARAMETERS BY CHANGING THICKNESS

TABLE IV

EXTRACTED CIRCUIT PARAMETERS BY CHANGING SIZE

Fig. 3 shows the transmission responses of the proposed

RGSs, whose simulations are conducted for different dielec-

tric constants of the SFMC. The loss tangents of SFMCs are

slightly different because of commercial availability. The cir-

cuit parameters extracted for three different dielectric constants

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SHIN et al.: A NOVEL RESONANT GROUND STRUCTURE BASED ON A CAVITY-BACKED DGS 323

Fig. 5. Photographs and measurements of the fabricated RGSs and DGSs. (a)Bottom-view photos of the DGS (left) and the RGS (right). (b) Transmission re-

sponses for the dumbbell-shape defect. (c) Transmission responsesfor the spiral-shape defect.

are listed in Table II. As the dielectric constant increases, the

effective capacitance increases so that the resonant frequency

moves to a lower band and the Q factor increases. It is noted

that the capacitance is mainly affected by a dielectric constant

of the SFMC.

Fig. 4 shows the transmission responses of the proposed

RGSs which are simulated for different thicknesses of the

SFMC. The circuit parameters extracted for three substrate

thicknesses are listed in Table III. As the thickness of theSFMC increases, the effective inductance increases so that the

resonant frequency decreases and the Q factor also decreases.

Table IV show the extracted parameters of the proposed RGSs

for three different SFMC sizes. The perpendicular direction of

the SFMC does not make any effect on the current distribution

as shown in Fig. 2(b). Thus, the only longitudinal variation of

the SFMC is analyzed as given in Table IV. It is noted that

the thickness and the size of the SFMC have an impact on the

inductance rather than the capacitance.

In addition, the gap of the proposed RGS mainly makes effect

on the effective capacitance. Since the gap effect in the proposed

RGS is similar to that in the DGS [5], the gap property of the

proposed RGS is not dealt with in this letter.

III. MEASURED R ESULTS AND DISCUSSION

In order to verify the above mentioned properties of the pro-

posed RGS, the proposed RGS and the DGS are fabricated and

measured. Fig. 5(a) shows the bottom-views of the DGS and the

RGS. Fig. 5(b) shows the transmission responses of the DGS

and the RGS with the same dumbbell-shaped defect. The mea-

sured results show good agreements with the simulated ones,except that each resonant frequency is slightly higher than the

simulated one due to fabrication error. Frequency characteris-

tics of the proposed RGS at all frequencies behave well, while

those of theDGS become unstable at high frequency band due to

high back-radiation. The Q factor of the proposed RGS is 2.31

and that of the DGS is 0.729. Thus, the Q factor increases 3.2

times compared to the almost same size of the DGS.

The RGS concept can easily be applied to the DGS with

different defect shapes. The RGS with a spiral-shaped defect

is also fabricated and compared to the spiral-shaped

DGS . The same design parameters are set for the

, except for the SFMC which has a height of

1.27 mm, a dielectric constant of 6.15, and a loss tangent of 0.0027. The spiral-shaped defect has a gap 0.5 mm. Fig. 5(c)

shows the transmission responses of the DGS and the RGS with

the same spiral-shaped defect. The measured results agree with

the simulated ones very well. The Q factor of the is

7.4 times higher than that of the . It may be expected

that the RGS has a few times higher Q factor than the DGS

depending on the defect shape.

IV. CONCLUSION

In this letter, a new RGS based on a cavity-backed DGS,

which can be modeled by a parallel LC resonance circuit, is

introduced. It is demonstrated that the proposed RGS, whichshows an excellent resonance characteristic, features a high Q

factor and no back-radiation by comparing with the DGS. It is

expected that the proposed RGS can be widely applied to var-

ious microwave and millimeter-wave circuits such as filters, di-

viders, amplifiers, and so on.

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