Miniaturized dual-band dielectric resonator antenna for IEEE 802.16d fixed WiMAX applications

Miniaturized Dual-Band Dielectric Resonator Antennafor IEEE 802.16d Fixed WiMAX Applications

Runa Kumari, Santanu Kumar Behera

Department of Electronics and Communication Engineering, National Institute of Technology,Rourkela, India

Received 1 September 2011; accepted 10 January 2012

ABSTRACT: A miniaturized dual-band C-shaped dielectric resonator antenna (DRA) with

partial ground plane is presented for IEEE 802.16d fixed WiMAX applications at 3.5 and

5.8 GHz. The design starts with dimensioning a single band cylindrical DRA, which has

been transferred to get a dual-band ring-shaped DRA. One portion of the ring-shaped DRA

is removed for forming a C-shaped DRA to get a more compact antenna. For easy fabrica-

tion, the compact DRA dimensioned as 60 � 50 � 6.6 mm3is excited by a microstrip

line feeding. The design parameters are inner and outer radii of the C-shaped antenna and

air gap (between DR and ground) to control both the resonating frequency and the quality

factor. The result shows peak gain around 3.26 and 5.55 dBi at 3.5 and 5.8 GHz, respec-

tively. The obtained results indicate very good agreement between the simulated and meas-

ured results. VC 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE 22:682–689, 2012.

Keywords: dielectric resonator antenna; dual band; WiMAX; microstrip line feed

I. INTRODUCTION

Nowadays, the IEEE 802.16 WiMAX standard allows

data transmission using multiple broadband frequency

ranges. The original 802.16a standard specified transmis-

sions in the range 10–66 GHz, but 802.16d allowed lower

frequencies in the range 2–11 GHz. For WiMAX applica-

tions, different bands are available in different parts of the

world, but the frequencies commonly used for 802.16d

fixed WiMAX applications are 3.5 and 5.8 GHz [1]. The

lower frequencies used in this specification means that the

signals suffer less from attenuation, and therefore, they

provide improved range and better coverage. For low fre-

quency WiMAX applications, the important requirement

is the antenna miniaturization.

The dielectric resonator antenna (DRA) offers advan-

tages like low cost, ease of manufacture, wider impedance

bandwidth, and high radiation efficiency [2, 3]. DRAs can

be designed with different shapes to accommodate various

design requirements [4–8]. DRAs can also be excited with

different feeding methods, such as probes, microstrip

lines, slots, and coplanar lines. As compared to microstrip

antenna, DRA has much wider impedance bandwidth due

to their many advantageous features. These include their

compact size, light weight, simple structure, and versatil-

ity in their shape and feeding mechanism. Among the

different shapes of DRA, the cylindrical-shaped DRA

offers greater design flexibility, where the ratio of radius

to height (R/hR) controls the resonant frequency and the

Q-factor [5]. Fabrication of cylindrical-shaped DRA is

also simpler than other shaped DRA [9–11]. Various

modes can be easily excited within the cylindrical-shaped

DRA, which results in either broadside or omnidirectional

radiation patterns [12, 13]. Analytical studies carried out

on cylindrical dielectric resonators (DRs) have demon-

strated that the Q-factor could be reduced by removing a

central portion of the dielectric material to form a ring.

By applying perturbation theory [2], the removal of

dielectric material would result in lowering the Q-factor

and an increase in the resonant frequency. The impedance

bandwidth can be enhanced using low-dielectric-constant

materials for the resonator, but the size of the DR will be

increased slightly [14]. Further, the bandwidth and reso-

nant frequency of the ring-shaped DRA can be improved

by proper choice of antenna parameters [15]. It has been

discussed that there is generally an increase in the reso-

nant frequency and a decrease in the Q-factor, when an

air gap is introduced, which can provide a good imped-

ance matching significantly [16–19].

This article mainly demonstrates the design of a micro-

strip line fed, dual-frequency C-shaped compact DRA,

Correspondence to: R. Kumari; e-mail: [email protected]

VC 2012 Wiley Periodicals, Inc.

DOI 10.1002/mmce.20627Published online 27 March 2012 in Wiley Online Library

(wileyonlinelibrary.com).

682

made up of C-shaped DR supported by a substrate and

partial ground plane for IEEE 802.16d fixed WiMAX

applications. In this article, DRA volume miniaturization

for compact antenna design is also discussed. The initial

design of the proposed antenna is considered as a simple

cylindrical-shaped DRA. The desired operating frequency

of cylindrical-shaped DRA is chosen as 5.8 GHz. The

shape of the antenna has been modified by taking partial

ground plane for WiMAX and WLAN applications. The

modified C-shaped DRA is then approximated for facili-

tating its fabrication. The design procedure of the pro-

posed DRA is presented. Some parametric studies have

been performed to investigate the characteristics of DRAs

with different shapes (cylindrical- and C-shaped DRA)

and different values of inner radius (6, 9, and 2 mm) and

air gap between ground and DR. Measured S-parameter

values are also compared with the simulated results.

Finally, the extensive simulations have been carried out to

investigate the different characteristics (gain, directivity,

and radiation features) of the proposed antenna, and the

obtained results are presented in the following sections.

II. NUMERICAL ANALYSIS ON C-SHAPED DRA

The cylindrical-shaped DRA is characterized by height

(hR), radius (R), and a dielectric constant eR. The cylindri-

cal shape offers one degree of freedom more than the

hemispherical shape because of the aspect ratio RhR

. The

impedance band width and quality factor (Q) of cylindri-

cal DRA are given by Drossos et al. [20].

An analytical study has been carried out on cylindrical

DRs, which demonstrated that the Q-factor could be

reduced by removing a central portion of the cylindrical-

shaped dielectric material [21, 22]. By applying perturba-

tion theory, the removal of dielectric material would result

in reduction of the Q-factor and rise in the resonant

frequency.

The resonant frequency of the ring-shaped DRA having

outer radius R and inner radius r can be resolved using eq. (1):

fR ¼c

2pffiffiffiffiffieRp

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffip

2hR

� �2

þ X0

R

� �2s

(1)

where c is the speed of light in free space, and X0 is the

solution to:

J1 X0ð ÞY1 X0ð Þ

¼J1

rR X0

� �Y1

rR X0

� � (2)

where J1(X) and Y1(X) are the first-order Bessel functions

of the first and second kind, respectively (Ref. 2; Table I).

For rR � 0:7

X0 ¼ 3:56þ 5:13r

R

� �� 13:07

r

R

� �2

þ28:2r

R

� �3

(3)

The ring-shaped DRA is made up of cylindrical DRA,

where a central cylindrical section of radius r has been

detached. The effect of increasing the ratio of inner to outer

radius rR

� �of the ring-shaped resonator on resonant fre-

quency and Q-factor can be seen by applying eqs. (1)–(3).

Increasing rR

� �ratio results in intensification in Q-factor.

However, to maintain the same resonant frequency, the

outer radius R must be increased.

If the volume to surface ratio is minimized, then the

quality factor is reduced. Thus, by removing a portion of

ring-shaped DRA, a C-shaped DRA is designed to get

minimum quality factor. The volume of cylindrical- or ring-

shaped DRAs can be reduced by removing sectors of mate-

rial [23–25]. For the sectored ring-shaped DRA, the resonant

frequency fvpm of the TMvpmþd mode is given by [26]

fvpm ¼c

2pRffiffiffiffiffieRp

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiX2

vp þpR

2hR

2mþ 1ð Þ� �2

s(4)

where Xvp is the solution for

J0v Xvp

� �¼ 0 (5)

It is clear that the annular ring-shaped DRA has higher

resonant frequency than a simple cylindrical-shaped DRA of

equal outer radius.

III. ANTENNA GEOMETRY AND DESIGN

In this article, the design of DRA is initiated with the res-

onant frequency assortment for the desired fixed WiMAX

applications. The dielectric materials and dimensions for

the resonator and substrate are carefully chosen. The nu-

merical analysis (described in previous section) has been

used to normalize the ratio of inner to outer radius (r/R)

for DR. The air gap (g) and inner radius are presumed.

The resonant frequency for the DRA is extracted from the

S-parameter versus frequency plot. If the resulted values

are suitable for desired applications, then only the other

parameters (namely, gain, directivity, and radiation pat-

terns) are extracted. This methodology gives an easy and

simple procedure to realize the design of proposed DRA.

The configuration of the C-shaped DRA is shown in

Figure 1. Among the different shapes of DRA, the cylin-

drical-shaped DRA offers greater design flexibility. Thus,

the design of proposed antenna is initialized by a simple

cylindrical-shaped DR, where the ratio of radius to height

controls the resonant frequency and Q-factor.

The proposed DRA design starts with a cylindrical-

shaped DR made from Teflon with dielectric constant

eR ¼ 2.1, having radius R ¼ 19 mm and height hR ¼ 5 mm.

Microstrip line feeding offers the advantages of easy,

simplest, and cost-effective fabrication of DRA. So, the

TABLE I Values of X0 for Various Ratios of rR

rR 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

X0 3.8317 3.9409 4.2358 4.7058 5.3912 6.3932 7.9301 10.522 15.7376 31.4292

Miniaturized Dual-Band DRA 683

International Journal of RF and Microwave Computer-Aided Engineering DOI 10.1002/mmce

DRA is excited by a 50 X microstrip line feeding, which

has dimensions Lf ¼ 57.75 mm and Wf ¼ 2 mm. The DR

along with the feed line is mounted on the top side of FR4

substrate measuring �60-mm long by 50-mm wide with a

thickness hs of 1.6 mm, a relative dielectric constant (es) of

4.4, and a loss tangent (tan d) of 0.001 (shown in Fig. 1c).

A partially printed ground plane with dimensions 50 � 13.5

mm2 (Lg � W) was present on the bottom side of the sub-

strate as shown in Figure 1d. The air gap (g) between DR

and ground is 5.5 mm. As the cylindrical-shaped DRA (Fig.

1a) is resonanting (with VSWR < 2) at only 5.8 GHz fre-

quency, the shape of the DRA is modified to achieve the

desired dual-band resonant frequency.

A circular core of dielectric material with radius r ¼ 6

mm is detached from cylindrical-shaped DRA to get a

ring-shaped DRA as shown in Figure 1b, which resonates

at 3.5 GHz as well as at 5.8 GHz. The ring-shaped DRA

is reformed to miniaturize the size of antenna by remov-

ing one portion of ring-shaped dielectric material. Finally,

the proposed antenna consists of a well-designed C-shaped

DR having outer radius R ¼ 19 mm, inner radius r ¼ 6

mm, thickness hR ¼ 5 mm and Rr ¼ 23 mm as shown in

Figure 1c. Figures 1e and 1f show the front view and rear

view photographs of fabricated prototype C-shaped DRA

on FR4 substrate with partial ground plane.

IV. RESULTS AND DISCUSSION

A. S-Parameter CharacteristicsThe simulation studies of S-parameter versus frequency

characteristics for the proposed C-shaped DRA have been

carried out using CST Microwave Studio software. The

simulated S-parameter versus frequency curves of the pro-

posed antenna have been presented in Figure 2. The reso-

nant frequencies are found to be 3.5 and 5.8 GHz, which

are the desired frequencies for IEEE 802.16d fixed

WiMAX applications. The parametric studies of the DRA

with respect to different shapes, its inner radius r and air

Figure 1 (a) Cylindrical-shaped DRA, (b) ring-shaped DRA,

(c) C-shaped DRA front-view, (d) C-shaped DRA rear-view, (e)

front view of fabricated C-shaped DRA, and (f) rear view of fab-

ricated DRA. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]

Figure 2 Simulated S-parameter plot of different shaped

DRAs. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

Figure 3 Simulated S-parameter plot of a C-shaped DRA for

different values of inner radius ‘‘r.’’ [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

684 Kumari and Behera

International Journal of RF and Microwave Computer-Aided Engineering/Vol. 22, No. 6, November 2012

Figure 4 Simulated S-parameter plot of a C-shaped DRA

for different values of air gap g between DR and ground. [Color

figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

Figure 5 Simulated and Measured S-parameters of the C-shaped

DRA. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

Figure 6 Simulated radiation patterns of C-shaped DRA and same antenna without DR. (a) E-Plane at 3.5 GHz; (b) E-plane at 5.8

GHz; (c) H-plane at 3.5 GHz; and (d) H-plane at 5.8 GHz. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

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International Journal of RF and Microwave Computer-Aided Engineering DOI 10.1002/mmce

gap g (between DR and ground) are performed and pre-

sented in Figures 2–4.

The radius of the basic cylindrical-shaped DRA is taken

as 19 mm. By drilling out a cylindrical region of radius r(which is nearly equal to 1/3 of the cylinder’s radius R)

from the center and then by removing a portion of the ring-

shaped resonator, a C-shaped DRA has been devised.

Figure 2 shows the variation of S-parameter (S11) with

different shapes of DR. It can be perceived from the fig-

ure that S11 decreases as the shape diverges from simple

cylindrical to modified ring-shaped DRA, whereas the res-

onant frequencies as well as the S11 is almost identical for

both the ring-shaped and C-shaped DRA. In comparison

to general cylindrical DRA, the S-parameter (S11) of the

proposed C-shaped DRA is less than �10 dB and exactly

at our desired resonant frequencies (3.5 and 5.8 GHz).

The variation of resonance frequency with inner radius

‘‘r’’ is shown in Figure 3. It has been observed from the

figure that the resonant frequency shifts toward right

from the desired frequency as the inner radius varies from

6 to 12 mm.

Similarly in the next design step, to improve the posi-

tion of resonant frequency, an air gap has been introduced

in between DR and ground. Thus, a parametric study has

been accomplished by varying the air gap to achieve

an optimum antenna performance. Figure 4 shows the

simulated S11 with different values of air gap g. For the

case g ¼ 5.5 mm, the desired resonant frequency with

S11 (dB) below �10 dB is perceived. From Figure 4,

it can be concluded that the resonant frequency of the

C-shaped DRA varies linearly with air gap (g).

The measurement of the fabricated C-shaped DRA is

performed using an E8363B network analyzer. Figure 5

shows the simulated and measured S-parameter (dB) as

the function of frequency. From Figure 5, it has been per-

ceived that the proposed antenna operates in 3.46–3.61

and 5.66–6.14 GHz bands, where the center frequencies

are at 3.5 and 5.8 GHz.

B. Radiation PatternsIn this proposed DRA, we have used an electrically quite

long microstrip line feeding to excite the C-shaped DRA.

Figure 7 Simulated radiation patterns of a ring-shaped and C-shaped DRA. (a) E-Plane at 3.5 GHz; (b) E-plane at 5.8 GHz; (c) H-plane

at 3.5 GHz; and (d) H-plane at 5.8 GHz. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

686 Kumari and Behera

International Journal of RF and Microwave Computer-Aided Engineering/Vol. 22, No. 6, November 2012

The radiation patterns of C-shaped DRA and same antenna

without DR at different resonant frequencies are shown in

Figure 6 to illustrate the contribution of feed line.

Figure 7 shows the simulated far field radiation pat-

terns of the proposed ring-shaped DRA and C-shaped

DRA for both E-plane and H-plane, at resonant frequen-

cies 3.5 and 5.8 GHz.

It can be observed from these figures that radiation

patterns for ring- and C-shaped DRA are almost same.

The proposed DRAs have same polarization plane and

almost identical radiation patterns at the two operating

frequencies, whereas the H-plane radiation patterns are

omnidirectional over the entire frequency range.

C. Gain and Directivity PerformanceFinally, the gain and directivity of the proposed DRAs are

investigated. Figure 8 represents the gain versus frequency

as well as the directivity versus frequency for the pro-

posed ring-shaped and C-shaped DRAs.

It has been noticed from Figure 8 that the gain and

directivity of the ring-shaped DRA are almost similar to

the gain and directivity of the C-shaped DRA at the

resulted resonant frequencies. The peak gains at the two

resonant frequencies 3.5 and 5.8 GHz are 3.26 and 5.55

dBi, respectively. Similarly, the directivities of 3.95 and

5.73 dBi are achieved at 3.5 and 5.8 GHz frequencies,

respectively. The gain and directivity of the proposed C-

shaped DRA as well as ring-shaped DRA are found to

match with the characteristics of antenna used for fixed

WiMAX applications.

V. CONCLUSIONS

This article presents an approach for miniaturizing the

shape of DRA for dual-band applications. The final design

achieved after some geometrical and structural parameter-

izations is validated by careful S-parameter measurement.

The overall investigation results in a microstrip line fed,

dual-band C-shaped DRA by removing some portion of

the ring-shaped DRA mounted on a vertical ground plane

edge for wireless communication. The proposed C-shaped

DRA yields the desired resonant frequency with very

good radiation patterns, gain and directivity. The radiation

Figure 8 Simulated gain and directivity versus frequency of the proposed DRA. (a) Gain at 3.5 GHz; (b) directivity at 3.5 GHz; (c) gain at

5.8 GHz; and (d) directivity at 5.8 GHz. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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International Journal of RF and Microwave Computer-Aided Engineering DOI 10.1002/mmce

characteristics of the proposed antenna are also evaluated.

Within the frequency range from 3.46 to 3.61 GHz and

5.66 to 6.14 GHz, the proposed DRA exhibits S11 less

than �10 dB. The peak gains of 3.26 and 5.55 dBi are

found at 3.5 and 5.8 GHz, respectively. The simulated and

measured results present a good agreement. The proposed

C-shaped DRA design is practically suitable for IEEE

802.16d fixed WiMAX applications.

ACKNOWLEDGMENTS

The authors thank Mr. Rajeev Jyoti, Group Head Antenna

Systems Group, Space Applications Center (SAC) ISRO,

Ahmedabad, India for providing simulation and measure-

ment facilities in his Antenna Design Laboratory. They also

thank Prof. R.K. Mishra, Department of Electronic Science,

Berhampur University, India for his valuable suggestions

during preparation of the article.

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688 Kumari and Behera

BIOGRAPHIES

Runa Kumari is born in Odisha,

India. She received the B.E. and

M.Tech. (Engg.) degrees in Electron-

ics and Communication Engineering

from Biju Pattnaik University of

Technology, Orissa, India in the year

2003 and 2008, respectively. She is

currently working toward the Ph.D.

degree in Microwave Laboratory, Department of Electron-

ics and Communication Engineering, National Institute of

Technology Rourkela, India. Her current research area is

in Dielectric Resonator Antennas, compact antenna design

and antenna arrays. She is a Student Member of IEEE

(USA).

Santanu Kumar Behera received

the B.Sc. (Engg.) degree from UCE

Burla, Sambalpur University in the

year 1990, M.E. and Ph.D. (Engg.)

from Jadavpur University in the year

2001 and 2008, respectively. He is

presently working as an Associate

Professor in the Department of Elec-

tronics and Communication Engineering, National Institute

of Technology Rourkela, India. His current research interest

includes Planar Antenna; Dielectric Resonator Antenna and

Metamaterials. Dr. Behera is a Life Member of IETE

(India), Computer Society of India, Society of EMC Engi-

neers (India), ISTE (India), and Member of IEEE.

International Journal of RF and Microwave Computer-Aided Engineering DOI 10.1002/mmce

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