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ETRI Journal, Volume 31, Number 3, June 2009 2009 Paitoon Rakluea et al. 271

This paper presents a novel multiband microstrip-fed

right angle slot antenna design technique for multiple

independent frequency bands. The new technique uses

various slot sizes at various appropriate positions. We first

propose a tri-band slot antenna consisting of three right

angle slots. Then, a quad-band slot antenna is developed

with four right angle slots which achieves slant 45 linear

polarization, omnidirectional pattern coverage, good

antenna gain, and acceptable impedance bandwidths over

all the operating frequency range. Moreover, an open-

circuited tuning stub is introduced to achieve good

impedance matching. Both proposed antennas are

designed on a ground plane of RT/duroid 5880 substrate

with a thickness of 1.575 mm. The real measurable results

show that the desired frequencies used in wireless

communication systems, namely, WLAN and WiMax, are

efficiently achieved.

Keywords: Tri-band antenna, quad-band antenna,

multiple independent frequency bands, tuning stub, slant

45 linear polarization.

Manuscript received Nov. 23, 2008; revised Apr. 4, 2009; accepted Apr. 13, 2009.

Paitoon Rakluea (phone: +66 2595 1763, email: [email protected]), Noppin

Anantrasirichai (email: [email protected]), and Kanok Janchitrapongvej (email:

[email protected]) are with the Department of Information Engineering, King Mongkuts

Institute of Technology Ladkrabang, Bangkok, Thailand.

Toshio Wakabayashi (email: [email protected]) is with the School of Information

Science and Engineering, Tokai University, Japan.

I. Introduction

The steadily increasing popularity of Internet access and

multimedia applications has driven a rapid development of

wireless communication systems. It has been noted that a

wireless communication device provides the ability tointegrate

multiband. Therefore, a multiband antenna is attractive in many

commercial applications as it is designed to have a single

radiator with a capability to transmit and receive multiple

frequencies. Nevertheless, a multiband antenna may not

sufficiently cover the required operating bands. Therefore, anantenna which is able to operate with multiple independent

frequency bands is required. This antennashould also provide

ease in controlling the desirable resonance frequencies,

impedance bandwidths, radiation patterns, and polarizations.

These are obviously becoming the most important factors for

the applications of antennas inbothcontemporary and future

wireless communication systems.

Many researchers in academia and industry have introduced

multiband antennas such as Sierpinski fractal antennas [1]-[3]

and printed inverted-F antennas (PIFAs) [4]-[7]. However,

these antennas are highly complex in their structures.Therefore, they are hard to design and manufacture, and they

have difficulty in achieving good impedance matching over the

entire operating frequency range. The printed monopole slot

antenna in [8] proposed a widened bandwidth to support

multiband operation, unlike the multiple independent

frequency band antenna in our research, for which each

bandwidth is sufficient for only one operating band. Many

other papers have presented multiband microstrip antennas,

such as the microstrip-fed slot antenna [9], the coplanar-

waveguide-fed monopole antenna [10], and the planar antenna

[11]. These antennas are highly suitable for wireless

Multiband Microstrip-Fed Right Angle Slot AntennaDesign for Wireless Communication Systems

Paitoon Rakluea, Noppin Anantrasirichai, Kanok Janchitrapongvej, and Toshio Wakabayashi

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272 Paitoon Rakluea et al. ETRI Journal, Volume 31, Number 3, June 2009

communication systems due to their attractive features in terms

of planar conformal construction, light weight, low cost, lack of

soldering points, and ease of fabrication. However, multiple

independent frequency bands are not achieved by these

antennas.In this paper, we propose a novel multiband microstrip-fed

right angle slot antenna. This antenna is simple to design

using various right angle slot sizes at appropriate positions to

achieve multiple independent frequency bands which can

control the desirable resonance frequency, impedance

bandwidth, polarization, and radiation pattern. First, a tri-

band slot antenna consisting of three right angle slots is

described, and then a quad-band slot antenna is developed

with four right angle slots.The polarization of the antennas is

also achieved in slant 45o linear polarization for installation

in various environments where the antennas are applied.

Furthermore, good antenna gain and omnidirectional pattern

coverage over the operating bands have been observed. In

this study, a technique to improve the impedance matching

for all bands of operating frequencies is introduced which

uses a tuning stub.

The proposed antennas were analyzed by using the finite-

difference time-domain (FDTD) method. The prototypes of the

antennas were realized and measured. Simulation results were

compared with measurements performed on the antenna

prototypes. The antennas were developed to be used for

wireless communication systems, such as WLAN and WiMax,

which are applied in many devices, such as desktop computers,laptop computers, and in-building access points.

The rest of the paper is structured as follows. Sections II and

III describe the designs and measurement of the tri-band and

quad-band microstrip-fed right angle slot antennas. The

conclusion is given in section IV.

II. Tri-band Microstrip-Fed Right Angle Slot Antenna

1. Structure of the Proposed Antenna

The two slot arrangements on a ground plane are short-endedand open-ended configurations. The open-ended configuration in

[12] was successfully designed to widen the bandwidth for a

multi-operation band, while the bandwidth of the short-ended

configuration is sufficient for only one operation band. The slot

antenna with the short-ended configuration was designed for an

individual operation band which can suppress the interference

frequency from the nearest operating band.

The basic structure of the proposed tri-band microstrip-fed

right angle slot antenna is shown in Fig. 1. The antenna consists

of three different right angle slot sizes at various appropriate

positions on the ground plane of the microstrip line. The

Fig. 1. Structure of the tri-band antenna.

Lg

Wg

z

x

y

Ground plane

Ant.#2

Ant.#3

h

Microstrip line Substrate

Ant.#1

RT/duroid substrate has a dielectric constant rand a thickness

h. The width of the microstrip line is Wmand the dimension of

the ground plane isLgWg.

The first, second, and third slots of antenna are denoted by

Ant. #1, Ant. #2, and Ant. #3, to generate first resonant

frequency (f1), second resonant frequency (f2), and third

resonant frequency (f3), respectively, as shown in Fig. 1.

The off-center feeding is proposed to improve impedance

matching. A 50-ohm microstrip line is designed to excite the

antenna. For efficient excitation of the slots, the microstrip line

terminates in an open circuit [13].

The FDTD method [14], [15] was used in the simulation.

The Yee cell sizes along thex,y, andzdirections are defined as

follows: dx=dy=0.1mmand dz=0.1575mm. The total size of

the FDTD grid is 400dx450dy 20dz.The exciting source isa baseband Gaussian pulse with a pulse width of 25 ps. To

explain the evolution process of the tri-band antenna design

[16], we begin with the single-band and dual-band antennas.

In the following, it is of interest to investigate the behavior of

the designed single-band and dual-band antennas to satisfy the

required specifications.

2. Single-Band and Dual-Band Antennas

Figure 2 shows the geometry of a single-band antenna which

consists of a single right angle slot (Ant. #1) on the ground plane of the microstrip line. The microstrip line is designed

with Wm = 5 mm on the substrate of dielectric constant r= 2.2,

and thickness h = 1.575 mm.

Based on the operating frequency, the overall effective inner

slot length (A1+B1) is designed to be g/2, where g is the

guided wavelength in the slot. LengthsA1 andB1 are equalized

with g/4 length. This technique reduces thesizeof the antenna

by approximately 20% compared to the conventional

microstrip-fed slot antenna [17]. Here, S1 denotes the width of

the slot in which the impedance bandwidth can be affected by

changing the slot width. The lengthLm1 of the microstrip line

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ETRI Journal, Volume 31, Number 3, June 2009 Paitoon Rakluea et al. 273

Fig. 2. Geometry of the single-band right angle slot antenna.

S1

B1

A1

Lm1

r1

Wmy

x

z

Fig. 3. Simulated return loss for the single-band antenna:

A1+B1=50.8,A1=B1=25.4, S1=0.8, r1=2.1,Lm1=17 (mm).

1.5 2.0 2.45 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

-5

-10

-15

-20

-25

-30

-35

-40

Frequency (GHz)

Returnloss(dB)

terminates in an open-circuit stub beyond the edge of the slot,

and r1 is the off-set distance from the center axis of the

microstrip line. The parameters Lm1 and r1 are adjusted to

satisfy the desired impedance matching.

The simulated return loss of the single-band antenna is

shown in Fig. 3. The resonant frequency which was obtained

from the simulation is 2.45 GHz with the return loss of -37 dB.

The -10 dB bandwidth ranges between 2.4 GHz to 2.5 GHz.Figure 4 illustrates the geometry of a dual-band microstrip-

fed right angle slot antenna. By using the same design concept,

the second right angle slot (Ant. #2) is added to generate the

second resonant frequencyf2 at 3.5 GHz. The parameters of the

second right angle slot consist ofA2,B2, S2, r2, andLm2 for

which the optimized values are given in Table 1.

The simulated resonant frequencies for the dual band

antenna are 2.45 GHz and 3.5 GHz with thereturn losses of

-36 dB and -31 dB, respectively, as shown in Fig. 5. The

impedance bandwidths range between 2.4 GHz to 2.5 GHz

and 3.4 GHz to 3.6 GHz, respectively.

Fig. 4. Geometry of the dual-band right angle slot antenna.

Lm2

r2

Lm1

S1

B1

A1

r1

Wm

S2

A2

B2x

y

z

Table 1. Optimized parameters of multiband microstrip-fed rightangle slot antenna for various operating bands.

An+ Bn = g/2

Frequency range

(GHz)

Width of slot Sn

(g)

Off-set dist.

rn (mm)Lmn (g)

2.42.4835 0.0080.012 23

2.52.7 0.20.35 24.50.20.4

3.43.6 0.020.025 1.52.5

4.95.1 0.0050.02 23

5.155.35 0.010.025 0.52.5

0.10.2

5.475.725 0.0250.03 1.53.5

5.75.9 0.0050.025 140.050.1

n = 1, 2, 3, and 4 (n=slot number)

Fig. 5. Simulated return loss for the dual-band antenna:

A1+B1=50.8, A1=B1=25.4, S1=0.8, r1=2.1, Lm1=17,

A2+B2=34,A2=B2=17, S2=1.5, r2=2.1,Lm2=7 (mm).

1.5 2.0 2.45 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

Frequency (GHz)

-40

-35

-30

-25

-20

-15

-10

-5

0

Returnloss(dB)

3. Tri-band Antenna

Figure 6 illustrates the geometry of a tri-band microstrip-fed

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Fig. 6. Geometry of the tri-band right angle slot antenna.

S3

x

y

z

r3S1

Lm3

Lm2

Lm1

B1

A3B3

B2

S2

A2

A1

r2

r1

Wm

Fig. 7. Simulated return loss for the tri-band antenna: A1+B1=50.8,A1=B1=25.4, S1=0.8, r1=2.1,Lm1=17,A2+B2=34,A2=B2=17,

S2=1.5, r2=2.1, Lm2=7, A3+B3=19.8, A3=B3=9.9, S3= 0.9,

r3=1.5,Lm3=2 (mm).

1.5 2.0 2.45 3.0 3.5 4.0 4.5 5.0 5.5 5.86.0 6.5

Frequency (GHz)

-40

-35

-30

-25

-20

-15

-10

-5

0

Returnloss(dB)

right angle slot antenna. Here, the third right angle slot

(Ant. #3) is added to get the third resonant frequency f3 at

5.8 GHz with parametersA3,B3, S3, r3, andLm3. Theoptimized

parameters of the tri-band microstrip-fed right angle slot

antenna are given in Table 1.Table 1 shows the optimized parameters of the multiband

microstrip-fed right angle slot antenna for some of the

most commonly used operating frequencies in wireless

communication systems. The values were selected after several

simulations.

Using these parameters, the simulated resonantfrequencies

for the tri-band antenna are 2.45 GHz, 3.5 GHz, and 5.8 GHz

with the return losses of -38 dB, -36 dB, and -23 dB,

respectively, as shown in Fig. 7. Those impedance bandwidths

are approximately 100 MHz (2.4 GHz to 2.5 GHz), 200 MHz

(3.4 GHz to 3.6 GHz), and 400 MHz (5.6 GHz to 6.0 GHz).

Fig. 8. Geometry of the tri-band right angle slot antenna with

tuning stub.

dstub

x

y

z

Wstub

lstub

Obviously, the obtained simulated impedance bandwidths can

cover the 2.4 GHz WLAN band for thefirst resonant frequency,

the 3.5 GHz WiMax band for thesecond resonant frequency,

and both the 5.8 GHz WLAN and WiMax bands for thethird

resonant frequency.

4. Tri-band Antenna with Tuning Stub

The tri-band microstrip-fed right angle slot antenna with

tuning stub can be introduced to achieve good impedance

matching, which is defined as a return loss level less than

-30 dB for all bands of operating frequencies as shown in Fig. 8.

A single open-circuittuning stub is connected by a shunt withthe microstrip line. The parameters of the single tuning stub

consist of thelength (lstub), thewidth (Wstub), and thedistance

from the end edge of the microstrip line to the center of dstub.

After intensive simulations, the optimized parameters of the

single tuning stub were found to be dstub = 11.5 mm, lstub =1 mm,

and Wstub = 3 mm. The effect on the return lossof using the

single tuning stub is shown in Fig. 9. The return loss decreases

from -38 dB to -40 dB at 2.45 GHz, from -36 dB to -45 dB at

3.5 GHz, and from -23 dB to -37 dB at 5.8 GHz. Thus, the

impedance matching can be improved by using a short tuning

stub connected by a shunt with the feed line without much

change in the resonant frequencies and bandwidths. To further

verify the validity of the proposed designs, the tri-band antenna

with the tuning stub based on these parameters was fabricated

and measured.

5. Equivalent Circuit of Tri-band Antenna

A diagram of the tri-band microstrip-fed right angle slot

antenna is shown in Fig. 10. Total input impedance (Zin) of the

antenna is summarized as

,in stub slotf1 slotf2 slotf 3 Z Z Z Z Z = + + + (1)

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Fig. 9. Simulated return loss of the tri-band antenna with tuning

stub and without tuning stub.

1.5 2.0 2.45 3.0 3.5 4.0 4.5 5.0 5.5 5.86.0 6.5

Frequency (GHz)

0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

-55

Returnloss

(dB)

Tri-band antenna with single tuning stubTri-band antenna without single tuning stub

whereZstubis the input impedance of the microstrip open stub.

The impedances of the slot radiators of resonant frequencyf1, f2,

andf3 areZslotf1, Zslotf2,andZslotf3,respectively.

By using thetransmission line theory, the input impedance of

the microstrip open stub can be obtained by

stub

tan( )

tan( )

m mc mmc

mc m m

Z jZ Z Z

Z jZ

+ =

+ , (2)

where m,Zmc, andZm are the length of the microstrip line, the

characteristic impedance of the microstrip line, and the input

impedance of the open-ended microstrip line, respectively, as

given by [17].

Also, the impedance of the slot radiator at the input of the

transformer is

2slotf s Z n Z = . (3)

The input impedance of the slot radiator (Zs) is

1 1 ( ) ,S S S S Z Y G jB= = + (4)

where, as given in [18], Gs and Bs can be approximately

determined by

20

2 ,SS PGV

= (5)

2cot( / 2),S S S

SC

B k LZ

= (6)

wherePS, V0, kS, L'S,andZsc are the radiated power from the slot,

the voltage of the slot radiator, the wave number, the effective

length, and the characteristic impedance of the slot, respectively.

Coupling between the microstrip line and the right angle slots

by an ideal transformer with a turn-ration:1 is considered, as

given in [19], determined by

Fig. 10. Equivalent circuit of the tri-band right angle slot antenna.

Gm

Bm

Zm

Zstub

Zslotf1

Zslotf2

Zslotf3

Zin

Zmcm

n:1

Zsc1s1 Bsf1 Gsf1

Zsf1

Zsc2s2 Bsf2 Gsf2

Zsf2

Zsc3s3 Bsf3 Gsf3

Zsf3

Fig. 11. Equivalent circuit of the tri-band right angle slot antenna

with tuning stub.

Gm1

Bm1

Zm1

Zmc1m1

Zmc2m2

Bm2 Gm2

Zstub1

Zstub2

Zm2

Zmc0m0

ZstubT

ZslotTZin

Zmcn

Zsc

. (7)

A diagram of the tri-band antenna with thesingle tuning stub

is shown in Fig. 11. The input impedance Zinis summarized as

in stubT slotT Z Z Z = + , (8)

whereZstubT is the input impedance of the microstrip open stub

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Fig. 12. Photographs of the tri-band microstrip-fed right angle

slot antenna with single tuning stub: A1+B1=50.8,A1=B1=25.4, S1=0.8, r1=2.1, Lm1=17, A2+B2=34,

A2=B2=17, S2=1.5, r2=2.1, Lm2=7, A3+B3=19.8,

A3=B3=9.9, S3=0.9, r3=1.5, Lm3=2, dstub=11.5, lstub=1,and Wstub=3 (mm).

(a) Top view (b) Bottom view

Fig. 13. Measured and simulated return loss of the tri-band right

angle slot antenna with single tuning stub.

1.5 2.0 2.45 3.0 3.5 4.0 4.5 5.0 5.5 5.86.0 6.5

0-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

Frequency (GHz)

Returnloss(dB)

Simulated

Measured

(Zstub1)in parallel with the single tuning stub (Zstub2). TheZslotT

can be obtained from the sum of the impedance of the slot

radiation at resonant frequenciesf1, f2, and f3as

slotT slotf1 slotf2 slotf3 Z Z Z Z = + + . (9)

6. Measurement Results

The proposed antenna was fabricated on an RT/duroid 5880

substrate with a dielectric constant of 2.2 and a groundplane size (WgLg) of 8 cm 9 cm as shown in Fig. 12. The

return loss of the antenna was measured using an HP8720-C

Network Analyzer. The reasonable agreement between the

measured and the simulated return loss was confirmed as

shown in Fig. 13. The measured -10 dB impedance

bandwidths were about 140 MHz (2.36GHz to 2.5 GHz), 300

MHz (3.3GHz to 3.6 GHz), and 480 MHz (5.52GHz to 6.0

GHz) with theminimum return losses of -33 dB, -37 dB, and

-32 dB at resonant frequencies of 2.43 GHz, 3.43 GHz, and

5.79 GHz, respectively. It is clearly seen that three operating

bandwidths are obtained. In this case, the bandwidths

Fig. 14. Measured radiation patterns for tri-band antenna: (a)f1 =

2.43 GHz, (b)f2 = 3.43 GHz, (c)f3 = 5.79 GHz, and (d)

polar coordinates.

030

60

90

120

150180

210

240

270

300

330 30

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

3300

-30

-20

-10

0

-30

-20

-100

-30

-20

-10

0

030

60

90

120

150180

210

240

270

300

330 30

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

3300

-30

-20

-10

0

-30

-20

-10

0

-30

-20

-10

0

(a)

(b)

030

60

90

120

150180

210

240

270

300

30

60

90

120

150180

210

240

270

300

3300

30

60

90

120

150180

210

240

270

300

3300

-30

-20

-10

0

-30

-20

-10

0

-30

-20

-10

0

(c)

x-yplane x-zplane y-zplane

z

x

y

(d)

E

E

330

cover WLAN and WiMax operation.

Figure 14 plots the measured radiation patterns in the

azimuthal plane (x-y plane) and the elevation planes (x-zand

y-z planes). The radiation patterns are normalized by themaximum of the E and E components at 2.43 GHz, 3.43

GHz, and 5.79 GHz. In the x-z and y-z planes, the

measurements show that the radiation levels of the E

component are nearly as high as those of the E component.

In thex-y plane, the E component levels are still high but the

radiation of the E component features are at low levels.

It is necessary for the various environments in which the

antenna would be installed for it to exhibit slant 45o linear

polarization. One of the ways to achieve slant 45o linear

polarization is to use a right angle slot with equal horizontal

and vertical axes (An=Bn). In all planes of interest in Fig. 14, the

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radiation patterns for the E and E components are found to be

relatively close to slant 45o

linear polarization. The first and

second resonant frequencies in the x-y plane show the

maximum radiation between angles of 20o to 40o and 200o to

220

o

, which is slant -45

o

linear polarization.Moreover, the thirdresonant frequency in the x-y plane exhibits the maximum

radiation between angles of 330o

to 350o

and 150o

to 170o,

which is slant +45olinear polarization.

Obviously, the effectiveness of the polarization properties

depends on the shapes of the right angle slots. The radiation

patterns of the tri-band are nearly omnidirectional. The

radiation pattern at 2.43 GHz is symmetrical over the

bandwidth. The asymmetry of the radiation patterns at 3.43

GHz and 5.79 GHz is mainly the result of the crosstalk

between thesecond and third slots. The maximum gains of the

antenna are 2.3 dBi, 3.51 dBi, and 5.48 dBi at 2.43 GHz,

3.43 GHz, and 5.79 GHz, respectively. Furthermore, each slot

operates in the dominant mode for the operating frequencies of

the tri-band antenna.

III. Quad-Band Microstrip-Fed Right Angle Slot

Antenna

1. Quad-Band Antenna and Its Characteristics

Figure 15 shows the geometry of a quad-band antenna.

The antenna consists of four right angle slots at various

appropriate positions on a ground plane. It follows the designconcept of the tri-band antenna; however, the additional fourth

right angle slot is added to generate the fourth resonant

frequency (f4). The parameters of the fourth right angle slot are

A4,B4,S4, r4, andLm4.

The resonant frequencies obtained by the simulation are 2.45

GHz, 3.5 GHz, 5.09 GHz, and 5.7 GHz with thereturnlosses

of -39 dB, -28 dB, -23 dB, and -25 dB, respectively, as shown

in Fig. 16. The impedance bandwidths are approximately 100

MHz (2.4 GHz to 2.5 GHz), 200 MHz (3.4 GHz to 3.6 GHz),

500 MHz (4.7 GHz to 5.2 GHz), and 310 MHz (5.62 GHz to

5.93 GHz), which can cover the 2.4 GHz WLAN band, 3.5GHz WiMax band, 5 GHz WLAN band, and 5.8 GHz WLAN

and WiMax bands. As seen in Fig. 17, the simulated resonant

frequencies are 2.6GHz, 3.5 GHz, 5.25 GHz, and 5.5 GHz

with the returnlosses of -24 dB, -25 dB, -22 dB, and -20 dB,

respectively. The impedance bandwidths are approximately

200 MHz(2.5 GHz to 2.7 GHz), 200 MHz (3.4 GHz to 3.6

GHz), 360 MHz (5 GHz to 5.36 GHz), and 280 MHz (5.45

GHz to 5.73 GHz) which can cover the WiMax band and the

WLAN band. These results demonstrate that multiple

independent frequency bands can be attained using various

appropriate positions and sizes of right angle slots, as required

Fig. 15. Geometry of the quad-band right angle slot antenna.

x

y

z

S4

B4

A4 Lm4

r4

Fig. 16. Simulated return loss for the quad-band antenna:

A1+B1=50.8, A1=B1=25.4, S1=0.8, r1=2.1, Lm1=17,

A2+B2=34, A2=B2=17, S2=1.5, r2=1.9, Lm2=7, A3+B3=23.3, A3=B3=11.6, S3=0.9, r3=2.2, Lm3=4.9, A4+

B4=20.2,A4=B4=10.1, S4= 0.7, r4=2.3,Lm3=1.7 (mm).

1.5 2.0 2.45 3.0 3.5 4.0 4.5 5.0 5.5 5.8 6.0 6.5

Frequency (GHz)

0

-5

-10

-15

-20

-25

-30

-35

-40

Returnloss(dB)

of the multiband microstrip-fed right angle slot antenna.

2. Effects of Crosstalk and Ground Plane Size

The fourth band is affected by the problem of crosstalk at

high frequencies, which is related to the spacing between slots.Figure 18 plots the crosstalk in the fourth band of spacing

between the third slot and the fourth slot at 1 mm (0.026g4),

2.5 mm (0.066g4), and 5 mm (0.132g4) as a function of the

frequency. The crosstalk is reduced to an acceptable value of

-20 dB for the spacing between slots at 5 mm (0.132g4). It is

evident that the crosstalk in the fourth band is strongly

influenced by the narrow space which refers to the guide

wavelength calculated at the fourth resonant frequency.

The multiband microstrip-fed right angle slot antenna with

dimensions WgLg = 8 cm 9 cm is used to approximate the

infinite ground plane in order to achieve good impedance

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Fig. 17. Simulated return loss of the quad-band antenna:

A1+B1=47.4, A1=B1=23.7, S1=3, r1=1.6, Lm1=14.9,

A2+B2=34, A2=B2=17, S2=1.5, r2=1.5, Lm2=7.8, A3+

B3=22, A3=B3=11, S3=0.5, r3=2, Lm3=4.9, A4+B4=20,A4=B4=10, S4=0.9, r4=3,Lm4=2.3 (mm).

1.5 2.0 2.6 3.0 3.5 4.0 4.5 5.25 5.6 6.0 6.5

0

-5

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-15

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-30

Frequency (GHz)

Returnloss(d

B)

Fig. 18. Effect of crosstalk in the fourth band with varying spacing

between the third and fourth slots.

5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5

Frequency (GHz)

1 mm2.5 mm5 mm

0

-5

-10

-15

-20

-25

-30

-35

Crosstalk(dB)

Fig. 19. Effect of varying the ground plane size.

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

Frequency (GHz)

WgLg = 8 cm 9 cmWgLg = 7 cm 8 cm

0

-5

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-35

-40

R

eturnloss(dB)

matching and the radiation pattern. The effect of varying the

ground plane size is shown in Fig. 19. The minimum

dimensions of the ground plane are 7 cm 8 cm.

Nevertheless, decreasing the ground plane size hasan impact

on the impedance matching and the radiation pattern.

Fig. 20. Geometry of the quad-band right angle slot antenna with

(a) single tuning stub and (b) double tuning stub.

dstub1lstub1

Wstub1

dstub1lstub1

Wstub1

lstub2

dstub2

Wstub2x

yz z

y

x

Fig. 21. Comparison of simulated return loss of the quad-band

antenna with tuning stub and without tuning stub.

1.5 2.0 2.45 3.0 3.5 4.0 4.5 5.0 5.5 5.8 6.0 6.5

Frequency (GHz)

0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

Returnloss(dB)

Quad-band antenna with double tuning stub

Quad-band antenna with single tuning stubQuad-band antenna without tuning stub

3. Quad-Band Antenna with Tuning Stubs

The proposed geometries of the quad-band microstrip-fed

right angle slot antenna with tuning stubs are shown in Fig. 20.

The dimensions of the antenna without a tuning stub are

chosen with the simulation resonant frequencies at 2.45 GHz,

3.5 GHz, 5 GHz, and 5.7 GHz as previously shown in Fig 16.

Tuning stubs can be introduced to achieve good impedance

matching for all the bands of operating frequencies. The single

and double tuning stubs are set as shown in Fig. 20. The

specification dictates that a return loss level lower than -30 dB

is required. The first design is created by using the single tuningstub with the dimensions dstub1, lstub1, and Wstub1 of 15.3mm, 4.5

mm, and 0.5 mm, respectively. In the second design, the

double tuning stub is introduced with an additional second

tuning stub with the dimensions dstub2, lstub2, and Wstub2 of 14.8

mm, 4.8 mm, and 0.6 mm, respectively.

The simulated return loss for the quad-band with tuning

stubs is shown in Fig. 21. The plot reveals that the obtained

simulated resonant frequencies of the quad-band antenna with

thesingle stub are 2.45GHz, 3.5 GHz, 5.07 GHz, and 5.8 GHz

with the return losses of -47 dB, -29 dB, -29 dB, and -26 dB,

respectively. The simulated resonant frequencies of the quad

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Fig. 22. Photos of the quad-band right angle slot antenna with

double tuning stub: A1+B1=50.8, A1= B1=25.4, S1=0.8,

r1=2.1, Lm1=17, A2+B2=34, A2=B2=17, S2=1.5, r2=1.9,Lm2=7, A3+B3=23.2, A3=B3=11.6, S3=0.9, r3=2.2,

Lm3=4.9, A4+B4=20.2, A4=B4=10.1, S4=0.7, r4=2.3,

Lm4=1.7, dstub1=15.3, lstub1=4.5, Wstub1=0.5, dstub2=14.8,

lstub2=4.8, Wstub2=0.6 (mm).

(a) Top view (b) Bottom view

band antenna with the double tuning stub are 2.45GHz, 3.5

GHz, 5.07 GHz, and 5.88 GHz with thereturnlosses of -46.1

dB, -39.1 dB, -32.8 dB, and -37.4 dB, respectively. The

antenna with the double tuning stub achieves the impedance

bandwidth of 100 MHz (2.4 GHz to 2.5 GHz), 200 MHz (3.4

GHz to 3.6 GHz), 630 MHz (4.6 GHz to 5.23 GHz), and 440

MHz (5.64 GHz to 6.08 GHz).

From these results, it is found that the desired level of the

return loss is not satisfied with the single tuning stub, but the

double tuning stub performs better than the desired level at all

of the resonant frequencies. The tuning stubs affect the return

losses of the first and second resonant frequencies without

changing the impedance bandwidth, whereas the tuning stubscan improve the antenna impedance bandwidth at the third and

the fourth resonant frequencies. The fourth resonant frequency

shifts to aslightly higher frequency.

4. Measurement Results

Figure 22 shows photos of a fabricated quad-band

microstrip-fed right angle slot antenna with the double tuning

stub. The ground plane size (WgLg) is 8 cm 9 cm. The

measured and simulated return losses of the quad-band antenna

with the double tuning stub are shown in Fig. 23. Themeasured resonant frequencies are 2.46 GHz, 3.5 GHz, 4.9

GHz, and 5.88 GHz with theminimum return losses of -35dB,

-30 dB, -32 dB, and -31 dB, respectively. The measured results

of the resonant frequencies correlate well with the simulated

results. Although the measured results of thereturn losses at the

resonant frequencies are not as good as the simulated results,

their levels are lower than -30 dB. The measured impedance

bandwidths are 100 MHz (2.4 GHz to 2.5 GHz), 380 MHz

(3.3 GHz to 3.68 GHz), 630 MHz (4.54 GHz to 5.17 GHz),

and 500 MHz (5.5 GHz to 6 GHz), which cover the WLAN

and WiMax bands.

Fig. 23. Measured and simulated return loss of the quad-band right

angle slot antenna with double tuning stub.

1.5 2.0 2.45 3.0 3.5 4.0 4.5 5.0 5.5 5.86.0 6.5

Frequency (GHz)

0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

Returnloss(dB)

SimulatedMeasured

Figure 24 shows the measured radiation patterns for both E

and E components in thex-y plane,x-zplane, andy-zplane.

Each slot operates in the dominant mode for the operating

frequencies of the quad-band antenna. Also, the radiation

patterns are found to have a relative slant 45o linear

polarization. It is clear from the measured results that the

radiation levels of the E components are nearly as high as

those of the E components in thex-zplane andy-z plane. In

thex-y plane, the E components levels are still high, but the

radiation of the E components exhibits low levels. The

polarizations at the first and the second resonant frequencies

are slant -45olinear. The polarizations at the third and the fourth

resonant frequencies are slant +45o linear.Figure 24 demonstrates that the radiation patterns of the

quad-band antenna are nearly omnidirectional and can achieve

good antenna gain over the operating bands. Furthermore, the

maximum gains of the antenna are 2.8dBi, 3.4 dBi, 5.3 dBi,

and 3.9 dBi at 2.26 GHz, 3.5 GHz, 4.9 GHz, and 5.88 GHz,

respectively. Still, the lowergain values at the fourth frequency

band are due to the effect of crosstalk between the third and

fourth slots. However, the gain at the fourth frequency band is

sufficient for most wireless applications. In this antenna, the

spacing of the third and fourth slots is approximately 2.5 mm

(0.066g4). It was used for size reduction of the quad-bandantenna.

IV. Conclusion

A novel microstrip-fed right angle slot antenna design

technique was proposed. The tri-band and quad-band antennas

are designed by appropriately positioning different right angle

slots. Multiple independent frequency bands are achieved with

asimple structure. Simulations demonstrated that the operating

frequency bands were achieved independent of each other, and

they exhibited insignificant crosstalk effects between the

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Fig. 24. Measured radiation patterns for the quad-band antenna:(a) f1=2.46 GHz, (b) f2=3.5 GHz, (c) f3=4.9 GHz, (d)

f4=5.88 GHz, and (e) polar coordinates

030

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adjacent slots. The tri-band antenna with a single tuning stub

and the quad-band antenna with a double tuning stub achieved

good impedance matching for all bands of operation. From the

measured and simulated return loss, it was confirmed that the

proposed antennas provide desirable resonant frequencies and

impedance bandwidths. From the measured radiation patterns,

it was also observed that the radiation patterns of the proposed

antennas were almost omnidirectional, with reasonable gain

and slant 45 linear polarization. The proposed antenna

design technique can be further developed to support five (or

more) frequency bands and provide flexibility of design for

both linear and circular polarizations. The proposed antennas

are certainly advantageous for wireless communication andcan make a significant contribution to the next generations of

mobile communication systems.

References

[1] C.T.P. Song, P.S. Hall, and H.G. Shiraz, Perturbed: Sierpinski

Multiband Fractal Antenna with Improved Feeding Technique,

IEEE Trans. Antennas Propag., vol. 51, no. 5, May 2003, pp.

1011-1017.

[2] A. Patnaik et al., Neurocomputational Analysis of a Multiband

Reconfigurable Planar Antenna,IEEE Trans. Antennas Propag.,

vol. 53, no. 11, Nov. 2005, pp. 3453-3458.

[3] J. Anguera et al., Broad-Band Triple-Frequency Microstrip Patch

Radiator Combining a Dual-Band Modified Sierpinski Fractal

and a Monoband Antenna,IEEE Trans. Antennas Propag., vol.

54, no.11, Nov. 2006, pp. 3367-3373.

[4] D.M. Nashaat, H.A. Elsadek, and H. Ghali, Single Feed

Compact Quad-Band PIFA Antenna for Wireless

Communication Applications,IEEE Trans. Antennas Propag.,

vol. 53, no. 8, Aug. 2005, pp. 2631- 2635.

[5] M. M. Vzquez et al., Integrated Planar Multiband Antennas for

Personal Communication Handsets, IEEE Trans. Antennas

Propag., vol. 54, no. 2, Feb. 2006, pp. 384-391.

[6] A.C.K. Mak et al., Reconfigurable Multiband Antenna Designs

for Wireless Communication Devices, IEEE Trans. Antennas

Propag., vol. 55, no. 7, July 2007, pp. 1919-1928.

[7] M. Tzortzakakis and R.J. Langley, Quad-Band Internal

Mobile Phone Antenna,IEEE Trans. Antennas Propag., vol. 55,

no. 7, July 2007, pp. 2097-2103.

[8] C.-I. Lin and K.L. Wong, Printed Monopole Slot Antenna for

Internal Multiband Mobile Phone Antenna, IEEE Trans.

Antennas Propag., vol. 55,no. 12, Dec. 2007, pp. 3690-3697.

[9] H. Tehrani and K. Chang, Multifrequency Operation of

Microstrip-Fed Slot-Ring Antennas on Thin Low-DielectricPermittivity Substrates,IEEE Trans. Antennas Propag.,vol. 50,

no. 9, Sept. 2002, pp. 1299-1308.

[10] W.C. Liu, Design of a Multiband CPW-fed Monopole Antenna

Using a Particle Swarm Optimization Approach, IEEE Trans.

Antennas Propag., vol. 53, no. 10, Oct. 2005, pp. 3273-3279.

[11] S.B. Chen et al., Modified T-Shape Planar Monopole Antennas

for Multiband Operation, IEEE Trans. on Microwave Theory

and Techniques, vol. 54, no. 8, Aug. 2006, pp. 3267-3270.

[12] S.K. Sharma, L. Shafai, and N. Jacob, Investigation of Wide-

Band Microstrip Slot Antenna,IEEE Trans. Antennas Propag.,

vol. 52, no. 3, Mar. 2004, pp. 865-872.

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[13] R. Garg et al., Microstrip Antenna Design Handbook, Artech

House, 2001.

[14] Y. Qain and T. Itoh,FDTD Analysis and Design of Microwave

Circuits and Antennas, Tokyo: Realize, 1999.

[15] D.M. Sullivan, Electromagnetic Simulation Using the FD-TDMethod, New York: IEEE, 2000.

[16] P. Rakluea et al., Analysis of Right Angle Microstrip Slot

Antenna,TENCON, Melbourne, Australia, Nov. 21-24, 2005.

[17] H.G. Akhavan and D. M. Syahkal, A Simple Technique for

Evaluation of Input Impedance of Microstrip-Fed Slot

Antennas, Proc. 9th Int. Antennas Propag. Conf., vol. 1, Apr.

1995, pp. 265-268.

[18] H. Kim and Y.J. Yoon, Microstrip-Fed Slot Antennas with

Suppressed Harmonics,IEEE Trans. Antennas Propag., vol. 53,

no. 9, Sept. 2005, pp. 2809-2817.

[19] C. Caloz et al., A Simple and Accurate Model for Microstrip

Structures with Slot Ground Plane, IEEE Microwave and

Wireless Components Lett., vol. 14, no. 4, Apr. 2004, pp. 133-135.

Paitoon Rakluea received the BIndTech (2nd

Class Honors) and MEng degrees from King

Mongkuts Institute of Technology Ladkrabang

(KMITL), Thailand, in 2000 and 2003,

respectively. In 2003, he joined the Department

of Electronic and Telecommunication

Engineering, Rajamangala University of

Technology Thanyaburi (RMUTT), Thailand, as an instructor. His

current research interests include printed antennas design for wireless

communication systems, antenna measurement systems, and wireless

networks.

Noppin Anantrasirichai received the

BIndTech from King Mongkuts Institute of

Technology Ladkrabang, and the MEng from

Chulalongkorn University in 1977 and 1985,

respectively. From 1997 to 2008, she was withKing Mongkuts Institute of Technology

Ladkrabang (KMITL) where she was involved

in antenna design research. She is currently an associate professor, and

her current research interest is in design and analysis of microstrip slot

antennas. In 2009, she joined the Institute for the Promotion of

Teaching Science and Technology (IPST) where she has been

involved in the development of curriculum and instruction media for

schools.

Kanok Janchitrapongvej received the BEng

in telecommunication engineering from King

Mongkuts Institute of Technology Ladkrabang

(KMITL), Bangkok, Thailand. He obtained his

MEng and DEng from Tokai University, Japan,

in 1977 and 1986, respectively. He has been

with the Department of Information

Engineering since 1977. He is currently an associate professor with

KMITL. From 2006 to 2008, he was the director of the research center

for communication technology at KMITL. His research interests

include audio and video equalizers, filter design, and the general area of

signal processing.

Toshio Wakabayashi received the BE and ME

degrees from Tokai University, in 1968 and

1970, respectively. He received the DE degree

from the same university in 1985. In 1970, he

joined the Faculty of Engineering, Tokai

University, and since then, as a faculty member,

he has engaged in research in the field of

electromagnetic waves, including computational electromagnetic fields,

microwave circuits and devices. He is a professor of the Department of

Communication Network Engineering, Tokai University. He is

currently involved in research on broadband planar antennas for mobile

communications. Dr. Wakabayashi is a member of the IEICE, theInstitute of Image Information and Television Engineers (ITE) and the

Japanese Cancer Association.

etrij.jun2009

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