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Novel Design Method of DualBand Antenna for WLAN Applications

Ding-Bing Lin1and Jian-Hung Lin

2

Institute of Computer and Communication, National Taipei University of Technology

No. 1, Sec. 3, Chung-Hsiao E. Rd. Taipei 106, Taiwan, Republic of China

Email:1dblin@en.ntut.edu.tw, 2s2418035@ntut.edu.tw

Abstract In this paper, we propose an alternativemethod to design the antenna that performs as well as

PIFA antenna. The advantage of the designed antenna

compared with patch antenna is that the antenna size can

be also reduced 50% as PIFA antenna. Also, another

advantages of the designed antenna compared with

monopole antenna as well as microstrip antenna are planar

and no dielectric loss. If we, on the structure of the

designed antenna, place an additional patch near the signal

probe feed. Then the antenna possesses the dual-band

characteristics, 2.4GHz2.48GHz and 5.15GHz 5.35GHz

for the bands of wireless local area networks (WLAN). In

other word, the designed antenna resonates at frequency

2.4GHz and 5.2GHz and possesses 8% and 11% bandwidth

respectively.

I ndex termsdual-band antenna, PIFA , WLAN .I. INTRODUCTION

With the rapid progress of wireless communication

systems which come in variety size ranging from small

hand-held devices to wireless local area networks. The

integration of different radio modules into the same

piece of equipment has created a need for multi-band

antenna. The antenna which can operate at two or more

frequency bands is more desirable and convenient.

Therefore, the design of compact and multi-band

antenna becomes a critical technique.

In numerous antenna structures, microstrip patchantenna is widely applied to design the antenna in ISM

band generally. It is because the advantages of the

microstrip patch antenna are low profile, lightweight and

low cost. Directly-fed microstrip patch antenna usually

have limited bandwidth about 2~4%. High dielectric

constant material is conducive to the reduction of

antenna size, but the problem of the radiation efficiency

and the limitation of the bandwidth will occur [1][2].

Therefore, on the consideration of the size and

bandwidth of the antenna, the microstrip patch antenna

structure is replaced with the PIFA (Planar Inverted F

Antenna) gradually. For PIFA antenna, the size andbandwidth of the antenna will be reduced 50% and

improved by introducing one more shorting pin than

microstrip patch antenna [3]. PIFA antenna is a major

structure in compact antenna, and there are detailed

discussions in [4]-[6]. Also the PIFA antenna is usually

used in the design of dual band antenna [7]-[9] and

diversity antenna [10]. The feature of the PIFA antenna

is its quarter wavelength of the resonant frequency. This

advantage compared with monopole antenna as well as

microstrip antenna is planar and no dielectric loss,

4 h

top plane

bottom plane

ground plane

Fig. 1 side view of the novel antenna structure

1L

2L

3L

gL

gW

aL

4W1W 2W1g

2g

h

3W

4Ltop plane

ottom plane

probe feedground plane

shorting pin

x

z

Fig. 2 dual band antenna configuration

respectively. So we propose an alternative method in this

paper to design the antenna that performs as well asPIFA antenna. And then, through the proposed antenna,

we design an antenna that possesses the dual-band

character used for the bands of WLAN.

II. ANTENNA STRUCTURE AND DESIGNMETHODOLOGY

Fig. 1 shows side view of the antenna structure

which is composed of two FR4 planes; the thickness of

the FR4 planes is 0.4mm, and the layout of the antenna

radiator on the top plane is shown in Fig. 2. The

dimension of the ground is 246 32 mm . The

geometrical parameters of the antenna are h = 6.8mm, L1

= 8mm, L2 = 17mm, L3 = 6mm, L4 = 30mm, La =21.5mm, Lg= 2.5mm, W1= 21mm, W2= 15.5mm, W3=

5.5mm, W4= 6mm, Wg= 6mm, g1= 2mm, g2= 8mm.

The probe is fed from cross position of the long and thin

radiator and the L-shape patch, while the short pin is

located at the center of the U-shape patch. Both patches

are coupled by an air gap with length Lg. The antenna

proposed in this paper provides two resonant frequencies

at 2.4GHz and 5.2GHz by the combination of two

resonators, one is the long and thin radiator and the other

is the L-shape patch.

mailto:dblin@en.ntut.edu.twmailto:s2418035@ntut.edu.twmailto:s2418035@ntut.edu.twmailto:dblin@en.ntut.edu.tw

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(a) (b)

Fig. 3 Current distribution at frequency (a) 2.4GHz (b) 5.2GHz.

The lower resonant frequency is determined by the

length of long and thin radiator and the length of air gap.

We set 4L = ! 4 of the lower resonant frequency

2.4GHz, then tuning the appropriate length of air gap

and the appropriate dimensions of the U-shape patch for

a good impedance match on the lower resonant

frequency. For the fixed length L4, increasing the length

of air gap is equivalent to reducing the length of the long

and thin radiator. The simulation result of the current

distribution for the frequency 2.4 GHz is shown in Fig.

3-(a). In which, the current distribution on the long and

thin radiator shows the same direction. Both the current

distributions on the U-shape patch and the L-shape patch

show the opposite direction and flow into the node of

short pin or probe feed. Hence the dominant radiationeffect comes from the current distribution on the long

and thin radiator.

The higher resonant frequency is dominated by the

dimension of L-shape patch. In our design, we

set 2 2 2L W + = of the higher resonant frequency

5.2GHz. Also, the air gap Lgand the U-shape patch are

needed to provide a good impedance on both resonant

frequencies. Observe the simulation results, shown in

Fig. 3-(b), of the current distribution for the higher

resonant frequency 5.2GHz. Both the current

distribution on long and thin radiator and L-shape patch

show the same direction, while the current distribution

of the U-shape patch shows the opposite direction andflows into the node of the short pin. The dominant

radiation effect comes from the current distribution on

both the L-shape patch and the long and thin radiator.

Hence, not only the dimension of the L-shape patch will

affect the higher resonant frequency but also the length

of the long and thin radiator.

Through the above descriptions, the design procedure

can be summarized as the following:

1. To decide the length of L4 such that the antenna

resonates at the lower resonant frequency.

2. Tuning the dimensions W1, Wg and L of U-shapepatch and set a sufficient long air gap to get good

impedance on the lower resonant frequency.

3. Place an additional L-shape patch near the signalprobe feed and decide the length of L2+W2such that

the antenna can also resonate at the higher resonant

frequency.

4. Tuning the length of long and thin radiator for finetuning the higher resonant frequency.

III. SIMUALTION AND MEASUREMENT RESULTSThe characteristics of the radiation can be

confirmed through the simulation software Ansoft

Ensemble using the analysis of moment method and the

automatic measurement system set up on an anechoic

chamber. Fig. 4 shows the simulation results of the

resistance and reactance of the dual-band antenna.

Figure 5 shows the simulation and measurement results

of return loss. The measurement results are good

agreement with the simulation results. And the designed

antenna resonates at frequencies 2.4GHz and 5.2GHz,

and the bandwidths are 200MHz and 600MHz

respectively. Figures 6 and 7 are the return loss for

different L2 and different Lgrespectively. The shorter L2

increases the higher resonant frequency from 5.08 GHz

to 5.4 GHz. In addition, the longer air gap L g, equivalent

to the shorter La for a fixed length L4, increases thehigher resonant frequency from 5.08 GHz to 5.34 GHz.

But, varying L2 and Lg has not impact on the lower

resonant frequency. Although the length Lg of air gap

will effect the impedance matching condition, that is

more insensitive than the dimension of U-shape patch. It

is found that the resonance frequencies can be adjusted

independently, which makes the design procedure

simpler.

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Fig. 4. The impedance of the designed antenna

Fig. 5. Return loss of the simulation and measurement

results

Fig. 6. The influence of the resonant frequency for

different L2

Fig. 7. The influence of the resonant frequency for

different Lg

The measured radiation patterns are shown in Figs.

8-11. The antenna gains are 3.46dBi and 7dBi at

frequencies 2.4GHz and 5.2GHz, respectively. The

simpler design procedure has been pointed out in the

previous section.

IV.CONCLUSIONThe novel design method of dual-band antenna is

introduced in this paper for wireless local area networksapplications at 2.4GHz and 5.2GHz bands. It can be

utilized to perform the multi-band or dual-band antenna

due to easy combination with other type antennas.Moreover, both the two resonant frequencies can be

adjusted independently through the variations of the

parameters of the antenna dimension. The characteristics

of the radiation can be confirmed through the simulation

software Ansoft Ensemble and the automatic

measurement system. Therefore, the two resonant

frequencies can be designed individually which makes

the design procedure simpler. The bandwidths of the

designed dual-band antenna were found for the lower

and upper resonant frequencies as 8% and 11%

respectively. The experimental results obtained show

good radiation characteristics for two operating bands of

a WLAN antenna.

REFERENCE

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substrate for wireless communication,# IEEE 2003Transactions on Antennas and Propagation , Vol 1, pp

435-438, June 2003.

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Transactions on Antennas and Propagation, Vol 45, pp837-842, May 1997.

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[6] Panayi P.K., Al-Nuaimi M.O., and Ivrissimtzis I.P.,"Tuning techniques for planar inverted-F antenna,#

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[7] Karmakar N.C., "Shorting strap tunable single feeddual-band stacked patch PIFA,#Antennas and Wireless

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[8] Karmakar N.C., "Shorting strap tunable dual-bandstacked PIFA,# IEEE 2003 International Symposium of

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[9] Fu-Ren Hsiao, Wen-Shyang Chen, Kin-Lu Wong,"Dual-frequency PIFA with a rolled radiating arm forGSM/DCS operation,# IEEE 2003 InternationalSymposium of Antennas and Propagation Society, Vol

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WLAN operation,# Electronics Letters, Vol 39, pp590-591, April 2003

Fig. 8. xz-plane at 2.4GHz Fig. 9. yz-plane at 2.4GHz

.Fig. 10. xz-plane at 5.2GHz Fig. 11. yz-plane at 5.2GHz