wideband dual-frequency double inverted-l cpw-fed

8/8/2019 Wideband Dual-frequency Double Inverted-L CPW-Fed

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Wideband dual-frequency double inverted-L CPW-fedmonopole antenna for WLAN application

W.-C. Liu

Abstract: A novel and simple wideband dual-frequency design of a coplanar waveguide (CPW)-fedmonopole antenna is proposed. The antenna comprises a planar patch element with a sidedL-shaped slit to become a double inverted-L monopole and is capable of generating two separateresonant modes with good impedance matching conditions. Prototypes of the proposed antennahave been constructed and studied experimentally. The measured results show good agreementwith the numerical prediction, and good dual-frequency operations with À10 dB impedancebandwidths of 7.3% and 35.1% at the resonant frequencies of 2.48 and 5.22 GHz, respectively,which cover the 2.4/5.2/5.8GHz WLAN operating bands. Also, good monopole-like radiationpatterns and antenna gains over the operating bands have been obtained.

1 Introduction

With wireless communications, such as the wireless localarea network (WLAN), having evolved at an astonishingrate during the last decade, there are various antennadesigns, which enable antennas with low-profile, light-weight, flush mounted and single-feed to fit the limitedequipment space of the WLAN devices. These antennas,with enhanced dual- or multi-frequency capabilities tosatisfy the IEEE 802.11 WLAN standards in the 2.4/5.2/5.8 GHz operating bands, have been developed andpresented in the literature. These antennas include theplanar inverted-F antennas (PIFAs) [1, 2], the chip antennas

[3, 4], and the planar monopole antennas [5–7]. Amongthese antennas, the planar monopole antennas haveespecially received much more interest than others owingto their potential in providing various required radiationfeatures of dualband or multiband, wide bandwidth, andlow profile for a communication system. However, suchkinds of antennas mostly need a large size of ground plane,which is often printed on the different side of the substratefrom the radiating plane, and thus a via-hole connection isalways necessary for feeding the signal and this increases themanufacture difficulty and cost.

Recently, a great interest in coplanar waveguide (CPW)-fed antennas has been found because of their manyattractive features such as wider bandwidth, better im-pedance matching, simplest structure of a single metalliclayer, no soldering point, and easy integration with activedevices or monolithic microwave integrated circuits. For theavailable designs, the CPW-fed square slot antenna reportedin [8] is capable of broad but single-band operation only,and the CPW-fed inductive slot antennas reported in [9]are capable of dual or multiband operation for WLAN

operations. However, they do not have broad bandwidthsand require greater complexity of antenna shape. For this,previous work [10] has presented a much simpler CPW-fednotched monopole antenna with enhanced bandwidth andis suitable for WLAN 2.4/5.2 GHz dualband operations.However, the antenna size, including the ground planes, isas large as 70Â 66mm2 to occupy much of the device space.

In this paper, a novel and simple wideband dual-frequency design of a double inverted-L planar monopoleis presented. The antenna is fed by a CPW line such thatonly a single-layer substrate is required for this antenna. Inaddition, the case of the proposed design with a morereduced size than that in [10] is not only capable of

providing the dual-frequency operation, but can achievebandwidth enhancement. Details of the antenna design aredescribed, and prototypes of the proposed antenna forWLAN operations at the 2.4, 5.2 and 5.8 GHz frequencieshave been constructed and tested.

2 Antenna design

The geometry of the proposed double inverted-L CPW-fedplanar monopole antenna for wideband and dual-frequencyoperations is shown in Fig. 1. For the design studied here,the antenna is etched on the same side of an inexpensiveFR4 substrate with the dielectric constant of 4.4 (eg) and the

substrate thickness of 1.6mm (h), while the other side iswithout any metallisation. A CPW transmission line, whichconsists of a signal strip thickness of w f and a gap distanceof d between the single strip and the coplanar ground plane,is used for feeding the antenna. Two equal finite groundplanes, each with dimensions of length Lg and width W g, aresituated symmetrically on each side of the CPW feed line.The basis of the antenna structure is a rectangular patchmonopole, which has the dimensions of length L and widthW , and is centred and connected at the end of the CPWfeed line. To achieve the desired dual-frequency operations,the patch is embedded with an L-shaped slit, whichcomprises both the horizontal and vertical sections withdimensions of l 1Âw1 and l 2Âw2, respectively, to drama-

tically form two inverted L-shaped monopoles. The slit issited with a distance of (l 3Àl 2) from the bottom of the patch.The major effect of the inserted slit is to produce two

The author is with the Department of Aeronautical Engineering, NationalFormosa University, 64, Wenhua Rd., Huwei, Yunlin, Taiwan, Republic of China

E-mail: [email protected]

r IEE, 2005

IEE Proceedings online no. 20050011

doi:10.1049/ip-map:20050011

Paper first received 14th January and in revised form 1st April 2005

IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005 505

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different current paths from the two separated invertedL-shaped monopoles, and thus dual resonant modes arecertainly excited. That is, the left-up larger invertedL-shaped monopole, which comprises a vertical strip of length L and width W Àw1, and a horizontal strip of lengthLÀl 1Àl 3 and width w1, is considered to mainly control thelower operating band of the proposed antenna. Alterna-tively, the upper resonant frequency is excited to be greatlydependent on the length of the right-down smaller invertedL-shaped monopole with a vertical strip of length l 3 andwidth w1Àw2, and a horizontal strip of length l 3Àl 2 andwidth w2.

To investigate the performance of the proposed antennaconfigurations in terms of achieving the wideband dual-frequency operations a commercially available moment

method code, IE3Dt, was used for required numericalanalysis and obtaining the proper geometry parameters inFig. 1, and then the optimal dimensions were determinedfrom experiments. The geometric parameters were adjustedcarefully and, finally, the antenna dimensions were obtainedto be L ¼ 22:48 mm, W ¼ 16:44 mm, Lg ¼ 18:9 mm,W g¼10:83mm, wf ¼3:0mm, w1¼9:53mm, l1 ¼ 3:35mm,w2 ¼ 7:95 mm, l2 ¼ 7:05 mm, l3 ¼ 9:66 mm, d ¼ 2:0 mm,

and s ¼ 1:

93 mm, where w f corresponds to the 50O CPWfeed line. Thus, in the following Section, prototypes of theproposed antenna were constructed, and the numerical andexperimental results of the input impedance and radiationcharacteristics are presented and discussed. In addition,owing to the fact that the experimental results show that thelength of l 2 and the distance of d both have a significanteffect on the impedance bandwidth of the proposedantenna, the influence will also be described.

3 Results and discussion

Figure 2 shows the simulated and measured return loss

against frequency for the proposed double inverted-L wide-band dual-frequency planar CPW-fed monopole antenna.It is clearly seen that two wide operating bandwidths are

2 3 4 5 6−40

−30

−20

−10

0

with L-shaped slit (measured)

with L-shaped slit (simulated)

without slit (measured)

frequency, GHz

r e t u r n

l o s s ,

d B

Fig. 2 Measured and simulated return loss for the proposed wideband dual-frequency antenna shown in Fig. 1 L ¼ 22:48mm, W ¼ 16:44mm, Lg ¼ 18:9mm, W g ¼ 10:83mm,

wf ¼3mm, w1¼9:53mm, l1¼3:35mm, w2 ¼7:95mm, l2 ¼ 7:05mm,

l3 ¼ 9:66mm, d ¼ 2:0 mm, s ¼ 1:93mm, h ¼ 1:6 mm, eg ¼ 4:4

longer path

shorter path

b

a

Fig. 3 Simulated surface current distributions on the radiating patch for the proposed antennaa 2.45GHz

b 5.25GHz

d Lg L

g

W g

W g

s

L

W

w f

50-Ω SMA

connector

xy

z

h

l 3

l 2

l 1

w 2

w 1

Fig. 1 Geometry of the proposed CPW-fed double inverted-L

planar monopole antenna

506 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005



obtained. The measured lower band achieves a À10dBimpedance bandwidth of 7.3% ranging from 2.39 GHz to2.57GHz with respect to the centre frequency at 2.48GHz,and the measured bandwidth for the upper mode reaches1.83 GHz (4.03–5.86GHz), or about 35.1% referred to thecentre frequency at 5.22GHz. Obviously, the antenna canoperate over the bands which cover the required band-widths of the IEEE 802.11 WLAN standards in the bandsat 2.4GHz (2400–2484 MHz), 5.2GHz (5150–5350 MHz),and 5.8 GHz (5725–5825 MHz). We compared the mea-sured data with the simulated results obtained from the

IE3Dt electromagnetic solver. The agreement seemedgood and a similar curve trend between the measurementand the simulative results is seen over the whole operatingbands beyond existing a slight frequency shift and afrequency discrepancy that may mainly be due to thefrequency response of the substrate permittivity. In addi-tion, to investigate the difference between with and withoutthe L-shaped slit, the frequency response of return loss forthe proposed antenna without the L-shaped slit is alsomeasured and plotted in Fig. 2. As a result, the slit-unembedded antenna only provides a much narrowerbandwidth of 700 MHz (4.78–5.48 GHz) with the bestimpedance matching condition of return loss less than –

11.3 dB over the higher frequency band. The excited surfacecurrent distributions, obtained from the IE3Dt simulation,on the radiating slit-loaded patch for the proposed antennaat 2.45 and 5.25 GHz, respectively, are presented in Fig. 3.As expected, the two resonant modes excited are, primarily,a result of the formed longer and shorter current paths. Forthe 2.45 GHz excitation, clearly, a larger surface distributionis observed for the longer path along the larger L-shapedmonopole, while for the 5.25 GHz operation, the surfacecurrent distribution is very focused at the smaller L-shapedmonopole. Furthermore, in the proposed design, the electriclength of the longer path is about 29.4 mm (i.e. ffi Lþ w1þ

l2 À l3), which is slightly less than one-quarter wavelength

of the operating frequency at 2.45GHz. Alternatively, thelength of the shorter path is about 14.8 mm (i.e.ffi w2 þ l2),which is also close to one-quarter wavelength of theoperating frequency at 5.25GHz. The deviation of theresonant length of the radiating element is additionally dueto the effect of the loading of the particular resonant part bythe remaining non-resonant part of the whole structure.

Figure 4 shows the effect of varying the vertical slit lengthas l2 ¼ 1, 5 and 7 mm on the proposed antenna’s impedancematching. It is observed that with a decrease in the length l 2,the lowest resonant frequency of the lower band increaseswhile the impedance bandwidth of the upper banddecreases. This is due to the fact that to decrease the lengthof l 2 will shorten the effective current path for the larger

inverted L-shaped monopole, while the length of current

path for the smaller inverted L-shaped monopole is slightlyaffected. Additionaly, an important feature of the proposedantenna is the influence of impedance matching causedfrom the coupling effects between the feed line and the

coplanar ground plane over the two desired operatingbands, especially over the higher operating band. For this,the effect of the gap distance d on the performance of theproposed dual-frequency antenna was also studied andpresented in Fig. 5. The obtained results indicate that thebandwidth of the higher band for the proposed design isreduced with increasing distance of d , while that of thelower band is not significantly changed. For comparison,

2 3 4 5 6−35

−30

−25

−20

−15

−10

−5

0

d = 2 mmd = 3 mmd = 5 mm

frequency, GHz

r e t u r n

l o s s ,

d B

Fig. 5 Measured return loss for the proposed antenna with variousgap distance (d) between the single strip and the coplanar ground

planeOther parameters are the same as in Fig. 2

Table 1: Measured performances of proposed dual-frequency design with various l 2 and d

l 2 (mm) d (mm) Lower band Upper band

f c (GHz) BW (%, GHz) f c (GHz) BW (%, GHz)

Antenna 1 7 2 2.48 7.3, 2.39–2.57 5.22 35.1, 4.03–5.86

Antenna 2 5 2 2.63 8.4, 2.51–2.73 5.31 30.9, 4.21–5.85

Antenna 3 1 2 2.83 9.9, 2.71–2.99 5.35 25.8, 4.48–5.86

Antenna 4 7 3 2.48 5.2, 2.41–2.54 5.56 19.8, 4.82–5.92

Antenna 5 7 5 2.47 8.5, 2.34–2.55 5.34 21.7, 4.53–5.69

L¼22.48mm, W ¼16.44mm, Lg ¼18.9mm, W g ¼10.83 mm, w f ¼3mm, w 1¼9.53mm, l 1¼3.35mm, w 2¼7.95mm, l 3¼9.66mm,

s ¼1.93mm, h¼1.6mm, eg ¼4.4

2 3 4 5 6−35

−30

−25

−20

−15

−10

−5

0

frequency, GHz

r e t u r n

l o s s ,

d B

I 2=7 mm

I 2=1 mm•

I 2=5 mm×

Fig. 4 Measured return loss for the proposed antenna with variouslengths (l 2) of the L-shaped slitOther parameters are the same as in Fig. 2




φ=90°

0°(+x)

180°(−x)

20 dB

0 dB

40 dB

−90°x−y plane

θ=0°

90°

(+x)

−90°

(−x)

0 dB

180°x−z plane

θ=0°

90°

(+y)

−90°

(−y)

180°y−z plane

x

z

E

x x x E

20 dB

0 dB

40 dB

20 dB

40 dB

Fig. 6 Measured radiation patterns at 2.45GHz for the proposed antenna studied in Fig. 2

φ=90°

0°(+x)

180°(−x)

− 20 dB

0 dB

− 40 dB

−90°x−y plane

θ=0°

90°(+x)

90°(+y)

−90°(−x)

−90°(−y)

−20 dB

0 dB

−40 dB

θ=0°

− 20 dB

0 dB

−40 dB

180°

y−z plane

180°

x−z plane

x

z

E

x x x E





Table 1 summarily gives the antenna parameters andmeasured centre frequencies and bandwidths for theconstructed prototypes with various lengths of l 2 and d .

Typical radiation characteristics of the frequencies acrossthe lower and upper bands for the proposed dual-bandantenna are also examined. Figures 6–8 show, respectively,the measured radiation patterns including the vertical (E y)and the horizontal (E f) polarisation patterns in the azimuthcut (x–y plane) and the elevation cuts (y–z plane and x–zplane) for the antenna at the lower band of 2.45 GHz, and atthe upper band of 5.25, and 5.75 GHz. It is seen that theobtained radiation patterns are not as good as those of aconventional ideal monopole antenna, which has a goodomni-directional pattern in the azimuth plane and conical

radiations in the elevation planes. However, the proposedantenna in general shows monopole-like radiation patternswith nearly omnidirectional radiation in the azimuthal planefor all measured frequencies. Owing to the symmetry in thestructure, rather symmetrical radiation patterns are seen inthe x–z and y–z planes as depicted in the plots. In addition, itis also found that the E y and E f components of the patternsin both x–z and y–z planes are seemed to be muchcomparable. This electromagnetic phenomenon is probablya result of the strong horizontal components of the surfacecurrent on the radiating patch. However, this characteristicwould be an advantage of better transmission capabilities forwireless communications, such as WLAN, in a multi-pathenvironment. Also note that measurements at other

operating frequencies across the bandwidth of each bandshow radiation patterns similar to those plotted here. Thatis, stable radiation patterns have been obtained for the

φ=90°

0°(+x)

180°(−x)

−20 dB

0 dB

−40 dB

−90°x−y plane

θ=0°

90°

(+x)

−90°

(−x)

−20 dB

0 dB

−40 dB

180°x−z plane

θ=0°

90°

(+y)

−90°

(−y)

−20 dB

0 dB

−40 dB

180°y−z plane

x

z

E

x x x E


2.1 2.2 2.3 2.4 2.5 2.6 2.71

2

3

4

frequency, GHz

a n t e n n a

g a i n ,

d B i

a

3.9 4.4 4.9 5.4 5.91

2

3

4

5

6

frequency, GHz

b

a n t e n n a

g a i n ,

d B i

Fig. 9 Measured peak antenna gain for frequencies across thelower band and the higher band for the proposed antenna studied inFig. 2a 2.39–2.95 GHz

b 4.03–5.86 GHz




proposed antenna. Finally, the measured antenna gainagainst frequency for the proposed antenna across the twobands is shown in Fig. 9. For the lower band, the maximumradiation gain of about 3.2 dBi is observed, with gainvariations less than 1 dBi, while for the higher band, thepeak antenna gain observed is 4.5 dBi, and the gainvariations are also less than 1 dBi.

4 Conclusion

A single-layer planar double inverted-L monopole antennabased on 50O CPW-fed technology for broadband dual-frequency operation has been presented, with experimentaland numerical results. Constructed prototypes suitable forspatial-diversity operation in WLAN communicationscovering the 2.4, 5.2 and 5.8 GHz bands have been studied,and good antenna performances of the operating frequen-cies across the three operating bands have been obtained.The measurements on the fabricated antenna showed goodagreement with the simulated data. Large effects of varyingantenna parameters and gap distance (d ) between the singlestrip and the coplanar ground plane on the antennaresonant frequencies and impedance bandwidths have alsobeen examined.

5 References

1 Song, C.T.P., Hall, P.S., Ghafouri-Shiraz, H., and Wake, D.: ‘Tripleband planar inverted F antennas for handheld devices’, Electron. Lett.,2000, 36, (2), pp. 112–114

2 Hsiao, F.R., and Wong, K.L.: ‘Compact planar inverted-F patchantenna for triple-frequency operation’, Microw. Opt. Technol. Lett.,2002, 33, (6), pp. 459–462

3 Choi, W., Kwon, S., and Lee, B.: ‘Ceramic chip antennausing meander conductor lines’, Electron. Lett., 2001, 37, (15),pp. 933–934

4 Dakeya, Y., Suesada, T., Asakura, K., Nakajima, N., and Mandai,H.: ‘Chip multiplayer antenna for 2.45GHz-band application usingLTCC technology’. Int. IEEE MTT-S Microwave Symp. Dig., June

2000, Vol. 3, pp. 1693–16965 Lin, S.Y.: ‘Multiband folded planar monopole antenna for mobile

handset’, IEEE Trans. Antennas Propag., 2004, 52, (7), pp. 1790–17946 Kuo, Y.L., and Wong, K.L.: ‘Printed double-T monopole antenna for

2.4/5.2GHz dual-band WLAN operations’, IEEE Trans. AntennasPropag., 2003, 51, (9), pp. 2187–2192

7 Lee, L.S., Hall, P.S., and Gardner, P.: ‘Compact wideband planarmonopole antenna’, Electron. Lett., 1999, 35, (25), pp. 2157–2158

8 Lin, X.C., and Wang, L.T.: ‘A broadband cpw-fed loop slot antennawith harmonic control’, IEEE Antennas Wirel. Propag. Lett., 2003, 2,(22), pp. 323–325

9 Angelopoulos, E.S., Stratakos, Y.E., Kostaridis, A.I., Kaklamani, D.I.,and Uzunoglu, N.K.: ‘Multiband miniature coplanar waveguide slotantennas for GSM-802.11b and 802.11b-802.11a wireless applica-tions’, IEEE Wirel. Commun. Netw., 2003, 1, pp. 103–108

10 Liu, W.C., and Wu, C.M.: ‘Broadband dual-frequency cpw-fed planarmonopole antenna with rectangular notch’, Electron. Lett., 2004, 40,(11), pp. 642–643


wideband dual-frequency double inverted-l cpw-fed

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