Modified PIFA and its array for MIMO terminals

Y. Gao, C.C. Chiau, X. Chen and C.G. Parini

Abstract: A study on a modified planar inverted-F antenna (PIFA), operating at 5.2GHz withinthe IEEE802.11a WLAN frequency band (5.15–5.35GHz) with a very small ground plane(7mm� 12mm) and its application to multiple input multiple output (MIMO) systems ispresented. While it inherits the attractive features of PIFAs, such as compact size and low profile,the modified antenna exhibits low coupling to the printed circuit board (PCB). A 2-element PIFAarray is implemented on a mobile handset PCB and an isolation better than 28dB is achieved inboth simulation and measurement.

1 Introduction

The use of multi-element antenna arrays at both thetransmitter and the receiver in a multiple input multipleoutput (MIMO) system can offer great benefits overtraditional wireless communication systems, enabling themto exploit fully the multipath scattering environment [1].The radio channels in MIMO systems operate on the samefrequency but the system relies upon there being richmultipath to ensure that the multiple decorrelated transmit-ting and receiving antennas and associated signal processingresolve a number of signals arriving via different routes[2, 3]. The essential requirement for a diversity antennaarray in a mobile terminal MIMO system is that there mustbe good isolation between the antenna elements in order toreceive different signals, even though they are closelylocated. It still remains a challenge to design multipleantennas on a compact mobile terminal, such as a handset,for MIMO communication systems.

A planar inverted F antenna (PIFA) is commonly used ina mobile handset owing to its compact size and goodperformance. Conventionally, the ground plane of a PIFAis the printed circuit board (PCB) of the mobile handset.Since the PIFA couples strongly to the ground plane andspreads radio frequency (RF) current on it [4], it is difficultto obtain good isolation when multiple PIFA elements areplaced closely on a handset PCB. In this paper, a modifiedPIFA with a small local ground plane is proposed andstudied. It has shown that it not only retains the uniquefeatures of a conventional PIFA, but also offers lowcoupling to the PCB of the mobile handset. In addition, a2-element modified PIFA array is implemented on a mobilehandset PCB and good isolation is achieved. The operatingfrequency chosen for this study is 5.2GHz, which covers theband of IEEE802.11a WLAN (5.15–5.35GHz) and thehigh speed wireless access network type (HiSWANa) [5].

This paper is set out as follows: Section 2 depicts theantenna configuration and design; Section 3 presents theanalysis for the single modified PIFA and the 2-element

PIFA array through both simulation and measurement;and some conclusions are drawn in the final Section.

2 Antenna design

The modified PIFA with a small ground plane constitutes astand alone structure, as shown in Fig. 1. The ground plane(7mm� 12mm), as small as the antenna, is located betweenthe PIFA and the PCB. As such, the PCB is no longer aground plane for the PIFA and the influence of the PCB onthe antenna is reduced dramatically. The height of the PIFAis reduced to 4mm, which is almost half (6–10mm) aconventional PIFA [4], in order to retain the compactness ofthe whole antenna structure. But a small height for thePIFA results in a narrow bandwidth and so a parasiticelement with height (HPE) of 3mmwas used to increase thebandwidth [6, 7].

The two modified PIFA elements were then mounted onthe PCB of a mobile handset (40mm� 100mm) with agap as shown in Fig. 2. These two PIFAs are separatelyplaced 20mm apart on the PCB. The relative permittivityof the dielectric substrate (PCB) is 4.7. The coaxial feed ofthe proposed PIFA was drilled through the substrateof the PCB without touching the PEC layer on the PCB asshown in Fig. 2a.

Computer aided design (CAD) was carried out by usingthe Computer Simulation Technology (CST) Microwave

RG402LG

small ground plane

8 mmWG

H

WS

LT

WT

HPE

3 mm

T

pE

Fig. 1 Configuration of the single PIFAThe dimensions for the operating frequency of 5.2GHz: LG¼ 12mm,LT¼ 10.5mm, WG¼ 7mm, WT¼ 5mm, WS¼ 5mm, H¼ 4mm,HPE¼ 3mm and T¼ 0.4mm

The authors are with the Department of Electronic Engineering, , Queen Mary,University of London, UK

E-mail: yue.gao@elec.qmul.ac.uk

r IEE, 2005

IEE Proceedings online no. 20045180

doi:10.1049/ip-map:20045180

Paper first received 20th December 2004 and in revised form 31st March 2005

IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 4, August 2005 255

StudioTM package, which utilises the finite integral techni-que (FIT) for electromagnetic computation [8]. Likeconventional PIFAs, the performance of the modifiedPIFA is dependent on a number of design parameters, suchas antenna height, length, width and the shorting plate. Themodified PIFA was designed carefully through simulationto operate at the 5.2GHz band. The optimised dimensionsof the proposed PIFA model are: LG¼ 12mm,LT¼ 10.5mm, WG¼ 7mm, WT¼ 5mm, WS¼ 5mm,H¼ 4mm and HPE¼ 3mm. The feed position with respectto the ground plane is shown in Fig. 1. The optimal gapbetween the small ground plane and the PCB is 1mm(gap¼ 1mm). A copper sheet with the thickness (T) of0.4mm is used for the proposed PIFA. A 50O RG402coaxial cable (d¼ 0.51mm, D¼ 1.7mm, epsilon¼ 2.1) isused as the feed of the proposed PIFA.

3 Characteristics of the modified PIFAs

3.1 Single element PIFA on the PCBThe single element PIFA was fabricated and mounted onthe PCB in the Antenna Measurement Laboratory atQueen Mary, University of London (QMUL). AHP8720ES network analyser was used to measure thereturn loss in an anechoic chamber. Figure 3 shows goodagreement between the simulated and measured returnlosses. The measured �10dB bandwidth (328MHz) isslightly narrower than the simulated one (360MHz). Thishas verified the computer model of the antenna and gaveconfidence to carry out a further simulation study using theCST Microwave StudioTM package.

The performance of the modified PIFA with differentgaps between the small ground plane and the PCB wasinvestigated next. Figure 4 illustrates the simulated returnloss curves for the different gaps. It is noticed that theantenna gain is substantially increased without sacrificingthe bandwidth very much when the gap is decreased. Thisresult is encouraging since the compactness of the PIFA can

be retained with a good performance. The highest antennagain is obtained when the gap is 1mm.

The characteristics of the conventional PIFA, such asreturn loss, bandwidth and gain at the frequency of interest,strongly depend on the ground plane size [9, 10]. When thesize of the PCB in a handset is changed the PIFA has to beredesigned. So, further simulations were conducted to studythe dependence of the modified PIFA on the groundplane. As shown in Fig. 5, when the width of the PCB(x-axis direction) is kept constant and the length of the PCB(y-axis direction) changed from 30mm to 100mm, thefluctuation of the centre resonant frequency is less than20MHz. Figure 5 also shows that the fluctuation of thecentre resonant frequency is less than 55MHz when thePIFA was located in a different position of the PCB. It isevident that the changes of the PCB’s size and the locationof the antenna do not affect the modified PIFA perfor-mance very much owing to its self-contained structure.

The RF current induced by the modified PIFA on thePCB is observed in simulation, as shown in Fig. 6. It isfound that the RF current is mostly confined in the locationof the antenna on the PCB, and the coupling between thePCB and the modified PIFA is reduced dramatically. Allthe results obtained in this Section gave us an increasedconfidence to add more antennas on the handset PCB forMIMO applications.

a

unit : mm

100

40

b

gap

epsilon = 4.7 PCB

Fig. 2 Configurations of the two PIFAs mounted on a handsetPCBa 3-D viewb Side view

measurementsimulation

4.6 4.8 5.0 5.2 5.4 5.6 5.8−50

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Fig. 4 Simulated return losses with different gaps

256 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 4, August 2005

3.2 Two-element PIFA array on the PCBThe 2-element modified PIFA array based on the optimisedconfiguration and dimension of the single element was alsosimulated and fabricated, Fig. 7. The S-parameters andradiation patterns were measured inside an anechoicchamber.

The simulated and measured S-parameters of Antenna 1and 2, as depicted in Fig. 8, are slightly different because of

the imperfection during the fabrication process of smallantennas. A slight shifting of the centre resonance frequency(less than 20MHz) and a small variation of the bandwidth(less than 70MHz) are observed in Fig. 8. However, aslisted in Table 1, the measured bandwidths of Antenna 1and 2 span more than 310MHz, where the lower resonantfrequency is less than 5.15GHz and the upper resonantfrequency is more than 5.35GHz. These bandwidths havecovered the band of IEEE 802.11a WLAN.

The proposed 2-element PIFA array has an isolationbetter than 28dB between each element in both simulationand measurement, Fig. 8. There are two reasons why suchgood isolation was obtained. First, there is a low couplingbetween the PCB and each modified PIFA as described inthe previous Section. Secondly, the coupling between thePIFAs that has been caused by the ground plane effect isreduced as each PIFA has its own ground planerespectively.

The simulated and measured radiation patterns ofAntenna 1 at the resonant frequency of 5.2GHz are plottedin Fig. 9. The measured and simulated results agree verywell, the ripples in the measured radiation pattern being dueto the effect of the feeding cable. The co- and cross-polarisation of the proposed PIFA array are typical of thoseof the conventional PIFAs.

A 2-element conventional PIFA with the same height(H), length (LT), width (WT), the same location, and thesame size of the PCB as the modified PIFAs was modelledfor comparison. The PCB was acting as the ground plane,i.e. the small ground plane and the parasite element beingremoved from the model. The S parameters of the2-element conventional PIFA array are shown in Fig. 10.The isolation (S12) between the two elements is now morethan 15dB, which is 13dB worse compared to the modified

scale

0 A/m 10

Fig. 6 RF current distribution on the PCB

100

unit : mm

y

x

z

86

20

40

a b

1

2

Fig. 7 2-element PIFA arraya Configurationb Photograph

S p

aram

eter

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B

frequency, GHz

simulated measured S11 (1)measured S11 (2)

measured S12simulated

0

−10

−20

−30

−404.6 4.8 5.0 5.2 5.4 5.6 5.8

S11 (1&2)

S12

Fig. 8 Measured and simulated S parameters of Antenna 1 and 2

0 10 20 30 40 50 60 70 80 90 1005.0

5.1

5.2

5.3

5.4

change PCB dimension in y directionlocate PIFA in x direction of the PCBlocate PIFA in y direction of the PCB

x

y

dimension, mm

freq

uenc

y, G

Hz

Fig. 5 Centre resonant frequency for different dimension PCB anddifferent locations for the PIFA on the PCB

Table 1: Measured and simulated the lower and upperresonant frequency, and bandwidth at �10dB of Antenna 1and 2

fL (GHz) fU (GHz) Bandwidth(GHz)

Simulated antenna 1&2 5.006 5.365 0.359

Measured antenna 1 5.081 5.392 0.311

Measured antenna 2 5.102 5.480 0.378

IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 4, August 2005 257

PIFA array. This is primarily due to more RF currentcoupled to the PCB in the conventional PIFA. In contrast,the small ground plane in the modified PIFA acts as part of

the radiator and so reflects most of the radiation backtowards the PIFA. Consequently, there is only a littlecurrent coupled to the PCB, and hence better isolation isobtained.

4 Conclusions

Extensive studies have been conducted both numericallyand experimentally on a modified PIFA with a smallground plane and its 2-element array mounted on a PCB ofa mobile handset operating at the 5.2GHz band. It isshown that the compactness and performance of the PIFAcan be well retained with a small gap between the smallground plane and the PCB. It is also demonstrated that the2-element modified PIFA array can achieve an isolationbetter than 28dB even with a separation of 20mm becausethe RF current on the PCB has been reduced dramaticallywith the introduction of the small ground plane between thePIFA and the PCB. These features make this modifiedPIFA array very suitable for the implementation on a smallMIMO terminal, such as a mobile handset.

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Fig. 9 Measured and simulated radiation patterns of Antenna 1 at 5.2 GHza xz plane E-cob xz plane E-crossc yz plane E-cod yz plane E-crosse Legend

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S p

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Fig. 10 Theoretical S parameters of the 2-element conventionalPIFA array

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5 Acknowledgments

The authors would like to thank Mr John Dupuy of theDepartment of Electronic Engineering, QMUL for his helpin the measurement and fabrication of the antenna. Theauthors would like to acknowledge Computer SimulationTechnology (CST), Germany, for the complimentary licenseof the Microwave StudioTM package.

6 References

1 Paulraj, A.J., Gore, D.A., Nabar, R.U., and Bolcskei, H.: ‘Anoverview ofMIMO communicationsFa key to gigabit wireless’, Proc.IEEE, 2004, 92, (2), pp. 198–218

2 Foschini, G.J.: ‘Layered space-time architecture for wireless commu-nication in a fading environment when using multi-element antennas’,Bell Labs Tech. J., 1996, 1, (2), pp. 41–59

3 Dong, L., Choo, H., Heath, R.W., and Ling, H.: ‘Simulation ofMIMO channel capacity with antenna polarization diversity’, IEEETrans. Wirel. Commun., Revised Jan 2004

4 Wong, K.: ‘Planar antennas for wireless communications’ (JohnWiley& Sons, New York, 2003)

5 Adachi, F.: ‘Challenges for broadband wireless technology’. Int. Tech.Conf., Hong Kong, Jan 2003

6 Stoiljkovic, V., and Wilson, G.: ‘A small planar inverted-f antennawith parasitic element for WLAN applications’. IEE 10th Int. Conf.Antennas Propag., April 1997

7 Fujimoto, K., Henderson, A., Hirasawa, K., and James, J.: ‘Smallantennas’ (John Wiley & Sons, 1987)

8 Computer Simulation Technology (CST), User’s manual 5, in CST-Microwave Studio, 2003

9 Huynh, M., and Stutzman, W.: ‘Ground plane effects on planarinverted-F antenna (PIFA) performance’, IEE Proc., Microw.Antennas Propag., 2003, 150, (4), pp. 209–213

10 Abedin, M.F., and Ali, M.: ‘Modifying the ground plane and its effecton planar inverted-F antennas (PIFAs) for mobile phone handsets’,IEEE Antennas Wirel. Propag. Lett., 2003, 2, (15), pp. 226–229

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