Proceedings of Asia-Pacific Microwave Conference 2007

Multi-port Multi-band Small Antenna Design

Seunghwan Yoon (1), Changwon Jung (2), Young-eil Kim (2), and Franco De Flaviis (

(1) Electrical Engineering and Computer ScienceUniversity of California, Irvine, CA 92697 USA

(2) Samsung Advanced Institute of Technology, Yongin, Korea

Abstract- In this paper, a novel multi-port, multi-band smallplanar antenna is presented. It has three input ports; each ofthem has their own frequency bands, 2.4 GHz, 3.5 GHz and 5.5GHz. These three antennas are printed on a small area, 250 mm2.The proposed antenna is printed on a cheap organic materialsubstrate with other RF components. It requires enough isolationamong the antennas since each frequency band ought to operateat the same time. Polarizations of monopole antennas are allorthogonal as best as possible to minimize mutual couplings andopen stubs are used at feed points to block the other frequencyband signals.

The antenna proposed in this paper is a solution to the on-board configuration of handset system needed multiple wirelessservices and they can be used simultaneously.

I. INTRODUCTION

Antenna design is becoming a bottle neck of wirelessmobile handsets where multiple communication services areintegrated in the same piece of hardware. Today a typicalcellular phone can have multiple services such as GPS, DCS,BT, WLAN services that all rely on a multi-band antenna.Implementing the antennas in small mobile handsets is verychallenging due to the limited space and low costrequirements [1 - 4].

In this work, we propose a novel tri-port, tri-bands antennaoperating at 2.4 GHz, 3.5 GHz and 5.5 GHz that can be usedin cellular phone handset. The antenna we design used printedmonopole antenna based on a planar inverted F antenna(PIFA). The structure is printed on planar substrate which isan appropriate material to print any two-dimensional structureand also provide low cost and reliable solution for handset.The size of antenna should be small so that can be integratedin the handset effectively. Using standard printed circuit board(PCB) for the antenna also has the advantage that otherpassive components can be implemented on to the samesubstrate easily.

2.4 GHZ monopole antenna operates for WiBro (2.3 - 2.4GHz) and Wi-Fi (2.4 - 2.48 GHz), 3.5 GHz monopole antennais for Mobile-WiMAX (3.4 - 3.6 GHz), and, broadband 5.5GHZ monopole antenna is for IEEE 802.1 la (5.15 - 5.35 GHZ,5.725 - 5.825 GHZ) as shown in Fig. 1 (a).Combining three antennas having multiple services in a

limited space is very hard work due to their mutual couplings;however, it can be overcome by spatial orthogonality ofpolarization and choking neighbored undesired frequencybands.

In this paper, we show how the proposed idea can addresssome of the challenges above, and measured data show a

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small foot print antenna capable to operate at 2.4 GHz, 3.5GHz and 5.5 GHz simultaneously [5].

II. SPATIAL ORTHOGONALITY AND FILTERING FOR ISOLATION

PIFA has good electromagnetic characteristics as amonopole antenna and it is simple and easy to design,fabrication and tuning [6 - 7]. Above the reason, PIFA ischosen as a single element antenna of the multi-band andmulti-port antenna system. An element PIFA shows goodVSWR and omni-directional radiation pattern in its resonantfrequency band. However, if the antennas are close each other,undesired phenomenon happen. Mutual couplings among theantennas are serious problem in small multi-port, multi-bandantennas [7 - 9]. Because, antenna is a reciprocal system, theyabsorb radiant powers from neighbored antennas. If the

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Fig. 2. Undesired band rejection by shunt open stub. (a) Schematic, (b) s-parameters (2.4 GHz BRF: loc = 19 mm, 3.5 GHz BRF: loc = 12.8 mm, 5.5GHzBRF:loc =8.1mm).

couplings among the antennas are considerable, the antennasystem works as a transmission-line coupler as shown in Fig.1 (b). It is much undesired case for antenna, because badradiation makes worse antenna gain and transmitted highpower will affect the neighbored ports. It is an axiomatic factthat the farther distance among antenna, the smaller mutualcoupling. However, the space for fabrication is very limited inreal handset system. To minimize the coupling in small space,

i.e. maximize isolations among ports, polarity of monopoleantenna is considered before everything else.

In the planar substrate, only two vertical vector componentscan be exist, such as along x-axis and y-axis. If a metal line ofmonopole antenna printed along the x-axis, the other will beprinted along the y-axis. They are perfect orthogonal in theory,however, the electromagnetic fields from one of the verticalvector components can induce to the other near electricconductors. To make matters worse, one more vectorcomponent is needed on the two dimensional substrate in thisresearch.However, it is not absolutely impossible to build tri-port,

tri-band antenna on the planar substrate. Since each portoperates at different frequency. Therefore, undesired signalscan be blocked by attaching kind of filter circuit. Open stubsat each feed point of antenna can be a function of undesiredfrequency signal blocking filter as shown in Fig. 1 (c).

Fig. 2 shows the open stub which shunt to 50Qtransmission-line can choke undesired frequency signal and

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the each stub length is achieved by transmission-line circuitsimulation.

III. TRI-PORT TRI-BAND ANTENNA DESIGN

It starts from each individual antenna design. For a linearantenna, the resonant frequency is inversely proportional to itslength. Three different frequency band PIFAs are drawn andsimulated. If they operate well in specifications, we placedthem in a planned area of substrate. Although, the antennaarea is so limited; we need to place them as vertical as

possible to decrease mutual couplings. And then simulate thistri-port, tri- band antenna. But, most likely the results will notsatisfy the specifications even such as resonant frequencies.We can fix the desired resonant frequencies by severaliterative tunings with changing lengths of the monopoles.However, isolations among the ports still will not be inspecifications. After the resonances are in the properfrequencies, three more ports are added at antenna feed point.We will connect open stubs for better isolations among theports. Now, this tri-port, tri-band antenna system is total 6-portnetworks, three input port and three feed ports. Frequencyswept 6-port s-parameters can be extracted from the full wavesimulation. We use the s-parameter data to circuit simulator,

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and then connect the open stubs. By several iterative tuning ofopen stub lengths in the circuit simulator, we can achieveoptimum values of the open stubs. The transmission-linecircuit simulation is a fast method to tune the stub length,which has an analytic model.We can draw the antenna part and open stub circuit part

from the tuned geometric data. And analyze it accurately bythe full wave simulation. The final simulation results will beclose to the specifications what we expect. After check theradiation pattern, gain, if it meets the specification, we canmove to fabrication and measurement. We can see the wholedesign flow of the proposed antenna in Fig. 3.

IV. FABRICATION AND MEASUREMENT

The antenna is fabricated on FR4 substrate with a relativepermittivity £r= 4.4, dissipation factor tan6 = 0.02 at 2.4 GHz,and thickness of 0.8 mm. In the proposed design, we havethree metal lines having different lengths are printed such away that each ofthem share top side ofPCB with the others asshown as Fig. 4 (a). Bottom side of PCB where the monopoleantenna metal lines are over passed is removed.

In order to fit the size of the monopole antenna in smallareas, metal line of 2.4 GHz antenna, which is the longestelectric length in the proposed multi-port, multi-band antennasystem, is slightly bent. The shortest is 5.5 GHz monopoleantenna and the remained is 3.5 GHz monopole antenna.The open stubs blocking the undesired frequency signals are

also integrated on the same substrate. The only antenna area is250 mm2 on the selected substrate; thus it is suitable forhandset devices. The widths of monopole lines whereoptimized and are chosen to be around 0.5 mm at 2.4 GHzmonopole and 1.5 mm at 3.5 GHz and 5.5 GHz monopole.

Fig. 4 (b) and (c) shows the measured return losses andisolations of the proposed tri-port, tri-band monopole antennasystem. In-band return losses are below 10 dB, specification,for all three frequency band. The isolations are also below orclose to 15 dB, specification. Radiation patterns for eachfrequency result are shown in Fig. 4 (d), (e) and (f). Peakgains are around 4 dBi for 2.4 GHz and 3.5 GHz, and -3 dBifor 5.5 GHz. The first two frequency operations have linearpolarized radiation and omni-directional radiation patterns.The 5.5 GHz radiation pattern is not as optimal but still usable,

we suspect this is due to the coupling with the neighboringmonopoles, when the antenna operate at this frequency.

V. CONCLUSION

In the paper, we show how building multi-port, multi-bandantenna in a small two-dimensional space and we can achieveenough isolation among the ports with spatial orthogonalityand undesired band rejection. The proposed antenna has threeports and each port has its own resonant frequency band,respectively at 2.4 GHz, 3.5 GHz, and 5.5 GHz. In-band returnloss and isolation are all meet the specifications, in addition,radiation patterns are linearly polarized and omni-directionalat the first two frequency bands.

ACKNOWLEDGMENT

The authors would like to thank Communication andConnectivity Lab, Samsung Advanced Institute of Technology,Korea for supporting this work.

REFERENCES

[1] Seunghwan Yoon; Changwon Jung; Young-eil Kim; Franco De Flaviis"Triple-band Fractal Antenna Design for Handset System", Antennasand Propagation Society International Symposium 2007, IEEE, 10-15June 2007.

[2] Cheng-Nan Hu; Chen, W.; Book Tai "A compact multi-band antennadesign for mobile handsets", Microwave Conference Proceedings,APMC 2005, Asia-Pacific Conference Proceedings. Volume 4, 4-7 Dec.2005 Page(s): 4 pp.

[3] Yuehe Ge; Esselle, K.P.; Bird, T.S. "A Spiral-Shaped Printed MonopoleAntenna for Mobile Communications", Antennas and PropagationSociety International Symposium 2006, IEEE 2006, Page(s): 3681 -

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[6] Yu-Shin Wang; Ming-Chou Lee; Shyh-Jong Chung "Two PIFA-RelatedMiniaturized Dual-Band Antennas", Antennas and Propagation, IEEETransactions, Volume 55, Issue 3, Part 2, March 2007 Page(s):805 - 811

[7] Thaysen, J. "Mutual coupling between two identical planar inverted-Fantennas", Antennas and Propagation Society International Symposium,IEEE, Volume 4, 16-21 June 2002 Page(s):504 - 507

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Fig. 4. Measurement results of tri-port, tri-band small antenna. (a) Fabricated antenna, (b) Return loss, (c) Isolation, (d) Antenna #1 gain at 2.4 GHz, (e)

Antenna #2 gain at 3.5 GHz, (f) Antenna #3 gain at 5.5 GHz.

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