1005AgI1'AORUFI(OIREIE 9ONAPiBDKROMAAQISPRO 5,December 20-21, 2005, Johor Bahru, Johor, MALAYSIA

Small Aperture Radial Waveguide Slot Array Antenna Design for IndoorBluetooth Application

L. M. Choong, M. R. Ahmad, and T. A. RahmanWireless Communication Centre (WCC)

Faculty of Electrical EngineeringUniversiti Teknologi Malaysia81310 Skudai, Johor, Malaysia

Email: 'choonglaimun@hotmail.com, 2 {riduan, tharek} @fke.utm.my

Abstract - The Radial Waveguide Slot Array (RWSA)Antenna is known for its good characteristics such aslow profile, low cost, aesthetically pleasing, ease ofinstallation and simple structure. This research projectinvolves the design and development of the novellinearly polarized small aperture Radial WaveguideSlot Array (RWSA) Antenna for Indoor Bluetoothapplications operating based on ISM frequency bandstandard, with frequency range of 2.4 - 2.4835 GHz.Zeland Fidelity 4.0, an FDTD based antennasimulation is utilized to estimate the input impedanceand radiation pattern of the antenna design. The secondpart of this project includes the antenna prototypesfabrication and experimental evaluation. Finally, themeasurement analysis is compared with the theoreticalresult.

1. Introduction

The quarter-wave monopole antenna posedcertain disadvantage due to its physical limitation. Thephysical structure of monopole makes it susceptible tophysical damage as the antenna probe can be bent orbroken easily. Beside that, the moving and tilting ofthe antenna probe will change the radiation pattern andhence recalibration is necessary. The omni directionalradiation pattern of the monopole can penetrate thewall. This will reduce the propagation envelope and itsefficiency for a specific room or area for the indoorWLAN application.

Radial Line Slot Array (RLSA) which is knownfor its flat, low profile and rugged structure isconsidered as one of the potential options for theindoor WLAN application. RLSA was introduced byKelly K.C. in the 1960s [1]. Takada and severalauthors proposed the use of RLSA in the mobilesatellite communication [2 - 4]. Tharek and Farah Ayuproposed the linear polarized RLSA for indoor WLANapplication for 5.5GHz [5]. This design applied thesimilar concept to design the antenna in the morepopular 2.4GHz range for indoor WLAN andBluetooth applications.

This paper is organized as follows. In section 2,we explain the tools and procedures used in theantenna design. Section 3 presents the simulation and

measurement results and analysis of the RWSAantenna. Lastly, in section 4, we conclude the paper.

2. The Antenna Design

The Linear Polarized (LP) RWSA 2.4GHzantenna for indoor WLAN and Bluetooth applicationsis designed based on the small aperture RLSA 5.2GHzantenna [5]. The RLSA antenna structure consists oftwo radial metal plates adhered to a radial dielectricmaterial acting as a radial waveguide cavity. Theradiating slots pattern is at the upper plate while thefeed element is incorporated at the center its rear plate.Both plates are spaced d apart filled with a dielectricmaterial with dielectric constant of more than I toprovide a slow wave structure in the waveguide andthus avoiding grating lobes in the radiation pattern [5].The orientation of the slots will determine thepolarization whether it is linear or circular. The linearpolarization pattern is chosen for this design. Few slotsare needed to obtain a wide beam width of radiationpattern. The slots are arrayed so that the radiations areadded in phase in the beam direction [6]. Power feed atthe center by coaxial cable will transfer into a radiallyoutward traveling wave with a rotational symmetry toexcite the slots [7]. An area of certain radius is leftwithout any slots on the upper plate to allow theaxially symmetric traveling wave to stabilize whenentering the feeding structure [5]. The LP RWSA 2.4GHz antenna structure is shown in the Figure 1, whilethe proposed geometry of the LP RWSA 2.4 GHzantenna design is shown in Figure 2.

Pobolene Ai CoxasaMonopole4 1rv SIedRad

Copperla-e l epideI , Ihickness

Figure 1: The LP RWSA 2.4 GHz antenna structure.

0-7803-9431-3/05/$20.00 C2005 IEEE. 66

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x

PW

The radiating surface of the RWSA antenna is formed by4 discrete slots arranged at tangent of the array radius.

Figure 2: The proposed geometry of the LP RWSA 2.4 GHzantenna design.

3. Simulation and Measurement Analysis

The design is simulated using Zeland 3.0, aFinite Difference Time Domain (FDTD) based full-wave electromagnetic simulator. Simulation showedthat the design produce a semi-omnidirectional flowerlike radiation pattern (Figure 3) with the beamwidth ofmore than 100 degree and directivity of 5.06241dBi.This fits indoor WLAN and Bluetooth applicationswhich require a wide beamwidth. The measurementhas very good agreement with the simulation result interms of resonant frequency with the variation of0.002%. The bandwidth variation of the simulation andthe measurement is 52.6%, with the prototype having awider bandwidth. This wide variation is due to theinaccuracy in the simulation meshing when it comes tovery small structure like the SMA probe. Thesimulation and the measurement results are shown inFigure 3 and Figure 4.

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Figure 3: Radiation pattern.

10:'~ 0B -10@ -20F -30g -40D -50

-60-70-80-90

-100-110

Elevation Pattem Directivity Display----F 1=2.4(GHz) E-theta, phi=O (deg), PG=4.0560!

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|: ~103] . ; . . 6°:2. . . w . . ..f . ... w ....-90

7 ~~~~~~~-110-180-150-120-90 -60 -30 0 30 60 90 120 150 180

Elvation Angle (deg)

Figure 4: Beamwidth ofmore than 100 degree.

Figure 5: Return loss simulation and measurementcomparison.

The Received Signal Strength Index (RSSI)measurement was carried out on the RWSA 2.4GHzantenna prototype to check for its performance in theindoor WLAN with -1OOdBm as the noise bed. TheRSSI of the RWSA antenna covers the whole WLANoperating channel range from channel I to channel 11.This result revealed that the antenna can be applied forthe IEEE 801.1lb/g and Bluetooth applications. Theresult is compared to the monopole antenna operatingat the same frequency measuring at the same distanceshown in Figure 6. Overall, RWSA antenna shows abetter RSSI compare to monopole antenna.

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RWSA 2.4GHz Simulaffon and Prototype MeasurementCompaison

-5

., 15... ~~~ ~~~~~~~-*-Simulation-5 I Ii! Prototype25 I i.a , 1Frequency (Giz)

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ILSA vs. Monopole

0

-10

-20

-30

-40

-50

-70

80

.90

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.:

Figure 6: RSSI comparison for RLSA and monopole.

4. Conclusion

This paper has proposed the Linear PolarizedRWSA antenna as access point antenna for 2.4GHzbased on IEEE 802.1 lb/g and Bluetooth applications.FDTD electromagnetic fuill-wave simulator, Zeland 3.0has been used to analysis the operations of the designbased on theoretical parameter investigation. Theantenna has been fabricated and tested for its returnloss and RSSI performance. The simulation andmeasurements shows good result agreement. From theRSSI result, RWSA 2.4GHz covers the range ofWLAN and Bluetooth and it shows a better RSSIperformance than the monopole antenna. Theadvantages of the RWSA 2.4 GHz antenna itself andits performance enable it to be considered as anotheralternative for the monopole antenna.

References

[1] Kelly K. C., "Recent Annular Slot ArrayExperiments," in IRE International ConventionRecord, vol. 5, March 1957, pp. 144 - 152.

[2] Kechagias, K.; Vafiadis, E.; Sahalos, J.N., "On theRLSA Antenna Optimum Design for DBSReception," in IEEE Transactions onBroadcasting, vol. 44, issue 4, Dec. 1998, pp. 460-469.

[3] Davis, P.W.; Bialkowski, M.E., "ExperimentalInvestigations into a Linearly Polarized RadialSlot Antenna for DBS TV in Australia," in IEEETransactions on Antennas and Propagation, vol.45, issue 7, July 1997, pp. 1123 - 1129.

[4] Ando, M.; Sakurai, K.; Goto, N., "Characteristicsof a radial line slot antenna for 12 GHz bandsatellite TV reception," in IEEE Transactions onAntennas and Propagation, vol. 34, issue 10, Oct1986,pp. 1269 -1272.

[5] Farah Ayu I.K, Tharek, A.R, "TheoreticalInvestigations of Linearly Polarized Radial LineSlot Array (RLSA) Antenna for Wireless LANIndoor Application at 5.5 GHz," in 11thMediterranean Electrotechnical Conference,MELECON 2002, 7-9 May 2002, pp. 364 - 367.

[6] J.Hirokawa, M. Ando, N. Goto, "An Analysis ofSlot Radiation on a Parallel Plate Waveguide for aRadial Line Slot Antenna," in IEEE Trans. OnAntenna and Propagation, San Jose, CA., June1989, pp. 1452-1455.

[7] M. Takahashi, J. Takada, M. Ando, N. Goto,"Characteristics of Small Aperture, Single-layeredRadial Line Slot Antennas," in IEEE Trans. OnAntenna and Propagation, vol. 139, no. 1, Feb1992.

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