Proceedings of iWAT2008, Chiba, Japan

P118

UWB Switched-Beam Array Antenna EmployingUWB Butler Matrix

Yu-Chuan Su 1, Marek E. Bialkowski22 Feng-Chi E. Tsai 1, Kai-Hong Cheng1 Antenna Design Department, Wistron NeWeb Corporation

No. 10-1, Li-Hsin Road 1, Hsinchu Science Park, Hsinchu 300, Taiwan R.O.C.,{vincent su, eddie_tsai, kh cheng}@wneweb.com.tw

2School of ITEE, University of QueenslandSt Lucia, Brisbane, QLD 4072, Australia, meb@itee.uq.edu.au

1. Introduction

Recent years have seen a growing interest in ultra-wideband (UWB) technologies, whichaim at overcoming problems associated with a heavy utilization of radio spectrum by currentwireless communication standards. To tackle this problem, the US-FCC [1] has allocated aspectrum of 3.1 to 10.6 GHz for use of narrow pulse signal transmission. The wideband spectrumand the low power spectral density used in this transmission scheme introduce very littleinterference to the currently existing narrowband systems. However, the shortfall ofUWB is a smalloperating range. The range and performance can be enhanced by employing array antennas with anelectronically steered beam. When such array antennas are equipped with a suitable beam-formingalgorithm they are named smart antennas. One example of a simple smart antenna scheme is aswitched-beam scheme that employs a Butler matrix [5]. This has been demonstrated for narrow-band systems. The Butler matrix type beam-forming network is constituted by quadrature couplersand phase shifters. The extension of this smart antenna concept to UWB applications requires bothantennas and a beam-forming network to operate over UW frequency band.

In recent years, a lot of successful research has been reported with respect to UWBantennas. Examples include UWB planar monopoles [2], [3] and UWB tapered slot antennas (TSA)[4]. Considerably less successful has been the area ofUWB beam-forming networks.

In this paper, we show that the design of a UWB Butler matrix can be accomplished usingcouplers and phase shifters in microstrip/slot technology [6], [7]. As a result of the proposedapproach, we present the design of a 4-element switched-beam array antenna operating between 3.1and 10.6 GHz.

2. Design

Figure 1 shows the configuration of a 4-element UWB switched-beam array antenna. TheUWB array antenna employs four TSA antennas and a 4x4 Butler matrix that is formed by fourquadrature couplers and two 450 phase shifters. The design of individual radiating and beam-forming components is accomplished using CST Microwave Studio [8]. The aim is to obtain goodperformance of individual components and then of the full antenna system in the frequency band of3.1 to 10.6 GHz. The design assumes a 0.508 mm thick Rogers R04003 (&r 3.38) substrate forboth the antenna array and the beam-forming network.

2.1 TSA Antenna Design

FigurIe 2shows the TSA antenna configuration to achieve UWB performance. As observedin Figure 2, the antenna occupies a rectangular area of 59.9 mm x 59.9 mm. The inner flare edge isusing a portion of the elliptical shape curve whose radii are 50 mm and 19.75 mm for both theradiating and ground flares. The outer flare edge is using a portion of the elliptical shape curvewhose radii are 29.55 mm and 1 1.1 mm for both the radiating and ground flare. The radiating part is

978-:1-42=1- l 23-3/08/$25.00 t 2008 IEEE 199

47 mm long and the aperture width is 38.9 mm. The width of the microstrip line feed is 0.6 mm andthe length is 12.9 mm. The ground plane is extended by 1.8 mm in order to improve the impedancematching of the antenna.

The antenna's return loss performance is presented in Figure 3. The 10 dB return loss ofthis antenna spans from about 3.1 GHz to beyond 12 GHz. This UWB operation is confirmed byboth full EM simulations and measurements, and relatively close agreements are observed.

2.2 Quadrature Coupler DesignThe UWB coupler shown in Figure 4a consists of three conductor layers interleaved by two

dielectric layers. The top and bottom conductor layers include elliptically shaped patches. The twolayers are coupled via an elliptical slot, which is made in the conductor supporting the top andbottom dielectric layers. The choice of an elliptical shape for the microstrip patches and the slot wasfound to be advantageous in terms of obtaining high quality return loss, coupling and isolation overUWB. The microstrip lines forming the input/output ports of the coupler are designed to have 50Qcharacteristic impedance. Ports 1 and 2 are one side while ports 3 and 4 and on the other side of theground plane. Because the coupler is of backward wave type, ports 1 and 4 are the isolated ports. Sothere are ports 2 and 3.The initial dimensions are estimated assuming the odd and even impedances of coupled lines ofZOO= 20.7Q and ZOe = 120.5Q for a 3dB coupling [6]. The microstrip line width is chosen

Wm = 1.18 mm for the 50Q characteristic impedance, and the radius of curved line r = 4.8 mm.The final dimensions of the coupler are obtained by a manual iterative process involving CSTMicrowave Studio and are D1 =4.8mm, D2 7.4mm and D3=7.2mm.

2.3 450 Phase Shifter DesignThe chosen configuration ofUWB phase shifter is adapted from the design presented in [7].

It is the differential phase shifter in which the reference is formed by a 50Q microstrip line ofsuitable length while the phase shifting part is formed by a microstrip/slot transition. This transitionis the truncated form of the microstrip/slot coupler of Figure 4a, in which either microstrip ports 2and 4 or ports 2 and 3 are eliminated by cutting them at the junction with the upper and lowerelliptical conducting patches. These truncations lead to either configuration of Figure 4b or Figure4c. The operation of this phase shifter is similar to the loaded line type phase shifter for which thephase shift is related to the normalized shunt susceptance (or series reactance) loading atransmission line. The larger phase shift is accompanied by the smaller return loss and thus this typeof phase shifter is limited in use to small values of phase shift in the range of 450 [9]. This well-known fact was confirmed in [7]. The compromise between the return loss and the achievable phaseshift led to the phase shift between 300 and 480 for RL not less than 10 dB [7]. The inherent featureof the chosen transition is that it offers the specified phase shift over an UW frequency band.

The dimensions of the transition of Figure 4c for the 450 phase shift are: D1 =4.8mm, D27.3mm and D3=7.1mm, as obtained using CST Microwave Studio. Note that these dimensions arevery similar to those of the 3dB coupler. The phase shift is 450±30 and the RL is better than 10dBacross 3.3 and 10.6 GHz. The insertion loss is about 0.5 dB in the same frequency band.

3. Beamforming Network and Array Performance

The four (4) element switched-beam antenna system is designed and fabricated using a0.508 mm thick Rogers R04003 (&Fr =3.38) substrate. Here we report on simulation results. Firsttests concentrate on the 4x4 Butler matrix. A layout of the Butler matrix with ports indication ispresented in Figure 5. The simulation results shown in Figure 6a and 6b, as obtained with respect toport 1, indicate that the matrix operates relatively well in terms of insertion loss and phasecharacteristics,. The magnitudes of transmission coefficients deviate from the ideal value of -6 dB.However, the phase shifts between output ports, being approximately 45±7° and 135±6°, are closeto the ideal values of 450 and 135°.

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Figure7 shows the radiation plots for four beams (2L, IL, IR, 2R) at 7 GHz. The result wasobtained for the array element spacing of 20 mm. This corresponds to 0.2 2 0 at 3 GHz and 0.66 2 0at 10 GHz. The beams point approximately at -35°, -10°, 10°, and 350 from the array's broadsidedirection. They are accompanied by a relatively large level of cross polarization. This problem canbe overcome by the array elements mirroring arrangement, as shown in Figure 8.

4. Conclusion

An UWB switched-beam array antenna formed by four tapered slot antennas and a 4x4Butler matrix has been presented. The matrix uses novel 3dB couplers and 450 phase shifters inmicrostrip/slot technology to obtain ultra wideband performance. The obtained results confirmUWB performance of the designed antenna system.

Acknowledgments

The authors acknowledge the assistance of Dr A. Abbosh of the University of Queenslandin the initial phase of designing of the presented UWB switched beam antenna system.

1. 1XR

90' 90,Hybrid Hybrid

90' 90'

Hybrid Hybrid1I

R 1. 7 1R 18w Wf back

Figure 1: Array antenna employing Butler matrix Figure 2: Front and back views of TSA.

Port 8Port 7

Portort utPort6outus0

~-1Q~-20

SimuLlation

-G0 Port42 3 4 5 6 7 8 9 10 1 1 12 Inputs ~ Port2Pr3

Frequency (GHz) Port I

Port 3 Elliptical Patch Port 2 Port 3

D D, D

D1 D2 __ D1D~~~~~~~~~~~~~~~~~~~~~~~2 D D2D

Y

-H K w. Elliptical Slot Port 4 Port 1 Port I Port 4Port 1

(a) (b) (c)Figure 4: Layout of (a) coupler, and (b), (c) alternative transitions for use in a 450 phase shifter.

Parfield (Aray); breeb6i (Theta) ,afd (f_ 6)[1,;(AbPH-= )1llllllll

90 f*66H ff]6)[1](AbsPPf d0 )[2

240

Figure 7: Four radiation beams. Figure 8: Mirrored H-Plane TSA array.S-Pararneter Magnitude ih dB S-Pararneter Phase in Degrees

LUl I '' I stUt LM;9°X1 I Sfi~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~S65 ' # | m I r ~~~~~~~~~~~~~~~~~~~~J6 1 0

-- -- - --- -- -- -- -- -- -- --- - -- ---- -- -i

-20 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~~~~6

-1

(a) _(b )_40 Am _20002 .4 6 a iO 12S 2 6 8 10 1 2

Frequenlcy I GHz Frequenicy1 GHz

Figure 6: Simulated S-parameters for port 1: (a) magnitude and (b) phase.

References

[1] http://ftp.fcc.gov/Bureaus/Engineering_Technology/ News Releases/2002/nretO2O3.pdf[2] S.K. Padhi, S. Zagriatski, S. Crozier and M.E. Bialkowski, "Planar ring antennas for ultra-

wideband applications", Proc. of IEEE Interntl. Workshop on Antenna Technology (IWAT-2005), Singapore, pp. 333-336, May 2005.

[3] S. Padhi,, S. Zagriatski, S. Crozier, and M.E. Bialkowski, "Investigations into printed monopoleantennas for ultra-wideband (UWB) applications", Proc. 2005 IEEE AP-S USNC/URSISymposium, vol. 2A, pp. 651-654, 3-8 July 2005, Washington, D.C., USA.

[4] A.M. Abbosh and M.E. Bialkowski, "Tapered slot antenna for near-field microwave imaging",Proc. Interntl. Symp. on Antennas and Prop. (ISAP 2006), vol. 1, pp. 1-4, Singapore, Nov., 2006

[5] Y-C. Su, Development of UWB tapered slot antennas and Butler beamforming network, MEngThesis, University of Queensland, 2006.

[6] A.M. Abbosh and M.E. Bialkowski, "Design of ultra-wideband 3dB quadrature microstrip/slotcoupler", Microwave and Optical Technology Letters, vol. 49, No. 9, pp. 2101-2103, Sept. 2007.

[7] A.M. Abbosh, "Ultra-wideband phase shifters", IEEE Trans. on Microwave Theory and Techn.,vol. MTT- 55, No. 9, pp. 1935-1941, Sept. 2007.

[8] http://www.cst.com/Content/Products/MWS/Overview.aspx[9] D. Pozar, Microwave Engineering, 3rd ed. New York: Wiley, 2005.

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