Proceedings of the 39th European Microwave Conference
A Ka-Band Butler Matrix with Antenna Array Based on Micromachined Rectangular Coaxial Structures Yi Wang, Maolong Ke, Michael J. Lancaster School of Engineering, University of Birmingham, Birmingham, B15 2TT, U.K.
[email protected] [email protected] [email protected] Abstract — A Ka-band 4x4 Butler matrix feeding a 4-element linear antenna array has been presented in this paper. The Butler matrix is based on a rectangular coaxial structure, constructed using five layers of gold coated micromachined silicon slices. The patch antennas are of an air-filled microstrip type, and spaced by half a wavelength at 38 GHz to form the array. The demonstrated device is 26 mm by 23 mm in size and 1.5 mm in height. The measured return losses at all input ports are better than -10 dB between 34.4 and 38.3 GHz. The measured radiation pattern of one beam has shown good agreement with the simulations.
dielectric-free rectangular-coaxial transmission line and suspended patch antenna. As compared with planar-circuit based Butler matrices [6], [7] used at mm-wave frequencies, this device is low loss. More importantly, the Butler matrix is completely shielded and this eliminates the spurious interference with the radiation elements. A micromachined coaxial-type 4×4 phased array [8] has been reported before using sequential surface micromachining [3]. The approach reported here differs in that a reduced number of structural layers are used, release holes and dielectric supports are not required, and the final device is solid and dielectric free.
I. INTRODUCTION Micromachining techniques have been increasingly utilized for developing novel and high performance millimeter wave and sub-millimeter wave components and systems [1]. As the device footprint gets smaller, micromachining techniques have great advantages because of their capabilities of accurately defining structural dimensions, and more readily integrating with other monolithic microwave integrated circuits. In addition, they are potentially low cost. One application is three dimensional coaxial-line or waveguide based devices. Among the fabrication processes exploited for such devices are surface-micromachining on sequentially deposited metal layers [2], [3], bulk silicon micromachining [4], and thick photoresist photolithography [5]. Silicon deep reactive ion etching (DRIE) and multi-layered assembly have been adopted in this work.
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III II I Fig. 2. Exploded view of the device constructed of five layers.
II. DESIGN A. Butler matrix
Fig. 1. A diagram of a 4x4 Butler matrix with a linear array.
The device demonstrated is a 38 GHz Butler matrix with a linear antenna array used for a beam forming. It is based on a
978-2-87487-011-8 © 2009 EuMA
Butler matrix is a beam-forming network which is connected to a phase antenna array. The signal is distributed in the matrix through a combination of hybrid couplers, crossovers and phase shifters. Its beams can be directed either by switching or by using dedicated transmitter or receiver channels at the input ports. In this paper, a 4×4 Butler matrix feeding a linear antenna array at 38 GHz is demonstrated. The Butler matrix delivers signals at its output ports with equal magnitudes and a phase increment between them of -45°, 135°, -135°, or 45° depending on which input port is excited. The main components, for the 4×4 matrix, are four 90° hybrid couplers, two cross-overs, and two phase shifters.
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29 September - 1 October 2009, Rome, Italy
Fig. 1 shows the schematic diagram. The transmission line media is based on a micromachined rectangular coaxial structure. Similar to what have been reported on stub-line filters [5], cavity filters [9], and branch-line couplers [10], the transmission lines are formed by bonding five layers on top of each other. Here, each layer is a micromachined silicon slice coated with gold. Such an assembly for the Butler matrix is illustrated in Fig. 2. It contains a middle layer (III) defining the central conductors and the antenna radiators (see Fig. 3), sandwiched between two layers (II and IV) featuring the outer conductors, and finally enclosed by a top (I) and a bottom slice (V) also as the ground planes for the patch antenna. It should be noted that all unconnected pieces shown in the figures are in fact jointed to the main parts using beams with tethers at both ends. These are broken off after bonding. B LC2
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The rectangular coaxial structure is then reduced to a semicoaxial structure [10] with only one outer side-wall, or even a stripline-like structure. This is the case for the branch-line couplers (BLC), the cross-overs, and some interconnection lines in between. For instance, the coupler BLC1 at the inputs as in Fig. 3 has three semi-coaxial branches and one 50 Ω branch like a stripline. For the coupler BLC2, the four branches are all different transmission lines. The break of the symmetry makes it more difficult to realize a 90° hybrid. The dimensions of these branches are all optimized to give the simulated responses shown in Fig. 4. All the simulations in this paper are performed using CST Microwave Studio [11]. Two hybrid couplers can be cascaded to make a cross-over. Alternatively, an all-50Ω four port network could also make a cross-over, both circuits are shown in Fig. 5. They present similar responses. The latter is chosen in this design considering the 25 Ω low-impedance line in the hybrid-based cross-over would require a much widened centre conductor, which is difficult to implement. Fig. 6 shows the simulated responses of the two cross-overs used in the Butler matrix. 0
S-parameters (dB)
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Fig. 3. Cutting plane view of the middle layer (III).
Fig. 5. Circuit-model responses of the two different cross-over networks.
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Fig. 4. Simulated responses of the two types of branch-line couplers (BLC1 and BLC2).
The central conductors are all self-suspended by placing quarter-wavelength-length shorted stubs. Unlike some other micromachined coaxial structures [3], dielectric supports are not used. However, as a result, an all-coaxial structure is not viable, especially when a transmission-line loop is required.
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Fig. 6. Simulated responses of the two cross-overs (Cross1 and Cross2) as shown in Fig. 3.
The Butler matrix was designed as an 8-port device. The input ports are labelled P1 to P4 and the output P5 to P8. In all simulations, the conductivity is assumed to be 2.45×107 S/m, i.e. 60% that of gold. The simulated responses are shown in
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Fig. 7. For an input at P1, the output magnitude is between 6.8 dB and -6.3 dB. For an input at P2, this is between -7.2 dB to -6.1 dB. The insertion loss on each signal path is about 0.5 dB. The return losses at ports P1 to P4 are better than -16 dB and the inter-port isolations, Si,j (i•j, i=1…4, j=1…8), are below -22 dB. The maximum deviation from the specified phase increments is 4.0° for an input at P1, 9.6° at P2, -9.6° at P3, and -4.0° at P4. These deviations are insignificant and would only increase the side lobe level by 0.6 dB.
S-parameters (dB)
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Frequency (GHz) Fig. 7. Simulated responses of the Butler matrix as an 8-port device.
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microstrip type and 300 μm in thickness. This thickness is the same as the separation between the patch and its ground. The patch is suspended in air. This eliminates the dielectric loss and surface-waves, which usually trouble substrate-supported mm-wave antennas [12]. The matching of the patch element is achieved by simply connecting to the coaxial centre conductor with a quarter-wavelength line of the same cross-sectional dimensions. A single optimized patch is 3.27 mm long and 3.54 mm wide. The simulated gain is 9.4 dBi and the radiation efficiency is 96 %. The patch elements are spaced by λ/2 to form the array. In order to reduce the cross coupling between them, the width of each element is reduced to 3 mm. This makes the separation between patches over three times of the patch-ground distance. Fig. 8 shows the simulated beam patterns of the complete device including the Butler matrix and the antenna array. For the two inner beams, the estimated gain is 13.1 dBi, the side lobe level is -11.2 dB, and the cross-over point is -3.5 dB. The 3-dB beam width is 25.6°. This is very close to an ideal Butler matrix linear array with a side lobe at -11.3 dB and cross-over at -3.7 dB. For the outer beams, the gain is 11.5 dBi, the side lobe is -9.7 dB down, and the cross-over is 4.4 dB. The beam width is 29.8°. The calculated radiation efficiency of these two sets of beams is 89% and 85%, respectively. The beam angles are -42.5°, -13°, +13°, and +42.5°. For an ideal array, these are -48.6°, -14.5°, 14.5° and 48.6°.
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Fig. 9. Photo of the Butler matrix with antenna array.
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Angle (degree) IV. FABRICATIONS AND MEASUREMENTS
Fig. 8. Simulated beam patterns of the Butler matrix antenna array.
It is worth mentioning that the elongated and meandered feed-lines at the inputs are undesirable structures, and bent sideways solely to facilitate the measurements using the eastwest oriented on-wafer probes. Four pairs of shorted stubs are placed to support the central conductors. This addition slightly broadens the bandwidth and barely affects the output magnitude. The lengths of the phase shifters and the interconnecting lines have been tuned to offset the phase change caused by the stubs. B. Antenna array The rectangular coaxial output ports of the Butler matrix are fed into a linear patch antenna array. The patch element is of a
A 0.3 mm thick silicon wafer was etched using DRIE to define the pattern of each of the five layers. After releasing from the handle wafer, the silicon slices were placed on a tilted revolving platform and evaporated with gold to a thickness of 2 μm. The coated silicon slices were then bonded on top of each other using a flip-chip bonder. The tethers were removed after bonding. Fig. 9 shows a photo of a constructed device. It has a size of 26 mm by 23 mm and a height of 1.5 mm. The scattering parameters of the 4-port device were measured using a two-port network analyzer. A pair of ground-signal-ground-ground-signal-ground (GSG-GSG) probes were used, with two device ports connected to the analyzer and the other two loaded with 50 Ω terminations. Fig. 10 shows the measured scattering parameters. The return
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losses at all ports are below -10 dB between 34.4 and 38.3 GHz. The inter-port isolations are below -25 dB. Some symmetries in the S-parameters have not been maintained well, which may be caused by uneven etching quality coupled with alignment errors during assembly. Fig. 11 shows the patterns of one beam from the in-house built free-space probebased radiation measurement. Good agreement with simulations has been shown over the angular range obtained. This range is limited due to the proximity of the probe and the station platform. Insufficient shielding of these objects is also believed to be the cause of the observed ripples in the patterns. All measured patterns were shifted up by 43.8 dB for normalization.
S-parameters (dB)
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show good output balance (less than 0.7 dB deviation) and neat phase increment (maximum deviation of 9.6°). The calculated insertion loss is about 0.5 dB. The return losses are better than -16 dB. The simulated beam patterns agree well with the design specification. The predicted maximum gain is 13 dBi. The measured scattering parameters show the return losses are below -10 dB from 34.4 to 38.3 GHz. The uneven etching quality and possible alignment errors may have compromised the structural symmetry of the device, causing some asymmetry in the S-parameters. It is expected that the insertion loss of the device will be higher than the simulated, as some extra losses due to imperfections in bonding and metallisation are not fully considered. The measured radiation pattern over the obtained angular range is in good agreement with simulation. The design and fabrication technique proposed in this paper can be applied for higher-order Butler matrix based on rectangular-coaxial or waveguide structures, at higher mm-wave frequencies.
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ACKNOWLEDGEMENT
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This work was funded by U.K. Engineering and Physical Science Research Council (EPSRC) under EP/D059933/1.
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REFERENCES 30
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Frequency (GHz) Fig. 10. Measured scattering parameters of the Butler matrix antenna array.
Simulated 38 GHz Measured 35 GHz 36 GHz 37 GHz 38 GHz
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V. DISCUSSIONS AND CONCLUSION A 4×4 Butler matrix with a linear antenna array has been built using silicon micromachining and multi-layer assembly techniques. The Butler matrix is based on a dielectric-free rectangular coaxial structure. The radiation element is a suspended patch antenna. The Butler matrix is designed to
[1] I. Llamas-Garro, A. Corona-Chavez, “Micromachined Transmission Lines for Millimeter-Wave Applications”, 2006 Int. Conf. on Electronics, Communications and Computers, pp. 15, Feb. 2006. [2] J. R. Reid, E. D. Marsh, and R. T. Webster, “Micromachined rectangularcoaxial transmission lines,” IEEE Trans. Microwave Theory Tech., vol. 54, no. 8, pp. 3433–3442, Aug. 2006. [3] D. S. Filipovic, Z. Popovic, K. Vanhille, M. Lukic, S. Rondineau, M. Buck, et al, “Modeling, design, fabrication, and performance of rectangular μ-coaxial lines and components,” 2006 IEEE MTT-S Int. Microwave Symp. Dig., pp. 1393-1396, Jun. 2006. [4] M. T. Stickel, P. C. Kremer, G. V. Eleftheriades, “High-Q silicon micromachined cavity resonators at 30GHz using the split-block technique,” IEE Proc. Microw. Antennas Propag., vol. 151, no. 5, pp. 450-454, Oct. 2004. [5] M. J. Lancaster, J. Zhou, M. Ke, Y. Wang, K. Jiang, “Design and High Performance of a Micromachined K-Band Rectangular Coaxial Cable”, IEEE Trans. Microw. Theo. Tech., vol. 55, no. 7, pp. 1548-1553, 2007. [6] C. Tseng, C. Chen, T. A. Chu, “Low-Cost 60-GHz Switched-Beam Patch Antenna Array with Butler Matrix Network”, IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 432-435, 2008. [7] M. Nedil, T. A. Denidni, “Design of a new millimeter-wave Butler matrix”, 2006 IEEE AP-S Int. Symp. Dig., pp. 841-844, Jul. 2006. [8] M. V. Lukic, D. S. Filipovic, “Integrated cavity-backed Ka-band phased array antenna”, 2007 IEEE AP-S Int. Symp. Dig., pp. 133-136, Jun. 2007. [9] Y. Wang, M. Ke, M. J. Lancaster, “Micromachined 38GHz Cavity Resonator and Filter with Rectangular-Coaxial Feed-lines”, IET Microw., Antennas Propag., vol. 3, no. 1, pp. 125-129, Feb. 2009. [10] Y. Wang, M. Ke, M. J. Lancaster, F. Huang, “Micromachined Millimeter-wave Rectangular-Coaxial Branch-Line Couplers with Enhanced Bandwidth”, IEEE Trans. Microwave Theory Tech., to be published. [11] CST Microwave Studio, CST GmbH, Germany 2006. [12] P. Bhartia, K. V. S. Rao, R. S. Tomar, Millimeter-wave microstrip and printed circuit antennas, Artech House, Inc., 1991
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