A Millimeter Wave Switched Beam Planar Antenna Array Ali T. Alreshaid, Oualid Hammi, and Mohammad S. Sharawi Electrical Engineering Department
Kamal Sarabandi Electrical Engineering Department University of Michigan, An Arbor, MI 48109, USA,
[email protected]
King Fahd University of Petroleum and Minerals (KFUPM)
Dhahran, 31261, Saudi Arabia {Alreshaid, ohammi, msharawi}@kfupm.edu.sa Abstract—A millimeter wave (mm-wave) beam-forming network based on the Butler matrix is designed alongside with its planar antenna array. The designed system has 1 GHz of BW centered at 28.5 GHz. A planar 4x4 slot based antenna array is connected to the Butler matrix via optimized series feeds. The beams radiated from the antenna array were steered to four different locations; ±20˚ and ±45˚. The maximum gain achieved was 12 dB and the minimum was 8 dB. The complete system had a size of 43 x 35 x 0.13 mm3. This system can be used in handheld devices for the up-coming 5G standards.1
I.
INTRODUCTION
With a lifespan of less than a decade for the previous communication technologies, it is anticipated that the current 4G-LTE will reach a point where it will no longer provide sufficient data rates needed by 2020. Extensive measurements have been conducted at two mm-wave bands; 28 and 38 GHz, and the results were promising [1]. This will nominate mmwaves as a solution to provide wider BW and higher data rate communications. Works have provided complete integrated mm-wave solutions in literature. For example, a linear QuasiYagi antenna array was integrated with Butler and operated at 60 GHz with 5.8% BW was presented in [2]. A linear slot antenna array mounted on substrate-integrated waveguide (SIW) was operating with Butler network at 60 GHz with a BW of 4.5 GHz in [3]. The gain achieved was 22 dBi with a HPBW greater than 2 3 radian, while the radiating efficiency was 68%. Almost all works focused on 60 GHz band with 4element linear arrays. The most common linear antenna array was based on microstrip patches, like the one designed in [4]. The linear antenna array operated at 60.5 GHz with a maximum gain of 8.9 dB. No work has ever used 2dimensional antenna array based on microstrip slot antenna elements. In this work, a switched beam mm-wave planar antenna array is proposed. It consists of a 4x4 grid of slot antenna elements placed at the corner of the system's ground plane and operating at a center frequency of 28.5 GHz. A 4x4 Butler matrix is used to feed the array via optimized series feeds that feed every 4-antenna elements at a time to reduce the feeding complexity. 1
This work was supported by DSR at KFUPM under project NO. RG1332.
978-1-4799-7815-1/15/$31.00 ©2015 IEEE
II.
MM-WAVE SWITCHED BEAM ARRAY DESIGN
The complete system shown in Fig. 1 operates at a center frequency of 28.5 GHz and has a compact size of 35 x 43 x 0.13 mm3. The proposed antenna system consists of two major parts; the beam-forming feed network and the planar array. The Butler feed network is a reciprocal passive NxN structure. The major consisting elements are couplers and crossovers, which can be implemented by several designs. For this work, a planar design is required and chosen. The number of elements needed will depend on the number of output and input ports. For a 4x4 Butler network, four Hybrid couplers and two crossovers are needed. An ideal value of -6dB of received power is detected at all of the output ports when one of the input port is excited. Also, the phase difference between the output ports must follow certain patterns for the Butler to operate. One possible combination is to have phase differences of -45˚, 135˚, -135˚ and 45˚ between the output ports when ports 1, 2, 3 or 4 are excited, respectively. 35 mm 5.6 mm
50Ω @ 28.5 GHz
5.6 mm
4x4 Slot Antenna Array
43 mm
Crossover Port 1
Port 2
Port 3
Hybrid Coupler
Butler
Port 4
Fig. 1 Complete Switched Beam mm-wave Antenna Array. Figure inset shows the Top layer of the fabricated Antenna Array
For the system to operate at 28.5 GHz, the dimensions of the Hybrid coupler were optimized to 2.3 mm by 2.6mm. Two couplers are cascaded by a 1.05mm line to form the crossover.
2117
AP-S 2015
The microstrip based slot antenna is commonly used in applications where an antenna is placed on fast moving objects, like airplanes, since it will not interfere with the aerodynamic properties of the structures; the slot could be filled with a dielectric where it will stop the flow of air through it. It is chosen to be the radiating element instead of the patch antenna due to its wider BW and compact size. Its only drawback is the relatively lower gain that it has compared to the patch, which can be compensated by using antenna arrays. [5] The feeding mechanism is done through aperture coupling; the microstrip line will be slightly extended over an opening slot in the ground. The feeding point should be selected such that the capacitive load will cancel out the inductive part and we will have resonance. It is found that the feeding point will be almost 5% of a wavelength from one edge of the slot. The reflection coefficient of a single slot is showing a BW of almost 4.4 GHz as shown in Fig. 2.
Fig. 3 mm-wave Butler Matrix response when port 1 is excited; (a) magnitude, (b) phase
Fig. 4 Radiation Pattern when port (a) 1 (b) 2 is excited
IV. Fig. 2. Reflection Coefficient of a single slot antenna
III.
CONCLUSIONS
A beam-forming network based on Butler has been integrated with a 4x4 slot antenna array and operated at 28.5 GHz with a BW of 1 GHz. The system gain ranged from 8 to 11.4 dB and steered to four different angles; ±20˚ and ±45˚.
RESULTS AND DISCUSSION
The simulation results are shown for ports 1 & 2 only. Ports 3 & 4 are identical due to the structure's symmetry. When designing the Butler, there was a tradeoff between having equal power split at the output ports and having properly low input reflection coefficients at 28.5 GHz. The detected power levels at the outputs ranged from 5 to 9.5 dB for both inputs. When port 1 is excited, an average error of 9.8 degrees takes place while it is 19.7˚ for port 2. Although the phase and magnitude variations seem large, the effect of the beam location and array gain was comparable to the cases with ideal excitation. For the radiation patterns acquired from the system, the maximum gain achieved was 11.2 dB when port 1 is excited and it is directed toward θ = 20˚. While a gain of 8 dB is obtained from port 2 and tilted 45˚ from the z-axis. Both cases are shown in Fig. 4. The integrated system has shown a steady performance over 1 GHz of BW, which was limited by the phase factor, where the phase error considerably deviated from the tolerated range and increased the back-lobe radiation when the frequency extended beyond the 1 GHz bandwidth.
REFERENCES [1] T. S. Rappaport, S. Sun, R. Mayzus, H. Zhao, Y. Azar, K. Wang, G. N. Wong, J. K. Schulz, M. Samimi and F. Gutierrez, "Millimeter Wave Mobile Communications for 5G Cellular: It Will Work!," IEEE Access, pp. 335 - 349, 2013. [2] C. E. Patterson, W. T. Khan, G. E. Ponchak, G. S. May and J. Papapolymerou, "A 60-GHz Active Receiving Switched-Beam Antenna Array With Integrated Butler Matrix and GaAs Amplifiers," IEEE Transactions on Microwave Theory and Techniques, vol. no. 99, p. 1 –10, 2012. [3] F. F. He, K. Wu, W. Hong, L. Han and X.-P. Chen, "Low-Cost 60-GHz Smart Antenna Receiver Subsystem Based on Substrate Integrated Waveguide Technology," Microwave Theory and Techniques, IEEE Transactions vol: 60, issue: 4, pp. 1156 - 1165, 2012. [4] C.-H. Tesng, C.-J. Chen and T.-H. Chu, "A Low-Cost 60-GHz Switched-Beam Patch Antenna Array With Butler Matrix Network," IEEE Antennas and Wireless Propagation Letters, vol. 7, p. 432 –435, 2008. [5] C. A. Balanis, Antenna Theory Analysis and Design, John Wiley & Son, Inc., 1997.
2118
