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A switched beam antenna array with butler matrix network using substrate integrated waveguide technology for 60 GHz wireless communications
Int. J. Electron. Commun. (AEÜ) 70 (2016) 850–856
Contents lists available at ScienceDirect
International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
A switched beam antenna array with butler matrix network using substrate integrated waveguide technology for 60 GHz wireless communications Nishesh Tiwari ∗ , Thipparaju Rama Rao Department of Telecommunication Engineering, SRM University, Chennai 603203, India
a r t i c l e
i n f o
Article history: Received 30 October 2015 Accepted 21 March 2016 Keywords: Antenna design Butler matrix Millimeter wave Substrate integrated waveguide Switched beam 60 GHz
a b s t r a c t The 60 GHz band has a lot of potential for high speed multi-gigabit wireless communications. In this paper, a switched beam antenna array based on substrate integrated waveguide (SIW) technology is designed and simulated at 60 GHz band. The antenna array is fed by 4 × 4 planar butler matrix network in order to achieve the switched beam characteristic. The use of SIW technology offers a low cost and a planar design. Each of the components is designed and simulated in the electromagnetic simulation tool CST MWS. The components are integrated later to form the switched beam antenna. Then the designed switched beam antenna is fabricated for measurement. The simulated results agree well with the measured values which validates the proposed design. The return losses and isolations are lower than −10 dB from 57 GHz to 64 GHz for all of the input ports. The peak gain for the switched beam antenna array is 17.5 dBi at 60 GHz. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction In today’s world the bandwidth requirement for high speed communication is growing at a steady pace. The availability of high bandwidth in 60 GHz band is becoming a highly attractive option for high speed wireless communications allowing transfer of uncompressed data, voice and video at the speed of gigabit per second [1,2]. At millimeter wave frequency band the losses in the planar microstrip circuit is high. Therefore this requires more efficient technology like the substrate integrated waveguide (SIW) to be used, which provides advantages of the traditional rectangular waveguide such as low loss, high quality factor, complete shielding and capability of handling high power along with the combined advantage of planar circuit designs [3,4]. Numerous research works involving SIW have been reported for many years [3–6]. SIW structures have proved to be a good choice for the construction of millimeter wave filter, beamforming network (BFN) and multibeam antenna, which includes techniques like SIW based butler matrix, blass matrix, rotman lens etc. As compared to Blass and Nolen matrix, the Butler matrix requires the least number of couplers [7]. Butler matrix has been widely used in radar, warfare and satellite applications. In [4] Cheng et al. have developed and shown
∗ Corresponding author. E-mail addresses: [email protected] (N. Tiwari), [email protected] (T.R. Rao). http://dx.doi.org/10.1016/j.aeue.2016.03.014 1434-8411/© 2016 Elsevier GmbH. All rights reserved.
an optimized SIW based butler matrix with slot antennas with low side lobes. In [8] Tseng et al. have developed 60 GHz switched beam patch antenna array with butler matrix designed using microstrip lines which provides peak gain of 8.9 dBi. In [9] Patterson et al. have designed a switched-beam antenna with butler matrix on an organic liquid crystal polymer (LCP) platform. In [10] Mohamed Ali et al. have designed two layer butler matrix for Ku band applications. The design approach helps in reduction of overall size of the matrix and can be fitted in small size subsystems. In [11] Djerafi et al. have designed SIW butler matrix for operating frequency of 12.5 GHz. It uses a cruciform structure for coupler and crossover. Also, curved SIW transmission lines with added discontinuities are used as phase shifters in [11]. In [12] Tekkouk et al. have developed 60 GHz switched beam antenna array using dual layer ridge waveguide technology with side lobe level control circuit. In this paper, simple and modular design approach is used for the design of switched beam antenna at 60 GHz. At 60 GHz the length of the crossover is quite small which makes it almost impossible to have straight delay line to provide the required phase shift. One approach can be to cascade two 90◦ hybrid couplers to make a crossover such that the length of the crossover increases but this approach also increases the overall circuit size and complicates the BFN design. Another possible option is to use a curved delay line to achieve the required phase shift. Hence in this paper, uneven curved SIW phase shifters without any added discontinuities have been used to design a switched beam antenna for 60 GHz wireless communications. The switched beam antenna array structure
N. Tiwari, T.R. Rao / Int. J. Electron. Commun. (AEÜ) 70 (2016) 850–856
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is designed on a Rogers RT/Duroid 5880 substrate with a dielectric constant of 2.2 and thickness of 0.254 mm and copper cladding of 0.035 mm. It is designed, simulated and verified utilizing the electromagnetic simulation tool CST MWS. The design is fabricated and measured for validating the proposed switched beam antenna. 2. Design of butler matrix Among the entire available BFNs butler matrix is one of the most popular multiple BFN. The butler matrix in its conventional form is used for connection of 2n elements to beam ports of equal number i.e. 2n . The 4 × 4 butler matrix design has four input ports and four output ports. Exciting each input port provides a different output beam as the relative phases across the output ports change. The general block diagram of 4 × 4 butler matrix is shown in the Fig. 1. The 4 × 4 butler matrix has four 3 dB 90◦ hybrid couplers, two crossovers and phase shifters. 2.1. Hybrid coupler The Riblet short slot coupler [13] can be designed in SIW technology by removing the common side wall which is similar to design in rectangular waveguides. This is because SIW has electrical similarities to the rectangular waveguide. SIW has two rows of metallic vias embedded in the dielectric substrate which acts as waveguide by connecting the two parallel metal plates. The diameter of the via and the space between the vias should be chosen as per (1) and (2) respectively [14]. Dvia < g /5
(1)
S ≤ 2Dvia
(2)
where g is the guided wavelength, Dvia is the diameter of the via and S is the space between the vias. The effective width of the waveguide is given by (3) [15]. Weff = Wsiw − 1.08(Dv2ia /S) + 0.1(Dv2ia /Wsiw )
(3)
Fig. 2. Hybrid coupler.
where Weff is the effective width, Wsiw is the width of the SIW, Dvia is the diameter of the via which is 0.4 mm and S is the space between the vias which is 0.7 mm. The cut off frequency for the SIW in TE mode can be found by the following equation, √ fc,mn = c/2 εr
2
(m/a) + (n/b)
2
(4)
where fc is the cut-off frequency, m and n are the mode numbers, εr is the dielectric constant, a is the width of the waveguide and b is the height. The diagram of the 3 dB hybrid coupler is shown in Fig. 2, where Lc = 3.68 mm and Wc = 4.86 mm. The Riblet short slot coupler is basically a 3 dB coupler. It is structurally similar to two waveguides joined laterally. A portion of the common side wall is removed such that the coupling of the waves in both of the waveguides occurs. The output signals from the two output ports are equal in amplitude and 90◦ out of phase. It is also known as 3 dB 90◦ hybrid coupler. Port 1 is the input port, port 2 is the transmitted port, port 3 is the coupled port and port 4 is the isolated port. The geometry of the coupler is based on the even and odd mode analysis, where TE10 and TE20 is the even and odd mode respectively. The difference in phase between the two modes is given by (5) ˚ = (ˇe − ˇo )ls
(5)
where ˚ is the phase difference, ˇe and ˇo are the phase constants of TE10 and TE20 modes. Fig. 3 shows the S-parameters of the 3 dB coupler and also the phase at output ports of the coupler. From Fig. 3 it is observed that the return loss and isolation is below −10 dB from 57 GHz to 64 GHz. Also, S21 and S31 are almost equal in magnitude.
Fig. 1. Butler matrix block diagram with antenna array.
Fig. 3. Simulated S-parameters and phase of hybrid coupler.
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Fig. 4. Crossover. Fig. 6. Phase shifters (a) 0◦ and (b) 45◦ .
Similarly from Fig. 3 it is also observed that S21 and S31 have phase difference of around 90◦ throughout the entire 60 GHz band. 2.2. Crossover/0 dB coupler The SIW crossover is a 0 dB coupler and is shown in Fig. 4, where Lc = 6.93 mm and Wc = 4.86 mm. It is also a four port device in which the signal from port 1 is transferred to port 3 and signal from port 4 is transferred to port 2. The crossover is also designed using (5). The length of slot region or the common wall removed in this design is naturally larger as the difference in phase between the TE10 and TE20 modes required for 0 dB coupler is 180◦ . Fig. 5 shows the magnitude and phase at the ports of the crossover. From Fig. 5 it is observed that S11, S21 and S41 are below −10 dB. Also from Fig. 5 it is noted that at 60 GHz the phase is 129.2◦ .
length of the line. The phase shift is obtained in this design using curved transmission line as it is not possible to achieve the required phase shift of 0◦ and 45◦ using straight transmission line. In [11] the accuracy of the phase shifter is reported to be better than 5◦ after adding discontinuities to the curved SIW transmission line. In this work, good accuracy is observed without any added discontinuities in the curved SIW phase shifters. Fig. 6 shows the 0◦ and 45◦ phase shifter respectively.
2.3. Phase shifters Phase shifters are widely used in microwave networks. Delay lines are the simplest of the phase shifters. The phase delay of a transmission line is given by (6). ˚ = kz l
(6)
where ˚ is the phase delay, kz is the propagation constant of the propagating mode and l is the length of the line. The phase difference between the two delay lines of unequal width and equal length is given by (7). ˚ = (kz2 − kz1 )l
(7)
where ˚ is the phase delay, kz2 and kz1 are the propogation constants of the propagating mode at different width and l is the
Fig. 5. Simulated S-parameters and phase at output of crossover.
Fig. 7. Simulated phase difference between phase shifters and crossover.
Fig. 8. Simulated magnitude and phase of 0◦ and 45◦ phase shifter.
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Fig. 9. SIW butler matrix structure.
In Fig. 6, Lp = 1.84 mm, Wp = 2.96 mm for 0◦ phase shifter and for 45◦ phase shifter Lp = 1.9 mm, Wp = 2.96 mm. The width of the SIW in phase shifters is also increased slightly to achieve the phase shifts. The 45◦ phase shifter is more curved than the 0◦ phase shifter. In this design 45◦ phase shifter is projected more outward than 0◦ phase shifter. Fig. 7 shows the simulated phase difference between the phase shifters and the crossover. The accuracy of the phase shifters from the simulation results is observed to be as high as 1.38◦ . The simulated accuracy of the phase shifter is found to be better than 1.4◦ for 0◦ phase shifter and better than 4◦ for 45◦ phase shifter. The magnitude and phase of the 0◦ and 45◦ phase shifter is shown in Fig. 8. From Fig. 8 it is observed that the return loss for both phase shifters is below −10 dB for the entire 60 GHz band. Also, from Fig. 8 it is observed that the phase difference between the phase shifters is almost 45◦ at 60 GHz. 2.4. Butler matrix Fig. 11. Simulated phase at the output of butler matrix when port 1 is excited.
The butler matrix is designed by integrating the components designed above. The performance of the butler matrix is verified through simulation in CST MWS. Fig. 9 shows the designed butler matrix. Fig. 10 shows the magnitude at the ports of the butler matrix when port 1 is excited and Fig. 11 shows the relative phases at the output ports when port 1 is excited. Similarly, Fig. 12 shows the magnitude at the ports when port 2 is excited and Fig. 13 shows the relative phases at the output ports when port 2 is excited.
Fig. 10. Simulated S-parameters of butler-matrix when port 1 is excited.
From Fig. 10 it is observed that when port 1 is excited, the return loss is lower than −10 dB for the entire band from 57 GHz to 64 GHz. Similarly, S(2,1) is observed to be below −13 dB, S(3,1) is below −20 dB and S(4,1) is below −11 dB from 57 GHz to 64 GHz. The magnitudes of output at port 5, 6, 7 and 8 are also observed to be in acceptable range.
Fig. 12. Simulated S-parameters of butler-matrix when port 2 is excited.
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Fig. 15. Simulated radiation pattern of slot antenna.
Fig. 13. Simulated phase at the output of butler matrix when port 2 is excited.
Though equal power distribution would be ideal but due to crossovers and other losses some variation is observed in their magnitude. Further, from Fig. 11 it is observed that the relative phase differences between the output ports are around −45◦ , −90◦ and −135◦ for ports 6, 7 and 8 with respect to port 5 when port 1 is excited. The values mentioned above are ideal values for the butler matrix. Some phase variation from ideal values of phase difference is observed in the output ports of the butler matrix. This can be due to the design of butler matrix where the signal has to cross through crossover and also ideal phase shift is hard to be achieved using the curved delay line for phase shifting. However it is observed that they are within the acceptable range. From Fig. 12 it is observed that when port 2 is excited, the return loss is lower than −13 dB for the entire band from 57 GHz to 64 GHz. Similarly, S(1,2) is observed to be below −12 dB, S(3,2) is below −15 dB and S(4,2) is below −20 dB from 57 GHz to 64 GHz. The magnitudes of output at port 5, 6, 7 and 8 are also observed to be satisfactory. Further from Fig. 13 it is observed that the phase differences between the output ports are around 135◦ , −90◦ and 45◦ for ports 6, 7 and 8 with respect to port 5 which are the ideal values for a butler matrix. 3. Switched beam slot antenna The waveguide longitude slot antennas are popular for beam steering applications. The diagram of slot antenna is shown in Fig. 14, where Ws = 0.198 mm, Ls = 1.789 mm, Es = 2.1 mm and slots displacement from the center is 0.17 mm. Many researchers have worked on slot antennas for a long time [16,17]. The values chosen for the length, width and offset from the center of the waveguide plays a vital role in the design and performance of the antenna. The antenna was designed as per the design equations mentioned in [17] using SIW technology with cylindrical vias which act as the side wall of the waveguide.
Fig. 14. Slot antenna schematic.
The theoretical values are used in the design of the antenna slots and further it is optimized through CST MWS simulations. The designed antenna was simulated in the simulation tool for analyzing the radiation pattern so that a symmetric radiation pattern is observed. Fig. 15 shows the radiation pattern of the single antenna. E-plane and H-plane radiation pattern is shown in the figure. It is observed that the gain is 13.7 dBi. The beamwidth of the single antenna is wide in E-plane as compared to the H-plane. Fig. 16 shows the 3D radiation pattern of the slot antenna in CST MWS. Four of such antenna elements are used for connecting to the four output ports of the butler matrix. Further the microstrip to SIW transition is also integrated into the design. Fig. 17 shows the fabricated switched beam antenna with the dimension of 74.3 mm × 56 mm. Fig. 18 shows the simulated and measured S-parameters of the switched beam antenna when port 1 is excited. It is observed that S11, S21, S31 and S41 are all below −10 dB from 57 GHz to 64 GHz. Fig. 19 shows the simulated and measured S-parameters of the switched beam antenna when port 2 is excited in which S12, S22, S32 and S42 are all observed to be below −10 dB for the entire 60 GHz band. The simulated and measured S-parameters are found to be similar. Further the simulated and measured radiation pattern of the switched beam antenna is shown in Fig. 20. The measured gain is observed to be 16.6 dBi when port 2 and port 3 is excited. However, the measured gain is 17.5 dBi when port 1 and port 4 is excited. Further, it is observed that the main beam is directed toward the +15◦ and −15◦ when port 1 and port 4 is excited respectively. It shifts to −41◦ and +41◦ when port 2 and port 3 is excited respectively. The 4 beams together cover the total angle range of 112◦
Fig. 16. Simulated 3D radiation pattern.
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Fig. 17. Fabricated switched beam antenna.
Fig. 19. Simulated and measured S-parameters when port 2 is excited.
Fig. 18. Simulated and measured S-parameters when port 1 is excited.
considering the 3 dB beam-width. Further, it can be seen that the measured values and CST MWS simulation results are similar which validates the proposed switched beam antenna design. Table 1 shows the comparison with some other switched beam antennas available in literature for 60 GHz band. In [8] the switched beam antenna consists of patch antenna array and microstrip lines are used to design the BFN. In [9] the quasi yagi dipole antenna array is used along with the BFN designed using microstrip lines. Similarly, in [12] ridged waveguide technology is used to design the switched beam antenna consisting of slotted waveguide antenna array and a BFN. In this work the switched beam antenna is designed using SIW technology. The switched beam antenna consists of SIW slotted waveguide array and a BFN with uneven curved delay lines as phase shifters. One of the advantages of the proposed design is that since it is based on SIW technology it has low loss and it is efficient than microstrip lines because at millimeter wave frequencies the losses in microstrip lines are high.
Fig. 20. Simulated and measured radiation pattern when different ports are excited.
Also, the design proposed in this paper is light weight and compact as compared to the design in [12] because ridged waveguide is bulkier than single layer SIW. Further, the proposed design can be fabricated using single layer PCB manufacturing process so it is economical for mass production. The main drawback of the proposed design is that the side lobe level (SLL) is high as compared to the design in [12] which uses additional SLL control circuit for obtaining low SLL. Also, the gain is less as compared to [9,12]. However, overall the proposed switched beam antenna displays a good performance at 60 GHz and is suitable for high speed multi-gigabit communications.
Table 1 Comparison with other switched beam antenna for 60 GHz band. Switched beam antenna
[8]
[9]
[12]
This work
Beamforming network used Scan angle Antenna type Technology Peak gain
Butler matrix −40◦ to +40 Patch antenna Microstrip lines (PCB) 8.9 dBi
Butler matrix −40◦ to +40◦ Quasi-Yagi dipole Microstrip lines (LCP) 27.5 dBi
Butler matrix −43◦ to +43◦ Slotted wave-guide antenna Ridged wave-guide 26.25 dBi
Butler matrix −41◦ to +41◦ Slotted wave-guide antenna SIW (PCB) 17.5 dBi
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4. Conclusion A planar 60 GHz switched beam antenna array has been designed and simulated. The butler matrix used for the feeding network shows good performance over the 60 GHz band. The proposed design has been validated through simulated and measured results which agree well with each other. Good dispersion characteristics are seen in the designed matrix considering the reflection, isolation, insertion and phase characteristics in the 60 GHz band. The designed multi-beam antenna array is small in size. It is fabricated using low cost technology as the ones used for printed circuit boards. It is suitable for applications requiring beam steering in the 60 GHz band and could be a good candidate for system-onsubstrate applications. Acknowledgment Authors are very much obliged to ISRO, Government of India, for the assistance provided for the execution of this research work. References [1] Smulders P. Exploring the 60 GHz band for local wireless multimedia access: prospects and future directions. IEEE Commun Mag 2002;40:140–7. [2] Khan MN, Rizvi UR. Antenna beam-forming for a 60 GHz transceiver system. Arab J Sci Eng 2013;38:2451–64. [3] Bozzi M, Perregrini L, Wu K, Arcioni P. Current and future research trends in substrate integrated waveguide technology. Radioengineering 2009;18:201–9. [4] Cheng YM, Chen P, Hong W, Djerafi T, Ke W. Substrate integrated waveguide beamforming networks and multibeam antenna arrays for low-cost satellite and mobile systems. IEEE Antennas Propag Mag 2011;53:18–30.
[5] Wu LS, Mao JF, Shen W, Yin WY. Extended doublet bandpass filters implemented with microstrip resonator and full-/half-mode substrate integrated cavities. Prog Electromagn Res 2010;108:433–47. [6] Bakhtafrooz A, Borji A, Busuioc D, Safavi Naeini S. Novel two-layer millimeterwave slot array antennas based on substrate integrated waveguides. Prog Electromagn Res 2010;109:475–91. [7] Hall PS, Vetterlein SJ. Review of radio frequency beamforming techniques for scanned and multiple beam antennas. IEE Proceedings H Microw Antennas Propag 1990;137:293–303. [8] Tseng CH, Chen CJ, Chu TH. A low-cost 60-GHz switched-beam patch antenna array with butler matrix network. IEEE Antennas Wirel Propag Lett 2008;7:432–5. [9] Patterson CE, Khan WT, Ponchak GE, May GS, Papapolymerou J. A 60-GHz active receiving switched-beam antenna array with integrated butler matrix and GaAs amplifiers. IEEE Trans Microw Theory Tech 2012;60:3599–607. [10] Mohamed Ali AA, Fonseca NJG, Coccetti F, Aubert H. Design and implementation of two-layer compact wideband butler matrices in SIW technology for Ku-band applications. IEEE Trans Antennas Propag 2011;59:503–12. [11] Djerafi T, Fonseca NJG, Wu K. Design and implementation of a planar 4 × 4 butler matrix in SIW technology for wide band high power applications. Prog Electromagn Res B 2011;35:29–51. [12] Tekkouk K, Hirokawa J, Sauleau R, Ettorre M, Sano M, Ando M. Duallayer ridged waveguide slot array fed by a butler matrix with sidelobe control in the 60 GHz band. IEEE Trans Antennas Propag 2015, http://dx.doi.org/10.1109/TAP.2015.2442612. [13] Riblet HJ. The short-slot hybrid junction. Proc IRE 1952;40:180–4. [14] Wu Ke, Deslandes D, Cassivi Y. The substrate integrated circuits – a new concept for high-frequency electronics and optoelectronics. In: Telecommunications in Modern Satellite, Cable and Broadcasting Service TELSIKS, vol. 1. 2003. p. III–X. [15] Bozzi M, Georgiadis A, Wu K. Review of substrate-integrated waveguide circuits and antennas. IET Microw Antennas Propag 2011;5:909–20. [16] Yan L, Hong W, Hua G, Chen JX, Wu K, Cui TJ. Simulation and experiment on SIW slot array antennas. IEEE Microw Wirel Compon Lett 2004;14:446–8. [17] Elliott RS. An improved design procedure for small arrays of shunt slots. IEEE Trans Antennas Propag 1983;31:48–53.
Contents lists available at ScienceDirect
International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
A switched beam antenna array with butler matrix network using substrate integrated waveguide technology for 60 GHz wireless communications Nishesh Tiwari ∗ , Thipparaju Rama Rao Department of Telecommunication Engineering, SRM University, Chennai 603203, India
a r t i c l e
i n f o
Article history: Received 30 October 2015 Accepted 21 March 2016 Keywords: Antenna design Butler matrix Millimeter wave Substrate integrated waveguide Switched beam 60 GHz
a b s t r a c t The 60 GHz band has a lot of potential for high speed multi-gigabit wireless communications. In this paper, a switched beam antenna array based on substrate integrated waveguide (SIW) technology is designed and simulated at 60 GHz band. The antenna array is fed by 4 × 4 planar butler matrix network in order to achieve the switched beam characteristic. The use of SIW technology offers a low cost and a planar design. Each of the components is designed and simulated in the electromagnetic simulation tool CST MWS. The components are integrated later to form the switched beam antenna. Then the designed switched beam antenna is fabricated for measurement. The simulated results agree well with the measured values which validates the proposed design. The return losses and isolations are lower than −10 dB from 57 GHz to 64 GHz for all of the input ports. The peak gain for the switched beam antenna array is 17.5 dBi at 60 GHz. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction In today’s world the bandwidth requirement for high speed communication is growing at a steady pace. The availability of high bandwidth in 60 GHz band is becoming a highly attractive option for high speed wireless communications allowing transfer of uncompressed data, voice and video at the speed of gigabit per second [1,2]. At millimeter wave frequency band the losses in the planar microstrip circuit is high. Therefore this requires more efficient technology like the substrate integrated waveguide (SIW) to be used, which provides advantages of the traditional rectangular waveguide such as low loss, high quality factor, complete shielding and capability of handling high power along with the combined advantage of planar circuit designs [3,4]. Numerous research works involving SIW have been reported for many years [3–6]. SIW structures have proved to be a good choice for the construction of millimeter wave filter, beamforming network (BFN) and multibeam antenna, which includes techniques like SIW based butler matrix, blass matrix, rotman lens etc. As compared to Blass and Nolen matrix, the Butler matrix requires the least number of couplers [7]. Butler matrix has been widely used in radar, warfare and satellite applications. In [4] Cheng et al. have developed and shown
∗ Corresponding author. E-mail addresses: [email protected] (N. Tiwari), [email protected] (T.R. Rao). http://dx.doi.org/10.1016/j.aeue.2016.03.014 1434-8411/© 2016 Elsevier GmbH. All rights reserved.
an optimized SIW based butler matrix with slot antennas with low side lobes. In [8] Tseng et al. have developed 60 GHz switched beam patch antenna array with butler matrix designed using microstrip lines which provides peak gain of 8.9 dBi. In [9] Patterson et al. have designed a switched-beam antenna with butler matrix on an organic liquid crystal polymer (LCP) platform. In [10] Mohamed Ali et al. have designed two layer butler matrix for Ku band applications. The design approach helps in reduction of overall size of the matrix and can be fitted in small size subsystems. In [11] Djerafi et al. have designed SIW butler matrix for operating frequency of 12.5 GHz. It uses a cruciform structure for coupler and crossover. Also, curved SIW transmission lines with added discontinuities are used as phase shifters in [11]. In [12] Tekkouk et al. have developed 60 GHz switched beam antenna array using dual layer ridge waveguide technology with side lobe level control circuit. In this paper, simple and modular design approach is used for the design of switched beam antenna at 60 GHz. At 60 GHz the length of the crossover is quite small which makes it almost impossible to have straight delay line to provide the required phase shift. One approach can be to cascade two 90◦ hybrid couplers to make a crossover such that the length of the crossover increases but this approach also increases the overall circuit size and complicates the BFN design. Another possible option is to use a curved delay line to achieve the required phase shift. Hence in this paper, uneven curved SIW phase shifters without any added discontinuities have been used to design a switched beam antenna for 60 GHz wireless communications. The switched beam antenna array structure
N. Tiwari, T.R. Rao / Int. J. Electron. Commun. (AEÜ) 70 (2016) 850–856
851
is designed on a Rogers RT/Duroid 5880 substrate with a dielectric constant of 2.2 and thickness of 0.254 mm and copper cladding of 0.035 mm. It is designed, simulated and verified utilizing the electromagnetic simulation tool CST MWS. The design is fabricated and measured for validating the proposed switched beam antenna. 2. Design of butler matrix Among the entire available BFNs butler matrix is one of the most popular multiple BFN. The butler matrix in its conventional form is used for connection of 2n elements to beam ports of equal number i.e. 2n . The 4 × 4 butler matrix design has four input ports and four output ports. Exciting each input port provides a different output beam as the relative phases across the output ports change. The general block diagram of 4 × 4 butler matrix is shown in the Fig. 1. The 4 × 4 butler matrix has four 3 dB 90◦ hybrid couplers, two crossovers and phase shifters. 2.1. Hybrid coupler The Riblet short slot coupler [13] can be designed in SIW technology by removing the common side wall which is similar to design in rectangular waveguides. This is because SIW has electrical similarities to the rectangular waveguide. SIW has two rows of metallic vias embedded in the dielectric substrate which acts as waveguide by connecting the two parallel metal plates. The diameter of the via and the space between the vias should be chosen as per (1) and (2) respectively [14]. Dvia < g /5
(1)
S ≤ 2Dvia
(2)
where g is the guided wavelength, Dvia is the diameter of the via and S is the space between the vias. The effective width of the waveguide is given by (3) [15]. Weff = Wsiw − 1.08(Dv2ia /S) + 0.1(Dv2ia /Wsiw )
(3)
Fig. 2. Hybrid coupler.
where Weff is the effective width, Wsiw is the width of the SIW, Dvia is the diameter of the via which is 0.4 mm and S is the space between the vias which is 0.7 mm. The cut off frequency for the SIW in TE mode can be found by the following equation, √ fc,mn = c/2 εr
2
(m/a) + (n/b)
2
(4)
where fc is the cut-off frequency, m and n are the mode numbers, εr is the dielectric constant, a is the width of the waveguide and b is the height. The diagram of the 3 dB hybrid coupler is shown in Fig. 2, where Lc = 3.68 mm and Wc = 4.86 mm. The Riblet short slot coupler is basically a 3 dB coupler. It is structurally similar to two waveguides joined laterally. A portion of the common side wall is removed such that the coupling of the waves in both of the waveguides occurs. The output signals from the two output ports are equal in amplitude and 90◦ out of phase. It is also known as 3 dB 90◦ hybrid coupler. Port 1 is the input port, port 2 is the transmitted port, port 3 is the coupled port and port 4 is the isolated port. The geometry of the coupler is based on the even and odd mode analysis, where TE10 and TE20 is the even and odd mode respectively. The difference in phase between the two modes is given by (5) ˚ = (ˇe − ˇo )ls
(5)
where ˚ is the phase difference, ˇe and ˇo are the phase constants of TE10 and TE20 modes. Fig. 3 shows the S-parameters of the 3 dB coupler and also the phase at output ports of the coupler. From Fig. 3 it is observed that the return loss and isolation is below −10 dB from 57 GHz to 64 GHz. Also, S21 and S31 are almost equal in magnitude.
Fig. 1. Butler matrix block diagram with antenna array.
Fig. 3. Simulated S-parameters and phase of hybrid coupler.
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Fig. 4. Crossover. Fig. 6. Phase shifters (a) 0◦ and (b) 45◦ .
Similarly from Fig. 3 it is also observed that S21 and S31 have phase difference of around 90◦ throughout the entire 60 GHz band. 2.2. Crossover/0 dB coupler The SIW crossover is a 0 dB coupler and is shown in Fig. 4, where Lc = 6.93 mm and Wc = 4.86 mm. It is also a four port device in which the signal from port 1 is transferred to port 3 and signal from port 4 is transferred to port 2. The crossover is also designed using (5). The length of slot region or the common wall removed in this design is naturally larger as the difference in phase between the TE10 and TE20 modes required for 0 dB coupler is 180◦ . Fig. 5 shows the magnitude and phase at the ports of the crossover. From Fig. 5 it is observed that S11, S21 and S41 are below −10 dB. Also from Fig. 5 it is noted that at 60 GHz the phase is 129.2◦ .
length of the line. The phase shift is obtained in this design using curved transmission line as it is not possible to achieve the required phase shift of 0◦ and 45◦ using straight transmission line. In [11] the accuracy of the phase shifter is reported to be better than 5◦ after adding discontinuities to the curved SIW transmission line. In this work, good accuracy is observed without any added discontinuities in the curved SIW phase shifters. Fig. 6 shows the 0◦ and 45◦ phase shifter respectively.
2.3. Phase shifters Phase shifters are widely used in microwave networks. Delay lines are the simplest of the phase shifters. The phase delay of a transmission line is given by (6). ˚ = kz l
(6)
where ˚ is the phase delay, kz is the propagation constant of the propagating mode and l is the length of the line. The phase difference between the two delay lines of unequal width and equal length is given by (7). ˚ = (kz2 − kz1 )l
(7)
where ˚ is the phase delay, kz2 and kz1 are the propogation constants of the propagating mode at different width and l is the
Fig. 5. Simulated S-parameters and phase at output of crossover.
Fig. 7. Simulated phase difference between phase shifters and crossover.
Fig. 8. Simulated magnitude and phase of 0◦ and 45◦ phase shifter.
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Fig. 9. SIW butler matrix structure.
In Fig. 6, Lp = 1.84 mm, Wp = 2.96 mm for 0◦ phase shifter and for 45◦ phase shifter Lp = 1.9 mm, Wp = 2.96 mm. The width of the SIW in phase shifters is also increased slightly to achieve the phase shifts. The 45◦ phase shifter is more curved than the 0◦ phase shifter. In this design 45◦ phase shifter is projected more outward than 0◦ phase shifter. Fig. 7 shows the simulated phase difference between the phase shifters and the crossover. The accuracy of the phase shifters from the simulation results is observed to be as high as 1.38◦ . The simulated accuracy of the phase shifter is found to be better than 1.4◦ for 0◦ phase shifter and better than 4◦ for 45◦ phase shifter. The magnitude and phase of the 0◦ and 45◦ phase shifter is shown in Fig. 8. From Fig. 8 it is observed that the return loss for both phase shifters is below −10 dB for the entire 60 GHz band. Also, from Fig. 8 it is observed that the phase difference between the phase shifters is almost 45◦ at 60 GHz. 2.4. Butler matrix Fig. 11. Simulated phase at the output of butler matrix when port 1 is excited.
The butler matrix is designed by integrating the components designed above. The performance of the butler matrix is verified through simulation in CST MWS. Fig. 9 shows the designed butler matrix. Fig. 10 shows the magnitude at the ports of the butler matrix when port 1 is excited and Fig. 11 shows the relative phases at the output ports when port 1 is excited. Similarly, Fig. 12 shows the magnitude at the ports when port 2 is excited and Fig. 13 shows the relative phases at the output ports when port 2 is excited.
Fig. 10. Simulated S-parameters of butler-matrix when port 1 is excited.
From Fig. 10 it is observed that when port 1 is excited, the return loss is lower than −10 dB for the entire band from 57 GHz to 64 GHz. Similarly, S(2,1) is observed to be below −13 dB, S(3,1) is below −20 dB and S(4,1) is below −11 dB from 57 GHz to 64 GHz. The magnitudes of output at port 5, 6, 7 and 8 are also observed to be in acceptable range.
Fig. 12. Simulated S-parameters of butler-matrix when port 2 is excited.
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Fig. 15. Simulated radiation pattern of slot antenna.
Fig. 13. Simulated phase at the output of butler matrix when port 2 is excited.
Though equal power distribution would be ideal but due to crossovers and other losses some variation is observed in their magnitude. Further, from Fig. 11 it is observed that the relative phase differences between the output ports are around −45◦ , −90◦ and −135◦ for ports 6, 7 and 8 with respect to port 5 when port 1 is excited. The values mentioned above are ideal values for the butler matrix. Some phase variation from ideal values of phase difference is observed in the output ports of the butler matrix. This can be due to the design of butler matrix where the signal has to cross through crossover and also ideal phase shift is hard to be achieved using the curved delay line for phase shifting. However it is observed that they are within the acceptable range. From Fig. 12 it is observed that when port 2 is excited, the return loss is lower than −13 dB for the entire band from 57 GHz to 64 GHz. Similarly, S(1,2) is observed to be below −12 dB, S(3,2) is below −15 dB and S(4,2) is below −20 dB from 57 GHz to 64 GHz. The magnitudes of output at port 5, 6, 7 and 8 are also observed to be satisfactory. Further from Fig. 13 it is observed that the phase differences between the output ports are around 135◦ , −90◦ and 45◦ for ports 6, 7 and 8 with respect to port 5 which are the ideal values for a butler matrix. 3. Switched beam slot antenna The waveguide longitude slot antennas are popular for beam steering applications. The diagram of slot antenna is shown in Fig. 14, where Ws = 0.198 mm, Ls = 1.789 mm, Es = 2.1 mm and slots displacement from the center is 0.17 mm. Many researchers have worked on slot antennas for a long time [16,17]. The values chosen for the length, width and offset from the center of the waveguide plays a vital role in the design and performance of the antenna. The antenna was designed as per the design equations mentioned in [17] using SIW technology with cylindrical vias which act as the side wall of the waveguide.
Fig. 14. Slot antenna schematic.
The theoretical values are used in the design of the antenna slots and further it is optimized through CST MWS simulations. The designed antenna was simulated in the simulation tool for analyzing the radiation pattern so that a symmetric radiation pattern is observed. Fig. 15 shows the radiation pattern of the single antenna. E-plane and H-plane radiation pattern is shown in the figure. It is observed that the gain is 13.7 dBi. The beamwidth of the single antenna is wide in E-plane as compared to the H-plane. Fig. 16 shows the 3D radiation pattern of the slot antenna in CST MWS. Four of such antenna elements are used for connecting to the four output ports of the butler matrix. Further the microstrip to SIW transition is also integrated into the design. Fig. 17 shows the fabricated switched beam antenna with the dimension of 74.3 mm × 56 mm. Fig. 18 shows the simulated and measured S-parameters of the switched beam antenna when port 1 is excited. It is observed that S11, S21, S31 and S41 are all below −10 dB from 57 GHz to 64 GHz. Fig. 19 shows the simulated and measured S-parameters of the switched beam antenna when port 2 is excited in which S12, S22, S32 and S42 are all observed to be below −10 dB for the entire 60 GHz band. The simulated and measured S-parameters are found to be similar. Further the simulated and measured radiation pattern of the switched beam antenna is shown in Fig. 20. The measured gain is observed to be 16.6 dBi when port 2 and port 3 is excited. However, the measured gain is 17.5 dBi when port 1 and port 4 is excited. Further, it is observed that the main beam is directed toward the +15◦ and −15◦ when port 1 and port 4 is excited respectively. It shifts to −41◦ and +41◦ when port 2 and port 3 is excited respectively. The 4 beams together cover the total angle range of 112◦
Fig. 16. Simulated 3D radiation pattern.
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Fig. 17. Fabricated switched beam antenna.
Fig. 19. Simulated and measured S-parameters when port 2 is excited.
Fig. 18. Simulated and measured S-parameters when port 1 is excited.
considering the 3 dB beam-width. Further, it can be seen that the measured values and CST MWS simulation results are similar which validates the proposed switched beam antenna design. Table 1 shows the comparison with some other switched beam antennas available in literature for 60 GHz band. In [8] the switched beam antenna consists of patch antenna array and microstrip lines are used to design the BFN. In [9] the quasi yagi dipole antenna array is used along with the BFN designed using microstrip lines. Similarly, in [12] ridged waveguide technology is used to design the switched beam antenna consisting of slotted waveguide antenna array and a BFN. In this work the switched beam antenna is designed using SIW technology. The switched beam antenna consists of SIW slotted waveguide array and a BFN with uneven curved delay lines as phase shifters. One of the advantages of the proposed design is that since it is based on SIW technology it has low loss and it is efficient than microstrip lines because at millimeter wave frequencies the losses in microstrip lines are high.
Fig. 20. Simulated and measured radiation pattern when different ports are excited.
Also, the design proposed in this paper is light weight and compact as compared to the design in [12] because ridged waveguide is bulkier than single layer SIW. Further, the proposed design can be fabricated using single layer PCB manufacturing process so it is economical for mass production. The main drawback of the proposed design is that the side lobe level (SLL) is high as compared to the design in [12] which uses additional SLL control circuit for obtaining low SLL. Also, the gain is less as compared to [9,12]. However, overall the proposed switched beam antenna displays a good performance at 60 GHz and is suitable for high speed multi-gigabit communications.
Table 1 Comparison with other switched beam antenna for 60 GHz band. Switched beam antenna
[8]
[9]
[12]
This work
Beamforming network used Scan angle Antenna type Technology Peak gain
Butler matrix −40◦ to +40 Patch antenna Microstrip lines (PCB) 8.9 dBi
Butler matrix −40◦ to +40◦ Quasi-Yagi dipole Microstrip lines (LCP) 27.5 dBi
Butler matrix −43◦ to +43◦ Slotted wave-guide antenna Ridged wave-guide 26.25 dBi
Butler matrix −41◦ to +41◦ Slotted wave-guide antenna SIW (PCB) 17.5 dBi
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4. Conclusion A planar 60 GHz switched beam antenna array has been designed and simulated. The butler matrix used for the feeding network shows good performance over the 60 GHz band. The proposed design has been validated through simulated and measured results which agree well with each other. Good dispersion characteristics are seen in the designed matrix considering the reflection, isolation, insertion and phase characteristics in the 60 GHz band. The designed multi-beam antenna array is small in size. It is fabricated using low cost technology as the ones used for printed circuit boards. It is suitable for applications requiring beam steering in the 60 GHz band and could be a good candidate for system-onsubstrate applications. Acknowledgment Authors are very much obliged to ISRO, Government of India, for the assistance provided for the execution of this research work. References [1] Smulders P. Exploring the 60 GHz band for local wireless multimedia access: prospects and future directions. IEEE Commun Mag 2002;40:140–7. [2] Khan MN, Rizvi UR. Antenna beam-forming for a 60 GHz transceiver system. Arab J Sci Eng 2013;38:2451–64. [3] Bozzi M, Perregrini L, Wu K, Arcioni P. Current and future research trends in substrate integrated waveguide technology. Radioengineering 2009;18:201–9. [4] Cheng YM, Chen P, Hong W, Djerafi T, Ke W. Substrate integrated waveguide beamforming networks and multibeam antenna arrays for low-cost satellite and mobile systems. IEEE Antennas Propag Mag 2011;53:18–30.
[5] Wu LS, Mao JF, Shen W, Yin WY. Extended doublet bandpass filters implemented with microstrip resonator and full-/half-mode substrate integrated cavities. Prog Electromagn Res 2010;108:433–47. [6] Bakhtafrooz A, Borji A, Busuioc D, Safavi Naeini S. Novel two-layer millimeterwave slot array antennas based on substrate integrated waveguides. Prog Electromagn Res 2010;109:475–91. [7] Hall PS, Vetterlein SJ. Review of radio frequency beamforming techniques for scanned and multiple beam antennas. IEE Proceedings H Microw Antennas Propag 1990;137:293–303. [8] Tseng CH, Chen CJ, Chu TH. A low-cost 60-GHz switched-beam patch antenna array with butler matrix network. IEEE Antennas Wirel Propag Lett 2008;7:432–5. [9] Patterson CE, Khan WT, Ponchak GE, May GS, Papapolymerou J. A 60-GHz active receiving switched-beam antenna array with integrated butler matrix and GaAs amplifiers. IEEE Trans Microw Theory Tech 2012;60:3599–607. [10] Mohamed Ali AA, Fonseca NJG, Coccetti F, Aubert H. Design and implementation of two-layer compact wideband butler matrices in SIW technology for Ku-band applications. IEEE Trans Antennas Propag 2011;59:503–12. [11] Djerafi T, Fonseca NJG, Wu K. Design and implementation of a planar 4 × 4 butler matrix in SIW technology for wide band high power applications. Prog Electromagn Res B 2011;35:29–51. [12] Tekkouk K, Hirokawa J, Sauleau R, Ettorre M, Sano M, Ando M. Duallayer ridged waveguide slot array fed by a butler matrix with sidelobe control in the 60 GHz band. IEEE Trans Antennas Propag 2015, http://dx.doi.org/10.1109/TAP.2015.2442612. [13] Riblet HJ. The short-slot hybrid junction. Proc IRE 1952;40:180–4. [14] Wu Ke, Deslandes D, Cassivi Y. The substrate integrated circuits – a new concept for high-frequency electronics and optoelectronics. In: Telecommunications in Modern Satellite, Cable and Broadcasting Service TELSIKS, vol. 1. 2003. p. III–X. [15] Bozzi M, Georgiadis A, Wu K. Review of substrate-integrated waveguide circuits and antennas. IET Microw Antennas Propag 2011;5:909–20. [16] Yan L, Hong W, Hua G, Chen JX, Wu K, Cui TJ. Simulation and experiment on SIW slot array antennas. IEEE Microw Wirel Compon Lett 2004;14:446–8. [17] Elliott RS. An improved design procedure for small arrays of shunt slots. IEEE Trans Antennas Propag 1983;31:48–53.