2021 8th NAFOSTED Conference on Information and Computer Science (NICS) A Circularly Polarized Array Antenna for GPS Application Khuat Dinh Chinh, Tran Thi Lan* Faculty of Electronic and Electrical Engineering University of Transport and Communications Hanoi, Vietnam *ttlan@utc.edu.vn Corresponding author: Tran Thi Lan Abstract— The paper proposes a 2×2 array antenna for GPS (Global Positioning System) application in the L1 band (1.575 GHz) In which, the element antenna is a square patch antenna with a diagonal slot in the middle to create circular polarization (CP) The element antennas are fed sequentialphase feeding network to reduce the size of the array antenna To extend the dB axial ratio (AR) bandwidth (BW), parasitic patches are added to either side of the main radiating patch of the element antennas The antenna is designed using FR-4 material with compact size of 182×182×1.6 mm3, impedance BW 62.2 MHz (3.95 %), dB AR BW 30.3 MHz (1.92%) and high gain 10 dBi Especially, the polarization isolation between the right-handed CP (RHCP) and left-handed CP (LHCP) is higher than 24 dB These results show that the proposed antenna is really effective for GPS applications frequency bands are combined together to form an array antenna The antenna operates at multi-bands make its very useful, but it has some limitations such as very small AR BW, which does not meet the requirement greater than 20 MHz At the same time, the gain is quite low 6.5 dBi Single antenna only provides limited bandwidth and gain To improve these parameters, array antennas are an ideal candidate In [7], the antenna is designed with element antennas operating at L1 frequency, each element antenna is fed by proximity fed method The antenna has many good parameters such as peak gain 10.8 dBi, impedance bandwidth 80.3 MHz, AR BW 23 MHz However, the disadvantage of this antenna is that the size of the antenna is big 350×157×3.2 mm3 because of the configuration of the feeding network Therefore, this paper proposes an antenna to overcome the disadvantages of the above-mentioned antennas The proposed antenna has properties such as RHCP with a high polarization isolation, high gain, impedance BW and AR BW greater than 20 MHz, compact size, easy in fabrication To meet these characteristics, the proposed antenna is a 2×2 array antenna to increase the gain of the antenna, fed by sequential phase feeding network to reduce the antenna size The parasitic patches are added to increase the AR BW for the proposed antenna The paper is organized as follows: first, a RHCP element antenna fed by a 50 Ω microstrip line is given in Section II.A Section II.B shows effect of the diagonal slit on the element antenna An array antenna based on the element antennas is presented in Section III The design principle of the proposed antenna with a sequential feeding network is discussed in Section III.A; Section III.B presents the structure and simulation results of the array antenna A method of improving AR BW by adding parasitic patches next to the main radiating patch is presented in Section III.C Finally, some discussions and conclusions are given in Section IV Keywords— Circularly polarized, array antenna, GPS, high gain, sequential feeding I INTRODUCTION GPS antennas are used to receive right-handed circular polarization (RHCP) signals from satellites To avoid polarization loss, the user's GPS receiver antenna should be designed with RHCP [1] Furthermore, to support the current GPS encoding mechanism, the antenna needs to have impedance bandwidth and AR bandwidth greater than 20 MHz [2] To create circular polarization for an antenna, two currents orthogonal to each other need to be excited Two orthogonal currents can be generated with dual fed supplies 900 out of phase as using a hybrid power divider However, dual fed antennas are often large in size, in order to reduce the size of the feeding, a single fed technique is used to produce circular polarization [3] But, two corners of main radiating patch have to be truncated or a diagonal narrow slit is added in the center of the main radiating patch However, the antenna proposed in [4] has AR BW only 10.3 MHz, which does not meet the requirement greater than 20 MHz for GPS application as well as a low gain of 5.77 dBi To achieve higher bandwidth and gain, the antenna is fed by an aperture-coupled feeding technique [5] The antenna's bandwidth is quite good such as 570 MHz impedance BW (40.57 %) covering both L1 and L2 frequency bands, AR BW is 121 MHz (7.74 %) at L1 frequency band, gain at 1.575GHz is 8.11 dBi However, this antenna was utilized an inductor to create circular polarization Thus, the thickness of the antenna is approximately cm This makes the antenna difficult to integrate into user devices II DESIGN OF ELEMENT ANTENNA A Structure of the Element Antenna The element antenna is a square microstrip antenna, uses material FR-4 with dielectric constant εr = 4.3, thickness 1.6 mm as shown in Figure The resonator length a is approximately a half of wavelength at 1.575 GHz The antenna is fed by a 50 Ω microstrip line To produce circular polarization, a rectangular slit is further cut across the diagonal of the main radiating patch [8] The size of the slit is optimized so that the antenna has the largest AR BW Figure shows the current distribution on the element antenna There are two orthogonal currents to create circular A number of recent studies develop multi-band antenna models for GPS Typically, the antenna in [6] is designed to operate on three frequency bands L1, L2, L5 (1.176 GHz) Four element antennas of different shapes and operating XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE 978-1-6654-1001-4/21/$31.00 ©2021 IEEE 29 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) polarization When the phase of the source changes in turn 0˚, 90˚, 180˚, 270˚, the distribution of the surface currents is perpendicular to each other sequentially as shown in Fig B Effect of the Slit on Element Antenna Characteristics Fig shows the effect of the diagonal slit size on the element antenna performance From Fig 3(a), it can be seen that S11 tends to decrease as the length c increases However, AR tends to be larger as shown in Fig 3(b) Therefore, the optimal length of the slit c is chosen to be 12 mm (a) Figure 4: Effect of slit width d on S11 and AR (b) w Similarly, the effect of the slit width d on the element antenna is shown in Figure When d increases impedance BW increases while AR BW tends to decrease as shown in Fig a III PROPOSED ARRAY ANTENNA A Design of the Feeding Network The design principle of the sequential feeding network has been presented in [9] To transform impedance and shift phase sequentially, a traditional series power divider requires at least seven quarter-wave lines as shown in Figure ZL, Zn (n = 1÷7) represent terminal load impedance and characteristic impedance of transformer lines, respectively Zn is calculated to transfer the quarter of the power to each port, where the input impedance is constant: Z4= 2×(ZLZin)1/2 (1) To reduce the size, four lines with characteristic impedance Zi (i = 4, 5, 6, 7) are removed, as shown in Fig 6(a) Then, we have: ZL = 2×(ZLZT)1/2 (2.a) ZT = ZL/4 (2.b) To convert Zin1 to Zin, a line with the impedance Z0 is calculated as follows: Z0 = (ZinZT )1/2 (3) From (2.b) and (3), we have: Z0 = [(ZinZL)1/2]/2 (4) To equally divide the power to ports and shift phase sequentially, we can calculate according to the following equations: Z1 = ZL/3, Z2 = ZL/2, Z3 = ZL (5) Applying the above equations to build a sequential feeding network operating at 1.575 GHz with Zin = 50 Ω and ZL = 100 Ω, dimension parameters of the feeding network are calculated and optimized as shown in Fig (b) y z x m Fig Structure of element antenna: W = 90; a = 44.3; c = 12; d = 1.5; m = (unit: mm) Fig Current distribution on the patch surface in phases a) 0˚, b) 90˚, c) 180˚, d) 270˚ Port Z1 ~ Z5 Z4 Port Zin (a) Z2 ZL Z3 Z6 Port ZL Z7 Port ZL Port ZL = (b) Figure 3: Effect of slit length c on S11 (a) and AR (b) Fig Sequential power supply circuit used for series power divider 30 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) 11.63 MHz, not wide enough to meet the bandwidth requirements of the current GPS system (as abovementioned) Therefore, a solution to improve AR BW is necessary, which is presented in the next section ZT Port Z1 Z0 ~ Z2 Z3 Port Port Port ZL ZL ZL Port ZL C Design of Array Antenna with Parasitic Patches In order to improve AR BW, capacitive parasitic patches (PP) are placed next to the main radiating patch of each element antenna as shown in Fig The capacitive coupling between the parasitic elements and the main radiating patch can realize two orthogonal modes in the parasitic elements, leading to a new circularly polarized radiation at additional higher frequency band Thus, ARBW is improved Zin = a) Port W1 W2 R Port Port l w W0 W3 Wp g Port Port y b) z Fig Sequential feeding network: (a) Equivalent circuit; (b) Geometry (R = 14.5, L0 = 23, Wp = 3, W0 = 5.2, W1 = 5.6, W2 = 3, W3 = 0.7 (mm)) x Fig Structure of the array antenna with parasitic patches 44 182 23 (a) (b) Fig Effect of the length l on S11 and AR of the proposed array antenna: (a) S11, (b) AR y z x (a) (a) (b) Fig.10 Effect of the gap g on S11 and AR of the proposed array antenna: (a) S11, (b) AR (b) Fig The 2×2 array antenna (a) structure (top view), (b) S11 and AR B Design of Array Antenna The array antenna consists of four element antennas fed by the sequential feeding network The feeding network is connected to the element antennas quarter-wave microstrip segments with a characteristic impedance of 100 Ω (Fig (a)) The antenna has impedance BW of 68.6 MHz (Fig (b)), RHCP peak gain of 9.8 dBi However, AR BW is only (a) (b) Fig.11 Effect of the width of PP on S11 and AR of the proposed array antenna: (a) S11, (b) AR 31 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) antenna has a higher gain than the antennas in [4, 5, 6, 10] Although, the antenna [4] has a compact size (single antenna), but the bandwidth is narrow and the gain is low The antenna in [5] has a wide bandwidth, but its thickness is large and the structure is complicated due to the need to use a lumped inductor Stacked structure is also the disadvantage of the antenna [10], making the antenna difficult to fabricate as well as having a large thickness Meanwhile, the antenna in [6] has a very narrow bandwidth that is not sufficient for 20 MHz requirements for GPS applications The antenna in [7] has a higher gain than the proposed antenna, but its large size is difficult to integrate in vehicles, and the AR bandwidth is narrower than the proposed one Thus, it can be seen that the proposed antenna has high gain, bandwidth greater than 20 MHz, compact size, easy to manufacture, suitable for GPS receivers, especially applications on vehicles In the future, antenna fabrication and testing will be carried out The size effect of PP on the proposed array antenna is shown in Figs 8, and 10 When l and g increase, S11 is deeper The longer l is, the higher AR is, and AR reduces when g is bigger It can be seen that the length l and gap g affect AR BW more than width w The optimal size of PP is l = 45, w = 5, g = (mm) A comparison in S11 and AR in two cases with and without PP is given in Fig 12 It can be seen that 3dB AR BW is significantly improved from 11.63 to 30.3 MHz Radiation efficiency is high and approximately 90 % in both cases as shown in Fig 12 (b) The 2D and 3D radiation patterns of the proposed array antenna are shown in Figs 13 and 14 The peak gain is 10 dBi, especially, the polarization isolation between RHCP and LHCP is bigger than 24 dB, very good for GPS applications TABLE I Ref [4] [5] [6] [7] [10] The proposed (a) (b) Fig 12 A comparison in S11 and AR in two cases: with and without parasitic patches (w PP and wo PP) S11 BW (%) 2.3 40.6 1.7 5.11 2.2 3.95 A COMPARISION WITH OTHER STUDIES Gain [dBi] 5.77 8.11 6.5 10.8 6.1 10 Size (Width × Length × Height) [mm] 67×67×1.67 80.8×80.8×30 180×180×4.7 350×157×3.2 114×114×13 182×182×1.6 AR BW (%) 1.07 7.44 Very narrow 1.46 not mentioned 1.92 REFERENCES [1] B Rama Rao, W Kunysz, R Fante, K McDonald, “GPS/GNSS Antennas,” pp 11-17 Chapter 1, 2013 [2] B Barker, J Betz, J Clark, J Correia, J Gillis, S Lazar, K Rehborn, and J Stration, “Overview of the GPS M code signal” in Proc ION Nat Tech Meeting, pp 542–549, 2000 [3] Zhi Ning, ChenDuixian, Liu Hisamatsu, Nakano Xianming Qing and Thomas Zwick,“Handbook of Antenna Technologies,” Springer Science+Business Media Singapore, 2015 [4] Dian Rusdiyanto, Fitri Yuli Zulkifli, Eko Tjipto Rahardjo, “A Circularly Polarized Microstrip Antenna for GPS Application as Small Boat Guidance,” IEEE Region 10 Humanitarian Technology Conference (R10-HTC), 2018 [5] Sungwoo Lee, Youngoo Yang, Kang-Yoon Lee, and Keum Cheol Hwang, “Dual-band Circularly Polarized Annular Slot Antenna With a Lumped Inductor for GPS Applications,” IEEE Transactions on Antennas and Propagation, Volume: 68, Issue: 12, Dec 2020 [6] Yuan Yao, Xuan Shao, Ying Li, Junsheng Yu, Xiaodong Chen, “Design of Multiband Circularly Polarized Antenna Arrays for GNSS Applications,” European Conference on Antennas and Propagation (EuCAP), 2018 [7] Tamer Elshikh, Ahmed Sayed, Alla M.Eid, Ahmed Alieldin “A Low Cost Circular Polarized Antenna Array for GPS Receivers,” EuCAP, 2019 [8] Constantine A Balanis, “Antenna Theory Analysis and Design, fourth edition”, pp 864, Chapter 14, 2016 [9] Ping Xu, Zehong Yan, Tianling Zhang, and Xiaoqiang Yang, “Broadband Circularly Polarized Slot Antenna Array Using a Compact Sequential-Phase Feeding Network,” Progress In Electromagnetics Research, 2014 [10] Satya Bhushan Shukla, Vidya.K.A, Thomas Chacko, Mukundan.K.K, “Single Feed Stacked Circularly Polarized Patch Antenna For Dual Band NavIC Receiver of Launch Vehicles” IEEE Indian Conference on Antennas and Propogation (InCAP), 2019 (a) (b) Fig 13 Simulated normalized 2-D patterns of the proposed antenna array at 1.575GHz: φ = 00 (a) and φ = 900 (b) Fig 14 Simulated 3D pattern of the proposed antenna array at 1.575GHz IV DISSCUSIONS AND CONCLUSIONS Table provides some information comparing the proposed antenna with previously published antennas The proposed 32 ... gap g on S11 and AR of the proposed array antenna: (a) S11, (b) AR (b) Fig The 2×2 array antenna (a) structure (top view), (b) S11 and AR B Design of Array Antenna The array antenna consists of... Tamer Elshikh, Ahmed Sayed, Alla M.Eid, Ahmed Alieldin ? ?A Low Cost Circular Polarized Antenna Array for GPS Receivers,” EuCAP, 2019 [8] Constantine A Balanis, ? ?Antenna Theory Analysis and Design,... Stacked structure is also the disadvantage of the antenna [10], making the antenna difficult to fabricate as well as having a large thickness Meanwhile, the antenna in [6] has a very narrow bandwidth