Prgress In Electrmagnetics Research C, Vl. 47, 173 179, 214 Bradband Circularly Plarized Slt Antenna Array Using a Cmpact Sequential-Phase Feeding Netwrk Ping Xu *, Zehng Yan, Tianling Zhang, and Xiaqiang Yang Abstract A bradband circularly plarized (CP) slt antenna array fed by an asymmetric cplanar waveguide (CPW) with stepped and inverted T-shaped strips is prpsed. Using fur square slt antenna elements with sequential rtatin blique feed and a mdified sequential-phase (SP) feeding netwrk, bradband CP can be achieved. The measured 1 db reflectin cefficient bandwidth and 3 db axial rati (AR) bandwidth are 55.4% (1.63 2.88 GHz) and 58% (1.65 3 GHz), respectively. Gd radiatin characteristics with gain mre than 6 dbic ver the perating band are btained by the prpsed antenna array with a cmpact size f 155 155.8 mm 3. Details f the prpsed antenna array design and experimental results are presented and discussed. 1. INTRODUCTION With the rapid develpment f wireless technlgy, mst satellite and mbile cmmunicatin systems use circularly plarized (CP) antenna arrays t vercme the prblems f mbility and adverse weather cnditins [1, 2]. Micrstrip CP antenna arrays have becme an excellent candidate wing t their light weight, simple structure, lw prfile, and ease f fabricatin [2 11]. But mst designs have narrw impedance and axial rati (AR) bandwidth [3 5]. In rder t btain better prpagatin characteristics, CP antenna arrays with gd perfrmance ver a wide frequency bandwidth are then required in many applicatins. In recent years, the designs f cplanar waveguide (CPW)-fed square slt antennas have received much attentin due t their wide bandwidth [2 1]. Varius structures have been prpsed t increase the bandwidth f square slt antennas [6 11]. And sequential rtatin techniques are generally used t imprve the bandwidth perfrmance f CP arrays [2, 3, 12, 13]. Prper sequential rtatin with the excitatin f sequential phases is an effective slutin t imprve the circularly plarized bandwidth. Several sequential-phase (SP) feed designs have been reprted [3, 12, 13]. Hwever, mst f them are cmplex and have a large size. In this letter, a bradband CP slt antenna array emplying a cmpact SP feeding netwrk is prpsed. The presented SP feed uses nly fur transfrmer segments, making the whle SP feed very cmpact and simple. An elliptically CP antenna element fed by an asymmetric CPW with stepped and inverted T-shaped strips is used t realize the prpsed antenna array. 2. ANTENNA ELEMENT CONFIGURATION AND DESIGN Figure 1 shws the gemetry f the prpsed square slt antenna element, which is printed n an FR4 substrate with a side length f G, a thickness f H, a relative permittivity f 4.4 and a lss tangent f.2. The square slt with a side length f L is lcated at the center f the tp layer, which is fed by a 5 Ω CPW transmissin line with a signal strip f width W f and tw identical gaps f width g. T Received 16 January 214, Accepted 24 February 214, Scheduled 25 February 214 * Crrespnding authr: Ping Xu (pingxu@mail.xidian.edu.cn). The authrs are with the Natinal Key Labratry f Science and Technlgy n Antennas and Micrwaves, Xidian University, Xi an, Shaanxi 7171, China.
174 Xu et al. G L Y X L 3 L 4 L 2 g S WS L 1 W f H Figure 1. Gemetry f the prpsed antenna element. (G = 6 mm, L = 4 mm, W f = 1.5 mm, W S =.2 mm, L 1 = 12.2 mm, L 2 = 11.55 mm, L 3 = 28 mm, L 4 = 8 mm, S = 1.5 mm, g =.2 mm, H =.8 mm). Antenna 1 Antenna 2 Antenna 3 Prpsed (a) Reflectin cefficient (db) -1-2 -3 Antenna 1 Antenna 2 Antenna 3 Prpsed 1. 1.5 2. 2.5 3. 3.5 Frequency (GHz) (b) Figure 2. Simulated reflectin cefficients f the antenna prttypes: (a) antenna prttypes; (b) simulated reflectin cefficient.
Prgress In Electrmagnetics Research C, Vl. 47, 214 175 9 Y X 18 27 Figure 3. Distributins f the surface current n the feed and grund f prpsed antenna element at 2.25 GHz in phase f, 9, 18 and 27. enlarge the impedance bandwidth, tw main structures have been embedded in the feeding structure: the stepped strip and the inverted T-shaped strip. The dimensins f the mdified structures are ptimized fr better impedance matching. Simulated reflectin cefficients f fur antenna prttypes with r withut the mdified structures are shwn in Figure 2. It is bserved that the frmer structure mainly affects the higher resnant mde, while the latter affects the lwer resnant mde. A wide impedance bandwidth with a smth curve trend cvering 1.5 3. GHz (abut 66.6%) can be btained by the prpsed antenna element. Simulated vectr surface current distributins f the prpsed antenna element at 2.25 GHz are shwn in Figure 3. It is bserved that the predminant vectr surface currents in 18 and 27 are ppsite in phase f and 9, respectively. And the current rtates in the left-hand directin viewed frm the +Z-directin. 3. THE COMPACT SEQUENTIAL-PHASE FEEDING NETWORK Figure 4 illustrates the equivalent circuit f SP feed using a traditinal serial pwer divider. Fr impedance transfrmatins and sequential-phase rtatin, it requires at least seven quarter-wave transfrmer segments [3, 12]. As shwn in Figure 4, Z, Z n (n = 1 7) and 9 represent the terminal lad impedances, the characteristic impedances and the electrical lengths f the transmissin line, respectively. Here, Z n Prt 1 Z 2, 9 Z 4, 9 Z 6, 9 Z 7, 9 Prt 5 Z in Z 1, 9 Z 3, 9 Z 5, 9 Z Prt 2 Prt 3 Prt 4 Z Z Z Figure 4. Equivalent circuit f SP feed using a traditinal serial pwer divider.
176 Xu et al. is calculated such that the SP feed diverts ne quarter f the pwer t each prt, while the input impedance Z in is invariant. S that Z 1 = 2 Z Z in. (1) In rder t reduce the size, fur segments with the characteristic impedance f Z i (i = 1, 3, 5, 7) is deleted, as shwn in Figure 5(a). Thus, we have Z = 2 Z Z in1, (2a) Z in1 = 1 4 Z. (2b) T transfrm Z in1 t Z in, a transmissin line segment with the characteristic impedance f Z 8 and the electrical length f 9 is inserted. S the equatins can be expressed as Using (2b), (3) is simplified t Z 8 = Z in1 Z in. (3) Z 8 = 1 2 Z Z in. (4) Fr equal dividing and sequential-phase rtatin, the fllwing equatins can be btained Z 2 = 1 3 Z, Z 4 = 1 2 Z, Z 6 = Z. (5) In this design, the SP feed is designed at the center frequency 2.25 GHz and Z in = Z = 5 Ω. The cnfiguratin and ptimized dimensins are shwn in Figure 5(b). Simulated results shw that the 1 db reflectin cefficient bandwidth cvers frm 1.55 GHz t 2.85 GHz (abut 59%). And the magnitude balance level amng the utput prts ( S j1, j = 2 5) is less than 1 db ver the same impendence band [12]. Prt 3 W1 W 2 R Prt 1 Z, 9 8 Z in1 Prt 2 Prt 3 Z in Z Z 2, 9 Z 4, 9 Z 6, 9 Z Prt 4 Z Prt 5 Z Prt 2 Y X Prt 4 (a) W L W 3 Prt 1 W p Prt 5 (b) Figure 5. Prpsed SP feed using a mdified serial pwer divider: (a) equivalent circuit; (b) cnfiguratin. (R = 11 mm, L = 17 mm, W p = 1.5 mm, W = 4.4 mm, W 1 = 7.4 mm, W 2 = 4.5 mm, W 3 = 1.6 mm). 4. BROADBAND CIRCULARLY POLARIZED ANTENNA ARRAY Figure 6 presents a 2 2 sequentially rtated planar antenna array using the prpsed elliptically CP slt antenna element and SP feed. The spacing between elements is taken t be 95 mm (abut.7λ, where λ is the wavelength f center frequency). In rder t imprve the transitin, via pins are used t
Prgress In Electrmagnetics Research C, Vl. 47, 214 177 Via pins Figure 6. Phtgraph f the prpsed antenna array prttype. cnnect the micrstrip line t the asymmetric CPW feed f antenna element [2, 3]. And a star-shaped grund plane is applied t imprve the impedance and AR bandwidth. The side length f the square antenna array is 155 mm (abut 1.16λ ). 5. EXPERIMENTAL RESULTS AND DISCUSSION Ansft HFSS is utilized in the design prcedure t ptimize structural parameters. An Agilent E571B vectr netwrk analyzer and a far-field measurement system have been used t measure the reflectin cefficient and far-field perfrmances f the prpsed antenna array, respectively. Simulated and measured reflectin cefficients f the prpsed antenna array are shwn in Figure 7. The mvement f resnant pints may be due t the unsteady substrate parameters f FR4 substrate. The measured 1 db reflectin cefficient bandwidth cvers 1.63 2.88 GHz (abut 55.4%). Figure 8 presents the simulated and measured gains and ARs f the antenna array. As can be fund, the measured 3 db AR bandwidth cvers 1.65 3 GHz (abut 58%). The measured gain is mre than 6 dbic ver the perating band. Nrmalized radiatin patterns at 2.25 GHz f the prpsed array are shwn in Figure 9. The discrepancy between simulated and measured results is mainly due t the test envirnment and the effect f the SMA cnnectr in the lwer layer. The patterns are bidirectinal 1 Gain Reflectin cefficient (db) -1-2 -3 Measured Simulated Axial rati (db) & Gain (dbic) 8 6 4 2 Simulated gain Measured gain Simulated axial rati Measured axial rati Axial rati -4 1. 1.5 2. 2.5 3. 3.5 Frequency (GHz) Figure 7. Simulated and measured reflectin cefficients f the antenna array. 1. 1.5 2. 2.5 3. 3.5 Frequency (GHz) Figure 8. Simulated and measured gains and axial ratis f the antenna array.
178 Xu et al. 33 3 33 3 3 6-2 3 6 27 ϕ= deg simulated -4 9 27-4 ϕ=9 deg simulated 9 24 12-2 24 12 21 33 18 15 3 LHCP RHCP 21 33 18 15 3 3 6-2 3 6 27 ϕ= deg measured -4 9 27-4 ϕ=9 deg measured 9 24 12-2 24 12 21 18 15 21 18 15 Figure 9. Simulated and measured nrmalized patterns f the antenna array at 2.25 GHz. Table 1. Cmparisn f the CP perfrmances and dimensins between the prpsed antenna array and antenna arrays in the previus wrks. Ref. [2] [3] Prpsed 3 db AR bandwidth 49% (1.15 1.9 GHz) 31% (5.1 7 GHz) 58% (1.65 3 GHz) 1 db reflectin bandwidth 52% (1.15 1.95 GHz) (S 11 < 15 db) 52% (4 6.825 GHz) 55.4% (1.63 2.88 GHz) Side length f square antenna 1.1λ (2 mm) 1.81λ (92 mm) 1.16λ (155 mm) Peak gain (dbic) 8 7.5 8.9 with left-hand circularly plarized (LHCP) radiatin in the upper-half space and right-hand circularly plarized (RHCP) radiatin in the lwer-half space. A cmparisn between the prpsed antenna array and the antenna arrays presented in [2, 3] has been shwn in Table 1. It is bserved that the prpsed antenna array has a gd CP perfrmance with a cmpact size. 6. CONCLUSION A 4-element CP slt patch antenna array emplying a micrstrip-line-t-asymmetric-cpw feeding netwrk is prpsed. Stepped and inverted T-shaped strips are embedded in the feeding structure t expand the bandwidth f the antenna element. Using a mdified SP feed, which cmprises nly fur segments fr impedance transfrmatins and sequential-phase rtatin, and via pins t imprve the transitin between micrstrip line and asymmetric CPW feed line, the prpsed antenna array can btain wide impedance and AR bandwidth. The measured 1 db reflectin cefficient bandwidth is 55.4% and 3 db AR bandwidth is 58%. Gd radiatin characteristics can be btained with gain mre than 6 dbic ver the perating band.
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