COMPACT CPW-FED SLOT ANTENNA USING STEPPED IMPEDANCE SLOT RESONATORS HARMONIC SUPPRESSION

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 12, December 2018, pp , Article ID: IJCIET_09_12_045 Available online at aeme.com/ijciet/issues.asp?jtype=ijciet&vtype= =9&IType=12 ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed COMPACT CPW-FED SLOT ANTENNA USING STEPPED IMPEDANCE SLOT RESONATORS AND A BAND-STOP FILTER FOR WIDEBAND HARMONIC SUPPRESSION J. Yeo School of Computer and Communication Engineering, Daegu University, Jillyang, Gyeongsan, Gyeongbuk38453, Korea ABSTRACT A compact slot antenna fed by a coplanar waveguide (CPW) transmission line for wideband harmonic suppression is presented. The antenna consists of a capacitively coupled CPW-fed rectangular slot combined with stepped impedance slot resonators (SIRs), which are symmetrically appended to the slot at both ends, and a band-stop filter embedded on the centerline of the CPW feed line. Two SIRs are first used to suppress up to the fourth harmonics of the fundamental resonant frequency of the slot antenna with an additional effect of size reduction. A band-stop filter is then applied on the centerline of the CPW feed line to extend the suppression up to the eighth harmonics. A prototype antenna resonating at 1 GHz was fabricated on an FR4 substrate. The input reflection coefficient of the antenna remains over -3 db at up to 8.25 GHz, and suppression of the harmonics is demonstrated at up to eight times the fundamental resonant frequency. In addition, the slot length of the proposed antenna is reduced by 38.3% compared to that of a conventional rectangular slot antenna. Keywords: Band-Stop Filter, Coplanar Waveguide (CPW), Slot Antenna, Stepped Impedance Slot Resonators, Wideband Harmonic Suppression. Cite this Article: J. Yeo, Compact CPW-Fed Slot Antenna Using Stepped Impedance Slot Resonators and A Band-Stop Filter For Wideband Harmonic Suppression, International Journal of Civil Engineering and Technology (IJCIET) 9(12), 2018, pp et/issues.asp?jtype=ijciet&vtype=9&itype e=12 1. INTRODUCTION In wireless communication systems, higher-order harmonics are generated by nonlinear devices or active circuits. Such harmonics can reduce the system efficiency and cause electromagnetic interference in other systems when they radiate through the antenna. Thus, a harmonic-suppression filter is necessary. In general, the easiest harmonic suppression method is to place a harmonic suppression filter between the antenna and the harmonic generator editor@iaeme.com

2 Compact CPW-Fed Slot Antenna Using Stepped Impedance Slot Resonators and A Band-Stop Filter For Wideband Harmonic Suppression However, when a separate harmonic-suppression filter is used, the size of the RF front ends and manufacturing cost are increased, and additional insertion loss may occur [1]. To solve this problem, various harmonic suppression techniques and harmonic-suppressed antennas have been studied [2]. Such antennas are combined with an integrated harmonic suppression filter. Planar slot antennas fed by a coplanar waveguide (CPW) feed line have been widely used because of advantages such as wide bandwidth, easy fabrication, and easy integration with RF components [3]. Several harmonic-suppression methods have been applied to CPW-fed slot antennas. Photonic-bandgap (PBG) structures with asymmetric square-shaped lattices and a cut-off frequency of 3.20 GHz were incorporated into the CPW feed line of a broadband loop slot antenna. The input reflection coefficient was greater than -10 db in the band of GHz [4]. Harmonics were suppressed in the frequency range of GHz. A compact harmonic-suppressed inductively coupled CPW-fed slot antenna operating at 4.5 GHz was designed by appending two stepped-impedance slot resonators (SIRs) at the ends of the original rectangular slot [5]. In this case, harmonics were suppressed at up to 10.8 GHz. A compact H-shaped slot antenna was combined with an open-ended CPW feed line for 2.4 GHz WLAN applications [6]. Higher-order harmonics were suppressed at up to 10 GHz. A CPW-fed slot-coupled circular slot loop antenna was designed with unequal lengths of the coupling slots [7]. In this design, the two coupling slots are connected directly to the feed line and located inside the radiating slot loop. The second and third harmonics were successfully suppressed by properly adjusting the two unequal coupling slot lengths. A compact half-ring-shaped slot antenna fed by a capacitively coupled CPW transmission line using two end-aligned SIRs was proposed [8]. The slot length of the antenna was reduced by 45% compared to a conventional rectangular slot antenna, and the second, third, and fourth harmonics were suppressed. However, in most studies, only the second, third and fourth harmonics are suppressed, and harmonics above the fifth order cannot be suppressed. A design method is presented for a compact CPW-fed slot antenna for wideband harmonic suppression at up to eight times the fundamental resonant frequency. This is accomplished by using SIRs and a CPW band-stop filter. Two SIRs are used to suppress up to the fourth harmonics of the fundamental resonant frequency of the slot antenna. Next, a band-stop filter is employed on the centerline of the CPW feed line to extend the suppression up to the eighth harmonics. A prototype antenna for wideband harmonic suppression was designed on an FR4 substrate to resonate at 1 GHz. Full-wave simulation results of the antenna were obtained with the commercial electromagnetic simulator CST Microwave Studio. 2. ANTENNA GEOMETRY AND DESIGN Figure 1(a) shows the geometry of the proposed antenna, and Figure 1(b) showsamagnified view of the band-stop filter at the centerline of the CPW feed line. The antenna consists of a CPW-fed rectangular slot antenna, two SIRs symmetrically appended to the rectangular slot at both ends, and the band-stop filter. The rectangular slot is capacitively coupled to the CPW feed line, and a T-shaped stub is added for impedance matching. The capacitively coupled CPW feed method has several advantages compared to the inductively coupled method, such as size reduction of the resonating slot length to half and elimination of the second harmonic generation [5]. The proposed antenna is printed on the top surface of an FR4 substrate with a dielectric constant of 4.4 and a thickness of h = 0.8 mm (loss tangent = 0.025). The length and width of the antenna are L = 85 mm and W = 55 mm, respectively. The length and width of the rectangular slot are l s = 64.2 mm and w s = 6 mm, respectively. The width of the centerline of the CPW feed line is w f = 3 mm, and the gap between the centerline and the ground of the editor@iaeme.com

3 J. Yeo CPW feed line is g f = 0.3 mm. These parameters are designed to match an input impedance of 50 Ω. The length of the CPW feed line is l f = 25 mm. The length and width of the T-shaped stub are l t = 11 mm and w t = 3 mm, respectively, and the gap between the stub and the top side of the rectangular slot is g t = 2.5 mm. The length and width of the two SIRs at both ends of the rectangular slot are l SIR = 8.9 mm and w SIR = 36 mm, respectively. (a) (b) Figure 1 (a) Geometry of the proposed compact CPW-fed slot antenna for wideband harmonic suppression and (b) magnified view of the CPW band-stop filter A band-stop filter is appended at the centerline of the CPW feed line to extend the harmonic suppression up to the eighth harmonics. The band-stop filter is a kind of onedimensional CPW PBG resonant cell [9]. The distance between the bottom of the band-stop filter and the antenna s input port is l BSF = 4.8 mm. The geometric parameters of the band-stop filter are as follows: l 1 = 8.8 mm, l 2 = 0.3 mm, g 1 = 0.3 mm, g 2 = 0.3 mm, w 1 = 2.4 mm, and w 2 = 0.3 mm. The design procedure for a compact CPW-fed slot antenna with wideband harmonic suppression is explained using the three antenna structures shown in Figure 2. The corresponding simulated input reflection coefficient and gain characteristics for the three antenna structures are presented in Figure 3. Figure 2(a) shows the conventional CPW-fed rectangular slot antenna, and Figure 2(b) shows the compact CPW-fed slot antenna with SIRs only. Figure 2(c) shows the proposed antenna with both SIRs and a CPW band-stop filter. The conventional capacitively coupled CPW-fed rectangular slot antenna without harmonic suppression capability was designed to resonate at 1 GHz. The length and width of the rectangular slot are 104 mm and 6 mm, whereas those of the antenna are 114 mm and 55 mm respectively. The first frequency band for a voltage standing wave ratio (VSWR)< 2 is GHz. There are several higher-order resonant frequencies at 3.38, 5.52, 7.44, 9.16, and GHz after the first resonant frequency of 1 GHz. (a) editor@iaeme.com

4 Compact CPW-Fed Slot Antenna Using Stepped Impedance Slot Resonators and A Band-Stop Filter For Wideband Harmonic Suppression (b) (c) Figure 2Three antenna structures for performance comparison: (a) conventional capacitively coupled CPW-fed rectangular slot antenna for reference, (b) compact CPW-fed rectangular slot antenna with two SIRs only, and (c) the proposed compact CPW-fed slot antenna with two SIRs and a CPW bandstop filter Figure 3Comparison of input reflection coefficient for the antenna structures in Figure 2 Two SIRs are symmetrically appended to the rectangular slot at both ends, and the length of the rectangular slot is reduced to 64.2 mm to resonate at 1 GHz, as shown in Figure 2(b). The addition of the two SIRs to the rectangular slot enables size reduction, and the rectangular slot length is decreased by about 38.3% compared to that in Figure 2(a). In this case, the input reflection coefficient remains over -3 db at up to 4.54 GHz after the first resonant frequency of 1 GHz. Therefore, harmonics at up to four times the first resonant frequency are suppressed by using the two SIRs. The band-stop filter at the center-line of the CPW feed line further extends the harmonic suppression up to the eighth harmonics, as shown in Figure 2(c). Figure 3 shows that the input reflection coefficient remains over -3 db at up to 8.25 GHz after the first resonant frequency of 1 GHz. Hence, the harmonics up to eight times the first resonant frequency are successfully suppressed by using both the SIRs and the CPW band-stop filter editor@iaeme.com

5 J. Yeo (a) (b) Figure 4(a) Geometry of a CPW band-stop filter for simulation and (b) input reflection coefficient characteristic of the CPW band-stop filter To validate its effectiveness, the CPW band-stop filter shown in Figure 4(a) was simulated, and its S-parameter characteristics are shown in Figure 4(b). It has a wideband band-stop characteristic with a criterion of -10 db in the frequency range of GHz. This guarantees the extension of harmonic suppression up to eight times the first resonant frequency. Figure 5Effect on input reflection coefficient characteristic of the proposed CPW-fed slot antenna from varying l BSF The effect of the location of the CPW band-stop filter on the antenna performance was investigated, as shown in Figure 5. For this purpose, the distance l BSF between the bottom of the band-stop filter and the antenna s input port was varied from 4.8 to 10.8 mm. As l BSF increases, the level of small resonance at around 3 GHz decreases, and the frequency limit over -3 db in the input reflection coefficient moves slightly toward lower frequency. Basedon the simulation results, l BSF = 4.8 mm was selected after considering both the level of the small resonance and the -3-dB frequency limit. 3. EXPERIMENT RESULTS AND DISCUSSION Figure 6 shows a fabricated prototype of the proposed antenna. Its performance was measured using an Agilent N5230A network analyzer and anechoic chamber, and the results are compared in Figure7. The measured first frequency band for VSWR < 2is GHz. Similar to the simulated result, the measured input reflection coefficient remained over -3 db at up to 8.25 GHz after the first resonant frequency of 1 GHz. The simulated and measured results agree well with each other editor@iaeme.com

6 Compact CPW-Fed Slot Antenna Using Stepped Impedance Slot Resonators and A Band-Stop Filter For Wideband Harmonic Suppression Figure 6Photograph of the fabricated antenna Figure 7Measured input reflection coefficient of the fabricated antenna The radiation patterns of the fabricated antenna in the y-z and z-x planes at 1 GHz are plotted in Figure8,and the measured patterns for both planes agree well with the simulated results. Figure 8Measured radiation patterns of the fabricated antenna in the y-z and z-x planes at 1 GHz 4. CONCLUSION A compact CPW-fed slot antenna was designed for wideband harmonic suppression at up to eight times the fundamental resonant frequency using SIRs and a CPW band-stop filter. First, two SIRs were symmetrically appended to the CPW-fed rectangular slot at both ends to suppress up to the fourth harmonics of the fundamental resonant frequency. Next, a band-stop filter was inserted at the centerline of the CPW feed line to further extend the suppression up to the eighth harmonics editor@iaeme.com

7 J. Yeo The fabricated antenna prototype showed a first frequency band of GHz for VSWR < 2.The input reflection coefficient remained over -3 db up to 8.25 GHz after the first resonant frequency of 1 GHz. The rectangular slot length was reduced by 38.3%compared to that of a conventional antenna. The proposed antenna could be used as a compact wideband harmonic suppression antenna for wireless power transfer and RF energy harvesting applications. REFERENCES [1] Radisic, V., Qian, Y. and Itoh, T. Novel architectures for high-efficiency amplifiers for wireless applications.ieee Transactions on Microwave Theory and Technology,46(11), 1998, pp [2] Horii, Y., and Tsutsumi, M.Harmonic control by photonic bandgap on microstrip patch antenna.ieee Microwave and Guided Wave Letters,9(1),1999, pp [3] Chen, H. D.Broadband CPW-fed square slot antenna with a wideband tuning stub.ieee Transactions on Antennas and Propagation, 51(8), 2003, pp [4] Lin, X. C., and Wang,L. T. A broadband CPW-fed loop slot antenna with harmonic control.ieee Antennas and Wireless Propagation Letters, 2, 2003,pp [5] Tu, W. H.Compact harmonic-suppressed coplanar waveguide-fed inductively coupled slot antenna.ieee Antennas and Wireless Propagation Letters, 7, 2008,pp [6] Sujith, R., Mridula, S., Binu, P., Laila, D., Dinesh, R., and Mohanan, P.Compact CPW-fed ground defected H-shaped slot antenna with harmonic suppression and stable radiation characteristics.electronics Letters,46(12),2010, pp [7] Liu, Y. W., Lu,Y. J., and Hsu, P.Harmonic suppressed slot loop antenna fed by coplanar waveguide.ieee Antennas and Wireless Propagation Letters, 13, 2014,pp [8] Yeo, J., and Lee,J. I. Compact CPW-fed half-ring-shaped slot antenna for harmonic suppression applications.microwave and Optical Technology Letters,57(5),2015, pp [9] Xue, Q., Mo,T. T., Shum,K. M., and Chan,C. H. Characteristics of a compact CPW resonant cellmicrowave and Optical Technology Letters,37(6), 2003, pp editor@iaeme.com