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Compact dual‐band circularly polarised antenna with omnidirectional and unidirectional properties

2018; Institution of Engineering and Technology; Volume: 12; Issue: 2 Linguagem: Inglês

10.1049/iet-map.2017.0658

1751-8733

Meng‐Shuang Wang, Xiaoqi Zhu, Yong‐Xin Guo, Wen Wu,

Antenna Design and Optimization

IET Microwaves, Antennas & PropagationVolume 12, Issue 2 p. 259-264 Research ArticleFree Access Compact dual-band circularly polarised antenna with omnidirectional and unidirectional properties Meng-Shuang Wang, Meng-Shuang Wang Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing, 210094 People's Republic of China Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576 SingaporeSearch for more papers by this authorXiao-Qi Zhu, Xiao-Qi Zhu Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing, 210094 People's Republic of China Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576 SingaporeSearch for more papers by this authorYong-Xin Guo, Corresponding Author Yong-Xin Guo eleguoyx@nus.edu.sg Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576 SingaporeSearch for more papers by this authorWen Wu, Wen Wu Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing, 210094 People's Republic of ChinaSearch for more papers by this author Meng-Shuang Wang, Meng-Shuang Wang Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing, 210094 People's Republic of China Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576 SingaporeSearch for more papers by this authorXiao-Qi Zhu, Xiao-Qi Zhu Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing, 210094 People's Republic of China Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576 SingaporeSearch for more papers by this authorYong-Xin Guo, Corresponding Author Yong-Xin Guo eleguoyx@nus.edu.sg Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576 SingaporeSearch for more papers by this authorWen Wu, Wen Wu Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing, 210094 People's Republic of ChinaSearch for more papers by this author First published: 09 January 2018 https://doi.org/10.1049/iet-map.2017.0658Citations: 11AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract A novel dual-band circularly polarised (CP) antenna is presented in this study. The proposed antenna is composed of a U-slot patch and four sequentially rotated shorted monopoles. Different from conventional omnidirectional CP antennas with shorted monopolar patches, a novel capacitively coupled technique is proposed to achieve good impedance matching, which makes it possible to generate unidirectional CP patterns simultaneously. This antenna can generate omnidirectional CP radiation patterns in the global positioning system L1 band (1575 ± 5 MHz) for navigation application and unidirectional CP radiation patterns in the 2.4-GHz industrial, scientific, and medical band (2.40–2.48 GHz) for municipal wireless networks. The dual-band performance can be tuned separately. Experimental results confirm the good performances of a compact antenna with the size of 0.26 λ0 × 0.26 λ0 × 0.026 λ0. The obtained 10-dB impedance bandwidths can cover the two required bands. The 3-dB axial ratio (AR) bandwidths cover the lower band and the upper band from 2.425 to 2.46 GHz. In addition, a wide 3-dB AR beamwidth of 200° is obtained at 2.45 GHz. The simple structure and good performances make this antenna a good candidate for handheld devices and vehicular applications. 1 Introduction Recently, omnidirectional circularly polarised (CP) antennas have attracted great attention because of the 360° full coverage. This feature can facilitate the tracking of satellites in navigation applications [[1]] and provide wide signal coverage in wireless communications [[2]], hence a variety of omnidirectional CP antennas have been reported at different operating frequencies, such as the global positioning system (GPS) bands [[3], [4]], radio-frequency identification (RFID) band [[5]], unmanned aerial vehicle (UAV) band [[6]] and wireless local area network (WLAN) band [[7]–[9]]. Various types of omnidirectional CP antennas have been proposed so far, including monopolar patch antennas with curved branches [[7]–[10]], tilted dipole array [[11]], bended monopoles [[12]], and dielectric resonator antennas [[13], [14]]. Among them, monopolar patch antennas have a low profile and compact size, which can be easily fabricated using the printed circuit board and via technology. For this type of antennas, the monopolar patch acts as an electric dipole and radiates vertically polarised components, while the curved branches form an electric loop and radiate horizontally polarised components. It is presented in [[3]] that by feeding a dipole and a loop radiator identically, the radiated vertically and horizontally polarised components are inherently orthogonal with the 90o phase difference, hence an omnidirectional CP radiation pattern can be obtained. Usually, the curved branches can be loaded either to the patches [[1], [7]] or to the ground planes [[8]–[10]]. Dual-band or multi-band antennas with both omnidirectional CP and unidirectional CP radiation patterns are able to perform the multi-function operation. However, such designs are rarely reported. Recently, a dual-band antenna with the two different types of radiation patterns is presented in [[15]], where a slotted patch with four curved branches is placed on electromagnetic band gap (EBG) cells. The omnidirectional and unidirectional CP waves are generated at the n = 0 and + 1 modes, respectively. Nonetheless, this antenna has a complicated structure. In this paper, we propose an alternative solution different from that in [[15]] with a simple and compact structure that can be easily realised. Moreover, most of the performances are better than those given in [[15]]. In the lower band, this antenna operates in the GPS L1 band (1575 ± 5 MHz) [[16]] with omnidirectional CP radiation patterns, which can provide a stable link with the GPS satellites. In the upper band, it operates in the 2.4-GHz industrial, scientific, and medical (ISM) band (2.40–2.485 GHz) with unidirectional CP radiation patterns, which can be used for municipal wireless networks. In this scenario, the CP waves can alleviate the multipath interference caused by scatters (e.g. building walls and ground surface). The dual-band performance can be tuned independently. By integrating the two functions into a single antenna portfolio, the proposed compact dual-band CP antenna can be a good candidate for handheld devices and vehicular applications [[17]]. 2 Antenna structure The configuration of the proposed antenna is shown in Fig. 1. It is composed of an asymmetric U-slot patch [[18]] and four sequentially rotated monopoles close to the patch. The monopoles are shorted to the ground plane through metallic pins. The total length of each monopole, including the height of shorting pin, is 37.35 mm ( = Ls + Lpd/2 + h), which is approximately a quarter guided wavelength at 1.575 GHz. The capacitive coupling between the U-slot patch and monopoles plays an important role in the impedance matching in the lower band [[7]]. This antenna is printed on an F4B255 (ɛr = 2.55, tan δ = 0.001) substrate with a thickness of 5 mm. The position of the feeding probe is slightly off the centre of the U-slot patch. The geometric parameters of the proposed antenna are listed in Table 1. Fig. 1Open in figure viewerPowerPoint Configuration of the proposed compact dual-band CP antenna (a) Top view, (b) Side view Table 1. Geometrical parameters of the proposed dual-band CP antenna (unit: mm) Dx Dy Ws Lp Lul 6 4.65 1.3 31.5 15.4 Du Df Rf Lu Lur 3.8 1.5 0.65 12.8 18.2 Wu Rp Lg Ls Lpd 1.3 0.5 50 31.1 2.5 3 Design procedure and operation principles Fig. 2 shows the design procedure of the proposed antenna structure. The procedure starts from Case-1, which is a centre-fed square patch antenna. In order to generate omnidirectional CP radiation patterns, four inverted-L shaped branches are loaded to the ground plane. However, Fig. 2b shows that the impedance matching the property of Case 1 is quite poor, and the minimum |S11| is only −1.4 dB at 1.62 GHz. Fig. 2c shows the simulated input impedances during the design procedure. It can be observed that the simulated reactances of Case 1 are purely inductive and the resonance is weak. To overcome this problem, conventional omnidirectional CP antennas usually utilise pins to short the patch to the ground plane to excite the TM01 mode or the monopolar patch mode [[7]–[9]]. However, this method is not suitable for our work because once the patch is shorted, it is difficult to reach our goal of achieving unidirectional CP radiation patterns simultaneously. In view of this issue, a novel antenna structure is proposed, as shown by Case 2. In this structure, four monopoles acting as curved branches are placed close to the patch and shorted to the ground plane. Fig. 2b shows clearly that the resonant frequency shifts to lower frequency and the impedance matching is improved greatly. The reason is that the capacitive coupling between the monopoles and the patch can effectively tune out the probe inductance which results in the impedance mismatch of Case 1 [[7]]. As shown in Fig. 2c, the sharp change of the simulated resistances and reactances of Case 2 reveals that the resonance is enhanced remarkably as compared to Case 1. Fig. 2d shows the simulated axial ratios (ARs) of Cases 1 and 2 in the azimuth plane. It can be seen that the ARs of Case 1 are lower than 3 dB in two bands, i.e. 1.50–1.554 GHz with co-polarisation of left-handed CP (LHCP) and 1.71–1.775 GHz with co-polarisation of right-handed CP (RHCP). It should be noted that the LHCP band is of no interest to us, since the GPS communication is realised using RHCP. While for Case 2, the RHCP band of Case 1 shifts downward and the GPS L1 band can be fully covered with the required polarisation. Fig. 2Open in figure viewerPowerPoint Design procedure of the proposed antenna and the simulation results (a) Structures of different antennas, (b)|S11|, (c) Input impedances, (d) ARs To reach our goal of achieving both omnidirectional and unidirectional CP patterns, an asymmetric U-slot is etched on the square patch, as shown by Case 3. Although U-slot antennas have been widely studied in the literature, this technique is employed here mainly for two reasons. First, it can excite a CP mode in the upper band with a wide impedance bandwidth without destroying the lower band current distribution [[15]]. Second, U-slot patch antennas can be centrally fed [[19]], which enables good impedance matching in the upper band and symmetric AR pattern in the lower band simultaneously. Fig. 2b shows that a wide bandwidth covering the 2.4 GHz ISM band is achieved by Case 3. Two resonance frequencies are observed in this band, which correspond to the two near-degenerated orthogonal modes for circular polarisation. Since the right arm of the U-slot is longer than the left arm, the co-polarisation of the CP waves at the broadside is RHCP. In addition, Fig. 2d reveals that the simulated ARs in the lower band are almost unaffected by the presence of the U-slot. For the purpose of analysing the operation principles for the omnidirectional and unidirectional CP properties, Figs. 3 and 4 show the simulated current distributions in the two bands, respectively. In Fig. 3, the simulated currents distribution on the U-slot patch, monopoles, feeding probe and pins at 1.575 GHz at t = 0 and t = T/4 are plotted, respectively, where T is the period of time. Generally, the current distribution is similar to that in [[12]]. Compared with the design in [[12]], the proposed antenna can eliminate additional feeding network because the monopoles are capacitively coupled to the U-slot patch. Fig. 3a shows that at t = 0, the dominant currents distribute on the monopoles and pins. The currents on the monopoles flow along the −φ direction, which behaves like an electric loop and radiates horizontally polarised field, i.e. the Eφ component. The currents on the pins flow along the −z direction and radiate vertically polarised field, i.e. the Eθ component. At t = T/4, the simulated surface currents on the monopoles and pins are quite weak. Since the currents on the monopoles and pins are in phase, the radiated Eφ and Eθ components are in phase quadrature [[3]], which leads to an omnidirectional CP pattern with co-polarisation of RHCP. To sum up, the shorted monopoles acts as curved branches in the lower band and the U-slot patch acts as a top-loaded monopole [[7]]. Fig. 3Open in figure viewerPowerPoint Simulated currents distributions on the patch, monopoles and pins at 1.575 GHz at different time instants (a) t = 0, (b) t = T/4 Fig. 4Open in figure viewerPowerPoint Simulated surface currents at 2.45 GHz at different time instants (a) t = 0, (b) t = T/4 In the upper band, Fig. 4 shows that the simulated surface currents on the U-slot patch are much stronger than those on the monopoles. Hence, unidirectional CP waves can still be generated by the asymmetric U-slot patch in spite of the surrounding monopoles. The anticlockwise rotation of the surface currents on the U-slot patch leads to the co-polarisation of RHCP in the broadside direction. 4 Parametric study and discussion In order to study the dual-band CP characteristics quantitatively, a parametric study of various geometric parameters is carried out using HFSS. Fig. 5 shows the simulated |S11| and ARs for different values of Ls. In the lower band, both the resonant frequency and the minimum AR frequency decrease as Ls increases. To achieve good impedance matching and AR performances in the GPS L1 band, the value of Ls is selected as 31.1 mm in this work. In spite of the slight deviation between the centre frequency and minimum AR frequency, the obtained 3-dB AR bandwidth can still cover the required GPS L1 band. Fig. 5b shows clearly that the simulated |S11| and ARs in the upper band are virtually unaffected by Ls. Hence, it can be concluded that the lower band resonance is primarily generated by the monopoles, which coincides well with the surface current distribution shown in Fig. 3. Moreover, the simulation also reveals that the lower band resonant frequency can be independently tuned from 1.47 to 1.71 GHz by adjusting Ls from 34 to 28.5 mm. These results are similar to that in Fig. 5 and are therefore not included here for brevity. The wide tuning range of the lower band suggests that after the upper band performance is optimised, the lower band can be easily tuned. Such feature is highly desirable as it greatly facilitating the design process. Fig. 5Open in figure viewerPowerPoint Simulated |S11| and ARs for different values of Ls (a) Lower band, (b) Upper band Fig. 6 shows the simulated antenna performance for various spacings Dx between the monopoles and the U-slot patch. In the lower band, both of the simulated resistances and reactances decrease remarkably as Dx increases. This phenomenon is mainly caused by the reduced coupling between the monopoles and the U-slot patch. Fig. 6b shows that both the resonant frequency and the minimum AR frequency in the lower band shift up as Dx increases and the minimum AR frequency increases faster. When Dx = 6 mm, the best impedance matching and AR performance can be obtained in GPS L1 band. In the upper band, Fig. 6c shows that the simulated resonant and minimum AR frequencies slightly shift downward as Dx increases, and the effect of Dx is much less significant than that in the lower band, which reveals that the monopoles only have a minor effect on the upper band performance. Fig. 6Open in figure viewerPowerPoint Simulated antenna performance for different values of Dx (a) Input impedances in the lower band, (b) |S11| and ARs in the lower band, (c) |S11| and ARs in the upper band Fig. 7 shows the simulated |S11| and ARs for different lengths Lur of the right arm of the U-slot. As shown in Fig. 7a, the lower band performance is virtually unaffected by Lur. In the upper band, the lower resonant frequency decreases as Lur increases, whereas the upper one remains unchanged. Hence, the impedance bandwidth is broadened. However, the AR performance will deteriorate as Lur increases. As a tradeoff, the value of Lur is selected as 18.2 mm in this work. Besides, the simulation also reveals that Lu and Du have a similar effect to Lur, which is not included here for simplicity. Hence, it can be verified that the upper band is basically excited by the U-slot patch, which agrees well with the design procedure. Generally, Fig. 7 reveals that by adjusting the size and position of the U-slot, the upper band performance can be tuned, keeping the lower band CP performance unaffected. Fig. 7Open in figure viewerPowerPoint Simulated |S11| and ARs for different values of Lur (a) Lower band, (b) Upper band At the optimised parameter values listed in Table 1, the simulated 10-dB impedance and 3-dB AR bandwidths in the lower band are 10 MHz (1.57–1.58 GHz) and 55 MHz (1.56–1.615 GHz), and the results in the upper band are 127 MHz (2.388–2.515 GHz) and 28 MHz (2.425–2.453 GHz), respectively. In the upper band, the proposed design can achieve a wide AR beamwidth, which is a highly desirable feature for CP antennas in wireless communications [[20]]. As an example, Fig. 8 shows the simulated AR beamwidths in the xoz plane for different values of Dx and the case without monopoles. It can be observed that compared with the case without monopoles, the proposed structure can broaden the AR beamwidth by 10%. In addition, the best AR beamwidth is obtained at Dx = 6 mm. Thus, it can be inferred that the simulated ARs at low elevation angles can be improved by the surrounding monopoles, which could be intuitively interpreted by the surface current distribution. It is shown in Fig. 4a that at t = 0, the surface currents on the U-slot patch mainly distribute along the −x direction, while the surface currents on the two x-directed monopoles flow along the reverse direction, i.e. the + x-axis. Thus, the resultant Eθ pattern in the xoz plane can be expressed as the subtraction between the radiation patterns of them [[20]]. At low elevation angles, the Eθ components radiated by the U-slot patch and the two monopoles are relatively low. At t = T/4, the simulated surface currents on the U-slot patch and the two y-directed monopoles are distributed along the −y and + y directions, respectively. Hence, the effective radiation aperture of the U-slot patch is reduced by the out-of-phase currents on the monopoles, which will generate an Eφ pattern with a wide beamwidth in the xoz plane. Moreover, the Eφ components at low elevation angles radiated by the U-slot patch are decreased because of the cancellation of the two monopoles. As a result, the resultant Eφ pattern is quite approximate to the Eθ pattern, and a wide AR beamwidth is achieved. Fig. 8Open in figure viewerPowerPoint Simulated upper band AR beamwidths in the xoz plane for different values of Dx and the case without monopoles 5 Measurement results To validate the proposed design, a prototype is fabricated and measured. Fig. 9 shows the photograph of the prototype, the simulated and measured |S11|, ARs and peak gains in the two bands. A reasonable agreement is obtained between the simulated and measured results. The slight deviation could be attributed to the fabrication tolerance. As shown in Fig. 9a, the measured 10-dB impedance and 3-dB AR bandwidths in the lower band are 11 MHz (1.572–1.583 GHz) and 60 MHz (1.563–1.623 GHz), respectively. The measured minimum AR is 0.36 dB at 1.585 GHz, which implies good polarisation purity. The measured peak gain of 0.91 dBic is close to the simulated value of 1.43 dBic. In the upper band, Fig. 9b shows that the measured 10-dB impedance bandwidth is 135 MHz (2.386–2.521 GHz) and the measured |S11| across the 2.4 GHz ISM band are below −15 dB, which coincides well with the simulation. The measured 3-dB AR bandwidth is 35 MHz (2.425–2.46 GHz) and the minimum AR is 0.57 dB at 2.445 GHz. In addition, the measured peak gains across the 2.4 GHz ISM band are above 5 dBic. Fig. 9Open in figure viewerPowerPoint Simulated and measured |S11|, ARs and gains the proposed dual-band CP antenna (a) Lower band, (b)Upper band Fig. 10 shows the simulated and measured ARs at different elevation angles in the upper band. The simulated results are related to the optimised parameters. It can be observed that the proposed antenna can achieve wide AR beamwidths in both xoz and yoz planes. In the xoz plane, the simulated and measured 3-dB AR beamwidths are 210° and 222°, respectively, and the results in the yoz plane are 205° and 225°. The slight deviation could be attributed to the measurement setup. Fig. 10Open in figure viewerPowerPoint Simulated and measured ARs at different elevation angles at 2.45 GHz Fig. 11 shows the measured radiation patterns in the two bands. At 1.575 GHz, an omnidirectional radiation pattern with RHCP is obtained, and the measured gain ripple in the azimuth plane is only 1.83 dB, which indicates a good omnidirectional property. At 2.45 GHz, a unidirectional radiation pattern with RHCP is obtained at the broadside and the measured gain is 6.8 dBic. In addition, Fig. 11b shows that the cross polarisations are very low over a wide angular range, hence a wide AR beamwidth can be achieved. In the xoz and yoz planes, the measured 3-dB AR beamwidths are more than 200°. Fig. 11Open in figure viewerPowerPoint Simulated and measured radiation patterns in the two bands (a) 1.575 GHz, (b) 2.45 GHz A comprehensive performance comparison between the reported works and the proposed antenna is shown in Table 2. It can be seen that the antenna in [[10]] occupies a large space and has a high profile in spite of the wide impedance bandwidths. Besides, the radiated unidirectional patterns are linearly polarised. For the design in [[21]], the radiated omnidirectional waves are linearly polarised, and the gain is only −5.4 dB. Among the listed designs, only the antenna in [[15]] and the proposed antenna can realise both omnidirectional CP and unidirectional CP properties. However, the antenna in [[15]] has a large size and the antenna structure is complex. In the lower band, the two designs exhibit similar performances. In the upper band, the proposed design with a compact size of 0.26λ0 × 0.26λ0 × 0.026λ0 can achieve wider impedance and AR bandwidths, a higher gain and wide AR beamwidth. Table 2. Comparison of antenna performances between reported works and the proposed antenna Antenna type Slotted patch + parasitic patches + curved ground [[10]] Spiral patch-slot antenna + EBG structure [[15]] Shorted patch + corner truncated patch [[21]] U-slot patch + bended monopoles (this work) dimension (λ03) π × 0.30 × 0.30 × 0.045 0.42 × 0.60 × 0.015 0.18 × 0.18 × 0.04 0.26 × 0.26 × 0.026 complexity low high high low f0, GHz, O/U 4.42/5.74 1.789/2.053 2.38/5.8 1.575/2.45 polarisation O/U C/L C/C L/C C/C RLBW, %, O/U 5.1/14.9 0.7%/2.2 2.8/9.0 0.7/5.5 ARBW, %, O/U 2.0/— NA/∼1 −/2.5% 3.8/1.4 gain, dBic, O/U 1.1/7.1 1.4/5.8 −5.4/3.28 0.91/6.8 AR beamwidth U, deg. — NA ∼90 200 f0: centre frequency, O: omnidirectional radiation patterns, U: unidirectional radiation patterns, L: linear polarisation, C: circular polarisation, RLBW: 10 dB return loss bandwidth, ARBW: 3 dB AR bandwidth, NA: not available. 6 Conclusion A compact dual-band CP antenna is presented in this paper. Since the conventional matching method using shorted monopolar patches is not suitable for our work, a novel capacitively coupled technique is proposed to achieve the impedance matching, which also enables this design to generate unidirectional CP patterns simultaneously. The design procedure and the operation principles to realise the omnidirectional and unidirectional CP radiation patterns in the two bands are explained. The obtained dual-band performance can be tuned separately. This feature provides the proposed antenna with more design flexibility. There is still room for the improvement of the 3-dB AR bandwidth in the upper band, which will be a topic of our future research. The two types of CP radiation patterns can be applied for navigation and municipal wireless networks, respectively, which makes the proposed design a good candidate for handheld devices and vehicular applications. 7 Acknowledgments This work was supported by the National Natural Science Foundation of China under grant no. 61372012. 8 References [1]Park, B.C., Lee, J.H.: 'Omnidirectional circularly polarized antenna utilizing zeroth-order resonance of epsilon negative transmission line', IEEE Trans. Antennas Propag., 2011, 59, (7), pp. 2717– 2721 [2]Hsiao, F.R., Wong, K.L.: 'Low-profile omnidirectional circularly polarized antenna for WLAN access points', Microw. Opt. Technol. Lett., 2005, 46, (3), pp. 227– 231 [3]Li, B., Liao, S.-W., Xue, Q.: 'Omnidirectional circularly polarized antenna combining monopole and loop radiators', IEEE Antennas Wirel. Propag. 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