Open Access
Add to BibSonomy
Abstract
We implemented a novel compact antenna by applying a metasurface with stereo elements (stereo-MS) as the superstrate of a patch antenna. The stereo-MS, an array of stereo patches printed on a grooved dielectric substrate, enabled the footprint miniaturization and bandwidth enhancement of the patch antenna. The overall size reduction of the stereo-MS antenna is over 38% compared with the conventional plane metasurface (plane-MS) antenna working in the same frequency range. A prototype antenna working at 5.3 GHz was designed, fabricated, and measured. Experiments demonstrated the fractional impedance bandwidth of the antenna was 44.5% at criteria |S11 |< −10 dB, covering the frequencies 4.18 to 6.56 GHz, and the average gain about 6.9 dBi in the band. Experimental results were found in very good agreement with the design, which confirms the functionality of stereo-MS in antenna minimization. Our antenna features a compact size (0.409 $\lambda _0^2$) and low profile (3.024 mm). The stereo-MSs provide a new way for the size miniaturization of microwave and optical devices, such as antennas.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Microstrip patch antennas are widely used as radiating elements in modern communication systems for the merits of light weight, low-profile, and low cost. However, the patch antennas suffer from a narrow impedance bandwidth and low gain. These inherent drawbacks greatly restrict their application. Many techniques have been developed to enhance the bandwidth of microstrip patch antenna, for example, U/E-shaped slot in the patch [1,2], aperture coupling [3], L-shaped probe feeds [4], and stacked patches [5]. All the methods can improve the bandwidth to some extent. Unfortunately, they are at the expense of antenna’s height. Therefore, it is a challenge to achieve an antenna with a compact size and low profile while it has a wide bandwidth.
Metasurface (MS) is the 2-D counterpart of metamaterial, which is in extensive studies. Because of its exotic electromagnetic properties, MS is applied to improve the performance of antennas [6]. When integrated with microstrip patch antennas, MS shows the ability to enhance gain, enlarge bandwidth, and improve the radiation efficiency of the antennas [7–9]. Recently, a lot of efforts have been devoted to miniaturize the size of antennas by MS. A common way to realize a compact antenna is to place the MS under the antenna as a reflector, which can achieve a profile of less than a quarter-wavelength. Another method is to use the MS to excite the low-frequency resonances, for example, the compact antenna realized by multilayered MS [10]. By applying MS with complementary split-ring resonators (CSRR), the antenna has an overall footprint of only 0.33λ0×0.33λ0. Also, the MSs are used in design low-profile wideband antennas [11]. For example, as the artificial ground planes of antennas, the MSs with interdigitated capacitor are investigated to achieve low profile antennas [12]. The thickness of the antenna is reduced to less than a quarter-wavelength. An increase in the equivalent capacitance of the MS leads to lower resonant frequencies [13]. A further reduction of MS antennas can be realized by eliminating the air gap between the radiation patch and MS [14–17] or using a high dielectric constant substrate that may be narrow the bandwidth and the reduce radiation efficiency of MS antennas.
In this work, we implement a new compact and low-profile MS antenna by applying the MS fabricated by stereo elements as a superstrate. The antenna is composed of a hexagonal patch radiator sandwiched between the stereo-MS superstrate and a ground plane. By using stereo-MS superstrate, the MS antenna reduces its size by 38.5% compared with the same antenna with plane-MS superstrate. Meanwhile, the stereo-MS superstrate enlarges the bandwidth of the antenna. Our antenna provides a new way for compact, low profile, and wideband antennas that are highly desired in communication systems.
2. Metasurface with stereo element
The previous studies demonstrate that the minimization of patch antenna by conventional plane-MS is associated with the excitation of surface waves on MS [18]. To design a MS with surface modes at a small electric size, a possible way is making the MS by a stereo element. The stereo elements enlarge the electric dimension of elements at a given unit cell size, leading to an electrically small unit cell to the conventional plane-MS.
Considering the MS made of wrinkled metallic patches whose unit cell is shown Fig. 1(a). In practice, good conductors such as copper and gold are used as the metal layer. Because of their high conductivity, the penetration depth is usually at sub-micron scale at microwave frequencies. Therefore, these good conductors are often considered as the perfect electrical conductors (PEC). For simplicity, we suppose the metal patches are made of PEC and they are patterned on the grooved dielectric substrate FR4B whose dielectric constant and loss tangent are ɛr1 = 2.65 and tan δ = 0.001, respectively. The depth of the groove is hg = 0.75 mm, very small compared with the working wavelength λ0.
Fig. 1. Schematics of the unit cell of (a) proposed stereo-MS element, and (b) conventional plane-MS element. (c)-(d) Dispersive curves of MS with stereo element and MS with plane element. The results are obtained at parameters: h1 = 1.5, hg = 0.75, g1 = 0.5, g2 = 0.8, p = p1 = 8, all in millimeters.
We calculated the dispersion of stereo-MS along its surface by eigenmode solver of commercial software HFSS. Here, only the TM mode was considered. In the calculations, the unit cell was enclosed in an air box, and periodic boundary conditions were used to mimic an infinite MS. Figure 1(c)-1(d) show the results of the calculations. The dispersion curves are below the light line, as displayed in blue balls in Fig. 1(c). With the frequency increase, the dispersion curve deviates gradually from the light line and reaches a saturation status, where the slope of the curves achieves zero. The dispersion curve has the same form of surface plasmon. The cut-off frequency or the surface plasmon resonance frequency is about 8.2 GHz. As a comparison, we plot the dispersion curve for the conventional plane-MS of the same dimension in red stars. The cut-off frequency is about 10.2 GHz, higher than the frequency of stereo-MS. We conclude the stereo element reduces the cut-off frequency of MS.
The cut-off frequency reduction can be considered also as a plane-MS using a substrate with higher permittivity. As an example, displayed in Fig. 1(b), we simulated the stereo-MS by a plane MS where its substrate permittivity takes ɛr = 4.6. The calculated dispersion curve is plotted in Fig. 1(d) in red stars. We see the dispersion curve is the same as that of the stereo-MS, which indicates the stereo-MS effectively increase the permittivity of the substrate. This in turn lowers the dimension of the antenna.
In antenna applications, MS is used as a substrate or superstrate. Here, we consider the MS as the superstrate, as schematically displayed in Fig. 2(a). The stereo-MS and its dielectric substrate (FR4B) are on the top of other dielectric (Rogers RO4003) that is the substrate of the patch antenna. The material parameters of Rogers RO4003 are dielectric constant ɛr2 = 3.38 and loss tangent tan δ = 0.0027. Our MS and ground plane can be considered as a cavity, and its radiation modes are studied by the characteristic mode analysis (CMA) [19,20]. The radiation modes will have a big value of modal significance. In the analysis, the thicknesses of two dielectrics are h1 = 1.5 and h2 = 1.524 mm, respectively, and other parameters are the same as those used in Fig. 1. Simulation results are plotted in Fig. 2(b) where the modal significance of the first four modes is given. As shown in Fig. 2(b), modes 1 and 2 are at resonance with the same modal significance at the frequency 4.75 GHz. The other two modes have the same resonant frequencies 5.5 GHz and share a similar trend of variation against frequency. The modal significance shows that the cavity can radiate EM waves efficiently when the feed source excites these modes. In contrast, for plane-MS shown in Fig. 2(c), the modes 1 and 2 have a big significance value at 6.85 GHz, while modes 3 and 4 at even higher frequencies, as displayed in Fig. 2(d). The results imply that the stereo-MS can significantly reduce the frequency of radiation modes of the cavity, while the bandwidth of the antenna can be enhanced if all these cavity radiation modes are excited at the same time.
Fig. 2. (a) and (c) The schematics of simulation setup. The substrate 1 and 2 are the dielectrics FR4B and Rogers RO4003, respectively. (b) and (d) The modal significances of the stereo-MS and conventional plane-MS. The calculations are performed by commercial solver CST Studio Suite.
3. Antenna design with stereo-MS
3.1 Antenna configuration and performance
We designed an antenna using a stereo-MS superstrate, which is schematically displayed in Fig. 3. A radiator is placed between the two dielectrics, and no air gap is between the dielectrics. The patch radiator is a hexagonal metallic patch with three slots. The slots and an extended stub are designed for impedance matching [21]. A 50 Ω coaxial probe is soldered to the extended stub. The whole antenna has a lateral dimension of 0.32$\lambda _0^2$ and thickness of 0.053λ0, where λ0 = 56.6 mm is the wavelength of the central working frequency of the antenna.
Fig. 3. Configuration of the stereo-MS antenna. The optimized dimensions are L = 33, p = 8, g1 = 0.5, g2 = 0.8, l = 13.2, w = 13.5, wc = 6.05, lm = wm = 3.5, ls = 3.6, ws = 0.4, lf = 8.5, all in millimeters.
With stereo-MS superstrate, our antenna achieved a more compact size and wideband. As a demonstration, we compare our antenna and the one with plane-MS superstrate. The plane-MS antenna is similar to the stereo one except the stereo-MS replaced by plane one as shown in Fig. 1(b). To keep the central working frequency, the unit size of the plane-MS takes p1 = 10.2 mm. Figure 4(a) displays the return loss |S11| of two antennas. We see the stereo-MS antenna has wider impedance bandwidth than the plane-MS antenna. At the criteria of |S11| < −10 dB, the frequency band of the stereo-MS antenna spans from 4.15 to 6.46 GHz (fractional bandwidth 45%) while the plane-MS antenna spans 4.34–6.26 GHz (36%). The use of stereo-MS gets a further size reduction of about 38.5% than the plane-MS one. We did a parametric study of stereo-MS on impedance bandwidth. The simulated results are plotted in Figs. 4(b)–4(c), respectively. Decreasing the width g1 of the slot and the width g2 of the groove cause a lower resonant frequency. Simultaneously, g2 has little effect on antenna performance, which provides useful guidance for practical design.
Fig. 4. Comparison of |S11| (a) and gain (d) between reference antenna, patch antenna and stereo-MS antenna. Simulated |S11| of the stereo-MS antenna with (b) a different g1 and (c) a different g2. Simulated radiation patterns in (e) E- (f) H-planes at 4.4, 5.5 and 6.3 GHz.
The stereo-MS antenna has a good radiation performance in a wider frequency range. Figure 4(d) gives the simulated gain of the MS-based antenna and the patch antenna. It is clear that the use of the stereo-MS superstrate significantly improves the gain of the patch antenna. Within the frequency band of |S11| < −10 dB, the gain of the antenna is around 7.34 dBi. The antenna has a stable and higher gain compared with the patch antenna. However, our antenna has a little lower gain than the plane-MS antenna, which is due to the latter has a larger total footprint leading to a bigger radiation aperture.
Figures 4(e) and 4(f) present the simulated radiation patterns of the stereo-MS antenna in E- and H-planes at resonant frequencies of 4.4, 5.5, and 6.3 GHz. From the simulated results, we observe broadside radiation patterns across the achieved bandwidth. The half-power beam widths (HPBWs) in the E-plane are 83°, 79°, and 64° at the frequencies 4.4, 5.5, and 6.3 GHz, respectively. The HPBWs in H-plane are 84°, 69°, and 65° at the frequencies. Moreover, the front-to-back (FB) ratio over the operating frequencies is greater than 14 dB and up to 62 dB. The simulation results demonstrate our antenna having a broadband radiation performance.
3.2 Operation mechanism of the antenna
Figure 4(a) shows multiple radiation resonances are excited in our stereo-MS antenna, which widens the working bandwidth. To understand the working mechanism underlying, free-source CMA is performed on our antenna. In the analysis, the simulation setup of the antenna is the same as that in Fig. 2(a). Besides, the radiate patch is in the between of the stereo-MS and dielectric substrate 2.
We first calculate the modal significances of the stereo-MS antenna. The results are shown in Fig. 5(a). It can be seen that modal significance is greater than 0.9 at some frequencies from 4.3 to 6.78 GHz, which means that at those frequencies the radiation modes can be excited easily. The frequencies at the peaks of modes 1, 4, and 7 are close to three resonances of our antenna [see Fig. 4(a)]. To demonstrate that these modes are related to the resonances of the stereo-MS antenna, we evaluate the current distributions of the characteristic modes and that of our antenna at resonance. The mode currents of characteristic modes are illustrated in Figs. 5(b)–5(d), and current distributions of our antenna at resonances are in Figs. 5(e)–5(g). One can find the distributions of the mode currents is the same as that of surface currents of the antenna at resonance. However, for other characteristic modes, the current is orthogonal to the surface currents (not shown here). Therefore, those three radiation modes contribute to the radiation of our antenna.
Fig. 5. (a) Modal significances of the stereo-MS antenna; (b)–(d) Simulated modal currents (CST). Jn represents the modal surface current of mode n. (e)-(g) Surface currents (HFSS) of the stereo-MS antenna at 4.4, 5.6 and 6.3 GHz.
The radiation modes displayed in Figs. 5(b)–5(d) have a different origin. To illustrate this fact, we firstly analyze the radiation modes of the patch antenna by CMA. Figure 6(a) shows the modal significance of the first two radiation modes. We can see the 2nd radiation mode has the peak value at 6.54 GHz; the frequency is close to the 7th radiation mode of our antenna [see Fig. 5(a)], indicating it is caused by patch antenna. To make a confirmation, we checked the modal current of the radiation patch and the stereo-MS antenna. The results are displayed in Figs. 6(b)–6(c). It is clear that in the two cases the current distributions are almost the same, implying the 7th radiation mode of our antenna indeed comes from the radiation patch. Thus, the other two radiation modes are associated with cavity modes. We plot their modal currents, and the results are plotted in Figs. 6(d) and 6(e). One can find that the modal current of the stereo-MS antenna is the same as that of the cavity modes shown in Figs. 5(b) and 5(c), which confirms the 1st and 4th radiation modes of the stereo-MS antenna origins the cavity modes. At radiation resonances, the electric field distributions in the stereo-MS antenna indicate they operate, respectively, in TM10 and TM20 modes [22] as displayed in Figs. 6(f) and 6(g). A small frequency shift between the radiation modes of the cavity and the stereo-MS antenna may be due to the effect of the radiation patch.
Fig. 6. (a) Modal significances and (b) modal currents of patch antenna; (c) Modal currents of Stereo-MS antenna. The modal currents corresponding to (d) J1 and (e) J3 of cavity; Simulated E-field distributions of stereo-MS antenna in y = 16.4 mm plane at (f) 4.40 and (g) 5.56 GHz.
4. Experiment and discussion
Experimentally, a prototype antenna was fabricated and tested. Figure 7 shows the image of the fabricated antenna. It has an overall size of 37 mm×37 mm, which is about 0.64λ0×0.64λ0 at the center operating frequency of 5.3 GHz. The stereo-MS was fabricated by 18 µm thick copper layer and 0.025 µm thick gold layer. It was plated on the grooved dielectric FR4B. The radiation patch was made on the Rogers RO4003 substrate by standard PCB technology. Four nylon nut-spacer-screw sets are applied to fix the stereo-MS and the patch antenna. They improve the mechanical robustness of the antenna. Due to these nylon screws at the corners, we slightly change the antenna dimension to L = 37 mm, wc = 6.55 mm and ls = 5.5 mm; other parameters remain unchanged. In the test, the reflection coefficient of the antenna was measured by a Vector Network Analyser (PNA-X, N5247A) via a 50 Ω coaxial cable. The gain and radiation patterns were measured in an anechoic chamber.
Figure 8(a) plots the measured |S11| and realized boresight gain of our antenna. The experimented results show the impedance bandwidth (S11<-10 dB) is about 2.38 GHz, covering the frequencies from 4.18 to 6.56 GHz, at fractional bandwidth of about 44.5%. The results are in good agreement with the simulation ones, which are 2.29 GHz and 43%, respectively. The impedance bandwidth slightly shifted to a higher frequency. We find the discrepancies between the results are due to the fabrication impaction of the metal layer, which can be observed in Fig. 7(a). These defects reduce the effective size of the stereo-MS element, leading to the curve a small shift to a higher frequency.
Fig. 8. (a) The simulated and measured reflection coefficients for |S11|, as well as the gain. (b) E-plane and (c) H-plane radiation patterns at 5.3 GHz.
The maximum gain measured is up to of 7.7 dBi at 4.5 GHz. The average gain in the working band is 6.9 dBi, consistent with the simulation one of 7.58 dBi. Figures 8(b) and (c) show the measured and simulated radiation patterns in both the E- and H-planes at 5.3 GHz. In the E-plane, HPBW is 65° in experiments, while it is 75° in simulation. In the H-plane they are 66° and 68°, respectively. The experiments and simulations show a good agreement.
We compare performance of our antenna and other wideband MS-based antennas reported. The results are summarized in Table 1. From the table, we can see that our antenna has the smallest footprint, lowest profile and widest impedance bandwidth. In terms of antenna gain, our antenna has a very good performance. It is worth mentioning that the higher gain reported in [17], [19] and [23] is at the expense of antenna size. All the antennas have broadside radiation patterns.
Table 1. Propertomparison among different antennas with the proposed designa
5. Conclusion
We have introduced metasurface with stereo elements for patch antenna miniaturization. The stereo-MS effectively reduces the surface plasmon resonance frequency of the plane-MS in the same dimension, which helps the size reduction of conventional plane-MS antennas. By applying stereo-MS into a patch antenna, we implement a compact antenna with an overall size of 0.64λ0× 0.64λ0×0.053λ0, which realizes another size reduction of about 38.5% from conventional plane-MS antenna working at the same frequency. Our antenna also improves its bandwidth and gain remarkably. The antenna achieves an impedance bandwidth of 44.5% at |S11| < −10 dB, and the average gain about 6.9 dBi in the band. Measurements demonstrate the good performances of our antenna, which are in good agreement with simulations. The working mechanism of our antenna and its effects on the antenna performance are discussed. Our design provides a new way for antenna minimization, which is promising for various wireless communication systems. The method can be also applied to size minimization of the devices working in THz, infrared, and optical frequency range.
Funding
Priority Academic Program Development of Jiangsu Higher Education Institutions; Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Waves; National Natural Science Foundation of China (61771237).
Disclosures
The authors declare no conflicts of interest.
References
1. C. K. Hsu and S. J. Chung, “Compact antenna with u-shaped open-end slot structure for multi-band handset applications,” IEEE Trans. Antennas Propag. 62(2), 929–932 (2014). [CrossRef]
2. H. Malekpoor and S. Jam, “Miniaturised asymmetric E-shaped microstrip patch antenna with folded-patch feed,” IET Microw. Antennas Propag. 7(2), 85–91 (2013). [CrossRef]
3. S. D. Targonski and D. M. Pozar, “Design of wideband circularly polarized aperture-coupled microstrip antennas,” IEEE Trans. Antennas Propag. 41(2), 214–220 (1993). [CrossRef]
4. K. M. Mak, H. W. Lai, and K. M. Luk, “A 5G wideband patch antenna with antisymmetric l-shaped probe feeds,” IEEE Trans. Antennas Propag. 66(2), 957–961 (2018). [CrossRef]
5. D. Sun, W. B. Dou, L. Z. You, X. Q. Yan, and R. Shen, “A broadband proximity-coupled stacked microstrip antenna with cavity-backed configuration,” IEEE Antenna Wireless Propag. Lett. 10(3), 1055–1058 (2011). [CrossRef]
6. S. X. Ta and I. Park, “Compact wideband circularly polarized patch antenna array using metasurface,” IEEE Antenna Wireless Propag. Lett. 16, 1932–1936 (2017). [CrossRef]
7. C. F. Zhou, S. W. Cheung, Q. L. Li, and M. Li, “Bandwidth and gain improvement of a crossed slot antenna with metasurface,” Appl. Phys. Lett. 110(21), 211603 (2017). [CrossRef]
8. G. R. Feng, L. Chen, X. S. Xue, and X. W. Shi, “Broadband surface-wave antenna with a novel non-uniform tapered meta-surface,” IEEE Antenna Wireless Propag. Lett. 16, 2902–2905 (2017). [CrossRef]
9. H. Malekpoor and S. Jam, “Improved radiation performance of low profile printed slot antenna using wideband planar AMC surface,” IEEE Trans. Antennas Propag. 64(11), 4626–4638 (2016). [CrossRef]
10. T. W. Yue, Z. H. Jiang, A. H. Panaretos, and D. H. Werner, “A compact dual-band antenna enabled by a complementary split ring resonator loaded metasurface,” IEEE Trans. Antennas Propag. 65(12), 6878–6888 (2017). [CrossRef]
11. W. E. I. Liu, Z. N. Chen, X. M. Qing, and F. H. Lin, “Miniaturized wideband metasurface antennas,” IEEE Trans. Antennas Propag. 65(12), 7345–7349 (2017). [CrossRef]
12. T. W. Yue, Z. H. Jiang, and D. H. Werner, “Compact, Wideband Antennas Enabled by Interdigitated Capacitor Loaded Metasurfaces,” IEEE Trans. Antennas Propag. 64(5), 1595–1606 (2016). [CrossRef]
13. X. M. Li, J. J. Yang, F. Yun, M. X. Yang, and M. Huang, “Compact and broadband antenna based on a step-shaped metasurface,” Opt. Express 25(16), 19023–19033 (2017). [CrossRef]
14. Y. M. Pan, P. F. Hu, X. Y. Zhang, and S. Y. Zheng, “A low profile high gain and wideband filtering antenna with metasurface,” IEEE Trans. Antennas Propag. 64(5), 2010–2016 (2016). [CrossRef]
15. S. X. Ta and I. Park, ““Low-profile broadband circularly polarized patch antenna using metasurface,” IEEE Trans. Antennas Propag. 63(12), 5929–5934 (2015). [CrossRef]
16. J. Chatterjee, A. Mohan, and V. Dixit, “Broadband circularly polarized H-shaped patch antenna using reactive impedance surface,” IEEE Antenna Wireless Propag. Lett. 17(4), 625–628 (2018). [CrossRef]
17. W. Liu, Z. N. Chen, and X. M. Qing, “Metamaterial-based low-profile broadband mushroom antenna,” IEEE Trans. Antennas Propag. 62(3), 1165–1172 (2014). [CrossRef]
18. F. Costa, O. Luukkonen, C. R. Simovski, A. Monorchio, S. A. Tretyakov, and P. M. de Maagt, “TE Surface Wave Resonances on High-Impedance Surface Based Antennas: Analysis and Modeling,” IEEE Trans. Antennas Propag. 59(10), 3588–3596 (2011). [CrossRef]
19. F. H. Lin and Z. N. Chen, “Low-profile wideband metasurface antennas using characteristic mode analysis,” IEEE Trans. Antennas Propag. 65(4), 1706–1713 (2017). [CrossRef]
20. Y. K. Chen and And C. F. Wang, “Characteristic Modes: Theory and Applications in Antenna Engineering”, Wiley Publishing, 2015.
21. W. W. Yang, J. Y. Zhou, Z. Q. Yu, and L. S. Li, “Single-fed low-profile broadband circularly polarized stacked patch antenna,” IEEE Trans. Antennas Propag. 62(10), 5406–5410 (2014). [CrossRef]
22. C. A. Balanis, “Antenna theory: analysis and design”, 2nd ed. Wiley, New York, NY, USA, 1997.
23. W. Liu, Z. N. Chen, and X. M. Qing, “Metamaterial-based low-profile broadband aperture-coupled grid-slotted patch antenna,” IEEE Trans. Antennas Propag. 63(7), 3325–3329 (2015). [CrossRef]
24. X. M. Li, J. J. Yang, Z. G. Chen, P. S. Ren, and M. Huang, “Design and characterization of a miniaturized antenna based on palisade-shaped metasurface,” Intl. J. Antennas Propag. 2018, 1–9 (2018). [CrossRef]
