Abstract
A palisade-shaped metasurface (PSMS) is presented to miniaturize the micropatch antenna. With the aid of the metasurface, a footprint miniaturization is obtained, and the dual resonant modes are produced simultaneously. Furthermore, through analyzing the dispersion curve of the metasurface to optimize the structure, the proposed antenna achieves a compact structure with a maximum size of 0.38; a low profile of 0.08 (where is the free-space wavelength at 5.0 GHz) exhibits a 19.6% impedance bandwidth () and an average gain of 6.76 dBi across the bandwidth.
1. Introduction
Metasurfaces (MSs), which are artificial metamaterials with repetitive microstructured metallic or dielectric inclusions, have attracted considerable attention in recent years [1–4]. MSs can be divided into several types, including electromagnetic band-gap (EBGs) structures, reactive impedance surfaces (RISs), and high impedance surfaces (HISs) or artificial magnetic conductors (AMCs) [5–8]. Owing to their exotic electromagnetic properties, MSs have been widely applied in the design of microwave absorbers, thin planar lens, filters, and so on [9–11].
Recently, great interest has been devoted to miniaturization of antenna using MSs. In [12], the authors demonstrated that RIS substrate-assisted antenna has the advantages of enhanced bandwidth, miniaturized size, and reduced antenna back radiations. It has been illustrated that a slotted-slit-microstrip patch on a RIS substrate was able to further reduce the antenna size and improve the radiation performance [13]. In comparison to the conventional single-feed circularly polarized (CP) antennas, the combination of fractal MS and fractal resonator has been explored for compact CP antennas [14]. Several patch antennas loaded with resonators and a RIS have been proposed and comprehensively investigated [15]. The RIS is employed to store the magnetic energy and increases the inductance value of the patch-type resonance, which provides decreased resonance frequencies of patch in this way enabling antenna miniaturization. MSs can also be used to reduce the profile of antenna. For example, a single-feed CP patch antenna based on MSs [16] has been implemented to achieve a profile of less than 0.056 and 23.4% axial ratio bandwidth. Low-profile antennas comprising a dipole antenna above a HIS are reported in [17], and the TE surface wave resonances are analyzed thoroughly. Furthermore, MSs can improve the performance of antenna, such as broaden impedance bandwidth, enhance gain, and achieve polarization conversion [18–21]. Recently, a class of compact and wideband MS-based antennas have been achieved by applying various interdigitated capacitive (IC) MSs, and it is shown that the polarization characteristics of the antenna can be adjusted by these IC MSs [22].
In this paper, a compact metasurface antenna constructed by palisade-shaped metasurface (PSMS) and a monopole is proposed. Numerical simulation shows that the PSMS can not only improve the performance of the antenna but also move the working frequency to the lower frequency side which give rise to a miniaturization of the antenna. In what follows, the structure of the metasurface is introduced, and the performance of the PSMS antenna is analyzed in detail.
2. Antenna Configuration and PSMS Unit Analysis
2.1. Antenna Configuration and Metasurface Design
The proposed compact antenna is enabled by placing a PSMS underneath a planar monopole, of which a schematic view is shown in Figure 1(a). The PSMS is composed of a 3 by 4 periodic array of patches with slot-loaded palisade structures printed on a grounded dielectric substrate. The monopole is comprised of a rectangular patch fed by a 50 Ω microstrip line printed on substrate and a partial ground plane with the pairs of square stubs printed on the other side, as shown in Figure 1(b). The monopole antenna was placed 0.7 mm above the PSMS; the dielectric substrate of Rogers RO3003 (, ) and Glass Epoxy () is used for the two components, respectively. In addition, the resulting PSMS antenna has a total footprint of 0.38 with a low profile of only 0.08 at frequencies around 5 GHz. Optimized geometry parameters are shown in the caption of Figure 1. The design of the PSMS antenna consists of three steps. The first is the design of the monopole. The second is the design of the unit cell of the metasurface, which has a lower resonant frequency than the monopole, thus achieving a miniaturized electrical volume. The last is the optimization of the metasurface-loaded antenna.
The design of the PSMS is the key point of the antenna. It begins with determining the electromagnetic response of an infinite array of unit cells which is enclosed by an air box, as depicted in Figure 2(a). The simulations were performed using Ansys’ HFSS software. The unit cell is illuminated by a normally incident plane wave with electric field polarized along the x axis. To mimic an infinite PSMS, periodic boundary conditions are applied to the unit cell in the simulation domain. An equivalent circuit for the proposed unit cell is shown in Figure 2(b). Its inherent resonance frequency is determined by where and represent the equivalent inductance and capacitance of the MS, respectively. represents the inductance of substrate which is mainly determined by the dielectric constant and the thickness of substrate. It should be noted that the resonant frequency will be decreased when increasing and , but the profile and resonant bandwidth of the unit cell will become larger and narrower which is not desirable. In this paper, we attempt to make effort to decrease the resonant frequency by increasing the value of .
Reflection phase and magnitude which indicate the input-match frequency band and the relative level of Radio Frequency (RF) energy that is absorbed by the unit cell are plotted in Figure 3(a). The final optimized dimensions of the unit cell are set as slot gap mm, finger gap mm, and fingers length mm. The influence of on the reflection phase and the magnitude is shown in Figure 3(b). It is observed that a miniaturized electrical dimension can be achieved by decreasing since the resonant frequency shifts to the lower frequency side, but this will give rise to a narrower fractional bandwidth. The fractional bandwidth is defined as the frequency region of the reflection phase in the range ± 90°. The reflection phase and the corresponding reflection magnitude diagrams for an increase of the substrate height are shown in Figure 3(c), which indicates a positive correlation between a wider bandwidth and a lower resonant frequency. However, since increasing of substrate thickness will lead to an unacceptable increase in the antenna’s height profile, we set mm and mm in the following simulation as a compromise between compact size, lower frequency, and wider fractional bandwidth.
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2.2. Performance of the PSMS Antenna
To study the effect of the PSMS on the impedance matching, the reflection coefficient of the monopole and the PSMS antenna is simulated and compared in Figure 4(a). It is observed that the working frequency of the PSMS antenna shifts downward to the lower frequency side compared with the monopole only. Moreover, the PSMS antenna expresses 19.6% impedance bandwidth spanning from 4.57 to 5.56 GHz (). Hence, a good impedance matching is obtained when the monopole antenna is loaded with the PSMS structure. We have studied how is affected by the slot gap and the substrate height . The calculated results are shown in Figures 4(b) and 4(c). It can be seen that resonant frequency shifts toward a lower frequency with a decrease in the or an increase in the . However, it should be noted that the increase of and decrease of are both accompanied by bandwidth decreasing. These properties shown in Figures 4(b) and 4(c) provide a useful guideline for practical design, which are consistent with the prediction in Figures 3(b) and 3(c), respectively. Simulated gain of the monopole alone is around 2 dBi whereas the PSMS antenna has a simulated gain at broadside of around 6.76 dBi in the frequency band of 4.57–5.56 GHz, as shown in Figure 4(d). This means that a nearly 5 dBi enhancement is obtained when the monopole is loaded with the PSMS.
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Figure 5 shows the simulated radiation patterns of the PSMS antenna in both the E- and H-planes. For comparison, a monopole alone is simulated, and its radiation patterns are also plotted in the same figure. We can see that the proposed PSMS antenna has the majority of its energy radiated towards the +z half space, since the gain of the PSMS antenna increased in the forward direction. The simulated half-power beam widths (HPBWs) of the PSMS antenna are found to be 84°, 81°, and 64°at 4.6, 5.0, and 5.4 GHz in the E-plane. In the same frequencies, the HPBWs are 78°, 70°, and 54° in the H-plane, respectively. Additionally, we will take 5 GHz for operate frequency as an example to investigate the FB (front-to-back) ratio, efficiency, and impedance matching properties of the PSMS antenna. The PSMS antenna has a simulated FB ratio greater than 4.8 dB, while the FB ratio of the monopole is only 0.9 dB when , as presented in Figure 6(a). Across the entire bandwidth, the simulated radiation efficiency and antenna efficiency of the PSMS antenna are 87% and 78% as also demonstrated in Figure 6(b), indicating that the proposed antenna is well designed with high efficiency. The PSMS antenna can exhibit a good impedance matching performance over the targeted band, and the simulated input resistance and reactance of the proposed antenna for several microstrip line lengths are shown in Figure 6(c). Results show that the input resistance increases and the input reactance decreases slightly when the becomes larger, which makes the matching more critical.
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3. Radiating Mechanism of the PSMS Antenna
To understand the radiating mechanism of the PSMS antenna, the simulated E-field distributions on the plane of mm at 4.83 and 5.48 GHz are depicted in Figures 7(a) and 7(b), respectively. Here, the 4.83 and 5.48 GHz are first and second resonant frequency of the PSMS antenna (i.e., the blue line in Figure 4(a)). For comparison purposes, the expected E-field distribution of the TM10 and the antiphase TM20 modes based on the cavity model for a conventional rectangular microstrip antenna are also shown in the same pictures. It is found that the E-field distributions of the proposed PSMS antenna at the two resonant frequencies are very similar to the TM10 and TM20 modes of a conventional patch antenna, except for the radiation emitted from the gaps between PSMS cell array. In addition, the existence of the radiating gaps will decrease the quality factor compared to a complete rectangular patch antenna, which benefits the antenna impedance bandwidth enhancement. These results agree well with [23].
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When the height of the PSMS substrate is much smaller than the wavelength in a vacuum and the width of PSMS array, the resonances can be qualitatively explained by a simple transmission line model. The dispersion relations of a unit cell were calculated by the eigenmode analysis of HFSS, and the simulation model is displayed in Figure 8(a). The unit cell was enclosed by an air box on the bottom, and a perfectly matched layer (PML) boundary was assigned on the top of the air box. Periodic boundary conditions are assigned to the corresponding faces to mimic an infinite periodic structure. The parameters and indicate the period of unit cell in the x and y directions, respectively. The resonant frequencies for TM10 and TM20 modes can be expressed as [23] where represents the propagation constant, and and represent the number of unit cells in the x and y directions ( and in this paper). The PSMS array has an additional extended length at the x and y directions owning to the fringing field at the open edges of the unit cell array. The extended length in the two directions can be calculated, and it is given by [24] where and are the height and permittivity of the PSMS substrate, and and are the effective width of the unit cell array in the x and y directions. The propagation constant in the extended region for the x and y directions are given by
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The simulated dispersion diagrams in the x and y directions are shown in Figures 8(a) and 8(b), respectively. The calculated resonant frequencies for the TM10 and the antiphase TM20 modes are 4.62 and 5.61 GHz by using (2) and (3). The resonance frequencies of TM10 and TM20 modes are indicated by the intersection points of the dispersion curves and the vertical lines which correspond to and , respectively, as shown in Figure 8. We can see that the predicted resonances for TM10 and TM20 (corresponding 4.62 and 5.61 GHz) are very close to the simulated first and second resonant frequencies (4.83 and 5.48 GHz) of the PSMS antenna. In brief, the dispersion analysis discussed above demonstrated that the radiation mechanism of the proposed antenna can be pretty good explained by transmission-line model.
To illustrate why the broadside radiation of the TM20 mode can be generated for the proposed antenna, the current distribution on the top surface of the PSMS at the second resonant frequency around 5.5 GHz is simulated, as shown in Figure 9(a). We can observe that the surface current is nearly in-phase and aligned along the x direction, which is different from the out-of-phase surface current of typical microstrip antenna operating in the same mode; this property can partly explain the broadside radiation. There is another reason to explain the broadside radiation of the TM20 mode, which is the radiation emitted from the palisaded slots of the array. The electric field distribution on the top surface of the PSMS at 5.5 GHz is shown in Figure 9(b). It can be seen that strong E-fields are emitted from the slots of the unit cells, except the central slot along the x direction. As mentioned above, the corresponding radiation can be attributed to the magnetic current induced by the electric fields. On the other hand, the PSMS here can be considered as a refractor, receiving the radiation from the area on the source antenna where it covers and reradiating it to the other side, which generates broadside radiation.
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The measured performance of the proposed antenna in this paper and the metasurface antenna operating at around 5 GHz is summarized in Table 1. As compared to those reported in previous works, we can see that our antennas achieve a good compromise between gain, antenna footprint, and operational frequency bandwidth.
4. Conclusions
In summary, a miniaturized single-fed monopole antenna loaded with the PSMS is proposed. Dispersion analyses along with parametric studies were carried out to reveal the working principle of the antenna. The antenna preference can be flexibly adjusted by tuning the width of the slot gap and the height of substrate . These properties are consistent with the prediction from the studied reflection phase and magnitude of a PSMS unit cell. It is found that the E-field distributions of the proposed PSMS antenna at the two resonant frequencies are very similar to the TM10 and TM20 modes of a conventional patch antenna, except for the radiation emitted from the gaps between PSMS cell array. The PSMS-enabled antenna was shown to exhibit a miniaturized feature as well as an improved antenna gain. The miniaturized PSMS antenna has a great application potential in a variety of wireless communication systems.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this article.
Acknowledgments
Our work was funded by the National Natural Science Foundation of China (Grant nos. 61461052, 11564044, 61863035) and was supported by the Key Program of Natural Science of Yunnan Province (Grant no. 2015FA015), the Spectrum Sensing and borderlands Security Key Laboratory of Universities in Yunnan (C6165903), and Yunnan University’s Research Innovation Fund for Graduate Students.