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Abstract
A leaky-Vivaldi antenna covered with metasurface (LVAM) is proposed in this paper. The traditional Vivaldi antenna covered with metasurface realizes backward frequency beam-scanning from $- {41^\circ }$ to ${0^\circ }$ in the high-frequency operating band (HFOB) and retains aperture radiation in the low-frequency operating band (LFOB). In the LFOB, the metasurface can be regarded as a transmission line to realize a slow-wave transmission. In the HFOB, the metasurface can be considered a 2D periodic leaky-wave structure to realize a fast-wave transmission. The simulated results show that LVAM has the -10 dB return loss bandwidths of 46.5% and 40.0%, and the realized gain of 8.8–9.6 dBi and 11.8–15.2 dBi cover the 5 G Sub-6 GHz band (3.3–5.3 GHz) and the X band (8.0–12.0 GHz), respectively. The test results are in good agreement with the simulated results. As a dual-band antenna covering the 5 G Sub-6 GHz communication band and military radar band, the proposed antenna can guide the future integrated design of communication and radar antenna systems.
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1. Introduction
As a typical traveling wave antenna, the Vivaldi antenna is widely used in ground penetrating radar [1], microwave imaging [2], mobile communication [3], and wireless power transfer [4] due to its ultra-wideband, high directivity, and accessible design. However, the Vivaldi antenna also has some drawbacks, such as low gain and a split pattern at high frequencies. Besides, the radiation mode of the traditional Vivaldi antenna is monotonous, which is not flexible in engineering applications. The development of metamaterials and metasurfaces [5–8] provides a new way to solve these problems.
To improve the gain of Vivaldi antenna, traditional methods such as etching periodic slots on both arms of the antenna [9], loading a metal patch on the antenna aperture as a guide [10], or expanding the dielectric substrate of different shapes on the aperture as a dielectric lens [11] are widely used. As a new type of lens, metamaterial lens [12] has more obvious gain improvement and more flexible design than the traditional dielectric lens. In [13], a high-gain Vivaldi antenna loaded with a metamaterial lens over 20-40 GHz is proposed. A metamaterial lens attached to the aperture of the Vivaldi antenna can reconstruct the wavefront of the electromagnetic wave, turning the spherical wave into a quasi-plane wave, thus enhancing the antenna's gain and directivity. To realize the different radiation modes of Vivaldi antenna, some new design ideas have been proposed in recent years. Different from traditional array antennas [14], some Vivaldi array antennas with special arrangements can achieve wider scanning angles [15] or flexible beam control [16]. In [17], a novel metamaterial half lens mounted on the Vivaldi antenna aperture realizes beam deflection and frequency beam scanning. In [18], FSS is used to achieve beam deflection and bidirectional radiation of Vivaldi antennas. Ren et al. realized a large frequency ratio Vivaldi antenna design by constructing multiple small tapered slot antennas on the traditional Vivaldi antenna with a large size [19]. Sang et al. designed a high-gain Vivaldi antenna loaded with a 3-D phase-adjusting unit lens [20]. The lens is made of flexible material, and its operational characteristics can be controlled by bending. Yuan et al. realized switchable monopole and Vivaldi radiation modes by loading PIN diodes on the Vivaldi antenna feeder [21].
Leaky antenna can be divided into uniform leaky antenna, quasi-uniform leaky antenna, and periodic leaky antenna. The composite right-handed leaky wave antenna [22] provides a new idea for the leaky wave antenna to realize the forward to backward continuous beam scanning [23]. Periodic leaky wave antennas can also realize continuous beam scanning from forward to backward by extraordinary means [24]. In recent years, 1D leaky-wave antennas based on substrate integrated waveguide (SIW) [25,26] and spoof surface plasmon polariton (SSPP) transmission line [27] have become a research hotspot due to their excellent characteristics such as low power loss, high efficiency, and wide beam scanning angle [28]. 2D leaky antennas are more [29] flexible in design than 1D leaky antennas. In [30], a low-profile metasurface antenna based on TM leaky wave radiation is proposed. This metasurface leaky wave antenna can only realize fixed-line radiation, not frequency beam scanning. In [31], a holographic leaky wave antenna with multi-beam radiation fed by the Vivaldi antenna is designed. The size of the holographic leaky wave antenna is much larger than that of the Vivaldi antenna, which is not conducive to the miniaturization and integration of the antenna.
Compared with the 2D leaky antenna mentioned above, the antenna proposed in this paper has a compact structure and can realize two radiation modes: aperture radiation and leaky radiation shown in Fig. 1. The metasurface designed in this paper transmits slow waves at low frequencies and fast waves at high frequencies, resulting in generating two radiation modes. This design is also the first to use metasurface coverage to excite Vivaldi antenna leakage radiation, which is of great significance to developing and applying Vivaldi antenna. This design idea has the potential to be applied to the design of THz and optical band Vivaldi antennas [32–35].
2. Antenna design
2.1 Basic theory
The frequency beam scanning characteristics of leaky wave antennas are derived from the phase constant $\beta ,$ which varies with frequency. The beam direction of the ${n_{th}}$ space harmonic can be predicted by [36].
In this paper, the leakage wave metasurface (LWM) radiates -1 order harmonic. It can be seen that the deflection angle of the beam ${\theta _{ - 1}}$ can be adjusted by the period p. The electromagnetic waves trapped in the Vivaldi antenna's aperture can be considered slow waves. The electromagnetic wave is transmitted in the Vivaldi aperture in master mode, and the metasurface coverage excites -1 order harmonic radiation. To study the leaky properties of the proposed antenna, the function of phase constant β concerning frequency needs to be derived. Based on the two-port network of transmission lines, the ABCD transmission matrix can be obtained as follows [37]:
For a nontrivial solution, the determinant of the above matrix must vanish:
Or since AD-BC = 1,
Since $\gamma = \alpha + j\beta $, we have that
For an ideal transmission line $\alpha = 0$, referring to Eq. (4), we can reduce Eq. (5) to:
According to the conversion between S-parameters and ABCD matrix in a two-port network:
We can obtain the relationship between the phase constant $\beta $ and the S-parameters:
According to Eq. (9), the dispersion curve of the periodic unit can be retrieved by S-parameters.
2.2 Geometric structure
The simple traditional Vivaldi antenna was chosen as the primary antenna, and its structure is shown in Fig. 2(a). The partial curve of the ellipse is used as the contour of the antenna gradient slot, making the design more straightforward and controllable. The leaky metasurface consists of a 6 × 8 single-layer rectangular patch square array with the exact dimensions of the Vivaldi antenna, as shown in Fig. 2(b). The side view of the LVAM is shown in Fig. 1, where the metasurface is placed above the Vivaldi antenna, and the thickness of the air layer is h mm. The dielectric substrate used by LVAM is Rogers RO4003C, with a dielectric constant of 3.38 and a thickness of 1.524 mm. The dimensions of the LVAM are as follows (in millimeters) : ${w_1} = 50.74$, ${a_1} = 160,\; {b_1} = 59.4,\; \; {y_1} = 120,\; \; {x_1} = 160$.
2.3 Metasurface design process and analysis
Figure 3(a) shows the basic unit of LWM. We use a full wave simulation method to obtain the basic parameters of the unit in CST Microwave Studio 2020. The boundary conditions of the unit shown in Fig. 3(b) are set as follows: the electromagnetic wave propagation direction is set as the OPEN boundary (along the x axis), the electric field vibration direction is set as the PEC boundary (along the y axis), and the magnetic field vibration direction is set as the PMC boundary (along the z axis). The dimensions of the LWM unit are as follows (in millimeters) : ${p_x} = 20$, ${p_y} = 20,\; m = 11,\; \; n = 4.$
The unit dispersion curve can be obtained by post-processing the S-parameters. Since electromagnetic waves propagate forward along the x-axis on LWM, we only need to discuss the dispersion properties of the phase constant $\beta $ along the x-direction. $\beta $ referred to in the paper defaults to ${\beta _x}$. We studied the influence of the change of unit structure parameters on the dispersion curve in Fig. 4. As shown in Fig. 4(a)-(d), it can be seen that ${p_x}$ and ${p_y}$ are the dominant influences on the dispersion curve. The difference is that the dispersion curve varies linearly with ${p_x}$ and nonlinear with ${p_y}$.When the length m of the rectangular patch is too large, it will result in an open stopband. The dispersion curve is insensitive to width n.
Fig. 4. Dispersion curves for different unit structure parameters: (a) patch length m, (b) width n, the period (c) ${p_x}$ along the x-axis, and (d) ${p_y}$ along the y-axis.
1D leaky antenna is usually based on a two-port microstrip transmission line. The transmission line has good transmission characteristics to ensure the high radiation efficiency of the leakage wave. The dual ports are designed to absorb the reflected waves at the end for impedance matching. The Vivaldi antenna's gradient slot is a natural load at high frequencies. LWM needs to have good transmission characteristics to achieve high-efficiency leakage radiation. The equivalent relative impedance can be retrieved from the S-parameters, as shown in Fig. 5(a). It can be seen that the value of the relative impedance is close to 1 within 8-12 GHz, which is well-matched with the air. Besides, impedance deteriorates around 11.7 GHz, which results in poor radiation performance. This is not a flaw in the optimal design, but a consideration of the quality of the beam scanning across the entire band. Figure 5(b) shows the dispersion curve of the final LWM unit. It can be seen that LWM presents left-handed leakage wave characteristics at 8-12 GHz. According to the dispersion curve, LWM also has right-handed leakage characteristics after 12 GHz. But in the actual antenna simulation, the radiation characteristics of the right-hand leakage wave are inferior. The Vivaldi antenna has unstable electromagnetic wave transmission characteristics in different frequency bands, especially at high frequencies. Electromagnetic waves of different frequency bands have different radiation regions, which will directly affect LWM’s radiation. However, the dispersion curve of the LWM unit is obtained under stable boundary conditions, and its results can only be used as a reference. They cannot accurately describe the radiation characteristics of LWM.
The spacing h between LWM and Vivaldi antenna affects the antenna $|{S_{11}}|$ and gain. When h decreases, $|{S_{11}}|$ deteriorates in HFOB, as shown in Fig. 6(a). This is because the closer LWM is to the Vivaldi antenna, the greater the influence on antenna input impedance. When h is about 4 mm, the leakage radiation characteristic of LVAM is the most obvious. When $h = 4\textrm{}mm$, the antenna gain curve is optimal in the entire operating frequency band, as shown in Fig. 6(b). The electromagnetic field on the Vivaldi antenna is confined to the antenna aperture, and the farther away from the aperture, the weaker the electromagnetic field becomes. The coupling strength and operating frequency of Vivaldi antenna and metasurface are related to h. If h is too small, the coupling strength will affect the antenna impedance matching. If h is too large, the coupling strength is very small, and the energy cannot be radiated out through the coupling.
Fig. 6. Curve of (a) return loss ${S_{11}}$ and (b) realized gain with different thicknesses h of the air layer.
To further elaborate on the radiation mechanism of LWM, its local electric and magnetic field distributions are given in Fig. 7. We know that LWM works in the -1 mode, which can be regarded as the secondary radiation generated by LWM under the excitation of Vivaldi antenna. In Fig. 7(a), the edge radiation of the leaky wave unit is the major contributor to LWM radiation. The edge field can be decomposed into horizontal and vertical directions as follows:
Fig. 7. (a) The electric field distribution of LWM in the yoz plane and the (b) magnetic field distribution in the xoz plane.
Due to the symmetry of LWM, the components of the edge electric fields in the Z direction cancel each other out. In contrast, the components in the Y direction are superimposed on each other, which can be described as:
When the electromagnetic wave passes through the rectangular patch, it can excite the tangent component of the magnetic field along the X direction on the surface, as shown in Fig. 7(b). In general, electromagnetic waves will become discontinuous and generate radiation when passing through the LWM unit in the propagation process. Because the phase changes when the electromagnetic wave propagates forward, the initial phase of the radiation from the patches at different positions is different. The secondary radiation generated by LWM can be analogous to the beam deflection of an array antenna when it is excited by a source with a particular phase difference.
In fact, the LWM can be independently loaded directly above or below the Vivaldi antenna to achieve a mirrored radiation characteristic. Alternatively, LWMs can be loaded above and below the antenna to attain symmetrical dual-beam scanning, whose scanning characteristics are consistent with those of a single beam. Figure 8(a) and (b) respectively show the radiation characteristics of LVAM when loaded with double-layer LWMs and single-layer LWMs. Double-layer LWMs and single-layer LWM have the exact working mechanism, and their radiation is independent, so only single-layer is discussed in this paper. It is worth mentioning that the radiation characteristics of double-layer LWMs are worse than those of single-layer LWM in directivity and gain.
Fig. 8. Electric field distribution and 3D radiation far field of (a) double-layer LWMs and (b) single-layer LWM.
2.4 Array analysis of the proposed antenna
In this section, the array performance of the proposed antenna is evaluated. In Fig. 9, a 1 × 4 linear array is presented. The S-parameters, radiation patterns, realized gain and total efficiency of antenna arrays are analyzed by simulation. We find that the basic performance of array antenna can be consistent with that of single antenna under Equal-amplitude and in-phase excitation.
Because of the symmetry of a single antenna in the array, only the S-parameters of antenna 1 and antenna 2 are given in the analysis of antenna S-parameters. As shown in Fig. 10, it can be seen that the s11 of antennae-1 and antennae-2 are basically the same and the working band is consistent with that of the proposed single antenna. The isolation between antennas is below -30 dB. For the array, directional radiation is still maintained in the low frequency operating band, while the frequency beam scanning characteristic is also maintained in the high frequency operating band, as shown in Fig. 11(a) and (b). It can be seen in Fig. 12(a) that the gain of the array antenna is 6 dB larger than that of a single antenna. The total efficiency of antennae-1 remains around 90% within the operating band, as shown in Fig. 12(b).
Fig. 11. Radiation patterns of the array when the ports are excited with Equal-amplitude and in-phase. (a) aperture radiation and (b) leaky radiation of antenna.
Fig. 12. (a) The realized gain curves of single antenna and array antenna. (b) the total efficiency of antenna-1 in array antenna.
3. Simulation and measurement results
The proposed antenna was manufactured and measured to verify the feasibility of the simulation. In Fig. 13(a), it can be seen that LVAM has two operating bands, including LFOB and HFOB. There is a small stop band in the whole band, which is the transition band. Within the LFOB, the electromagnetic wave is transmitted in a slow wave through the aperture radiation of the Vivaldi antenna. In HFOB, electromagnetic waves are transmitted as fast waves through LWM radiation leakage waves. In the transition band, two radiation modes coexist and are not obvious. In the effective working band, LVAM inherits the Vivaldi and leaky wave antenna's excellent radiation efficiency characteristics, which can be seen in Fig. 13(b). The radiation efficiency and total efficiency of LVAM are close to 90%. The loading of LWM also dramatically enhances the primary gain and directivity of the Vivaldi antenna. The average gain of LVAM is about 9 dBi in LFOB and about 13 dBi in HFOB. The measured S-parameters and gain curves are in good agreement with the simulated ones. The error is mainly caused by inaccurate PCB processing and inaccurate hand-made air layer supporting foam thickness.
Fig. 13. (a) Simulated and measured ${S_{11}}$ of the proposed antenna. (b) Simulated and measured realized gain and efficiency of the proposed antenna.
The radiation pattern of the proposed antenna is measured and compared with the simulated results. Figure 14(a) and (b) show the test scenario of LVAM and the assembled template. We measured the E-plane and H-plane radiation patterns of LVAM at 4.3 GHz to verify the end-emission characteristics of the antenna in LFOB. The measured results and simulated results are compared in Fig. 15, from which it can be seen that the measured results are in good agreement with the simulated results. We also measured the radiation patterns of LVAM at 8 GHz, 9 GHz, and 12 GHz to verify the frequency beam scanning characteristics of the antenna in HFOB. The measured results and simulated results are shown in Fig. 16, from which it can be seen that the measured results are consistent with the simulated results. In the measurement process, the error mainly comes from the assembly of LVAM. There is an error in the thickness of the foam strut made by hand in place of the air layer, which causes the frequency shift of the beam scanning characteristics. In addition, if the test antenna is not aligned, the zero point of the radiation pattern will be offset.
Fig. 15. The radiation pattern of the (a) E plane and (b) H plane of the proposed antenna at 4.3 GHz.
Fig. 16. The simulated and measured radiation patterns of the proposed antenna at 8 GHz, 9 GHz, and 12 GHz.
A comparison of the proposed antenna with a similar type is shown in Table 1. As can be seen from the table, compared with leaky-wave antennas of different categories, the proposed antennas have advantages in structural dimension, operating frequency and average efficiency. The proposed antenna has both directional radiation and leaky wave radiation, which is superior to leaky wave antenna with a single radiation mode. However, the beam scanning angle of the proposed antenna is not satisfactory among similar leaky wave antennas, which is the problem to be solved in the following work.
Table 1. Comparison with Similar Works
4. Conclusion
The contribution of this paper is to expand the radiation mode of Vivaldi antenna by using metasurface induced leakage wave radiation at high frequency. The antenna realized directional radiation and frequency beam scanning in the low frequency and high frequency operating bands respectively. The proposed antenna has a compact structure and a low profile, which facilitates integration. The disadvantage of this design is that the metasurface loading causes a stopband in the working band of the Vivaldi antenna. In the stopband, the antenna impedance is not matched, the radiation is poor, and the radiation characteristics are in a transitional state. Besides, the leakage band of the proposed antenna can be adjusted by adjusting the size of the metasurface element. The design method in this paper has reference significance for THz Vivaldi's design.
Funding
Fundamental Research Funds for the Central Universities (2022CXZZ101, CCNU22JC018).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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