Abstract
This paper presents the effects of substrate dielectric constants on the performance characteristics of a circularly polarized (CP) metasurface-based patch antenna. The antenna structure is a modified patch with a step-like truncation sandwiched between a metasurface composed of a 4 × 4 lattices of periodic metallic patches and a ground plane. The effects on the performance variations are evaluated for two principal cases that include a uniform dielectric constant and a nonuniform dielectric constant for the upper and lower substrates of the antenna. Through careful computational analysis, the effects of the substrate dielectric constant on the antenna performance in terms of bandwidth and gain were investigated, and the results demonstrate that the antenna performance improves with a decrease in the substrate dielectric constant. For a uniform substrate material with dielectric constants of εr1 = εr2 = 2.2, the fabricated antenna with an overall size of 54 mm × 54 mm × 3.0 mm (0.76λo × 0.76λo × 0.042λo at 4.24 GHz) demonstrates the following measured performance characteristics: a −10 dB impedance bandwidth of 3.75–5.24 GHz (33.14%), a 3 dB axial ratio (AR) bandwidth of 3.85–4.64 GHz (18.61%), a radiation efficiency >93%, and a peak gain of 8.96 dBic within the AR bandwidth.
1. Introduction
Nowadays, microstrip patch antennas are commonly used in many wireless communication applications owing to the advantages of being low-profile, lightweight, and low-cost while boasting easy fabrication and ease of integration [1–5]. A conventional microstrip patch antenna generally consists of a printed metallic patch on one side of a dielectric substrate and a ground plane on the other side [6]. The resonance frequency of such antennas is a function of the dimension of the patch and the permittivity and thickness of the substrate. Nonetheless, the principal demerit of microstrip patch antennas is their low performance in terms of narrow bandwidth, which acts as a limitation for many applications [7–12].
The substrate permittivity (εr) combined with the thickness (h) of a microstrip patch antenna affects the antenna performance characteristics such as resonance frequency, gain, impedance, and axial ratio (AR) bandwidths [13], and polarization. The substrate, in addition to providing mechanical strength to the overall antenna design, also allows surface waves to propagate through it. High-permittivity substrates reduce the antenna size at the cost of the gain, bandwidth, and cost-effectiveness of the antenna. Accordingly, choosing the most appropriate substrate material with a suitable permittivity is a prime necessity in microstrip antenna design. Because the drawbacks of a microstrip antenna, such as low gain and restricted bandwidth, can be mitigated by selecting suitable substrate materials, the dielectric constant of a substrate is a significant variable in enhancing the gain and bandwidth of a microstrip patch antenna [14, 15].
A metasurface, a two-dimensional counterpart of a metamaterial, is a surface distribution of specially formed subwavelength scattering elements that can shape and control electromagnetic waves [16–20]. The performance of the metasurfaces is governed by the specific geometrical shape of the scattering elements distributed on the dielectric substrate. These metasurfaces can provide the unique characteristics of an ultra-low profile and enhanced performance in the bandwidth, gain, radiation pattern, and polarization state of the antenna [21–23]. In this paper, we present the effects of the substrate dielectric constant on antenna performance, particularly the bandwidth and radiation gain of a single-feed step-like truncated patch antenna loaded with a metasurface sandwiched between two dielectric substrates. The effects on the antenna performance are evaluated in two categories of dielectric substrates that include uniform and nonuniform dielectric constants. The results demonstrate that the antenna performance increases with decreasing permittivity. The antenna designed with the same dielectric material (εr1 = εr2 = 2.2) for both the lower and upper substrates are fabricated and measured, and attained the following performance: An impedance bandwidth for |S11| ≤ −10 dB of 3.75–5.24 GHz (33.14%), a 3 dB AR bandwidth of 3.85–4.64 GHz (18.61%), a radiation efficiency >93%, and a peak gain of 8.96 dBic within the AR bandwidth at an antenna size of 54 mm × 54 mm × 3.0 mm (0.76λo × 0.76λo × 0.042λo at 4.24 GHz).
2. Antenna Geometry
The geometry of the proposed antenna is shown in Figure 1. The antenna consists of a driven patch, metasurface, ground plane, 50 Ω SMA connector, and two dielectric substrates with the same thickness of h1 and h2 and dielectric constants of εr1 and εr2, respectively. The metasurface is a 4 × 4 lattices of square patches printed on the top surface of the upper substrate with periodicity = 13 mm and a gap between the patches .
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The size of the metasurface unit cell and the number of unit cells constituting the metasurface determine the surface wave resonant frequency of the metasurface [25, 26]. The surface wave resonant frequency of the metasurface according to the size and number of metasurface unit cells can be obtained using a dispersion diagram. Figure 2 is a dispersion diagram according to the design parameter P of a unit cell with with εr1 = εr2 = 2.20 and h1 = h2 = 1.575 mm substrate. Figure 2(a) is the 1st mode and 2(b) is the 2nd mode. It can be seen that the 3 × 3 metasurfaces with P = 15 mm, the 4 × 4 metasurface with = 13 mm, and the 5 × 5 metasurfaces with = 11 mm have almost the same surface wave resonant frequency. Figure 3 shows the gain and axis ratio of 3 × 3, 4 × 4, and 5 × 5 metasurfaces antennas designed to have the same surface wave resonant frequency. The 3 × 3 metasurfaces antenna with P = 15 mm has a low bandgap frequency, which results in a low antenna gain at the center frequency and poor AR characteristics. On the other hand, the 5 × 5 metasurfaces antenna with = 11 mm has a high antenna gain at the center frequency, but the 3 dB AR band is shifted to a higher frequency, owing to the small size of the unit cell. In this paper, considering fabrication, a 4 × 4 metasurfaces using unit cells with = 13 mm and , which has the best performance at the center frequency when applied to a substrate with εr1 = εr2 = 2.20, was selected. The details of the metasurface design and mechanism are presented [24–27]. The radiating element is a square patch with a step-like truncation printed on the top surface of the lower substrate and sandwiched between the metasurface and the ground plane. The outer conductor of the coaxial line is connected to the ground plane, and the inner conductor of the coaxial line extends through the lower substrate to connect to the extended strip of the radiating patch [28]. The extended strip is a stub attached to the driven patch to improve the impedance matching [29]. For different values of εr1 and εr2, each antenna was optimized with ANSYS high-frequency simulation software to show the effects of the substrate dielectric constant on the antenna gain, impedance bandwidth, and AR bandwidth.
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3. Antenna Performance
To simplify, the effects of the substrate dielectric constant on the antenna performance are summarized into two categories, those of a uniform dielectric constant and those of a nonuniform dielectric constant.
3.1. Uniform Dielectric Constant (εr1 = εr2)
In the case of a uniform dielectric constant material, the same dielectric constant is utilized for both the upper and lower substrates. Table 1 shows a summary of the performance variations of the antenna as a function of the antenna’s dielectric constant. An observation of the table provides a good understanding of the effects of different dielectric constant substrates on the antenna performance in terms of impedance bandwidth, AR bandwidth, and gain. In the context of impedance bandwidth, it can be seen from the table that the proposed antenna operates with a wide impedance bandwidth when the dielectric constant is 1.06 for both the lower and upper substrates. At this value, the impedance bandwidth generated is 47.55%, as shown in Figure 4. The figure also confirms that when the dielectric constant is changed to a value of 2.2 for both substrates, the dielectric effect is noticeable with a shift in the entire reflection coefficient curve to low frequencies. This shift is also accompanied by a nearly 15% reduction in the impedance bandwidth. Similarly, at a dielectric constant of 3.38, the |S11| curve shifts more toward lower frequencies compared with the aforementioned case and demonstrates an increased reduction in the impedance bandwidth of approximately 18%.
In the context of the AR bandwidth, the proposed antenna with a dielectric constant of 1.06 for both substrates generates a broad AR bandwidth of 21.5% with two AR minimum points. The first minimum point is generated by the patch whereas the second minimum point is generated from the metasurface [26]. In all three cases, exactly the same number of AR minimum points is generated, as shown in Figure 5. Nevertheless, different AR bandwidths are produced, as seen in Table 1. When the dielectric constant is changed from a low value of 1.06 to a high value of 2.2 for all substrates, a shift towards low frequencies is observed with a slight reduction in the AR bandwidth. When the dielectric constant is 3.38, more shifts are achieved, but the reduction in AR bandwidth almost remains the same.
The aforementioned results of the reflection coefficient and AR show the same trend with changes in the dielectric constant for both substrates. The first significant trend is the shift of the |S11| and AR curves to low frequencies with increasing dielectric constant. An increase in the dielectric constant produces an increase in the effective patch size or radius that results in a corresponding shift to low frequencies. Another notable trend is the reduction of bandwidth with an increasing dielectric constant. The deterioration in bandwidth can be attributed to the increased total quality factor and a decrease in the fringing fields as the dielectric constant increases.
The predominant effect of the dielectric constant on the antenna performance is shown in Figure 6, which shows the gain of the antenna as a function of frequency for different dielectric constants. It is evident from both Figure 6 and Table 1 that with low dielectric constant values such as 1.06 for the lower and upper substrates, high gain values above 10.0 dBic are achievable. When the dielectric constant value increases, the expected shift to low frequencies is exhibited; however, the radiation gain deteriorates accordingly. For instance, when the dielectric constant is changed from 1.06 to 2.2, the gain drops from 10.5 dBic to 8.96 dBic, which is equivalent to a decrease of approximately 1.5 dBic. Therefore, decreasing the dielectric constants for both substrates generate increasing gain and vice versa. The rise in gain at a low dielectric constant is because the patch size at resonance increases electrically, and consequently, the radiating area eventually enhances the antenna gain. Furthermore, the dielectric substrate provides a medium for the propagation of surface waves that deplete some of the power available for radiation, reducing the radiation gain. With high dielectric constant materials, there are increased dielectric losses and reduced gain values.
3.2. Nonuniform Dielectric Constant (εr1 ≠ εr2)
A combination of substrates with different permittivities for the lower and upper substrates is employed in the antenna with nonuniform dielectric constants. The performance of the antenna is evaluated with the lower and upper substrates having different dielectric constants in one case, and then, swapping the dielectric constants of the lower and upper substrates in the other case. Therefore, two pairs of dielectric constants are considered. The first pair consists of dielectric constant values of 1.06 and 2.20. In Table 2, the first row assigns εr1 = 1.06 for the lower substrate and εr2 = 2.20 for the upper substrate. This particular case generates a wide impedance bandwidth of 44.54% and an appreciable AR bandwidth of 17.39% with a moderate electrical size of the antenna. The generated gain values are also high with a peak value of 9.31 dBic. In row 2 of Table 2, the dielectric constants of row 1 are swapped, and the lower substrate has a dielectric constant of εr1 = 2.20, whereas the upper substrate has a dielectric constant of εr2 = 1.06. Compared with the preceding case, the antenna shows a reduction in the impedance bandwidth with other improvements demonstrated by a slight increment in the AR bandwidth and broadside gain. Therefore, with εr1 = 2.20 and εr2 = 1.06, the antenna shows a better useable CP bandwidth and gain values above 10.0 dBic. From Figure 7, which shows the reflection coefficient profile of the different dielectric constant combinations, the |S11| with εr1 = 1.06 and εr2 = 2.2 demonstrates multiple resonances within its profile that produce a wider bandwidth compared with the |S11| with εr1 = 2.20 and εr2 = 1.06 that shows just two resonances. This is because surface waves are strongly excited within the metasurface and produce better impedance bandwidth when the upper substrate has a dielectric constant of εr2 = 2.2. Figure 8 shows the AR of the antenna with various dielectric constant combinations. The figure visibly compares the AR profiles and shows two AR minimum points and a shift of approximately 0.6 GHz between the AR curves for εr1 = 2.20 and εr2 = 1.06, and εr1 = 2.20 and εr2 = 1.06. The shifting or movement along the frequency scale in the figure is because both the aforementioned cases have the same physical patch size but different electrical and effective sizes. The gain curves of the different dielectric constant combinations are represented in Figure 9. It can be observed that the case with εr1 = 2.20 and εr2 = 1.06 produces higher gain values than the case with these dielectric constants interchanged. When the upper substrate with the printed metasurface has the lower dielectric constant of 1.06, the effective size of the metasurface is increased, giving rise to a larger effective aperture area that improves the antenna gain. In contrast, when the upper substrate has a higher dielectric constant of 2.2, there is an increase in the dielectric losses and a high propagation loss of the surface waves within the dielectric medium that reduces the radiation gain.
In Table 2, the second pair of dielectric constant values are 1.06 and 3.38. The main advantage obtained by employing εr1 = 1.06 and εr2 = 3.38 is the reduction in the electrical size of the antenna, which is accompanied by a reduction in the useable CP bandwidth of the antenna. Nonetheless, in the inverse situation with εr1 = 3.38 and εr2 = 1.06, the antenna size is reduced without significantly losing the useable CP bandwidth and high gain values are produced by the antenna. By using combinations of dielectric constant values and interchanging their positions, the antenna size is reduced without considerable loss in performance. For example, in the case with εr1 = 2.20 and εr2 = 1.06, the electrical size is reduced while the bandwidth is nearly maintained at the optimum level and the gain values are above 10.0 dBic.
4. Experimental Setup
Figure 10 shows a prototype that was fabricated and measured with a dielectric constant of 2.2 for both the top and bottom substrates. The antenna was etched on a Roger 5880 substrate with a height h1 = h2 = 1.5748 mm and with the following design parameters: , P = 13 mm, = 12.8 mm, = 14 mm, = 12.5 mm, d = 7.8 mm Fp = 4.6 mm, Fl = 10.5 mm, and . To measure the reflection coefficient of the antenna, an Agilent E8362 B Network Analyzer and a 3.5 mm Coaxial Calibration Standard (GCS35M) were used. The measured reflection coefficient showed consistency with the simulated results that ranged from 3.66 to 5.23 GHz with an impedance bandwidth of 35.32%. Figure 11 is a graph of the simulated and measured reflection coefficients. For far-field radiation pattern measurements, a full anechoic chamber with dimensions of 5.5 m () × 5.5 m (L) × 5.0 m (H) at the Electromagnetic Wave Technology Institute (EMTI) Yongsan, Korea was utilized. During the radiation pattern measurement process, a standard wideband horn antenna was used for transmission while the fabricated antennas were used for reception. The transmitter-receiver separation distance was 2.9 m. The horn antenna was fixed while the fabricated antenna was rotated from −180° to 180° at a scan angle of 1° and a speed of 3° s−1. The AR, broadside gain, and radiation patterns were measured and recorded. The measured AR in the broadside direction as shown in Figure 12 ranges from 3.72 to 4.55 GHz and exactly overlaps the simulated AR with an AR bandwidth of 20.07%. The measured broadside gain shows the same profile as the simulated gain. Both the measured gain and simulated gain have a peak gain of 8.89 dBic, as presented in Figure 13. The radiation patterns were measured at two frequency points of 3.9 GHz and 4.35 GHz and plotted in Figure 14. The measurements indicate that the antenna produced good broadside unidirectional left-hand (LH) CP radiation patterns that are identical to their simulated counterparts. The measured patterns showed good symmetry in both the xz- and yz-planes.
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5. Conclusions
In this paper, the effects of the substrate dielectric constant on the performance of a modified square patch with a step-like truncation incorporated into the metasurface are presented. A concise and comprehensive study on the effects of the substrate dielectric constant on the antenna performance is demonstrated. The analysis establishes that the substrate dielectric constant has a significant effect on the antenna performance. With decreasing dielectric constant, the antenna performance is improved. Furthermore, by using substrates with different combinations of permittivity, the antenna size is reduced without significant degradation in antenna performance. A sample antenna is fabricated and measured with a low dielectric constant of 2.2 for both the top and bottom substrates for verification. The antenna showed a high gain with a peak value of 8.96 dBic and produced symmetrical LHCP unidirectional radiation patterns. The antenna has the merits of a low cross-polarization level, high gain, and wide bandwidth with a low profile.
Data Availability
All the data are included within the manuscript.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported in part by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2022R1F1A1065324); in part by the National Research Foundation of Korea(NRF) grant funded by the Korea government (MSIT) (No. 2021R1A4A1030775); and in part by the Institute of Information & communications Technology Planning & Evaluation (IITP) grant funded by the Korea government (MSIT) (No. 2022-0-00704-001, Development of 3D-NET Core Technology for High-Mobility Vehicular Service).