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Abstract
This paper introduces an innovative, compact, and high-gain metasurface antenna, covering both the 24 GHz millimeter wave (mmWave) radar band and the 5 G n257 and n258 bands. The proposed metasurface antenna consists of a wideband stacked patch antenna and a dual-layer metasurface to focus its radiation beams for multiple mmWave bands. The operating frequency can be slightly shifted by altering the distance between the feeder and the metasurface. The distribution of the metasurface unit cells is designed based on a simplified phase compensation formula. The dimension of the fabricated feeder is 6 mm × 6 mm, and the metasurface occupies a 65 mm × 65 mm radome area. Experimental results demonstrate a wide bandwidth from 23.5 GHz to 29.1 GHz for the feeder, and impressive maximum gains of 19.7 dBi and 19.5 dBi for the lower band and higher band of the metasurface antenna are achieved simultaneously. The frequency reconfiguration ability was characterized by a 750 MHz frequency shift with every 1 mm distance adjustment. The compact size and high gain performance of the proposed design underscore its potential for practical applications in millimeter wave joint communication and radar sensing systems.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the past decade, the requirement for integrating communication and detection functions into a single device has led to the rapid development of joint communication and radar/radio sensing (JCAS) systems [1]. These systems have been thoroughly studied and are now widely used in various applications, including roadside-to-vehicle networks, autonomous driving, indoor positioning, and unmanned aerial vehicles [2,3]. Recently, JCAS within the millimeter-wave spectrum has become a popular research area due to the benefits of the large bandwidth and small antenna size [4,5]. As a result, the imperative arises for the design and development of antennas capable of simultaneously covering all operational millimeter wave bands [6]. In addition, extended detection and communication ranges play a crucial role in numerous scenarios, such as driving safety and user experience enhancement [7–9]. Consequently, the requirement for a high-gain, multi-band, and miniature antenna becomes essential to meet the operational demands of mobile JCAS systems.
There are generally two methods to design antennas for JCAS systems. The first method is using planar antennas to cover different millimeter bands around 24 GHz or 77 GHz in one aperture [6,10–14]. For instance, [6] used a tapered plumb-shaped patch to achieve about an 11 GHz bandwidth at 80 GHz, [10] used a bowtie-like dipole to cover a very wide frequency range, [11] used a coupled dipole to cover 6 GHz −26 GHz, [12] used a rectangular monopole antenna to cover an ultra-wide band (UWB) for vehicle communication, [13] used a dipole, and [14] used a groundless patch to achieve an ultra-wide band but bring in backscattering. However, the gains of these antennas are still not high enough for mobile applications due to the limitation of their available space. The other trend in antenna design is the use of metasurfaces, particularly reflect array antennas [15–19], since they can generate pencil-like beams and achieve high gain. This is accomplished by introducing a small phase shift in the resonant frequency for each element, which alters the phase of the reflected or transmitted field and focuses the energy into a very narrow beam [20,21]. These antennas have been extensively used for radar and vehicle-borne JCAS system applications [22]. Furthermore, metasurfaces have been realized to operate in multiple bands [23–30]. As metasurfaces require only a simple feed instead of a feeding network to stimulate the antenna elements, they are considered to be low-cost and lightweight [31].
Despite the promising potential of reflectarray metasurface antennas, several challenges still persist. The first challenge is the design of the feeder. There are various options for the feeder of the metasurface, including horns, waveguides, SIW, and patch antennas [32–35]. However, horns and waveguides are characterized by large profiles. In [36], several patches as a feeder for different bands are proposed, but this approach still needs a large PCB area for placing the patches. Meanwhile, classic patch antennas are very sensitive to the location of the feeding point [37]. The second challenge is the design of the metasurface reflector. Most reflectarray antennas require a substantial area not only for the metasurface but also for the front and back sides of the reflector. Given that the feeder antenna generally must be integrated with the reflector, the integration of the RF circuit with the feeder and reflector becomes problematic. Finally, the cost of the metasurface antenna would be a concern for the industry. Complex unit cell shapes, multiple layers, and numerous vias may decrease the yield rate and increase the fabrication cost, which is a critical consideration for mass production.
To achieve the characteristics of low cost, compact size, and high gain, this paper presents a metasurface antenna designed to operate in the 24 GHz mmWave radar band, as well as the 5 G n257 and n258 bands. The concept of the proposed antenna is illustrated in Fig. 1. This design incorporates a wideband stacked patch antenna, serving as the feeder of a dual-layer metasurface. The feeder’s beam exhibits a relatively broad pattern, accompanied by comparatively low gain. However, after interaction with the metasurface, the beam undergoes a transformation, resulting in a narrower pattern and enhanced gain. For the 24 GHz mmWave radar application, the antenna is designed to function effectively within the frequency range of 24.0 GHz to 24.25 GHz. For the n257 band, the frequency span extends from 26.5 GHz to 29.5 GHz, while for the n258 band, it ranges from 24.25 GHz to 27.5 GHz. Given that different nations have varying regulations regarding frequency band allocation, the antenna is designed to be reconfigurable to different working channels without significant modifications. Therefore, it is suitable for use in the millimeter wave JCAS system.
This paper is organized as follows. Section 2 details the use of stacked patches as the feeder. To expand the bandwidth, a U-shaped slot is integrated into the lower patch, and a ring slot is added to the parasitic patch. These modifications enable the feeder to operate within a range of 23.5 GHz to 29.1 GHz. In Section 3, a phase compensation formula is introduced for metasurface design, accounting for phase changes in oblique incident waves. The backside of the reflector is not occupied therefore can be used to place the RF circuits. The capability for frequency reconfiguration is explored in Section 4. Section 5 presents the fabrication and evaluation of a prototype. The feeder of the prototype occupies 6 mm × 6 mm on the printed circuit board (PCB), and the reflector requires a 82 mm × 82 mm space on the topside of the PCB. Since a grounded coplanar waveguide (GCPW) is designed to connect the feeder and RF circuit, it is straightforward to integrate the RF circuits on the backside of the PCB to the feeder. The metasurface requires an area of 65 mm × 65 mm, which can be a part of the radome. Experimental results show that the proposed antenna achieves maximum gains of up to 19.7 dBi and 19.5 dBi for the lower band and higher band, respectively.
2. Feeder design
To stimulate the metasurface lens, a wideband feeder is required to emit electromagnetic waves towards it. The evolution of the feeder is presented in Fig. 2(a). Feeder1 is a single layer patch antenna, which connects to the RF circuit through a microstrip. However, the patch antenna, with a bandwidth of approximately 10%, falls short in covering three bands. The traditional patch antenna structure is inherently limited by its narrow bandwidth. To mitigate this constraint, various techniques can be implemented to expand the bandwidth, such as integrating a U-slot on the patch and adding parasitic patches. The core principle behind these modifications is to introduce additional resonance through these structures [38]. Another issue arises from a microstrip line, which is on the same layer as Feeder1 and is used to connect the antenna with the RF circuits. However, the radiation from the microstrip line will affect the radiation pattern and interfere with the operation of the metasurface. As shown in Fig. 2(c) and (d), there are some wrinkles and unsmooth places on the pattern of Feeder1.
Fig. 2. The evolution of the feeder. (a) 3D model. (b) The reflection coefficient. (c) H-plane pattern. (d) E-plane pattern.
To address these issues, a stacked patch antenna, referred to as Feeder 2, is designed. The microstrip line is replaced by GCPW and relocated from the topside to the backside of the feeder, minimizing the impact of the microstrip on the radiation pattern. A 0.2 mm via is used to connect the metal patch and GCPW since they are not on the same layer. A parasitic patch is positioned on top of the first patch, which can broaden the bandwidth and enhance the pattern. Fig. 2(c) shows that the pattern of Feeder2 is much smoother than Feeder1.
However, the bandwidth is still insufficient to cover the working band due to the impact of the via. In Feeder3, a U-slot is strategically utilized to achieve resonance at a high-frequency band, with both arms playing a significant role in this effect. Given the potential impact of the U-slot on the radiation pattern, it is judiciously incorporated into the lower patch instead of the upper patch. Furthermore, to establish resonance at a lower frequency band, a loop slot patch is tactically positioned, which forms the basis of the proposed feeder. As depicted in Fig. 2(b), the proposed antenna is capable of covering a range from 23 GHz to 29.1 GHz, exhibiting four resonances at 23.3 GHz, 25.0 GHz, 27.0 GHz, and 28.3 GHz, respectively. Fig. 2(c) and Fig. 2(d) shows that the pattern of the proposed feeder exhibits a significantly higher degree of smoothness compared to that of Feeder1.
3. Metasurface lens design
3.1 Unit cell design
Lots of patterns exist for the unit cell design of metasurfaces, encompassing rings, patches, slots, strips, crosses, and composite structures [39]. The presence of vias between upper and lower patterns is an option as well. These diverse patterns exhibit unique performance characteristics in terms of resonance, polarization, bandwidth, and coupling effects. The primary objective of the unit cell, however, is to induce a specific phase shift and/or magnitude change in incident waves [40], thereby enabling the conversion of a spherical wave into a plane wave and achieving high gain. Certain designs demonstrate superior performance characteristics, such as wide bandwidth and dual polarization. However, the complexity and excessive layering of these structures present manufacturing challenges and can lead to increased costs. As illustrated in Fig. 3(a), this paper proposes a simple dual-layer unit cell, with rectangular metal elements on the topside and backside. No via is needed for this structure, therefore it is low-cost and easy to fabricate.
Fig. 3. The design of unit cell. (a) 3D structure of unit cell. (b) The phase response of the transmitting wave of unit cells vs. frequency with a change in rotation angle.
The rotation of the unit cell has been proven as an effective method for altering the phase of the incident wave [41,42]. Assuming the Angle = 0° as the orientation where the long side of the element is parallel to the long side of the topmost patch of the feeder, the simulation result of unit cell is illustrated in Fig. 3(b). The result shows that varying the rotation angle results in different phase shifts. A significant advantage of the rotation patch method is its ability to achieve the necessary phase shift in a straightforward manner. After optimization, the parameters of the unit cell in Fig. 3(a) were determined to be P = 4.1 mm, L = 2.8 mm, W = 1.8 mm, and t = 2.5 mm.
3.2 Transmit array design
The design process of the transmit array starts with the verification of the phase distribution of the incident wave on the metasurface lens surface. It is crucial that the phase shift of a wave incident vertically is distinct from that of an oblique incident wave. The formula for phase distribution, which is derived from the path delay, cannot be directly applied due to these differences. Therefore, certain modifications are indispensable. Considering that the phase change for an oblique incident wave does not linearly correlate with the incident angle, achieving precise compensation is a challenge. However, this paper proposes a novel approach by substituting the focus distance with a specific value; it is possible to perfectly compensate for the phase changes induced by oblique incidence. The rotation angle of the nth row and mth column elements in the metasurface can be obtained by
For the frequency bands under consideration in this paper, a value of a0 = 4.5 is optimal for a metasurface thickness of 2.5 mm. It is noteworthy that metasurfaces primarily modulate the phase of incident waves, rather than drastically altering their initial propagation direction. Consequently, it is crucial to design the feeder structure and meta lens as a Fabry–Pérot (FP) resonator, as depicted in Fig. 4. The Fabry–Pérot (FP) cavity resonator, which resembles a parallel-plate waveguide enclosed at both ends by reflectors, is instrumental in enhancing antenna performance, especially in terms of gain and directivity [43]. Therefore, this paper proposes such a structure that utilizes the metal surrounding the feeder and dual-layer metasurfaces as the two reflectors of the Fabry-Pérot (FP) cavity. The metasurfaces are used to modulate the phase of the radiation waves, enabling the achievement of high gain and good directivity. Another notable advantage of this structure is its dual-layer metasurface configuration, suggesting that the Fabry-Pérot (FP) cavity will resonate at two separate frequencies. Consider a metasurface with a0 = 4.5, an array size of 16 × 16, and H = 32 mm, as depicted in Fig. 4(b). The full-wave simulation results, illustrating the phase distribution of the E-field above the metasurface, are presented in Fig. 5 for both 24 GHz and 28 GHz bands. Upon examination, it is evident that the phase remains uniform across the majority of the area.
Fig. 5. E-Field phase distribution above metasurface when a0 = 4.5 the array size is 16 × 16 (a) 24.3 GHz. (b) 28.6 GHz.
An important aspect to consider is the angle of the central element, denoted as Angle (0,0), which serves as the initial orientation for the entire metasurface. Since the rotation angle of a metasurface element will also decrease its transmission coefficient and hence further influence the antenna gain, Angle (0,0) = 90° is selected to keep most of the long sides of the elements in the center align perpendicular to the long side of the topmost patch of the feeder. Consequently, the metasurface antenna achieves maximum gain in both the 24 GHz and 28 GHz bands. However, as the angle decreases, the maximum gain gradually diminishes. Finally, at an angle of 0°, the peak gain becomes nearly imperceptible in the gain-frequency response. These findings underscore the critical role of the initial angle in optimizing metasurface performance.
4. Simulation and frequency adjustment verification
4.1 Distance and thickness adjustment
The primary motivation for emphasizing the adjustability of the antenna stems from the varying frequency allocations across different countries and regions. For instance, n257 covers a 3 GHz bandwidth, and n258 spans 2.25 GHz, while the bandwidth for a single channel will only occupy 50 MHz, 100 MHz, 200 MHz, or 400 MHz [44]. Few governments allocate the entire spectrum for civilian telecommunications. As a result, antennas must be adaptable to local frequency assignments. In this scenario, the Fabry-Pérot (FP) antenna offers a significant advantage of accommodating different frequencies by altering the distance (H) between two plates, thereby enhancing its flexibility. To validate the adjustable characteristics of the FP antenna, extensive simulations were conducted to analyze gain performance at varying distances. The simulation results in Fig. 6 illustrate a noticeable shift in frequency with the maximum gain across varying distances. If H = 28 mm, the peak gain will occur at 23.3 GHz and 27.1 GHz. When H = 30 mm, the peak gain frequency will be 25.6 GHz. Another frequency can not be observed since it is lower than 23 GHz. When increasing the H to 32 mm, the peak gain frequency will shift to 24.3 GHz. There is a about 750 MHz frequency shift for 1 mm distance change.
The influence of dielectric substrate thickness on the metasurface is examined in Fig. 7. The results indicate distinct gain performances may be achieved corresponding to different thickness levels. For example, at a thickness of 3 mm, the metasurface achieves a peak gain of approximately 22.8 dBi. In contrast, at a thickness of 2 mm, the maximum gain is roughly 2 dB lower than that at 3 mm, but it exhibits a wider bandwidth at both 24 GHz and 28 GHz.
Fig. 7. Gain-frequency response for different thickness. (a) Gain-frequency responses for 2 mm, 2.5 mm, 3 mm, and 3.5 mm. (b) 3D patterns at 28 GHz band.
As the thickness further increases, the peak gain point shifts towards lower frequencies, with the 24 GHz band shifting more rapidly than the 28 GHz band. Generally, when the thickness is less than 3 mm, an increase in the substrate thickness tends to result in a higher gain. But if it exceeds 3 mm, the gain will decrease. A noticeable 4 dB difference is observed between substrates of 3.5 mm and 3 mm. Another significant observation is the side lobe level, as depicted in Fig. 7(b), which exhibits a marked increase along with the increase of thickness at 28 GHz band. As mentioned in Fig. 3(b), the rotation of unit cell with a thickness of 2.5 mm, will cover about 100 degrees phase change. However, if the thickness is larger than 3 mm, the rotation of the patch cannot cover so much phase shift, therefore it cannot effectively compensate for the required angle across a broad frequency range.
4.2 Pattern adjustment
The metasurface pattern is systematically investigated by varying the coefficient a0 in Eq. (1) at thicknesses of 2 mm and 2.5 mm, respectively. As illustrated in Fig. 8(a), for larger values of a0, the area of the horizontal unit cells increases (Angle (n,m) between 90° and 110°), implying an extension in the focal length. These modifications will influence both the gain and bandwidth of the antenna. Thus, this paper proposes the normalized gain bandwidth product (NGBP) as follows
Fig. 8. Pattern adjustment. (a) Horizontal unit cells (high-lighted elements whose Angle(n, m) between 90° and 110°) (b) Gain-Frequency response for different a0.
As a0 increases, the bandwidth of the lower band decreases, while that of the higher band expands. Specifically, with a metasurface thickness of 2 mm, the impact on the lower band is more obvious compared to the higher band. In contrast, for a thickness of 2.5 mm, the higher band is more significantly influenced. a0 = 3.5 yields the maximum value for a 2 mm thickness, whereas a0 = 4.5 is optimal for a 2.5 mm thickness, as illustrated in Fig. 8(b), with color indicating the gain level. Although the gain value experiences minor fluctuations with different a0 values, defining the bandwidth based on gain (greater than 15 dBi) and side lobe level (less than −8 dB) unveils substantial changes.
5. Testing results and discussion
In the context of mobile applications for JCAS, the dimensions of radar systems are constrained,as highlighted by previous research [45]. Notably, Bosch front radar premium measures 110 mm × 143 mm, while Bosch front radar plus is even more compact at 56 mm × 76 mm [46]. The size constraint is of paramount importance as numerous devices require a stable placement on mobile platforms. To address this constraint and integrate metasurface lenses into the radome effectively, a compact metasurface antenna has been manufactured. This prototype contains 16 × 16 unit cells and a size of 65 mm × 65 mm metal area and 82 mm × 82 mm dielectric area with a thickness of 2.5 mm. This diversification allows for adaptable and efficient deployment, ensuring compatibility with the size requirement in mobile applications. Additionally, a feeder with dimensions of 6 mm × 6 mm has been fabricated and integrated into a metal reflector with dimensions of 82 mm × 82 mm.
5.1 Test result of the feeder patch antenna
A feed patch antenna is fabricated as shown in Fig. 9(a) and tested in a microwave chamber. Two layers of Rogers RT5880 are used as the substrate, with thicknesses of 0.783 mm and 0.254 mm, respectively. Fig. 9(c) presents the reflection coefficient test results of the feed patch antenna. The magnitude value of the measured reflection coefficient is less than −10 dB in the range of 23.5 GHz to 29.1 GHz, which is consistent with the simulation results. Consequently, users can select the working band of this antenna within a 5.6 GHz frequency range, following the regulations of different nations or regions.
Fig. 9. Test of the feeder prototype. (a) Prototype of the feeder (b) The dimensions of the prototype, L1 = 5.7 mm, W1 = 3.7 mm, L2 = 4.6 mm, W2 = 2.6 mm, L3 = 0.8 mm, W3 = 3.1 mm, L4 = 2.8 mm, W4 = 2.3 mm. (c) Test result of reflection coefficient.
5.2 Dual-band gain-frequqency response
A metasurface composed of 16 × 16 elements with a thickness of 2.5 mm is fabricated as shown in Fig. 10(a). This is then assembled with the feed patch antenna to form an antenna assembly, as illustrated in Fig. 10(b). The radiation pattern of this assembly is tested using a microwave chamber, as shown in Fig. 10(c). The gain response, along with the frequency, is presented in Fig. 11(a). It achieves maximum gain in the lower band at 23.9 GHz for H = 32.3 mm and in the higher band at 28.7 GHz for H = 24 mm, and it can work in a 1.9 GHz bandwidth that the gain is more than 15 dBi when H = 31.8 mm. The maximum gain of H = 31.8 mm is 19.7 dBi at 24.2 GHz and 19.5 dBi at 28.5 GHz. The radiation pattern of this antenna assembly is displayed in Fig. 11(b). When compared to the gain of the feeder, as depicted in Fig. 11(b), the implementation of the metasurface results in a substantial gain enhancement of 14 dB.
Fig. 10. Prototype of the proposed antenna and the test environment. (a) Metasurface. (b) Metasurface-feeder combination. (c) Anechoic chamber and test platform.
Fig. 11. Test result of gain frequency response. (a) Gain frequency response for different metasurface-feeder distance. (b) Radiation pattern for H = 31.8 mm.
5.3 Reconfigurable frequency along the distance from the feeder
The concept of the F-P antenna is demonstrated in Fig. 4 and discussed in section 4.1. A slight change in distance results in a shift of the gain-frequency response along the frequency axis. As illustrated in Fig. 12(a), the peak gain frequency point of the 16 × 16 metasurface antenna will be 24.5 GHz, 25.3 GHz, and 26 GHz for different distances with the gain fluctuation less than 1 dB. The conclusion is, in the test distance range, a frequency shift of approximately 750 MHz occurs for every 1 mm change in distance in the 24 GHz band. When substantial adjustments in distance are made, the gain-frequency response demonstrates a periodic behavior. Specifically, an increase or decrease in distance results in a similar gain-frequency response within the 24 GHz band. Similarly, a distance adjustment induces the same periodicity within the 28 GHz band. As illustrated in Fig. 12(b), the frequency exhibiting the maximum gain is precisely 23.5 GHz when the distance (H) is adjusted to 11 mm, 24 mm, and 33 mm, respectively. Moreover, in Fig. 12(b), it is important to note that as the profile size decreases, two peak gain frequencies remain observable, and the gap between these frequencies widens.
Fig. 12. Test result for distance adjustment. (a) Different profile with shifting peak gain frequency. (b) Different profiles with the same peak gain frequency.
5.4 Comparison of other works
Table 1 presents a comparison of various dual-band metasurface antennas from previous studies. The first innovation of this work is the use of a single patch antenna feeder to realize a dual-band metasurface antenna. Only a 6 mm × 6 mm PCB area is used for the feeder, and grounded coplanar waveguide (GCPW) as the feeder line, which can be easily integrated with RF circuits. RF circuits can be conveniently laid out on the back side of the feeder, as depicted in Fig. 1. This contrasts with previous works that used a coaxial cable to stimulate the patch or directly used the waveguide, posing significant integration challenges.
Table 1. Comparison of dual-band metasurface antenna
The second innovation is that the proposed antenna demonstrates that the metasurface can be designed using a simple formula. Although the gain is lower than some other works, it only requires a small area and a simple structure to realize a high-gain antenna. This eliminates the need for numerous vias and complex multi-layer structures, as seen in works such as [23–27], making it suitable for mass-production products like vehicle radar.
The third innovation of the proposed antenna is the reconfiguration of frequency. The maximum gain frequency point of this antenna can be easily adjusted. This is crucial as it eliminates the need to redesign the entire antenna for different requirements in various countries or applications.
6. Conclusion
This paper introduces a frequency reconfigurable, dual-band metasurface antenna, offering several advantages due to its unique design features. The antenna design incorporates a Broadband Stacked Patch Feeder, which is seamlessly integrated with a 50 Ohm Grounded Coplanar Waveguide (GCPW). This integration simplifies the connection with RF circuits, eliminating the need for complex PCB fabrication processes. To broaden the bandwidth, the feeder utilizes a U-shaped slot patch and a parasitic ring slot patch, ensuring insensitivity to fabrication deviations. Meanwhile, the antenna employs a straightforward metasurface structure to achieve high gain across multiple bands without the need for complex structures, making it a cost-effective solution. A phase compensation formula is proposed for the distribution of the metasurface unit cells and is verified by the prototype. The frequency re-configuration ability of the antenna is realized by adjusting the distance between the feeder and the metasurface, with a 1 mm distance adjustment resulting in a 750 MHz average frequency shift. The fabricated metasurface antenna, designed according to the space requirements of vehicle radars, obtained maximum gains of 19.7 dBi and 19.5 dBi for the lower band and higher band, respectively. The proposed antenna, due to its ease of integration, low cost, compact size, and high gain, is suitable for mmWave joint communication and radar systems, among other multifunctional applications.
Funding
Fudan University (MOE innovation platform); National Natural Science Foundation of China (62235005).
Acknowledgement
This paper is supported in part by the National Natural Science Foundation of China under Grant 62235005 and in part by the by the project of MOE innovation platform.
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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