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近日，清华大学李越团队以「Low-loss beam synthesizing network based on epsilon-near-zero (ENZ) medium for on-chip antenna array」¹为题在Chip上发表研究论文，利用近零介电常数媒质取代结构复杂、损耗较大的馈电网络，实现片上天线阵列的馈电与低副瓣波束综合。第一作者为博士生李昊，通讯作者为李越。
片上毫米波天线阵列是单片集成毫米波射频电路系统中的重要组件。通过提升天线阵列的增益并抑制副瓣，可以对抗毫米波频段的高衰减并获得更好的抗干扰性能。传统的高增益、低副瓣天线阵列，往往基于馈电网络实现。在毫米波频段，一方面，馈电网络引入了较高的电磁波传输损耗，导致天线的辐射效率降低；另一方面，馈电网络一般具有较为复杂的空间结构，给高精度加工带来了一定的困难²。
为提升片上毫米波天线阵列的效率，研究团队提出了使用截止波导等效的近零介电常数（Epsilon-Near-Zero, ENZ）媒质取代复杂的馈电网络，实现对天线阵列的波束控制。ENZ媒质，即介电常数接近零的媒质，电磁波在该类媒质中具有近似无限大的波长，因此能够在其中激励均匀的场分布³。利用近零介电常数媒质的这种场均匀特性，研究团队设计了如图1所示的天线结构，通过激励截止波导等效ENZ媒质中的阶梯状谐振模式，实现对各槽天线单元的同幅同相激励。这种馈电方法可以与阵元位置优化结合，实现低副瓣、非衍射等不同种类波束的综合。
图1 | 基于ENZ媒质馈电的毫米波天线阵列示意图。(a) 天线与射频芯片集成示意图；(b) 天线结构示意图；(c) 天线的等效电路图。
为更好地与芯片集成并进一步提升天线效率，研究团队采用硅基微系统（silicon microelectromechanical systems, Si-MEMS）加工工艺⁴，加工了空气填充的低副瓣天线阵列样机。在这种加工工艺下，由于天线内电磁场被金属边界条件约束在空气腔内，介质损耗被彻底消除，使得天线能够实现极高的辐射效率。图2展示了天线样机照片以及实测的辐射性能结果。该天线在3.4倍波长的长度内，经实测实现了13.6 dBi的增益、−20dB以下的副瓣电平以及84.8%的效率。
图2 | 低副瓣毫米波天线阵列样机的实物照片与测试结果：(a) 天线样机实物照片；(b) 实测与仿真的天线辐射方向图；(c) 实测与仿真的端口辐射系数；(d) 实测与仿真的天线增益；(e) 实测与仿真的天线效率。
此外，研究团队在文章中还讨论了通过ENZ媒质馈电实现的贝塞尔波束与高定向性波束的实现方法，说明了所提出的基于ENZ媒质的波束综合馈电网络具有较强的普适性。综上，本工作基于ENZ媒质提出了一种片上集成的天线阵列的新型馈电方法，可用于毫米波波束综合应用，与传统方法相比，具有结构简单、效率高的优势，对单片集成毫米波系统中有重要意义。
Low-loss beam synthesizing network based on epsilon-near-zero (ENZ) medium for on-chip antenna array¹
On-chip millimeter wave antenna arrays are essential in radio frequency integrated circuits. High gains and low sidelobe levels are both demanded against high attenuations and interferences. Conventionally, such antenna arrays are composed using feeding networks, which both deteriorates the radiation efficiency by introducing high Ohmic and dielectric losses and cause fabrication difficulties as a result of its structural complexity².
To enhance the efficiency of on-chip millimeter wave antenna arrays, the research team proposes a new method of on-chip feeding network design for antenna arrays with the same amplitude and phase based on epsilon-near-zero (ENZ) medium instead of complex feeding networks. The ENZ medium is referred to as the medium with near zero permittivity. The electromagnetic wave features an infinite wavelength in ENZ medium so that a uniformly distributed electromagnetic field is excited in this medium³. Based on this field homogeneity, the research team proposes the antenna array configuration in which a cutoff waveguide is used to emulate the ENZ medium and feed the antenna elements with the same amplitude and phase. Moreover, this feeding method is combined with inter-element spacing optimization to synthesize various beams including low-sidelobe beams and Bessel beams.
For the convenience on chip scale integration and further improvement on antenna’s radiation efficiency, the research team utilizes the silicon microelectromechanical systems (Si-MEMS)⁴ micromachining technology for fabricating the antenna prototype. Using this technology, an air-filled antenna array is fabricated in which the electromagnetic power is constrained in air by full metallic boundaries. In this case, the dielectric loss is completely eliminated so that an extremely high radiation efficiency. The fabricated prototype is measured to have a peak gain of 13.6 dBi, low sidelobe level of -20.0 dB, and a high efficiency of 84.8%. The efficiency performances show supremacy when compared with previous works. In addition, the research team illustrates in the paper that this ENZ-based feeding method is feasible to synthesize highly directive beams and quasi-nondiffractive Bessel beams at broadside direction, validating that the proposed method is useful for synthesizing arbitrary beams at the broadside direction.
In summary, a novel feeding method is proposed based on ENZ medium for on-chip millimeter wave beam synthesizing antenna arrays. Compared with conventional approaches, the proposed one is promising in integrated millimeter wave systems for the merits of structural simplicity and low loss.
参考文献：
1. Li, H., Zhou Z., Zhao, Y. & Li, Y. Low-loss beam synthesizing network based on Epsilon-near-zero (ENZ) medium for on-chip antenna array. Chip 2, 100049 (2023).
2. Wu, Q. et al. Millimeter-wave planar broadband circularly polarized antenna array using stacked curl elements. IEEE Trans. Antennas Propag. 65, 7052-7062 (2017).
3. Liberal, I. & Engheta, N. Near-zero refractive index photonics. Nat. Photonics 11, 149–158 (2017).
4. Chang, L. et al. Low-sidelobe air-filled slot array fabricated using silicon micromachining technology for millimeter-wave application. IEEE Trans. Antennas Propag. 65, 4067-4074 (2017).
Chip是全球唯一聚焦芯片类研究的综合性国际期刊，是入选了国家高起点新刊计划的「三类高质量论文」期刊之一。
论文链接：
https://www.sciencedirect.com/journal/chip
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