Open Access
Add to BibSonomy
Abstract
This work presents a theoretical design and experimental demonstration of a novel miniaturized leaky-wave antenna (LWA) using composite waveguide based on substrate-integrated plasmonic waveguide (SIPW). The SIPW is designed by embedding hybrid dual spoof surface plasmon polariton (SSPP) structure into a three-layer substrate integrated waveguide (SIW). Due to the slow-wave effect of SIPW, the proposed miniaturized composite waveguide forms slowed phase velocity and decreased lower cutoff frequency. To excite backward-to-forward beam scanning mode and suppress the open stop-band, an asymmetric sinusoidal modulated structure is introduced to the surface of the composite waveguide. The experimental results indicate that the proposed SIPW-based LWA can achieve continuous beam scanning from the backward to the forward direction within the bandwidth of 10.6-13.7 GHz, passing through the broadside at 11.6 GHz.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Leaky wave antenna (LWA) is a kind of travelling-wave antennas, which have attracted much attention due to its unique advantages, such as high gain, low profile, and low cost. LWAs can achieve a high directivity without the need for a costly and complicated feed network as typically used in phased-array metasurfaces [1–5]. Besides, the inherent narrow bandwidth nature of the phased-array metasurfaces hardly meets the system requirements [6–8]. Hence, LWAs are more popular in the microwave band. Substrate integrated waveguides (SIWs) have the merits of both planar microstrip transmission lines and rectangular waveguides, including the advantages of low insertion loss, high-quality factor and easily integration with planer microwave circuits [9,10]. Hence, they have been widely used in the design of microwave circuits and systems [11–15]. In the past few years, some designs of SIW-based leaky-wave antennas have been proposed [16–19]. However, the large longitudinal and especially the large lateral dimensions of SIW-based LWAs limits their application in compact microwave circuits and systems [20].
Spoof surface plasmon polariton (SSPP) is a pattern of electromagnetic waves inspired on the surface of a periodic structure, whose electromagnetic behaviors are similar to surface plasmon polariton (SPP) [21,22]. The slow-wave features can be achieved if the phase velocity of the SSPP is slowed down. Therefore, the SSPP can confine electromagnetic waves to the subwavelength spatial dimensions. It has been promisingly applied in the miniaturization of microwave devices and systems, including antennas, filters, logic gates, sensors, and hologram [23]. Particularly, to reduce the longitudinal size of the SIW-based LWA, several LWAs have been investigated by combining SIW with a single SSPP structure [24–26]. However, a single SSPP can only reduce the dimension in either longitudinal or lateral direction dimension. To further reduce the dimensions of SIW-based LWA, half-mode substrate integrated waveguide (HMSIW) structure has been used and investigated [27–29]. As HMSIW employs one-half of the SIW along a symmetrical magnetic wall, the lateral dimension of LWAs is reduced by half. However, compared with the design based on SIW, the realized gain of HMSIW-based LWA is dropped due to the decrease of radiating aperture.
In this paper, we propose a miniaturized LWA with backward-to-forward beam scanning and suppressed open stop-band using a novel composite waveguide based on substrate-integrated plasmonic waveguide (SIPW). The SIPW is formed by integrating hybrid dual SSPP structure into a three-layer SIW. One SSPP is realized by using sub-wavelength periodic slots, and the other SSPP is based on metal patches and metallized blind via-holes that are loaded into normal SIW. In this way, we can achieve shorter guided wavelength and decrease the cutoff frequency. To excite the radiation mode and suppress the open stop-band, we introduce an asymmetric sinusoidal modulated structure to the profile of sub-wavelength periodic slots. The SIPW structure promotes the miniaturization of LWA with backward-to-forward beam scanning and suppressed open stop-band.
2. Design theory
2.1. Configuration of LWA
Figure 1 illustrates the configuration of the proposed SIPW-based LWA with size of 13.5 mm × 333 mm. To realize good impedance matching, linearly tapered microstrip lines are added at the ends of the LWA. The SIPW is formed by embedding hybrid dual SSPP structure into a three-layer SIW. The one SSPP structure is formed by etching the sub-wavelength periodic slots onto the top metal layer of the waveguide. The asymmetric sinusoidal modulated structure of these slots is sinusoidal modulated and there is a phase difference of π/2 between the upper and lower sinusoidal modulation curves. The modulated period is 12.6 mm and the modulation amplitude is 0.7 mm. Another hybrid SSPP structure in waveguide is designed through employing arrays of metal patches and metallized blind via-holes. Metallized blind via-holes are periodically embedded inside the bottom substrate, with the diameter r and separation distance w.
Fig. 1. Configuration of the proposed SIPW-based LWA. (a) Exploded 3-D view. (b) Transversal cross section of the SIPW-based LWA.
The whole design is based on three substrate layers. The top and bottom substrate are the Rogers RT5880 with a relative dielectric constant εr of 2.2, a dielectric loss tangent tanδ of 0.0009. The thickness of the both top and bottom substrates is determined to be 0.254 mm. The middle substrate, acting as an adhesive layer, uses Rogers RO4450F with a relative dielectric constant εr of 3.7, a dielectric loss tangent tanδ of 0.004 and a thickness of 0.1 mm. Metal patches are shorted to the bottom metal layer of the SIW by using metallized blind via-holes and metal via-holes connect top metal layer and bottom metal layer. The values of the parameters are fixed as follows: w = 0.7 mm, r = 0.2 mm and Lp = 9 mm.
2.2 Analysis of the SIPW
Figure 2 demonstrates the simulated dispersion curves of SIPW with different maximum slot length h0 for comparison. The period of periodic unit is p0 = 0.65 mm. As the phase constant β increases, the slopes of SIPW curves decrease gradually and the slow-wave feature emerges. It also can be found that with the increase of h0, the SIPW has stronger slow-wave effect and lower upper cutoff frequency. Besides, it is obvious that the phase constant of SIPW is sensitive to frequency variation as the frequency increases to the upper cutoff frequency.
To investigate the miniaturization performance of the composite waveguide, the lower cutoff frequency and phase velocity of the SIPW are plotted in Fig. 3. Here, we choose h0 = 7 mm. As shown in Fig. 3 (a), the lower cutoff frequency of the SIPW is reduced by 30% compared with the normal SIW. Therefore, a narrower waveguide with 30% lateral size reduction can be achieved for the same desired lower cutoff frequency compared with SIW. As can be seen from Fig. 3 (b), the phase velocity of the SIPW is reduced by 75% compared with the normal SIW. This implies that for the same desired electrical length, the physical length ratio between SIPW and SIW is 1:4. Therefore, by employing an SIPW instead of a conventional SIW, a shorter waveguide with a 75% reduction in longitudinal length can be achieved for the same desired electrical length.
Fig. 3. Miniaturization capability of SIPW. (a) Simulated transmission coefficients (S21) of normal SIW and SIPW. (b) Simulated normalized phase velocity of normal SIW and SIPW.
2.3. Suppression of open stop-band
The angle of the maximum radiation direction from the broadside of LWA can be determined as
According to Bloch–Floquet theorem, the infinite space harmonics are introduced by periodically modulating the periodic slots and the phase constants corresponding to nth harmonics is calculated as
The modulated period d is preliminary determined by one guided wavelength at the broadside and the wavenumber in free space k0 is larger than β-1. In order to make the main beam scan in both forward direction and backward direction, the n = -1 space harmonic is chosen to realize unidirectional radiation by introducing sinusoidally modulated profile to the periodic slots. In this way, the surface reactance of the waveguide varies sinusoidally and the LWA is capable of converting SSPP modes to radiating modes. When the main beam of the LWA is scanned through the broadside direction, the propagating electromagnetic waves have the opposite direction and same amplitude, most electromagnetic waves are reflected to input port. Therefore, the LWAs usually suffer from an open stop-band at the broadside.
When the phase constant is approximately zero within a frequency band, the LWA encounters an open-stopband as described by Eqs. (1) and (2). Referring to the dispersion curve for the -1 and -2 order space harmonics excited by the sinusoidally modulated structure in Fig. 4, it can be observed that the phase constant approaches zero within the frequency range of 11.6-11.8 GHz. Consequently, the open-stopband of the LWA occurs within this frequency region. In this region, the radiated power drops substantially and the high reflection coefficient is encountered. To overcome this disadvantage, an asymmetric sinusoidal modulated structure with the phase difference of π/2 is introduced between two sinusoidal modulation curves.
As shown in Fig. 5, the configuration of asymmetric sinusoidal modulated structure is composed of two sin curves defined as y1 = Asin(ωx)+B and y2 = Asin(ωx + Δφ)+C, respectively. The period is 12.6 mm. The simulated reflection coefficient and realized gain for different values of phase difference are depicted in Fig. 6. It is obvious that the reflection coefficient of LWA with the phase difference of π/4 exists an open stop-band from 10.6 to 12.2 GHz. For the case with Δφ = 3π/4 for the sinusoidal modulated structure, the realized gain is increased, and the reflection coefficient is decreased, but the open stop-band is not completely suppressed. When the phase difference of π/2 is introduced, the reflected waves along the waveguide are not in phase and electromagnetic waves can be effectively fed into the antenna for broadside radiation. The reflection coefficient is well below -10 dB in the whole operating band and the realized gain is stable. Thus, we choose the phase difference of π/2 for suppression of open stop-band.
Fig. 5. Heuristic diagram of sinusoidal modulation profile for asymmetric sinusoidal modulated structure.
Fig. 6. Simulated reflection coefficients (S11) and realized gains for different asymmetric sinusoidal modulated structures.
In order to show the mechanism of leakage clearly, Fig. 7 displays the simulated electric field patterns of the proposed LWA at different frequencies in the xz plane. It is clearly seen that the electromagnetic waves radiate into free space throughout the asymmetric sinusoidal modulated slots. As the frequency increases, the angle of the main beam changes continuously from the backward to the forward direction.
3. Results and discussion
In order to prove the feasibility of the proposed LWA, the prototype of the proposed LWA is fabricated and measured. The photograph of the prototype is shown in Fig. 8 together with a chamber with an anechoic far-field setup. To radiate out most of electromagnetic energy, the prototype includes 25 sinusoidal modulation periods. The LWA was positioned at the center of a turntable for far-field measurements. One port of the antenna under test was fed through a 3.5 mm SMA connector, while the other port was terminated with a 50 Ω matching load. A standard horn was employed as source and a reference antenna was utilized to measure the gain of the LWA. By comparing the power of reference antenna and LWA, the gain of the proposed LWA could be determined. The complete radiation pattern can be obtained by rotating the turntable.
By using Agilent E8383C network analyzer, the S-paraments are measured. As shown in Fig. 9, the result is in good consistence with the simulation one at the frequency band below 12.7 GHz. The difference between the measured and simulated S21 at higher frequencies is mainly attributed to the fabrication tolerance, the dielectric tolerance of substrate and the insertion loss of connectors. Nevertheless, the measured result shows that S11 is lower than -16 dB around broadside. In the operating band from 10.6 to 13.7 GHz where the main beam of LWA scans from the backward to the forward direction, the measured S21 is lower than -6 dB, meaning that most transmitted energy is radiated out by LWA.
The simulated and measured radiation patterns for the LWA working at various frequencies are shown in Fig. 10, where it is obvious that the simulated main beam is scanned from -25° to +61° within the operating band from 10.6 to 13.7 GHz, while the measured main beam is scanned from -27° to +62°. It demonstrates that the proposed LWA realizes unidirectional main beam scanning from the backward to the forward direction without blind point. Hence, the open stop-band is successfully suppressed. In the operating band, the simulated and measured realized gains keeps relatively stable and varies from 10 to 14 dBi. Good agreement is achieved between the simulated and measured results. The slight differences between them may be caused by the fabrication tolerance and assembly misalignment.
Fig. 10. Radiation patterns of the co-polarized fields for the LWA in the principal scanning plane (the xz plane).
The comparison between the proposed LWA and several published works is outlined in Table 1. The proposed LWA stands out for its narrowest lateral dimension, broadened bandwidth, and extensive beam scanning range spanning from backward to forward directions. This method enables the creation of LWAs capable of bidirectional beam scanning while effectively mitigating open stop-band effects within a miniaturized physical size. These findings hold significant promise for advancing the development of compact and efficient antenna systems.
Table 1. Comparison of the proposed LWA with other reported worksa
4. Conclusion
In conclusion, a miniaturized LWA with backward-to-forward beam scanning and suppressed open stop-band using SIPW is designed, analyzed, fabricated and measured. Compared with the normal SIW, the lateral and longitudinal dimensions of the SIPW-based LWA are ultimately reduced by 30% and 75%, respectively. The radiation of the proposed LWA is realized by adopting sinusoidal modulation curves to the profile of periodic slots. The open stop-band is suppressed by introducing a phase difference of π/2 between two sinusoidal modulation curves. The measured results demonstrate that the proposed LWA has the ability of unidirectional and backward-to-forward beam scanning from -27° to +62° within the broadened operating band of 26%. Compared with the previously reported designs, the proposed LWA features narrow lateral dimension, broadened relative bandwidth as well as wide and continuous beam scanning range. The proposed LWA shows potential applications in communications and radar systems. Additionally, the SIPW structure provides valuable design considerations for compact microwave circuits and systems.
Funding
National Key Research and Development Program of China (2022YFF0604801); National Natural Science Foundation of China (62271056, 62171186, 62201037); Natural Science Foundation of Beijing Municipality (L222042).
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.
References
1. H. Cheng, H. Yang, J. Wu, et al., “Leaky-Vivaldi antenna covered with metasurface with leaky wave radiation and aperture radiation,” Opt. Express 31(11), 17291–17303 (2023). [CrossRef]
2. Y. Li, J. Jin, Z. Yang, et al., “Low-RCS low-profile MIMO antenna and array antenna using a polarization conversion metasurface,” Opt. Express 31(23), 38771–38785 (2023). [CrossRef]
3. X. Song, X. Bai, and W. Zhu, “Reconfigurable metasurface for nearly full-range and continuous modulation of reflection, transmission, and absorption,” ACS Appl. Electron. Mater. 4(3), 1225–1231 (2022). [CrossRef]
4. L. Shao, D. Zhang, J. Liu, et al., “Single-layer non-interleaved metasurface for arbitrary vector beam conversion in triple bands,” ACS Appl. Electron. Mater. 4(1), 443–451 (2022). [CrossRef]
5. C. Zhang, X. Zhang, P. Xie, et al., “Filter-assisted metasurface for full-space wavefront manipulation and energy allocation,” ACS Appl. Electron. Mater. 3(10), 4465–4471 (2021). [CrossRef]
6. G. Wu, L. Si, H. Xu, et al., “Phase-to-pattern inverse design for a fast realization of a functional metasurface by combining a deep neural network and a genetic algorithm,” Opt. Express 30(25), 45612–45623 (2022). [CrossRef]
7. G. Cheng, L. Si, P. Tang, et al., “Topology optimization of the azimuth-rotation-independent polarization conversion metasurface for bandwidth enhancement,” Opt. Express 30(23), 41340–41349 (2022). [CrossRef]
8. L. Si, H. Jiang, X. Lv, et al., “Broadband extremely close-spaced 5 G MIMO antenna with mutual coupling reduction using metamaterial-inspired superstrate,” Opt. Express 27(3), 3472–3482 (2019). [CrossRef]
9. D. S. Eom and H. Y. Lee, “Substrate integrated waveguide transitions to planar transmission lines using lumped elements and their applications,” IEEE Trans. Microwave Theory Tech. 64(12), 4352–4361 (2016). [CrossRef]
10. A. P. Saghati and K. Entesari, “Ultra-miniature SIW cavity resonators and filters,” IEEE Trans. Microwave Theory Tech. 63(12), 4329–4340 (2015). [CrossRef]
11. R. V. Gatti and R. Rossi, “Hermetic broadband 3-db power divider/combiner in substrate-integrated waveguide (SIW) technology,” IEEE Trans. Microwave Theory Tech. 66(6), 3048–3054 (2018). [CrossRef]
12. F. Parment, A. Ghiotto, T.-P. Vuong, et al., “Double dielectric slab-loaded air-filled SIW phase shifters for high-performance millimeter-wave integration,” IEEE Trans. Microwave Theory Tech. 64(9), 2833–2842 (2016). [CrossRef]
13. L. Ye, Z. Chen, Y. Zhang, et al., “High performance multiple passband substrate integrated plasmonic filters,” IEEE Trans. Circuits Syst. II 70(4), 1445–1449 (2023). [CrossRef]
14. W. Kong, Y. Hu, J. Li, et al., “2-D orthogonal multibeam antenna arrays for 5 G millimeter-wave applications,” IEEE Trans. Microwave Theory Techn. 70(5), 2815–2824 (2022). [CrossRef]
15. L. Pan, Y. Wu, and W. Wang, “Bandpass filter with reconfigurable rejection and deep-upper-wideband harmonics suppression using spoof surface plasmon polaritons of hollowed-bow-tie cells,” IEEE Trans. Microwave Theory Techn. 71(4), 1826–1834 (2023). [CrossRef]
16. J. Liu, D. R. Jackson, Y. Li, et al., “Investigations of SIW leaky-wave antenna for endfire-radiation with narrow beam and sidelobe suppression,” IEEE Trans. Antennas Propag. 62(9), 4489–4497 (2014). [CrossRef]
17. M. Z. Ali and Q. U. Khan, “High gain backward scanning substrate integrated waveguide leaky wave antenna,” IEEE Trans. Antennas Propag. 69(1), 562–565 (2021). [CrossRef]
18. A. M. Malekshah, A. R. Attari, and M. S. Majedi, “Improved design of uniform SIW leaky wave antenna by considering the unwanted mode,” IEEE Trans. Antennas Propag. 68(8), 6378–6382 (2020). [CrossRef]
19. Q. Liu and J. Liu, “An SIW periodic leaky-wave antenna with dual-stopband and grating lobe simultaneously suppressed using a new slot-array within a half-period,” IEEE Trans. Antennas Propag. 22(10), 2502–2506 (2023). [CrossRef]
20. Q. Zhang and Y. Chen, “Spoof surface plasmon polariton leaky-wave antennas using periodically loaded patches above PEC and AMC ground planes,” Antennas Wirel. Propag. Lett. 16, 3014–3017 (2017). [CrossRef]
21. S. H. Nayem, N. Imtiaz, S. R. Joy, et al., “Theoretical limit and framework of dynamic modulation in spoof surface plasmon polariton interconnects,” Opt. Express 31(18), 29536–29557 (2023). [CrossRef]
22. Y. Yin, Q. Li, X. Chen, et al., “Wide-angle scanning leaky-wave antenna array based on double-layer double-row spoof surface plasmon polariton,” Antennas Wirel. Propag. Lett. 22(5), 1050–1054 (2023). [CrossRef]
23. F. J. Garcia-Vidal, A. I. Fernández-Domínguez, L. Martin-Moreno, et al., “Spoof surface plasmon photonics,” Rev. Mod. Phys. 94(2), 025004 (2022). [CrossRef]
24. D. F. Guan, Q. Zhang, P. You, et al., “Scanning rate enhancement of leaky-wave antennas using slow-wave substrate integrated waveguide structure,” IEEE Trans. Antennas Propagat. 66(7), 3747–3751 (2018). [CrossRef]
25. S. D. Xu, D. F. Guan, Q. Zhang, et al., “A wide-angle marrowband leaky-wave antenna based on substrate integrated waveguide-spoof surface plasmon polariton structure,” Antennas Wirel. Propag. Lett. 18(7), 1386–1389 (2019). [CrossRef]
26. S. D. Xu, L. Liu, D. F. Guan, et al., “A hybrid substrate integrated waveguide and spoof surface plasmon polariton leaky-wave antenna with open stopband suppressed,” Int J RF Mic Comp-Aid Eng 32(11), e22413 (2022). [CrossRef]
27. S. Wang, K. L. Chung, L. Du, et al., “Design and analysis of a compact frequency beam-scanning antenna based on composite FHMSIW/SSPP waveguide,” Antennas Wirel. Propag. Lett. 21(3), 546–550 (2022). [CrossRef]
28. D. René-Loxq, O. Lafond, M. Himdi, et al., “Reconfigurable half-mode SIW antenna using uniaxial field programmable microwave substrate structure,” IEEE Trans. Antennas Propag. 70(11), 11103–11108 (2022). [CrossRef]
29. Y. Geng, J. Wang, Y. Li, et al., “A Ka-band leaky-wave antenna array with stable gains based on HMSIW structure,” IEEE Trans. Antennas Propag. 21(8), 1597–1601 (2022). [CrossRef]
30. J. Chen, W. Yuan, W. X. Tang, et al., “Linearly sweeping leaky-wave antenna with high scanning rate,” IEEE Trans. Antennas Propag. 69(6), 3214–3223 (2021). [CrossRef]
31. J. Liu, “Periodic leaky-wave antennas based on microstrip-fed slot array with different profile modulations for suppressing open stopband and n = -2 space harmonic,” IEEE Trans. Antennas Propag. 69(11), 7364–7376 (2021). [CrossRef]
32. H. Wang and S. Sun, “An omnidirectional CPW leaky-wave antenna with steering conical beam,” IEEE Trans. Antennas Propag. 22(8), 2042–2046 (2023). [CrossRef]
33. S. Zhao and Y. Dong, “Circularly polarized beam-steering microstrip leaky-wave antenna based on coplanar polarizers,” IEEE Trans. Antennas Propag. 21(11), 2259–2263 (2022). [CrossRef]
