Introduction
Metasurfaces such as transmitarrays (TAs), reflectarrays (RAs), and Metasurface-empowered antennas, are attractive structures that can achieve high gain, wide bandwidth, and beam-reconfigurable performance, possessing planar, lightweight, and easily fabricated properties [1–8]. TAs, in particular, have received significant attention due to their ability to refract the radiation beam on the desired direction by compensating for the phase of the incident electromagnetic (EM) wave. Therefore, TAs play a decisive role in high-frequency applications such as mmWave base stations, automotive radar systems, and satellite communications [8–10].
In general, there are two types of TAs: passive and active. The former steers the main beam with phase shifters or switching circuits connected to the source antenna [11, 12]. However, this type of TA requires the integration of additional components on the radio-frequency (RF) integrated circuit. The interconnections between these components increase the complexity of the circuits and RF power consumption, leading to excessive insertion loss [13].
In contrast, the latter type metamorphoses its EM properties, that is, phase responses, in the spatial domain. This characteristic can overcome the disadvantages of a passive TA. Fig. 1 illustrates the operational principle of a liquid crystal-based transmitarray (LCTA), which belongs to the category of active TAs. Liquid crystal (LC) adjusts the phase of the wavefront after the TA receives an RF signal from the feed antenna. Consequently, the direction of the EM wave radiated from the TA can be reconfigured.
Recently, active TAs have been investigated by employing active components or metamaterials such as PIN diodes, varactors, and LCs [14–18]. While PIN diodes have an advantage in beamsteering performance, the undesirable power consumption resulting from changing the on/off states by controlling currents can decrease the efficiency of systems. On the other hand, varactors vary the capacitance by applying voltages without currents flowing through the bias circuit. Varactors also provide a continuous change in the phase response, while PIN diodes result in discrete responses. However, these types of active components need to be mounted on substrates, making them vulnerable to external collisions. Furthermore, active components have limited operating frequencies due to their self-resonance frequencies. Unlike active components, an LC transforms its permittivity with various DC bias voltages. Additionally, an LC has the advantage of operating at high frequencies since it lacks a self-resonance frequency. Furthermore, mass-produced LCs cost little.
For the reasons mentioned above, LC-based metasurfaces are currently under active study [16–23]. However, while RAs and active frequency selective surfaces with LC have been widely investigated in recent decades, the LCTA-related research field remains limited. Due to the narrow tuning range of the permittivity in LC, the beam coverage of LCTA faces a crucial limitation when it is implemented in real-world communication systems. In [18], for instance, only achieved the maximum scan angle of ±5°.
This is the first study to propose an LCTA focusing on the mmWave communication band with a two-dimensional (2D) beam scanning capability. To achieve a wide tunable phase shift range despite the limited variance of permittivity, the meander line embedded in a substrate-integrated waveguide (SIW) unit cell structure is implemented. The unit cell consists of receive–transmit antennas and transmission lines such as SIW and stripline, where the unit cell receives the RF signal from the feeding source and transmits it to the far field with phase compensation. Typically, the incident EM wave propagates conventional unit cells vertically (i.e., through LC thickness). In contrast, in the proposed unit cell, the EM wave travels along the transmission lines horizontally due to the antenna–SIW transitions in the unit cell (Fig. 2). Due to the horizontal propagation of the EM wave in the unit cell, the electrical length of the unit cell increases drastically more than in traditional unit cell structures.
In Section II, the design of the proposed LCTA unit cell is introduced with the simulated frequency responses. Section III rovides the measurement setup and results of radiation patterns. Finally, Section IV concludes the study.
LCTA Design Principles
1. Phase Tuning Range Expansion
In microwave theory, the phase delay ϕ of the transmission line is defined as:
where ω is the angular frequency (i.e., ω= 2πf), ℓ is the physical length of the transmission line, and μ and ɛ are the permeability and permittivity of the material filled in the transmission line, respectively [23]. To induce a phase variation in the two-port system at the same frequency, either the physical length or material property must be changed (i.e., Δϕ= βΔℓ or Δϕ= Δβℓ).
Hence, the LC-based unit cells vary their phase tuning range by changing their material properties. However, LC-based unit cells typically have a narrow phase tuning range due to the limited range of the dielectric constant. The tunable range of the dielectric constant of the LC used in this paper increases from 2.5 to 3.5 (Merck GT7-29001) where the loss tangent decreases from 0.012 to 0.0064 [21, 22]. In this case, the simplest way to expand the tuning range is by increasing the length of the wave propagation (Fig. 2(a)). Thus, Maasch et al. [18] proposed a fishnet structure with two thick layers of LC substrates. From (1) and (2), we can assume that using two layers of thick LC substrates can significantly increase the phase tuning range. Although this methodology can expand the tuning range, the fabrication process would be complicated because of the increased bias circuits and increased cost compared to using a thin single LC layer. Therefore, this paper proposes the meander line structure on the LC substrate to increase the tunable range without using an additional LC layer. Unlike the conventional structure, the incident wave propagates horizontally on the LC layer, extending the length of the transmission line from the thickness to the width of the substrate. Fig. 2(b) illustrates the proposed unit cell design concept.
2. Unit Cell Design
Figs. 3 and 4 show the exploded view and detailed geometry of the proposed unit cell design. After the antenna receives the EM wave from the feed source, shorting vias among the patch and slotted ground can guide the wave to the SIW structure [25]. The SIW structure prevents leakage of the EM wave that interferes with the adjacent unit cells and bias lines. To guide the wave along the meander line, the transition between a slot and a stripline is employed.
On the top and bottom layers, magnetoelectric dipole antennas are adopted to facilitate the guidance of the incident wave from the antenna to the SIW structure. Fig. 5(a) plots the surface current vector of the magnetoelectric dipole on the unit cell to clarify the polarization characteristic. The polarization of the dipole layer maintains its direction despite the transmission lines being embedded in the LC substrate. As shown in Fig. 5(b), the electric field is concentrated strongly on the gap between the patches. This field excites the TE10 mode of the SIW and guides the wave to the next layers. To utilize the meander line for a wider phase tuning range, the SIW and meander stripline are coupled through the slot transition. Fig. 5(c) shows the electric field among the unit cell cross-section on the E-plane. The notable aspect is the propagation path inside the LC layer, which propagates horizontally guided by the meander line structure.
Fig. 6 shows the simulated S21 responses with respect to the relative permittivity of the LC. The proposed unit cell design achieves a phase tuning range of 130° while maintaining the insertion loss level under 6 dB for the frequency point of 28 GHz. Fig. 7 depicts frequency responses affected by the number of bias lines. The number inside the bracket of the legend refers to the relative permittivity of the LC layer. The frequency responses of the unit cell regarding the number of bias lines are unchanged because the via walls block the E-field excitation on the bias lines (Fig. 5(b)).
Fabrication and Measurement Result
A fabricated example of the proposed LCTA sample is shown in Fig. 8. The radiating region of the LCTA is 52 mm × 52 mm, and the supplemental area consists of DC bias connections. The Taconic TLY-5 substrate (ɛr = 2.2 and tanδ = 0.0009) is used with a thickness of 0.25 mm for all substrate layers. Fig. 9 depicts the measurement setup of the proposed LCTA. The distance between the receiver horn and LCTA is 1.5 m, which satisfies the far field condition (i.e., distance >2D2/λ0 at 28 GHz where
D = 2 × 52 mm
In Fig. 11, the simulated and measured radiation patterns of the proposed design are shown. From this figure, it is evident that the proposed LCTA can steer the main beam direction to 30° not only in the E-plane but also in the H-plane. The 2D beamforming capability was achieved because its SIW structure isolates the DC connections between adjacent unit cells. The peak gains of the simulated and measured results are 14.5 and 6.1 dBi, respectively.
Table 1 shows the comparative results of previous studies on LCTA [16–18]. A comparison of operating frequency, LC thickness, number of LC layers, phase tuning range, beam scanning capability, and maximum steering angle is conducted. The figure of merit (FoM) is defined as the ratio of the maximum phase tuning range over the LC thickness:
The remarkable feature of the proposed LCTA is the 2D beam scanning capability with the thin single LC layer.
Conclusion
In this paper, an LCTA antenna achieving 2D beam scanning capability for mmWave communication is implemented. It is observed that manipulating the propagation path among the unit cell horizontally could extend the phase tuning range of the thin LC layer-based unit cell design. The narrow scanning range of conventional LCTAs is overcome by adopting the proposed method, steering the main beam to 30°. Despite the persistent practical limitations of LCTA, this paper presents for the first time the feasibility of a 2D beam-steering LCTA that has been both fabricated and measured. Additionally, the proposed LCTA unit cell employs a concept that broadens the phase tuning range while using a minimal amount of LC layer.
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