Introduction

Frequency reconfigurable antennas are widely utilized in modern wireless communication systems due to their ability to dynamically alter frequency characteristics and good electromagnetic compatibility. To satisfy the requirements of miniaturization, broadband, and functional diversification of wireless communication system development, frequency reconfigurable antenna is becoming more and more popular. A large number of frequency reconfigurable antenna have been reported1,2,3,4,5, and their reconstruction methods are various. The traditional methods of using PIN diodes and MEMS switches to achieve antenna frequency reconstruction are limited by the inherent size of the switch and the operating frequency band6,7,8,9. Because the theoretical design of the variable capacitance of the varactor diode is not quite consistent with the actual, and the experimental design is difficult, the application of the varactor diode in the frequency reconfigurable antenna is relatively limited. The use of LC materials to achieve frequency reconfigurability in the millimeter-wave band is more advantageous than the above methods. Despite the many advantages of LC, many studies based on LCs do not provide measurement results10,11. The above studies do not discuss the design of an antenna structure that can be applied to 5G millimeter wave12,13.

To solve some issues discussed above, in this paper a reconfigurable antenna with parasitic elements, electromagnetic band gap structure and LC is presented. The antenna can achieve wide bandwidth and continuous tunable in 5G millimeter wave band. The paper is organized as follows. Some introductions about the research of reconfigurable antenna are introduced in "Introduction" section. "Operating principle and antenna design" section introduces and analyzes the electromagnetic properties and structure of the reconfigurable antenna. The optimized antenna parameters and corresponding measured outcomes are presented in "Experimental results and discussion" section. Finally, "Conclusion" section provides a conclusion.

Operating principle and antenna design

Electromagnetic properties of liquid crystal materials

LC material is a kind of material between liquid and solid, which has both liquid flow characteristics and crystal anisotropy. It can be seen from Fig. 1 that under the control of different external control voltages, the director of the LC molecule determines the dielectric constant tensor in different deflection states14,15.

Figure 1
figure 1

The relationship between the orientation vector of the LC molecule and the applied electric field: (a) the LC molecule’s director; (b) alignment of the LC bulk without a bias voltage (V = 0); (c) with a bias voltage (0 < V < Vmax); and (d) with a saturation voltage (V ≥ Vmax).

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The relationship between the tensor and the director is as follows16:

$$ \begin{aligned} \overleftrightarrow \varepsilon & = \varepsilon_{ \bot } \cdot \overleftrightarrow 1 + \Delta \varepsilon \left( {\overrightarrow {n} \otimes \overrightarrow {n} } \right) \\ & = \varepsilon_{ \bot } \left[ {\begin{array}{*{20}c} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \\ \end{array} } \right] + \Delta \varepsilon \cdot \left[ {\begin{array}{*{20}c} {n_{x}^{2} } & {n_{x} n_{y} } & {n_{x} n_{z} } \\ {n_{x} n_{y} } & {n_{y}^{2} } & {n_{y} n_{z} } \\ {n_{x} n_{z} } & {n_{y} n_{z} } & {n_{z}^{2} } \\ \end{array} } \right] \\ \end{aligned} $$
(1)

Formula (1), represents the tensor product between the pointing vectors, where \(\overrightarrow{n}=\left(\mathit{cos}\theta ,0,\mathit{sin}\theta \right)\), which can be further simplified as:

$$ \overleftrightarrow \varepsilon = \left[ {\begin{array}{*{20}c} {\varepsilon_{ \bot } + \Delta \varepsilon \cos^{2} \theta } & 0 & {\Delta \varepsilon \sin \theta \cos \theta } \\ 0 & {\varepsilon_{ \bot } } & 0 \\ {\Delta \varepsilon \sin \theta \cos \theta } & 0 & {\varepsilon_{ \bot } + \Delta \varepsilon \sin^{2} \theta } \\ \end{array} } \right] $$
(2)

When the applied voltage is 0 V, does not deflect, and the dielectric constant tensor can be expressed as follows:

$$ \overleftrightarrow \varepsilon_{ \bot } = \left[ {\begin{array}{*{20}c} {\varepsilon_{ \bot } } & 0 & 0 \\ 0 & {\varepsilon_{\parallel } } & 0 \\ 0 & 0 & {\varepsilon_{ \bot } } \\ \end{array} } \right]\quad \quad \quad \overleftrightarrow \varepsilon_{\parallel } = \left[ {\begin{array}{*{20}c} {\varepsilon_{ \bot } } & 0 & 0 \\ 0 & {\varepsilon_{ \bot } } & 0 \\ 0 & 0 & {\varepsilon_{\parallel } } \\ \end{array} } \right] $$
(3)

When the applied voltage reaches saturation, the deflection angle becomes 90°. At this point, the LC molecule aligns parallel to the electric field direction, as depicted in Fig. 1c, and its dielectric constant tensor can be simplified to the following formula.

Figure 2 visually demonstrates the change of relative dielectric constant (\({\varepsilon }_{r}\)) and loss tangent angle (tanδ) of LC materials with voltage. It can be observed that when the voltage changes within the range of threshold voltage and saturation voltage, the \({\varepsilon }_{r}\) and tanδ will also change within the corresponding extreme value.

Figure 2
figure 2

A typical continuous tuning dielectric permittivity and loss tangent tan versus the bias voltage.

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It can be seen from the figure that the LC molecule will produce elastic deformation in the process of deflection, and the change of energy will deflect the director, thus realizing the frequency switching function. The tunability of general tunable materials is usually expressed by \({\varepsilon }_{r}\) of LC materials. The following formula is the expression of the tuning ability of nematic LC17:

$$ \tau_{LC} = \frac{{\varepsilon_{r,\parallel } - \varepsilon_{r, \bot } }}{{\varepsilon_{r,\parallel } }} $$
(4)

In the optical field, the anisotropy can be represented by the following formula when LC materials are used in the microwave field because the refractive index can replace the dielectric constant:

$$ \Delta n = n_{\parallel } - n_{ \bot } = \sqrt {\varepsilon_{r,\parallel } } - \sqrt {\varepsilon_{r, \bot } } $$
(5)

According to the Oseen Frank Energy and electromagnetic characteristics related to LC molecules, the famous Freedericksz transition voltage (Vth) is a voltage with no molecular reorientation can be reduced derived18. The threshold voltage can be defined as:

$$ V_{th} = \pi \sqrt {\frac{{2k_{11} }}{{\varepsilon_{0} \Delta \varepsilon_{r} }}} $$
(6)

Antenna design

Figure 3 shows the structure diagram and side view of each layer of the frequency reconfigurable antenna. The overall dimensions of the antenna is 20 × 25 × 7.635 mm3, and the layered structure is adopted. The antenna is mainly composed of three layers, and the specific parameters of each layer are shown in Table 1.

Figure 3
figure 3

Exploded view and dimension parameters of the proposed antenna. (pw:5.5, w:5.5, l: 5.73, wj:0.46, l4:8, w4:8. Unit: m).

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Table 1 Details of the layers.
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The top layer is the Taconic TLY5 (TM) dielectric substrate with a thickness of 0.381 mm, its dielectric constant is 2.2, and the loss tangent angle is 0.0009. The upper surface of the first layer of the dielectric substrate is etched with a parasitic patch element of 5.5 × 5.2 mm2, and its lower surface is a radiation patch and part of the feeding structure. The four cylindrical through holes with a diameter of 1 mm at the center of the dielectric substrate facilitate the injection of LC using a syringe and the discharge of air during the physical test. The middle layer is a high-frequency dielectric plate with the same \({\varepsilon }_{r}\) and tanδ as the former, and its thickness is 0.254 mm. The 7 × 7 × 0.254 mm3 cavity in the center of the dielectric substrate of this layer is used for storing LCs, and two electromagnetic band gap structures of the same size and the same distance are placed on both sides of a part of the microstrip structure. The bottom layer is made of a metal aluminum block with a flange through the hole on the side for fixing 2.4 mm RF connector. The cylindrical through-hole with a diameter of 1 mm on both sides of the whole structure is used to fix the multi-layer dielectric substrate. When an external control voltage is applied, an electric field can be formed between the inverted microstrip structure and the aluminum block to control the molecular deflection of the LC.

To further broaden the impedance bandwidth of the antenna, this design uses the principle that parasitic patches can generate new resonance points near the original frequency points and load parasitic patches on the upper part of the upper dielectric substrate. The impact of parasitic patch units on antenna performance is demonstrated in Fig. 4a. Additionally, the return loss has been effectively widened. To further understand the working principle of the parasitic patch, the parameters of the parasitic patch are analyzed. Figure 4b illustrates that a new resonant point can only be generated near the resonant frequency point when the size of the parasitic unit closely matches that of the radiation patch, thereby facilitating spread spectrum effects.

Figure 4
figure 4

Performance results of parasitic elements. (a) performance comparison. (b) effect of parasitic element length pw. (c) equivalent circuit diagram.

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The original microstrip patch antenna and the parasitic patch placed above the main patch form two RLC parallel resonant circuits with the ground, respectively. These two resonant circuits are capacitively coupled through the radiation field of the main patch. Figure 4c demonstrates the equivalent circuit of the additional antenna parasitic patch. By modifying the shape, size, and quantity of parasitic elements based on the principle of parasitic elements and equivalent circuits, it is possible to enhance the antenna bandwidth.

Experimental results and discussion

Figure 5 introduces the antenna test scheme and the corresponding physical test process. Deflection of the LC molecules is achieved by applying an external control voltage to both sides of the LC layers through the T-shaped biasing device. The voltage controls achieved the frequency reconfigurable capability.

Figure 5
figure 5

Measurement setup for characteristic evaluation.

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Figure 6 shows the return loss of simulation and measurement. It can be seen in Fig. 6a with the increase of external control voltage, the resonant frequency point of the antenna decreases from 32.5 to 32 GHz, realizing a shift of 500 MHz. The impedance bandwidth of the antenna remains unchanged throughout the tuning process. Compared with the simulation results in Fig. 6b, the changing trend of return loss is the same, but the offset is different. The measured resonant frequency changes slightly, which is mainly caused by the manufacturing process, measurement error, liquid crystal distribution uniformity and air tightness. In addition, the designed LC antenna did not include a directional layer, which also led to a decrease in antenna tuning capability. The experiment shows that the frequency reconstruction function of the antenna can be realized by adjusting the liquid crystal material with a variable dielectric constant by an electric field.

Figure 6
figure 6

Simulated and measured S-parameters of the fabricated antenna.

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Antenna resonant frequency versus bias voltage and liquid crystal thickness are shown in Fig. 7. The resonant frequency exhibits variations for varying voltage strengths from 0 to 20 V in Fig. 7a. Consistent with expectations, the resonant frequencies undergo tuning corresponding to the applied voltage. With an increment in the bias voltage strength, the \(\varepsilon_{\parallel }\) increases, whereas the \(\varepsilon_{ \bot }\) decreases. Consequently, the resonant frequency shifts towards lower frequencies with an increase in voltage. The threshold voltage value and saturation voltage value are 2 V and 16 V, respectively. Figure 7b illustrates the impact of frequency offset by manipulating the liquid crystal thickness (H) while maintaining \(\varepsilon_{ \bot }\). It is observed that an increase in H results in a decrease in the resonant frequency.

Figure 7
figure 7

Antenna resonant frequency versus bias voltage and liquid crystal thickness.

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Figure 8 shows the radiation pattern of LC-BYIPS-P01 liquid crystal material on the yz-plane and xy-plane. It can be seen from Fig. 8a that the half power beam width of the antenna is 40° and 42° respectively. And the maximum gain values are 6.25, 5.54 dB. The xy-plane has omnidirectional radiation characteristics. In conclusion, the reconfigurable antenna based on liquid crystal material can not only enable the frequency switching but also keep the radiation characteristics stable during the reconstruction process. Figure 9 shows the simulated peak gain and radiation efficiency. At a dielectric constant of 2.7, the radiation efficiency and gain in the operating band exceed 86.82% and 5.72 dB, respectively. At a dielectric constant of 3, the radiation efficiency and gain in the operating band exceed 92.23% and 4.86 dB, respectively.

Figure 8
figure 8

Radiation patterns of yz-plane and xy-plane as a function of anisotropic permittivity of LC.

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Figure 9
figure 9

Simulated peak gain and radiation efficiency.

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Two prior studies used the GT7 LC (Merck KGaA) 1, and K15 LC (Merck) 2. The electromagnetic properties of these two LCs are, εr,// = 3.2, εr,⊥ = 2.55 and εr,// = 3.1, εr,⊥ = 2.7. The measurement results obtained from the proposed antenna are compared with the simulations from previous studies 1,2, as no existing literature presents the tunable range and reconfigurable bandwidth of LC-based microstrip antennas. Table 2 displays the performance characteristics of the proposed antenna based on LC and the previous antennas. The bandwidths of the antennas are respectively increased by a factor of 2 and 1.5 compared to the LC-based antennas 1,2, while the effective sizes of the antennas are 61.4% and 24.6% of those of the previously proposed antennas. It is worth noting that the previously proposed LC-based antenna 13 does not fully utilize the performance potential of LC, resulting in an inability to operate at 5G millimeter wave.

Table 2 Compare the design with the antenna of recent years.
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Conclusion

A frequency reconfigurable antenna has been designed utilizing LC material. A novel nematic LC, LC-BYIPS-P01, has been introduced into the antenna design. The dielectric constant of the LC is modified by applying a DC voltage, leading to enhanced frequency reconfigurability. Simultaneously, the antenna can be miniaturized using multi-layer technology and slotting technology. Simulation and measurement results demonstrate that the antenna achieves frequency reconfigurability, with a reconfigurable bandwidth of 500 MHz. The proposed antenna is compared with previously developed antennas. The LC utilized in this study exhibits a broader range of adjustable dielectric constants, lower loss tangent, broader bandwidth of the designed antenna, smaller effective size, and improved operation in the 5G millimeter wave band. This design is well-suited for communication systems operating in the 5G millimeter-wave.