# Radiation Pattern Reconfigurable Waveguide Slot Array Antenna Using Liquid Crystal

Authors: Hongyu Shi; Jianxing Li; Shitao Zhu; Anxue Zhang; Zhuo Xu
Journal: International Journal of Antennas and Propagation
 (2018)

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## Abstract

In this paper, we proposed a radiation pattern reconfigurable waveguide slot array antenna using liquid crystal (LC). Together with the waveguide slot, the designed complementary electric-field-coupled resonator functions like a switch controlled by the dielectric constant of the LC, which can control the antenna element to radiate or not. Thus, the array factor and radiation pattern can be manipulated. The proposed antenna was simulated, fabricated, and measured. Its radiation direction can be reconfigured to 46° or 0° at about 15 GHz.


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## Body

## 1. Introduction

Liquid crystals (LCs) are mixtures in which some degree of anisotropy is present [1]. The nematic phase is one of the most common LC phases. Nematic LCs are uniaxial mediums with an optical axis along the direction of LC molecules or nematic axis. The dielectric constants, measured along (ε∥) or normal (ε⊥) to the nematic axis, are different. With a time-dependent external electric field, it is possible to induce a motion in nematic LCs. This motion usually concerns the rotation of the optical axis, which can tune the LC dielectric constant between ε∥ and ε⊥ for a certain linearly polarized electromagnetic wave. Such phenomenon is widely used for liquid crystal displays (LCDs), spatial light modulators (SLMs), and reconfigurable electromagnetic wave antennas [2–5] and devices [6–10].Compared with ferroelectric crystals (e.g., (Ba,Sr)TiO3), the nematic LC has a lower bias voltage, which makes it more suitable for portable reconfigurable electromagnetic wave device design [11–14]. In recent years, nematic LCs have been widely applied for tunable filters [15–18], for tunable phase shifters [19–21], for active metamaterials [22–25], and for frequency [26–28], polarization [21, 29], or radiation pattern reconfigurable [30–35] antennas. Phase shifts of LC phase shifters are usually about 150° and 300° in the microwave [7] and millimetre-wave [36, 37] bands, respectively. Some tunable phase shifters were used in antenna feeding structures to realize polarization or radiation pattern reconfigurable antennas [20, 21]. With a LC substrate, operating frequencies of antennas or frequency selective surfaces can be tuned [4, 27]. LCs were also used to control the reflection phase of grounded structured patches and realize radiation pattern reconfigurable reflect-array antennas from the microwave to submillimetre-wave band [33, 34]. LC-based composite right-/left-handed transmission lines were also designed for radiation pattern reconfigurable leaky wave antennas [24, 31]. These reconfigurable designs are all based on phase shifts resulting from the tunability of LCs’ dielectric constants. However, such a method requires continuous bias voltage control and has nonuniform amplitudes for different operating states, which results in a deviation from the practical radiation pattern.In this paper, a novel method for radiation pattern reconfigurable antenna design using LC was proposed. A nematic LC-loaded metasurface structure was designed. With a bias voltage of 0 V or 30 V for LC, the designed metasurface structure works in the “ON” or “OFF” state like a switch at a certain frequency. This switchable property only requires two discrete bias voltages leading to a simpler bias voltage control. By applying the proposed switchable metasurface for a waveguide slot array antenna, the antenna array factor can be controlled, and then the radiation pattern can be steered.


## 2. Antenna Design

### 2.1. Antenna Array Element Design

An antenna element was designed to obtain a switchable radiation property. Due to the limited tunability of LC, the designed antenna element should have a narrow bandwidth to avoid bandwidth overlap between each state, which would reduce the radiation pattern reconfiguration property of the waveguide slot array antenna. Thus, an electric-field-coupled resonator-based structure was used, which usually has a narrow bandwidth [38, 39]. The theoretical design rules for such a resonator were proposed in [40, 41].The proposed slot array element is a multilayered structure including a waveguide slot and a complementary electric-field-coupled (CELC) resonator, as shown in Figure1. The top layer is metasurface structures with a bias via hole. CELC resonators are closed to the LC layer. The geometric parameters were chosen as l=5.7mm, s=5.5mm, w=0.2mm, and g=0.2mm. The LC material is LC BNLC1100 provided by the BaYi Space Co. Ltd. The material parameters of LC BNLC1100 are ε∥=3.37, ε⊥=2.6, tanδ∥=0.014, and tanδ⊥=0.022. The unbiased dielectric constant of unaligned LC BNLC1100 is about 3 to around 15 GHz as measured by Agilent 85070E Dielectric Probe Kit. The thickness of the LC layer is 0.79 mm. The bottom layer is a slot with a=0.2mm and b=6.2mm. The slot has an offset of c=3.5mm from the centre line of the waveguide. The metasurface structure is right above the waveguide slot. The substrate of the top and bottom layers is 0.254 mm thick Taconic RF-60TC with a dielectric constant of 6.15 and a loss tangent of 0.002. The metallic via through the top and LC layers can improve the isolation between each element. The diameter of the metallic via is 0.6 mm. The distance between two adjacent metallic vias is 1 mm. Two columns of the metallic via are located on the middle and edge of the waveguide.Figure 1
Schematic diagram of the proposed antenna array element.The waveguide slot couples the guided wave from the rectangular waveguide into the LC-loaded metasurface. Then, the CELC resonator above the waveguide slot can be induced. At its resonance frequency, the CELC resonator is excited and radiates like slots. This situation is considered as state “ON” and the electric field distribution in the CELC resonator is shown in Figure2(a). The slots along the y-direction of the resonator generate x-polarized radiation. However, the electric field in the slots along the x-direction of the resonator are out of phase and cancel each other. Thus, they contribute little to the radiation. At frequencies away from the resonance frequency, the CELC resonator is only slightly induced and almost does not radiate as shown in Figure 2(b). This situation is considered as state “OFF.” The resonance frequency can be controlled by changing the dielectric constant of LC. Thus, at a certain frequency, the state of the resonator varies by the dielectric constant of LC.Figure 2
Electric field distributions for the CELC resonator: (a) state “ON,” (b) state “OFF.”
(a)
(b)The antenna element model was built up and simulated by a commercial software CST Microwave Studio. Because the LC bias voltage is loaded along thez-direction, the LC molecules are also along the z-direction. Thus, the LC dielectric constants (εLC) obtain a lower/higher value for the x-polarized wave radiated from the slot with/without a bias voltage.Figure3 shows the S-parameters with different εLC. The S11 parameters are all lower than −15 dB indicating a low reflection of the slot and metasurface structure. The dips of the S21 parameters are caused by the radiation of the structure. The radiation frequency goes higher with a smaller εLC. When εLC=3, the S21 parameter is −1.64 dB at 15 GHz, which would be much lower using multiple elements as an array. When εLC is greater or lower than 3, the S21 parameter is higher at 15 GHz. For example, when εLC is 3.4 or 2.6, the S21 parameter is higher than −0.2 dB at 15 GHz, which means the radiated power is much lower. Thus, around 15 GHz, when the difference between εLC with and without bias is higher than 0.4, the antenna element can function like a switch controlled by a bias voltage.Figure 3
Simulated S-parameters of the antenna element with differentεLC.The radiation pattern of the antenna element withεLC=3 in the yoz plane is shown in Figure 4. The antenna element is x-polarized, and the plane of polarization is the xoz plane. The copolarized gain is 4.46 dB, and the cross-polarized gain is below −38 dB. The width of the main lobe is about ±30°.Figure 4
Simulated radiation pattern of the antenna element.A parameter studies for key geometric parameters of the antenna element are shown in Figures5 and 6. As shown in Figure 5, with a bigger b, the resonance frequency has a red shift and S21 becomes lower, which means more power is leaked. As shown Figure 6(a), with a bigger l, the resonance frequency become lower and less power is leaked. s has a smaller impact on the resonance frequency. A bigger s leads to a slightly lower resonance frequency. Resonators with different geometric parameters have different resonance frequencies. However, at their resonance frequencies, the radiation patterns are similar due to the same resonant mode.Figure 5
SimulatedS21 parameters with different b.Figure 6
SimulatedS21 parameters with different (a) l and (b) s.
(a)
(b)


### 2.2. Waveguide Slot Array Antenna Design

The used rectangular waveguide is WR62 operating from 11.9 GHz to 18 GHz. The proposed antenna contains 18 longitudinal slots as shown in Figure7. The distance between each slot is e=λg/4=6.6mm, where λg is the guide wavelength at 15 GHz. Thus, the feeding phase difference for adjacent elements is ξ=90°.Figure 7
Element arrangement of the proposed waveguide slot array antenna.As mentioned above, the metasurface structure with LC functions like a switch with different bias voltages, which controls the antenna element from radiating or not. Thus, the arrangement of the slot array or the array factor can be controlled by the bias voltage, and then the radiation pattern can be steered in theyoz plane.The normalized array factor is(1)Aψ=1NsinNψ/2sinψ/2,where
(2)ψ=βdcosϕ+ξ.β is the wavenumber in the vacuum, d is the distance between each radiating element, and ϕ is the observation angle. When all the antenna elements are radiating, the waveguide slot array antenna is a uniform linear array (Configuration 1) with d=e=6.6mm and ξ=90°. When only nos. 1, 3, 5, 7, 9, 10, 12, 14, 16, and 18 of these elements are radiating, considering each adjacent two slots as one element (red circled), the waveguide slot array antenna is a five-element array (Configuration 2) with a distance between each element of d=4e=26.4mm and ξ=0°. The calculated radiation pattern for these two slot arrays in the yoz plane are shown in Figures 8(a) and 8(b). The plane of polarization is the xoz plane. The main lobe directions for Configuration 1 and Configuration 2 are 48° and 0°, respectively. The grating lobe is caused by the relative big distance between each element.Figure 8
Calculated radiation pattern for (a) Configuration 1 and (b) Configuration 2.
(a)
(b)


## 2.1. Antenna Array Element Design

An antenna element was designed to obtain a switchable radiation property. Due to the limited tunability of LC, the designed antenna element should have a narrow bandwidth to avoid bandwidth overlap between each state, which would reduce the radiation pattern reconfiguration property of the waveguide slot array antenna. Thus, an electric-field-coupled resonator-based structure was used, which usually has a narrow bandwidth [38, 39]. The theoretical design rules for such a resonator were proposed in [40, 41].The proposed slot array element is a multilayered structure including a waveguide slot and a complementary electric-field-coupled (CELC) resonator, as shown in Figure1. The top layer is metasurface structures with a bias via hole. CELC resonators are closed to the LC layer. The geometric parameters were chosen as l=5.7mm, s=5.5mm, w=0.2mm, and g=0.2mm. The LC material is LC BNLC1100 provided by the BaYi Space Co. Ltd. The material parameters of LC BNLC1100 are ε∥=3.37, ε⊥=2.6, tanδ∥=0.014, and tanδ⊥=0.022. The unbiased dielectric constant of unaligned LC BNLC1100 is about 3 to around 15 GHz as measured by Agilent 85070E Dielectric Probe Kit. The thickness of the LC layer is 0.79 mm. The bottom layer is a slot with a=0.2mm and b=6.2mm. The slot has an offset of c=3.5mm from the centre line of the waveguide. The metasurface structure is right above the waveguide slot. The substrate of the top and bottom layers is 0.254 mm thick Taconic RF-60TC with a dielectric constant of 6.15 and a loss tangent of 0.002. The metallic via through the top and LC layers can improve the isolation between each element. The diameter of the metallic via is 0.6 mm. The distance between two adjacent metallic vias is 1 mm. Two columns of the metallic via are located on the middle and edge of the waveguide.Figure 1
Schematic diagram of the proposed antenna array element.The waveguide slot couples the guided wave from the rectangular waveguide into the LC-loaded metasurface. Then, the CELC resonator above the waveguide slot can be induced. At its resonance frequency, the CELC resonator is excited and radiates like slots. This situation is considered as state “ON” and the electric field distribution in the CELC resonator is shown in Figure2(a). The slots along the y-direction of the resonator generate x-polarized radiation. However, the electric field in the slots along the x-direction of the resonator are out of phase and cancel each other. Thus, they contribute little to the radiation. At frequencies away from the resonance frequency, the CELC resonator is only slightly induced and almost does not radiate as shown in Figure 2(b). This situation is considered as state “OFF.” The resonance frequency can be controlled by changing the dielectric constant of LC. Thus, at a certain frequency, the state of the resonator varies by the dielectric constant of LC.Figure 2
Electric field distributions for the CELC resonator: (a) state “ON,” (b) state “OFF.”
(a)
(b)The antenna element model was built up and simulated by a commercial software CST Microwave Studio. Because the LC bias voltage is loaded along thez-direction, the LC molecules are also along the z-direction. Thus, the LC dielectric constants (εLC) obtain a lower/higher value for the x-polarized wave radiated from the slot with/without a bias voltage.Figure3 shows the S-parameters with different εLC. The S11 parameters are all lower than −15 dB indicating a low reflection of the slot and metasurface structure. The dips of the S21 parameters are caused by the radiation of the structure. The radiation frequency goes higher with a smaller εLC. When εLC=3, the S21 parameter is −1.64 dB at 15 GHz, which would be much lower using multiple elements as an array. When εLC is greater or lower than 3, the S21 parameter is higher at 15 GHz. For example, when εLC is 3.4 or 2.6, the S21 parameter is higher than −0.2 dB at 15 GHz, which means the radiated power is much lower. Thus, around 15 GHz, when the difference between εLC with and without bias is higher than 0.4, the antenna element can function like a switch controlled by a bias voltage.Figure 3
Simulated S-parameters of the antenna element with differentεLC.The radiation pattern of the antenna element withεLC=3 in the yoz plane is shown in Figure 4. The antenna element is x-polarized, and the plane of polarization is the xoz plane. The copolarized gain is 4.46 dB, and the cross-polarized gain is below −38 dB. The width of the main lobe is about ±30°.Figure 4
Simulated radiation pattern of the antenna element.A parameter studies for key geometric parameters of the antenna element are shown in Figures5 and 6. As shown in Figure 5, with a bigger b, the resonance frequency has a red shift and S21 becomes lower, which means more power is leaked. As shown Figure 6(a), with a bigger l, the resonance frequency become lower and less power is leaked. s has a smaller impact on the resonance frequency. A bigger s leads to a slightly lower resonance frequency. Resonators with different geometric parameters have different resonance frequencies. However, at their resonance frequencies, the radiation patterns are similar due to the same resonant mode.Figure 5
SimulatedS21 parameters with different b.Figure 6
SimulatedS21 parameters with different (a) l and (b) s.
(a)
(b)


## 2.2. Waveguide Slot Array Antenna Design

The used rectangular waveguide is WR62 operating from 11.9 GHz to 18 GHz. The proposed antenna contains 18 longitudinal slots as shown in Figure7. The distance between each slot is e=λg/4=6.6mm, where λg is the guide wavelength at 15 GHz. Thus, the feeding phase difference for adjacent elements is ξ=90°.Figure 7
Element arrangement of the proposed waveguide slot array antenna.As mentioned above, the metasurface structure with LC functions like a switch with different bias voltages, which controls the antenna element from radiating or not. Thus, the arrangement of the slot array or the array factor can be controlled by the bias voltage, and then the radiation pattern can be steered in theyoz plane.The normalized array factor is(1)Aψ=1NsinNψ/2sinψ/2,where
(2)ψ=βdcosϕ+ξ.β is the wavenumber in the vacuum, d is the distance between each radiating element, and ϕ is the observation angle. When all the antenna elements are radiating, the waveguide slot array antenna is a uniform linear array (Configuration 1) with d=e=6.6mm and ξ=90°. When only nos. 1, 3, 5, 7, 9, 10, 12, 14, 16, and 18 of these elements are radiating, considering each adjacent two slots as one element (red circled), the waveguide slot array antenna is a five-element array (Configuration 2) with a distance between each element of d=4e=26.4mm and ξ=0°. The calculated radiation pattern for these two slot arrays in the yoz plane are shown in Figures 8(a) and 8(b). The plane of polarization is the xoz plane. The main lobe directions for Configuration 1 and Configuration 2 are 48° and 0°, respectively. The grating lobe is caused by the relative big distance between each element.Figure 8
Calculated radiation pattern for (a) Configuration 1 and (b) Configuration 2.
(a)
(b)


## 3. Simulation and Measurement Results

The designed waveguide slot array antenna was simulated by CST Microwave Studio. The simulated S-parameters of the Configuration 1 and Configuration 2 (as described in the previous section) are shown in Figures9(a) and 9(b). The S11 parameters are lower than −15 dB. The S21 parameter of the Configuration 1 has only one dip of −36.8 dB at 14.96 GHz with εLC=3 for all the elements. However, for the Configuration 2, the S21 parameter has two dips of −12.88 dB and −12.23 dB at 14.97 GHz and 15.26 GHz, respectively. The lower and upper dips are caused by the antenna elements operating in two states with εLC=3 and εLC=2.6, respectively. These transmission dips indicate the radiation of the waveguide slot array antenna. In this paper, we focus on the radiation characteristics in the yoz plane at about 15 GHz.Figure 9
Simulated S-parameters of the antenna arrays: (a) Configuration 1; (b) Configuration 2.
(a)
(b)The simulated electric field distributions of the waveguide slot array antenna within one wavelength are shown in Figure10. The field distributions of other parts of the array antenna are similar. Ex-field distribution for Configuration 1 is shown in Figure 10(a). With εLC=3, all the elements are excited. However, as for the Configuration 2, only the elements with εLC=3 (nos. 1, 10, etc.) are excited and the elements with εLC=2.6 (nos. 2, 11, etc.) are not excited as shown in Figure 10(b). Thus, by controlling the εLC, the array factor of the waveguide slot array antenna can be controlled and the radiation pattern can be reconfigured.Figure 10
Simulatedx component of electric field distributions: (a) Configuration 1; (b) Configuration 2.
(a)
(b)Figures11(a) and 11(b) show the radiation patterns for Configuration 1 and Configuration 2, respectively, at 14.96 GHz. The array antenna is x-polarized, and the plane of polarization is the xoz plane. The main lobe directions of Configuration 1 and Configuration 2 are about 46° and 0°, respectively, with copolarized gains of 11.7 dB and 10.68 dB. The cross-polarized gains of Configuration 1 and Configuration 2 are −21.61 dB and −8.28 dB at 14.96 GHz. Thus, by controlling the LC bias voltage, radiation pattern of the proposed waveguide slot array antenna is reconfigurable. The grating lobe levels of Configuration 1 and Configuration 2 are −8.6 dB and − 4.21 dB, respectively. As described in the previous section, the grating lobe of Configuration 2 is caused by the relative big distance between the radiating elements.Figure 11
Simulated radiation patterns of the antenna arrays: (a) Configuration 1; (b) Configuration 2.
(a)
(b)The proposed reconfigurable waveguide slot array antenna was fabricated and measured for further demonstration. The photos of the fabricated sample are shown in Figure12(a). Figures 12(b) and 12(c) show the detail structure of the slots and CELC resonators. The holes on the upper and lower sides are used for assembly. The measured S-parameters confirm with the simulation results. The LC is unaligned, and the initial εLC is 3 as aforementioned. Two antenna states were measured. The first state is unbiased, and all the antenna elements are radiating. The second state has a bias voltage of 30 V on the red circled elements in Figure 7, and these elements are not radiating. After being biased with a 30 V DC voltage, the property of the sample trended to be stable after 7 minutes, which can be reduced by using a thinner LC layer. The fabricated antenna sample was measured by a vector network analyser of Agilent E8363b, and the bias network was fed by a DC source of Motech LPS-305.Figure 12
Photos of the fabricated sample. (a) waveguide slots array antenna, (b) slot layer, (c) metasurface layer.The measured S-parameter and radiation pattern are shown in Figures13 and 14, respectively. As shown in Figure 13(a), Configuration 1 is unbiased; at 15.01 GHz, the S11 parameter is lower than −14.5 dB and the S21 parameter is −17.67 dB. As for Configuration 2, the S11 parameter is below −15 dB, and the S21 parameter has two dips of −13.09 dB and −10.93 dB at 15.04 GHz and 15.35 GHz, respectively. At 15.04 GHz, the S21 parameter of Configuration 1 is −16.8 dB. As shown in Figure 14, the main lobe directions of Configuration 1 and Configuration 2 are 44° and 0°, respectively, with gains of 9.81 dB and 9.02 dB. The cross-polarized gains of Configuration 1 and Configuration 2 are below −20 dB. The grating lobe level of Configuration 1 and Configuration 2 is −9.17 dB and−3.62 dB, respectively. The plane of polarization is the xoz plane. Thus, the measurement radiation patterns confirm with the simulation results, and the differences are caused by the mechanical errors and measurement errors. By controlling the bias voltage on LC, the array factor can be controlled and a radiation pattern reconfigurable waveguide slot array antenna was realized.Figure 13
The fabricated sample and measured S-parameters of the antenna arrays: (a) Configuration 1; (b) Configuration 2.
(a)
(b)Figure 14
Measured radiation patterns of the antenna arrays: (a) Configuration 1; (b) Configuration 2.
(a)
(b)


## 4. Conclusions

In conclusion, using LC material, a switchable antenna element with a tuneable radiation power was designed. An 18-element waveguide slot array antenna was then proposed. By controlling the LC bias voltage, the array factor and the radiation pattern can be controlled. Two situations were demonstrated by both simulation and measurement. For these two situations, the measured radiation directions are 44° and 0° and the measured gains are 9.81 dB and 9.02 dB at 15.04 GHz. The grating lobe can be suppressed by reducing the distance between each element using transverse slot array antenna, which also has potentials for more beam-steering configurations. The side lobs can be suppressed by using a Chebyshev array antenna.



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*Source: 2164065-2018-09-09.xml*