Radiation Pattern Reconfigurable Waveguide Slot Array Antenna Using Liquid Crystal

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 15GHz.


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][3][4][5] and devices [6][7][8][9][10].
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.

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 Figure 1.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 7 mm, s = 5 5 mm, w = 0 2 mm, and g = 0 2 mm.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, tan δ ∥ = 0 014, and tan δ ⊥ = 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 2 mm and b = 6 2 mm.The slot has an offset of c = 3 5 mm 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.
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 Figure 2(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.
The antenna element model was built up and simulated by a commercial software CST Microwave Studio.Because the LC bias voltage is loaded along the z-direction, the LC molecules are also along the z-direction.Thus, the LC dielectric constants (ε LC ) obtain a lower/higher value for the x-polarized wave radiated from the slot with/without a bias voltage.
Figure 3 shows the S-parameters with different ε LC .The S 11 parameters are all lower than −15 dB indicating a low reflection of the slot and metasurface structure.The dips of the S 21 parameters are caused by the radiation of the structure.The radiation frequency goes higher with a smaller ε LC .When ε LC = 3, the S 21 parameter is −1.64 dB at 15 GHz, which would be much lower using multiple elements     3 International Journal of Antennas and Propagation as an array.When ε LC is greater or lower than 3, the S 21 parameter is higher at 15 GHz.For example, when ε LC is 3.4 or 2.6, the S 21 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 ε LC with and without bias is higher than 0.4, the antenna element can function like a switch controlled by a bias voltage.
The radiation pattern of the antenna element with ε LC = 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 crosspolarized gain is below −38 dB.The width of the main lobe is about ±30 °.
A parameter studies for key geometric parameters of the antenna element are shown in Figures 5 and 6.As shown in Figure 5, with a bigger b, the resonance frequency has a red shift and S 21 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.

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 Figure 7.The distance between each slot is e = λ g /4 = 6 6 mm, where λ g is the guide wavelength at 15 GHz.Thus, the feeding phase difference for adjacent elements is ξ = 90 °.
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 the yoz plane.
The normalized array factor is where β is the wavenumber in the vacuum, d is the distance between each radiating element, and ϕ is the observation

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 Figures 9(a The simulated electric field distributions of the waveguide slot array antenna within one wavelength are shown in Figure 10.The field distributions of other parts of the array antenna are similar.E x -field distribution for Configuration 1 is shown in Figure 10(a).With ε LC = 3, all the elements are excited.However, as for the Configuration 2, only the elements with ε LC = 3 (nos.1, 10, etc.) are excited and the elements with ε LC = 2 6 (nos.2, 11, etc.) are not excited as shown in Figure 10(b).Thus, by controlling the ε LC , the array factor of the waveguide slot array antenna can be controlled and the radiation pattern can be reconfigured.
Figures 11(a    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 ε LC 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.
The measured S-parameter and radiation pattern are shown in Figures 13 and 14, respectively.As shown in Figure 13  7 International Journal of Antennas and Propagation −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.

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.

Figure 1 :
Figure 1: Schematic diagram of the proposed antenna array element.

Figure 3 :
Figure 3: Simulated S-parameters of the antenna element with different ε LC .
) 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 −21.61dB 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.The proposed reconfigurable waveguide slot array antenna was fabricated and measured for further demonstration.The photos of the fabricated sample are shown in Figure 12(a).Figures 12(b) and 12(c) show the detail

Figure 13 :
Figure 13: The fabricated sample and measured S-parameters of the antenna arrays: (a) Configuration 1; (b) Configuration 2.