Patent Publication Number: US-2021167497-A1

Title: Liquid crystal antenna unit and liquid crystal phased array antenna

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority of Chinese patent application No. 201811140473.6, filed on Sep. 28, 2018, the content of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to the field of antenna technologies, and in particular, to a liquid crystal antenna unit and a liquid crystal phased array antenna including the liquid crystal antenna unit. 
     BACKGROUND 
     With the development of communication technologies, services such as “Satcom on the move” and the like are being widely used, which requires an antenna with the desired performance. Compared with an antenna based on mechanical scanning, a phased array antenna has advantages of high tracking precision, short response time and the like due to not including a mechanical control mechanism. A phased array antenna based on a material of liquid crystal may further have additional advantages of, for example, small size, low power consumption, and ease of integration with a control circuitry, and is therefore considered to be a promising solution. 
     SUMMARY 
     According to an aspect of the present disclosure, there is provided a liquid crystal antenna unit, which includes: 
     a first substrate; 
     a second substrate opposite to the first substrate; 
     a liquid crystal layer between the first substrate and the second substrate; 
     a transmission line on a first surface and arranged to extend in a first direction along the first surface, wherein the first surface is one of a surface of the first substrate proximal to the second substrate and a surface of the second substrate proximal to the first substrate; 
     a first antenna oscillator on the first surface and arranged as an elongated dipole extending in a second direction along the first surface, wherein the dipole includes two poles spaced apart from each other by a gap, and the first antenna oscillator is configured to couple an electromagnetic wave between the two poles and the transmission line at the gap; 
     a second antenna oscillator on a surface of the second substrate distal to the first substrate and at a position corresponding to the first antenna oscillator, wherein the second antenna oscillator has an elongated shape extending in the second direction along the surface of the second substrate distal to the first substrate, and a length of the second antenna oscillator is less than a length of the first antenna oscillator; and 
     a ground electrode on a surface of the first substrate distal to the second substrate, wherein the ground electrode includes an elongated electrode portion at a position corresponding to the first antenna oscillator, the elongated electrode portion extends in the second direction along the surface of the first substrate distal to the second substrate, and a length of the elongated electrode portion is greater than the length of the first antenna oscillator. 
     In some embodiments, the length of the second antenna oscillator is 0.7 to 0.9 times the length of the first antenna oscillator, and the length of the first antenna oscillator is less than the length of the elongated electrode portion. 
     In some embodiments, the first substrate and the second substrate include a same material, and the second substrate has a thickness that is 3 to 5 times as large as a thickness of the first substrate. 
     In some embodiments, the first substrate and the second substrate include different materials, respectively, and a thickness of the first substrate and a thickness of the second substrate satisfy the following relationship: 
     
       
         
           
             
               
                 
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     where H 1  is the thickness of the first substrate, H 2  is the thickness of the second substrate, ε 1  is a dielectric constant of the first substrate, and ε 2  is a dielectric constant of the second substrate. 
     In some embodiments, the second substrate is a composite board including N layers of materials, where N is an integer greater than or equal to 2. 
     In some embodiments, the second substrate is a composite board including N layers of materials, and the dielectric constant ε 2  of the second substrate is calculated according to the following formula: 
     
       
         
           
             
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     Ti is a thickness of an i-th layer of material, ε 2i  is a dielectric constant of the i-th layer of material, and N is an integer greater than or equal to 2. 
     In some embodiments, the second antenna oscillator, the first antenna oscillator, and the elongated electrode portion are arranged such that a center of the second antenna oscillator, a center of the gap in the first antenna oscillator, and a center of the elongated electrode portion are all on a straight line perpendicular to the first substrate or the second substrate. 
     In some embodiments, the first direction is perpendicular to the second direction. 
     In some embodiments, the transmission line includes a coplanar waveguide transmission line including a signal line and two ground wires respectively on both sides of the signal line. 
     In some embodiments, one end of the signal line is electrically connected to one end, which is proximal to the gap, of one pole of the two poles of the dipole, and one end of one ground wire, which is closer to the other pole of the two poles of the dipole, of the two ground wires, is electrically connected to one end of the other pole proximal to the gap. 
     In some embodiments, the first surface is the surface of the first substrate proximal to the second substrate, the liquid crystal antenna unit further includes an electrode structure on the second surface, and the second surface is the surface of the second substrate proximal to the first substrate. 
     In some embodiments, the first surface is the surface of the second substrate proximal to the first substrate, the liquid crystal antenna unit further includes an electrode structure on the second surface, and the second surface is the surface of the first substrate proximal to the second substrate. 
     In some embodiments, the electrode structure includes a plurality of electrodes parallel to each other and extending along the second surface in a direction perpendicular to the first direction. 
     In some embodiments, the ground electrode further includes an additional electrode portion extending from the elongated electrode portion in a direction perpendicular to the second direction along the surface of the first substrate distal to the second substrate. 
     In some embodiments, an orthographic projection of the first antenna oscillator on the first substrate, an orthographic projection of the second antenna oscillator on the first substrate, and an orthographic projection of the elongated electrode portion on the first substrate overlap each other. 
     In some embodiments, a thickness of the liquid crystal layer is less than 100 μm. 
     According to another aspect of the present disclosure, there is provided a liquid crystal phased array antenna, which includes a plurality of liquid crystal antenna units in an array, wherein each of the plurality of liquid crystal antenna units is the liquid crystal antenna unit according to any one of the foregoing embodiments of the present disclosure. 
     In some embodiments, the first substrates of the plurality of liquid crystal antenna units as a whole are a single substrate, and the second substrates of the plurality of liquid crystal antenna units as a whole are another single substrate. 
     These and other aspects of the present disclosure will be clearly understood from the exemplary embodiments described below, and will be set forth with reference to the exemplary embodiments described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details, features and advantages of the present disclosure will be described in the following exemplary embodiments, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  schematically illustrates a top view of a liquid crystal antenna unit according to an embodiment of the present disclosure; 
         FIG. 2A  schematically illustrates a cross-sectional view of the liquid crystal antenna unit as shown in  FIG. 1  taken along line AA′ in  FIG. 1 ; 
         FIG. 2B  schematically illustrates a cross-sectional view of the liquid crystal antenna unit as shown in  FIG. 1  taken along line BB′ in  FIG. 1 ; 
         FIG. 3  schematically illustrates an arrangement of a first antenna oscillator (also referred to as first antenna element) and a transmission line of the liquid crystal antenna unit shown in  FIG. 1 ; 
         FIG. 4  schematically illustrates a bottom view of the liquid crystal antenna unit shown in  FIG. 1 ; 
         FIG. 5  shows a two-dimensional far-field directional pattern of the liquid crystal antenna unit shown in  FIG. 1 ; 
         FIGS. 6A and 6B  schematically illustrate cross-sectional views of a liquid crystal antenna unit according to another embodiment of the present disclosure; and 
         FIG. 7  schematically illustrates a block diagram of a liquid crystal phased array antenna according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood that, although the terms of first, second, third, and the like may be used herein for describing various elements, components, regions, layers and/or portions, these elements, components, regions, layers and/or portions should not be limited by these terms. These terms are only used for distinguishing one element, component, region, layer or portion from another element, component, region, layer or portion. Thus, a first element, component, region, layer or portion discussed below may also be referred to as a second element, component, region, layer or portion, without departing from the teachings of the present disclosure. 
     Spatially relative terms such as “below . . . ,” “under . . . ,” “lower,” “beneath . . . ,” “above . . . ,” “upper” and the like may be used herein for ease of description to describe a relationship between one element or feature and another element(s) or feature(s) as illustrated in the figures. It should be understood that these spatially relative terms are intended to encompass different orientations of a device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, an element described as “under” or “below” or “beneath” other element(s) or feature(s) would then be oriented “above” the other element(s) or feature(s). Thus, the exemplary terms “under . . . ” and “beneath . . . ” may encompass both orientations “above . . . ” and “below . . . ”. Terms such as “prior to . . . ” or “before . . . ” and “after . . . ” or “next” may similarly be used, for example, for indicating an order in which light passes through elements. The device may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein should be interpreted accordingly. In addition, it should also be understood that when a layer is referred to as being “between” two layers, it may be the sole layer between the two layers, or one or more intervening layers may also be present. 
     The terms used herein are for the purpose of describing exemplary embodiments only and are not intended to limit the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to also include the plural forms, unless the context clearly indicates otherwise. It should be further understood that the terms “comprise” and/or “include” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It should be understood that when an element or layer is referred to as being “on,” “connected to,” “coupled to” or “adjacent to” another element or layer, it may be directly on, directly connected to, directly coupled to or directly adjacent to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to,” “directly coupled to,” or “directly adjacent to” another element or layer, there are no intervening elements or layers. However, in no case should the term “on . . . ” or “directly on . . . ” be interpreted as requiring that one layer completely cover the underlying layer, unless otherwise stated. 
     Embodiments of the present disclosure are described herein with reference to schematic illustrations (and intermediate structures) of idealized embodiments of the present disclosure. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the present disclosure should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the present disclosure. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure belongs. It should be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and/or the present specification and will not be interpreted in an idealized or overly formal sense, unless expressly so defined herein. 
     Embodiments of the present disclosure will now be described in detail below with reference to the accompanying drawings. 
       FIG. 1  schematically illustrates a top view of a liquid crystal antenna unit  100  according to an embodiment of the present disclosure. 
     As shown in  FIG. 1 , the liquid crystal antenna unit  100  includes a second substrate  120  and a second antenna oscillator (also referred to as second antenna element)  122 . The second substrate  120  is made of, for example, glass. The second antenna oscillator  122  is located on the upper surface of the second substrate  120 , and has an elongated shape extending in a second direction D 2  (e.g., the horizontal direction in  FIG. 1 ) along the upper surface of the second substrate  120 . 
       FIG. 1  further shows a transmission line  112  as indicated by dashed lines, a first antenna oscillator  118  and an electrode structure  124 . The transmission line  112  is indicated by the dashed lines because the transmission line  112  is not formed on the upper surface of the second substrate  120  but is formed under the upper surface of the second substrate  120  (see, for example,  FIGS. 2A and 6A ). The transmission line  112  is arranged to extend in a first direction D 1  (e.g., the vertical direction in  FIG. 1 ). In the present embodiment, the first direction D 1  is perpendicular to the second direction D 2 , but the present is not limited thereto. In some other embodiments, the first direction D 1  and the second direction D 2  may form any angle therebetween. Specifically, the transmission line  112  may be a coplanar waveguide (CPW) transmission line including a signal line  113  and two ground wires  115  and  117  respectively located at both sides of the signal line  113 . The electrode structure  124  includes a plurality of electrodes  125  that are parallel to each other and extend in a direction (which is, in this example, the second direction D 2 ) perpendicular to the first direction D 1 . In an embodiment, the plurality of electrodes  125  may be electrically insulated from each other. 
     It should be noted that the transmission line  112  shown in  FIG. 1  is exemplary, and that any other suitable type of transmission line may be used in other embodiments, such as a grounded coplanar waveguide (CPWG) and a conventional microstrip (which may also referred to as microstrip line). In an embodiment, the grounded coplanar waveguide may include the transmission line  112  and a ground electrode (i.e., grounded electrode)  114  as shown in  FIG. 4 . As described above, the transmission line  112  includes the ground wire  115 , the signal line  113 , and the ground wire  117  that are coplanar, and the ground electrode  114  is used for grounding. In an embodiment, the microstrip may include the signal line  113  of the transmission line  112  (i.e., the ground wire  115  and the ground wire  117  of the transmission line  112  may be omitted) and the ground electrode  114  as shown in  FIG. 4 . In this case, the signal line  113  may be connected to one of two poles of a dipole of the first antenna oscillator  118  (a more detailed description thereof will be given later), and the ground electrode  114  may be connected to the other of the two poles of the dipole of the first antenna oscillator  118 . It should be further understood that the electrode structure  124  shown in  FIG. 1  is exemplary, and that the electrode structure  124  may have any other suitable pattern in other embodiments. 
       FIGS. 2A and 2B  schematically illustrate cross-sectional views of the liquid crystal antenna unit  100  shown in  FIG. 1  taken along line AA′ and line BB’ in  FIG. 1 , respectively. The structure of the liquid crystal antenna unit  100  is more clearly shown in  FIGS. 2A and 2B . 
     Referring to  FIG. 2A , the liquid crystal antenna unit  100  may further include a first substrate  110  disposed opposite to the second substrate  120  and a liquid crystal layer  130  disposed between the first substrate  110  and the second substrate  120 , in addition to the above-described components. The first substrate  110  is made of, for example, glass. The liquid crystal layer  130  includes liquid crystal molecules (not shown), and has a typical thickness less than 100 μm, which may improve a response speed of the liquid crystal antenna unit  100  and reduce a size of the liquid crystal antenna unit  100 . The transmission line  112  including the signal line  113 , the ground wire  115  and the ground wire  117  is located on a first surface S 1  (e.g., the upper surface in  FIG. 2A ) of the first substrate  110 , and is arranged to extend in the first direction D 1  along the first surface S 1 . In the present embodiment, the first surface S 1  is a surface of the first substrate  110  proximal to the second substrate  120 , and a surface of the second substrate  120  proximal to the first substrate  110  is referred to as a second surface S 2 , correspondingly. The plurality of electrodes  125  of the electrode structure  124  are located on the second surface S 2 , and extend in a direction (which is, in this example, the second direction D 2 ) perpendicular to the first direction D 1  along the second surface S 2 . 
     Each of the liquid crystal molecules in the liquid crystal layer  130  is anisotropic and has different dielectric constants in the long axis direction and the short axis direction thereof. When in operation, the transmission line  112  may transmit (or transfer) electromagnetic wave signals to a receiver (not shown) and/or from a transmitter (not shown), and a voltage is applied to the electrodes  125  of the electrode structure  124  to form a bias electric field between the electrodes  125  and the ground wires  115 ,  117 . Due to the bias electric field between the electrodes  125  and the ground wires  115 ,  117 , the liquid crystal molecules are rotated such that a dielectric constant of the liquid crystal layer  130  varies with the rotation of the liquid crystal molecules. When an electromagnetic wave signal is transmitted along the transmission line  112  in the liquid crystal layer  130  with the changed dielectric constant, the electromagnetic wave signal is phase-shifted to a certain extent. Thus, a phase shift of the electromagnetic wave signal may be achieved by controlling the rotation of the liquid crystal molecules in the liquid crystal layer  130 , and the rotation may in turn be achieved by controlling the voltage applied to the electrode structure  124 . This is a known technique referred to as a liquid crystal phase shifter. For this purpose, the liquid crystal antenna unit  100  further includes a first alignment layer  132  disposed on the first surface S 1  and a second alignment layer  134  disposed on the second surface S 2 , as shown in  FIG. 2A . Each of the first alignment layer  132  and the second alignment layer  134  may be made of, for example, polyimide for setting an initial alignment of the liquid crystal molecules in the liquid crystal layer  130  without a bias electric field. Each of the first alignment layer  132  and the second alignment layer  134  may have a conventional small thickness such that it has substantially no influence on electromagnetic wave signals transmitted on the transmission line  112 . 
     It should be noted that the electrode structure  124  (and thus the electrodes  125  thereof) provided for the purpose of adjusting the dielectric constant of the liquid crystal layer  130  may even be optional in some embodiments, since a desired bias electric field may be established with only the transmission line  112 . For example, a voltage may be applied to the signal line  113  of the transmission line  112  such that the desired bias electric field is established between the signal line  113  and each of the ground wires  115 ,  117  of the transmission line  112 . The ground wires  115  and  117  are shown to be isolated from each other in  FIGS. 1 and 3 , but the present disclosure is not limited thereto. For example, in some embodiments, the ground wire  115  may be connected to the ground wire  117  by a connector (not shown). 
     Referring to  FIG. 2B , the liquid crystal antenna unit  100  may further include the first antenna oscillator  118  in addition to the above-described components. The first antenna oscillator  118  is located on the first surface S 1  and is arranged as an elongated dipole extending along the first surface S 1  in the second direction D 2 . As shown, the dipole includes two poles  118 A and  118 B separated from each other by a gap  119 . The first antenna oscillator  118  is configured to couple electromagnetic waves between the two poles  118 A,  118 B and the transmission line  112  at the gap  119 . 
       FIG. 3  schematically illustrates an exemplary arrangement of the first antenna oscillator  118  and the transmission line  112 . 
     Referring to  FIG. 3 , the transmission line  112  extends in the first direction D 1 , and the two poles  118 A,  118 B of the first antenna oscillator  118 , which is a dipole, extend in the second direction D 2 . One end of the signal line  113  of the transmission line  112  is electrically connected to one of the poles  118 A,  118 B (in this example, the pole  118 A) of the dipole on one side (the left side in  FIG. 3 ) of the gap  119  in the first antenna oscillator  118 , such that, for example, a side of the signal line  113  and a side of the pole (in this example, the pole  118 A) to which the signal line  113  is connected are flush with each other. Further, one end of one (in this example, the ground wire  117 ) of the two ground wires  115 ,  117  of the transmission line  112  is electrically connected to the other (in this example, the pole  118 B) of the poles  118 A,  118 B of the dipole on the other side (the right side in  FIG. 3 ) of the gap  119 , such that, for example, a side of the ground wire (in this example, the ground wire  117 ) and a side of the pole (in this example, the pole  118 B) to which the ground wire is connected are flush with each other. In a transmitting scenario, the transmission line  112  feeds an electromagnetic wave signal from a transmitter (not shown) to the first antenna oscillator  118 . Alternatively, in a receiving scenario, the first antenna oscillator  118  feeds a received electromagnetic wave signal to the transmission line  112 , and then the received electromagnetic wave signal is provided to a receiver (not shown) via the transmission line  112 . 
     It should be noted that although the first antenna oscillator  118  and the transmission line  112  are shown as being perpendicular to each other in the present embodiment, the first antenna oscillator  118  and the transmission line  112  may form any suitable angle therebetween (e.g., may be parallel to each other) in other embodiments. Further, other embodiments are possible although the first antenna oscillator  118  and the transmission line  112  are shown as being directly electrically connected to each other in the present embodiment. For example, the transmission line  112  may be electromagnetically coupled to the first antenna oscillator  118  through any suitable intermediate medium. 
     It should be further noted that the pattern of the first antenna oscillator  118  (and more particularly the two poles  118 A,  118 B forming the dipole) shown in  FIG. 3  is exemplary, and that in other embodiments the first antenna oscillator  118  may be a dipole having other patterns, such as a folded dipole formed by an elongated conductor. The folded dipole is known in the art, and such a folded dipole formed by the elongated conductor is also included in the “elongated” dipole of the present disclosure. 
       FIG. 4  schematically illustrates a bottom view of the liquid crystal antenna unit  100 . 
     Referring to  FIG. 4 , the liquid crystal antenna unit  100  may further include the ground electrode  114  for grounding, in addition to the above-described components. The ground electrode  114  includes an elongated electrode portion (e.g., a first electrode portion)  114 A extending in the second direction D 2  along a surface of the first substrate  110  distal to the second substrate  120 . The elongated electrode portion  114 A has a length L 3 . In this example, the ground electrode  114  further includes an additional electrode portion (e.g., a second electrode portion)  114 B extending from the elongated electrode portion  114 A in a direction (in this example, the first direction D 1 ) perpendicular to the second direction D 2  along the surface of the first substrate  110  distal to the second substrate  120 . 
     It should be noted that the ground electrode  114  shown in  FIG. 4  is exemplary, and in some embodiments the ground electrode  114  may cover the entire surface of the first substrate  110  distal to the second substrate  120 . In other words, the additional electrode portion  114 B may be optional in some embodiments. 
     Referring back to  FIG. 2B , the second antenna oscillator  122  has an elongated shape extending in the second direction D 2  along a surface of the second substrate  120  distal to the first substrate  110 . The second antenna oscillator  122  has a length L 2  smaller than a length L 1  of the first antenna oscillator  118 , and the length L 1  of the first antenna oscillator  118  is smaller than the length L 3  of the elongated electrode portion  114 A. With continued reference to  FIG. 2B , the second antenna oscillator  122  is located at a position corresponding to the first antenna oscillator  118  and on the surface of the second substrate  120  distal to the first substrate  110 , and the elongated electrode portion  114 A is located at a position corresponding to the first antenna oscillator  118  and on the surface of the first substrate  110  distal to the second substrate  120 . In other words, an orthographic projection of the second antenna oscillator  122  on the first substrate  110 , an orthographic projection of the first antenna oscillator  118  on the first substrate  110 , and an orthographic projection of the elongated electrode portion  114 A on the first substrate  110  overlap each other. In this case, a three-element directional radiator formed by the second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A may have a higher gain. For example, a frequency of an electromagnetic wave to be transmitted by the liquid crystal antenna unit  100  may be between 1 GHz and 100 GHz (i.e., a wavelength of the electromagnetic wave to be transmitted may be between 3 mm and 300 mm). In this case, the “elongated shape” of the second antenna oscillator  122  may be a rectangular parallelepiped (e.g., a cuboid), the length L 1  (i.e., a size in the horizontal direction in  FIG. 2B ) of the rectangular parallelepiped may be about one-half (½) of the wavelength of the electromagnetic wave to be transmitted, a thickness (i.e., a size in the vertical direction in  FIG. 2B ) of the rectangular parallelepiped may be between about 3 times a skin depth and about 35 microns, or between about 1 micron and about 35 microns, and a width of the rectangular parallelepiped (i.e., a size in a direction perpendicular to the cross-sectional view shown in  FIG. 2B ) may be less than about 0.1 times the wavelength of the electromagnetic wave to be transmitted. Further, the first antenna oscillator  118  or the elongated electrode portion  114 A may have a shape similar to the “elongated shape” of the second antenna oscillator  122 , as long as it suffices that the second antenna oscillator  122  has the length L 2  less than the length L 1  of the first antenna oscillator  118 , and that the length L 1  of the first antenna oscillator  118  is less than the length L 3  of the elongated electrode portion  114 A. 
     The term “corresponding to” as used herein in connection with the second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A means that, the second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A are positioned in relation to each other such that they form the three-element directional radiator, in which the second antenna oscillator  122  serves as a director, the first antenna oscillator  118  serves as an active oscillator, and the elongated electrode portion  114 A serves as a reflector. Specifically, the reflector is positioned at one side of the active oscillator to attenuate an electromagnetic wave transmitted from or emitted toward the one side, and the director is positioned at the other side of the active oscillator to enhance an electromagnetic wave transmitted from or emitted toward the other side. Such a directional radiator may operate in a similar manner to a known Yagi antenna (may also be referred to as Yagi-Uda antenna), the detailed description thereof is therefore omitted here. 
     In the example of  FIG. 2B , the first antenna oscillator  118  is positioned right above the elongated electrode portion  114 A, and the second antenna oscillator  122  is positioned right above the first antenna oscillator  118 . More specifically, the second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A are arranged such that a center of the second antenna oscillator  122 , a center of the gap  119  in the first antenna oscillator  118 , and a center of the elongated electrode portion  114 A are all located on a straight line, which is as indicated by the dashed line OO′, perpendicular to the first substrate  110  or the second substrate  120 . 
     The second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A are suitably arranged depending on the wavelength of the electromagnetic wave. For example, a suitable length L 1  of the first antenna oscillator  118  may be one-half of the wavelength of the electromagnetic wave to be transmitted. In some embodiments, the length L 2  of the second antenna oscillator  122  is 0.7 to 0.9 times the length L of the first antenna oscillator  118 . In an embodiment where the first substrate  110  and the second substrate  120  are made of a same material, the second substrate  120  has a thickness H 2  that is 3 to 5 times a thickness H 1  of the first substrate  110 . In an embodiment where the first substrate  110  and the second substrate  120  are made of different materials, the thickness H 1  of the first substrate  110  and the thickness H 2  of the second substrate  120  satisfy the following relationship: 
     
       
         
           
             
               
                 
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     where ε 1  is a dielectric constant of the first substrate  110 , and ε 2  a dielectric constant of the second substrate  120 . 
     In some embodiments, the second substrate  120  may be a composite board including multiple layers of materials (e.g., N layers of materials) to provide a suitable dielectric constant. In this case, the dielectric constant of the second substrate  120  may be a weighted average of the dielectric constants of the multiple layers of materials of the second substrate  120 . For example, the dielectric constant ε 2  of the second substrate  120  may be calculated by the following formula: 
     
       
         
           
             
               
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                 Ti 
               
             
             , 
           
         
       
     
     Ti is a thickness of the i-th layer of material, ε 2i  is a dielectric constant of the i-th layer of material, and N is an integer greater than or equal to 2. For example, the outermost layer of the second substrate  120  may be formed even of air. That is, the second antenna oscillator  122  is not directly on the surface of the second substrate  120  distal to the first substrate  110 , but is positioned at a distance above the second substrate  120 . In this case, the thickness H 2  of the second substrate  120  may be a distance between the lower surface of the second substrate  120  and the lower surface of the second antenna oscillator  122 , as shown in  FIG. 2B . In this case, the second antenna oscillator  122  may be fixed by means of any suitable support means (e.g., a pillar). 
     It should be noted that although an example arrangement of the second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A is described above with respect to  FIGS. 1 and 2B , other embodiments are possible. For example, the first antenna oscillator  118  may be offset by a distance in the first direction D 1  with respect to the elongated electrode portion  114 A, and the second antenna oscillator  122  may be further offset by a distance in the first direction D 1  with respect to the first antenna oscillator  118 . Alternatively or additionally, the first antenna oscillator  118  may be offset by a distance in the second direction D 2  with respect to the elongated electrode portion  114 A, and/or the second antenna oscillator  122  may be offset by a distance in the second direction D 2  with respect to the first antenna oscillator  118 . In practice, the second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A may be arranged in any suitable manner as long as they can form the three-element directional radiator. 
     The liquid crystal antenna unit  100  is realized to have a high amplitude gain by means of the directional radiator formed by the second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A. 
       FIG. 5  shows a two-dimensional far-field directional pattern of the liquid crystal antenna unit  100 . The directional pattern has been obtained from a software-implemented simulation in which the frequency of the electromagnetic wave is set to 30 GHz. According to the simulation results, an amplitude gain of the main lobe is 8.89 dBi in the main lobe direction of 0 degrees (e.g., the vertically upward direction in  FIG. 5 ). This amplitude gain is higher than the gain of a conventional patch antenna by 2 to 3 dB. Therefore, the liquid crystal antenna unit  100  advantageously achieves an improvement in gain. 
       FIGS. 6A and 6B  schematically illustrate cross-sectional views of a liquid crystal antenna unit  200  according to another embodiment of the present disclosure. 
     Similar to  FIG. 2A ,  FIG. 6A  shows an example arrangement of the transmission line  112  and one of the electrodes  125  of the electrode structure  124 . Unlike the liquid crystal antenna unit  100 , in the liquid crystal antenna unit  200 , the first surface S 1  is defined as a surface of the second substrate  120  proximal to the first substrate  110 , and the second surface S 2  is defined as a surface of the first substrate  110  proximal to the second substrate  120 . Thus, in the present embodiment, the transmission line  112  is located on the surface of the second substrate  120  proximal to the first substrate  110 , and the electrodes  125  of the electrode structure  124  are located on the surface of the first substrate  110  proximal to the second substrate  120 . 
     Similar to  FIG. 2B ,  FIG. 6B  shows an example arrangement of the second antenna oscillator  122 , the first antenna oscillator  118  and the elongated electrode portion  114 A. Unlike the liquid crystal antenna unit  100 , in the liquid crystal antenna unit  200 , the first antenna oscillator  118  is located on the surface of the second substrate  120  proximal to the first substrate  110 . 
     In addition to the differences described here, the details and variations of the liquid crystal antenna unit  100  described above with respect to  FIGS. 1 to 5  may also be applicable to the liquid crystal antenna unit  200 , and thus are not repeated here. 
       FIG. 7  schematically illustrates a block diagram of a liquid crystal phased array antenna  700  according to an embodiment of the present disclosure. 
     Referring to  FIG. 7 , the liquid crystal phased array antenna  700  includes a plurality of liquid crystal antenna units  710  arranged in an array. Each of the plurality of liquid crystal antenna units  710  may take the form of the above-described liquid crystal antenna unit  100  or liquid crystal antenna unit  200  or a variation thereof, and a detailed description thereof is thus omitted herein. The plurality of liquid crystal antenna units  710  may be applied with different voltages through a feeding network (not shown), respectively, so as to change a phase of an electromagnetic wave signal, thereby implementing the functions of beam scanning and beam forming. Advantageously, each of the liquid crystal antenna units  710  may provide a higher gain than that of a conventional patch antenna unit. 
     In some embodiments, the first substrates (e.g., the first substrates  110 ) of the plurality of liquid crystal antenna units  710  as a whole are formed to be a single (i.e., one) substrate, and the second substrates (e.g., the second substrates  120 ) of the plurality of liquid crystal antenna units  710  as a whole are formed to be another single substrate. This facilitates mass production of the liquid crystal phased array antenna. Alternatively, the liquid crystal phased array antenna  700  may be formed by combining a plurality of separate liquid crystal antenna units  710 . 
     The liquid crystal phased array antenna  700  may provide a high gain because each antenna unit  710  as described above may provide a high gain. The liquid crystal phased array antenna  700  may provide a higher gain than a conventional liquid crystal phased array antenna, in a case of a same antenna array size. This improves the communication performance. 
     The liquid crystal phased array antenna  700  may also have a small size because each antenna unit  710  may be formed to have a smaller size than a conventional patch antenna unit while still providing a same gain. The liquid crystal phased array antenna  700  may have a size of about ½ of a size of a conventional liquid crystal phased array antenna, in a case where they include the same number of antenna units. As such, a space for arranging the antenna may be reduced. 
     Variations to the disclosed embodiments may be understood and carried out by one of ordinary skill in the art in practicing the claimed subject matter, from a study on the drawings, the present disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The additional features recited in different dependent claims may be combined with each other to form new solutions in a case of no obvious conflict.