Patent Publication Number: US-9837720-B2

Title: Metamaterial antenna

Description:
CROSS REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY 
     This patent application claims benefit under 35 U.S.C. 119(e), 120, 121, or 365(c), and is a National Stage entry from International Application No. PCT/KR2013/004152, filed 10 May 2013, which claims priority to Korean Patent Application No. 10-2012-0096209, filed 31 Aug. 2012, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     Embodiments of the present invention relate to a metamaterial antenna, and more particularly, to a metamaterial antenna using a conductor cover of a wireless terminal. 
     BACKGROUND PART ART 
     In recent years, wireless terminals, such as mobile phones, smart phones, and personal digital assistants (PDAs), has been developed with an emphasis on the appearance design as well as a variety of functions, such as a voice call, a Global Positioning System (GPS), Digital Multimedia Broadcasting (DMB), data communication, the Internet, authentication, payment, and near field communication. Thus, in order to provide a refined design, a conductor cover may be formed at an exterior of the wireless terminal (for example, at a lateral side of the wireless terminal). In this case, the radiation efficiency of an embedded antenna of the wireless terminal may be degraded due to the conductor cover. That is, since the conductor cover formed at an exterior of the wireless terminal serves as an obstacle restricting or hindering electric waves radiated from the embedded antenna, the radiation efficiency of the embedded antenna may be degraded. Accordingly, there is a need for a method for preventing the radiation efficiency of an embedded antenna from being degraded while maintaining a refined design when a conductor cover is formed at the exterior of a wireless terminal. 
     SUMMARY 
     The embodiments of the present invention provide a metamaterial antenna capable of preventing the radiation efficiency of an embedded antenna from being degraded even if a conductor cover is formed at the exterior of a wireless terminal. 
     According to an aspect of the present invention, there is provided a metamaterial antenna including a conductor cover, a feed parallel inductor element, and at least one ground parallel inductor. The conductor cover may be formed at one side of a wireless terminal. The feed parallel inductor element may be formed to connect the conductor cover to a feed part. The at least one ground parallel inductor element may be formed to connect the conductor cover to at least one ground part. 
     According to another aspect of the present invention, there is provided a metamaterial antenna including a conductor cover, a feed parallel inductor element, a first ground parallel inductor element, and a second ground parallel inductor element. The conductor cover may be formed at one side of a wireless terminal. The feed parallel inductor element may be formed to connect one end of the conductor cover to a feed part. The first ground parallel inductor element may be formed to connect the other end of the conductor cover to a first ground part. The second ground parallel inductor element may be formed to connect the conductor cover to a second ground part between both ends of the conductor cover. 
     According to another aspect of the present invention, there is provided a metamaterial antenna including a conductor cover, a plurality of couple patches, a feed parallel inductor element, and at least one ground parallel inductor element. The conductor cover may be formed at one side of a wireless terminal. The plurality of couple patches may be formed to be spaced at a predetermined interval from the conductor cover. The feed parallel inductor element may be formed to connect one of the plurality of couple patches to a feed part. The at least one ground parallel inductor element may be formed to connect the remaining couple patches of the plurality of couple patches to a ground part. 
     According to another aspect of the present invention, there is provided a metamaterial antenna including a conductor cover, a couple patch, a feed parallel inductor element, and at least one ground parallel inductor element. The conductor cover may be formed at one side of a wireless terminal. The couple patch may be formed to be spaced at a predetermined interval from the conductor cover. The feed parallel inductor element may be formed to connect the couple patch to a feed part. The at least one ground parallel inductor element may be formed to connect the couple patch to a ground part. 
     According to the above-described aspects of the present invention, the radiation efficiency of an embedded antennal formed on a main board of a wireless terminal can be prevented from being degraded while maintaining the design of the wireless terminal provided by a conductor cover, using the conductor cover formed at the exterior of the wireless terminal as an antenna. In addition, since an antenna is additionally formed without using a separate space in the wireless terminal, multiple antennas can be implemented while maximizing the spatial use of the wireless terminal. 
     In addition, as the conductor cover serves as an antenna using the Epsilon Negative (ENG) construction, a resonant frequency and an input impedance of the metamaterial antenna can be easily adjusted through at least one of inductance values and positions of parallel inductor elements. 
     In addition, as the conductor cover is not directly connected to the main board of the wireless terminal, the main board of the wireless terminal is prevented from being damaged by an external surge signal. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating a metamaterial antenna in accordance with a first embodiment of the present invention. 
         FIG. 2  is a view illustrating an equivalent circuit of the metamaterial antenna in accordance with the first embodiment of the present invention. 
         FIG. 3  is a view illustrating a metamaterial antenna in accordance with a second embodiment of the present invention. 
         FIG. 4  is a graph showing a reflection coefficient of the metamaterial antenna in accordance with the first embodiment of the present invention shown in  FIG. 1 . 
         FIG. 5  is a graph showing a reflection coefficient of the metamaterial antenna in accordance with the second embodiment of the present invention shown in  FIG. 3 . 
         FIG. 6  is a view illustrating a metamaterial antenna in accordance with a third embodiment of the present invention. 
         FIG. 7  is a view illustrating a metamaterial antenna in accordance with a fourth embodiment of the present invention. 
         FIG. 8  is a graph showing a change in a resonant frequency according to a width of a slot in the metamaterial antenna in accordance with the fourth embodiment of the present invention. 
         FIG. 9  is a graph showing a change in resonant frequency according to a length of a slot in the metamaterial antenna in accordance with a fourth embodiment of the present invention. 
         FIG. 10  is a perspective view illustrating a metamaterial antenna in accordance with the fifth embodiment of the present invention. 
         FIG. 11  is a plan view illustrating the metamaterial antenna in accordance with the fifth embodiment of the present invention. 
         FIG. 12  is a view illustrating an equivalent circuit of the metamaterial antenna in accordance with the fifth embodiment of the present invention. 
         FIG. 13  is a graph showing a change in resonant frequency according to lengths of a first couple patch and a second couple patch of the metamaterial antenna in accordance with the fifth embodiment of the present invention. 
         FIG. 14  is a plan view illustrating a metamaterial antenna in accordance with a sixth embodiment of the present invention. 
         FIG. 15  is a perspective view illustrating a metamaterial antenna in accordance with a seventh embodiment of the present invention. 
         FIG. 16  is a plan view illustrating the metamaterial antenna in accordance with the seventh embodiment of the present invention. 
         FIG. 17  is a perspective view illustrating an equivalent circuit of the metamaterial antenna in accordance with the seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, detailed embodiments of metamaterial antennas according to the present invention will be described with reference to  FIGS. 1 to 17 . However, the exemplary embodiments of the invention are merely illustrative examples and the present invention is not limited thereto. 
     In describing the present invention, detailed descriptions that are well-known but are likely to make the subject matter of the present invention unclear will be omitted in order to avoid redundancy. The terminology used herein is defined in consideration of its function in the present invention, and may vary with an intention of a user and an operator or custom. Accordingly, the definition of the terms should be determined based on overall contents of the specification. 
     These inventive concepts are determined by scope of claims, and it would be appreciated by those skilled in the art that changes and modifications, which have not been illustrated above, may be made in these embodiments without departing from the principles and scope of the invention, the scope of which is defined in the claims and their equivalents. 
       FIG. 1  is a view illustrating a metamaterial antenna in accordance with a first embodiment of the present invention. 
     Referring to  FIG. 1 , a metamaterial antenna  100  includes a conductor cover  102 , a feed parallel inductor element  104 , and a ground parallel inductor element  106 . The metamaterial antenna  100  exhibits metamaterial properties through the feed parallel inductor element  104  and the ground parallel inductor element  106 , and details thereof will be described later. 
     The conductor cover  102 , for example, may be formed at a lateral side of a wireless terminal (not shown) with a predetermined length. In this case, the conductor cover  102  may be formed at one side or both sides of the wireless terminal (not shown). Both ends of the conductor cover  102  are fixed to a main board  110  of the wireless terminal. A ground  112  having a predetermined area is formed on the main board  110  of the wireless terminal, and on a region of the main board  110  where the ground  112  is not formed, an embedded antenna  114  is provided separately from the metamaterial antenna  100 . For convenience of description, the embedded antenna  114  is represented by a dotted line. For convenience of description, although the following description will be made only in relation to a conductor cover  102  formed at a left side of the wireless terminal (not shown), a metamaterial antenna may be implemented in the same manner using a conductor cover formed at a right side of the wireless terminal (not shown), and a metamaterial antenna may be implemented using at least one of conductor covers formed at both sides of the wireless terminal (not shown). Although the conductor cover  102  is illustrated as being formed at a lateral side of the wireless terminal (not shown), the present invention is not limited thereto. For example, the conductor cover  102  may be formed at any of a front side, a rear side, an upper side, and a lower side of the wireless terminal (not shown). 
     The feed parallel inductor element  104  is formed to connect one end of the conductor cover  102  to one end of a feed part  116 . The other end of the feed part  116  is spaced at a predetermined interval from the ground  112 . A feeding point  118  is formed at the other end of the feed part  116 . 
     The ground parallel inductor element  106  is formed to connect the other end of the conductor cover  102  to one end of a ground part  120 . In this case, the other end of the ground part  120  is connected to the ground  112 . 
     As described above, one end of the conductor cover  102  is connected to the feed part  116  through the feed parallel inductor element  104 , and the other end of the conductor cover  102  is connected to the ground part  120  through the ground parallel inductor element  106 , thereby using the conductor cover  102  as an antenna. Accordingly, radiation efficiency of the internal antenna  114  may be prevented from being degraded. 
     In general, when a conductor material is present around an antenna, the conductor material confines or restrains electric waves radiated from the antenna so as to limit an electrical volume of the antenna, thereby degrading the radiation characteristics of the antenna. As such, the conventional conductor cover is a simple conductor material, and causes the radiation characteristics of the embedded antenna  114  to be degraded. 
     Meanwhile, the conductor cover  102  in accordance with embodiments of the present invention serves as an antenna rather than a simple conductor material. In this case, it is possible to enhance the radiation efficiency of the embedded antenna  114  that may be degraded due to the conventional conductor cover. In this case, when a resonant frequency of the conductor cover  102  is adjusted to be same as a resonant frequency of the embedded antenna  114 , improved radiation efficiency is provided compared to when only the embedded antenna  114  is used. Meanwhile, the embedded antenna  114  is provided at a front end portion or a rear end portion of the main board  110 , and the conductor cover  102  is formed at a side of the main board  110 . Here, since the two antennas are provided perpendicular to each other, mutual interference hardly occurs between the internal antenna  114  and the conductor cover  102 . 
     Since the conductor cover  102  is designed in views of the design, and fixedly formed at the wireless terminal (not shown), it is not easy to change the structure of the conductor cover  102  in terms of resonance frequency adjustment and impedance matching. According to embodiments of the present invention, it is possible to use the conductor cover  102  as an antenna using a construction of Epsion Negative (ENG), which is a type of a metamaterial, without changing the structure of the conductor cover  102 . 
     Metamaterials are materials or electromagnetic structures artificially engineered to have electromagnetic properties that have not yet been found in nature, and having at least one of permittivity and permeability provided in a negative value. The metamaterial antenna  100  in accordance with embodiments of the present invention has negative permittivity due to the feed parallel inductor element  104  and the ground parallel inductor element  106 , thereby exhibiting metamaterial properties. Since electromagnetic waves propagated through the metamaterial has a negative phase velocity and a negative group velocity opposite to the propagation direction of the electromagnetic waves, the electromagnetic waves are propagated by following a Fleming&#39;s left-hand rule rather than following a Fleming&#39;s right-hand rule, exhibiting a left-handed property. Accordingly, the metamaterial antenna  100  has a zero-order resonance or a negative order resonance, so that a resonant frequency may be determined regardless of the antenna length. 
     That is, the resonant frequency of the metamaterial antenna  100  is determined by inductance values of the feed parallel inductor element  104  and the ground parallel inductor element  106 . Accordingly, in the resonant frequency matching and the impedance matching, there is no need to change the structure of the conductor cover  102 , and only the inductance values of the feed parallel inductor element  104  and the ground parallel inductor element  106  need to be adjusted. In detail, the resonant frequency and the input impedance of the metamaterial antenna  100  are adjusted by ratios of the inductances of the feed parallel inductor element  104  and the ground parallel inductor element  106 . As such, by using the ENG construction, the conductor cover  102  is easily used as an antenna. 
     According to embodiments of the present invention, the conductor cover  102  is used as an antenna, so that the radiation efficiency of the internal antenna  114  formed on the main board  110  of the wireless terminal is prevented from being degraded while maintaining the design of the wireless terminal provided by the conductor cover  102 . In addition, an antenna is additionally formed without using a separate space of the wireless terminal, so that multiple antennas are implemented while maximizing the spatial use of the wireless terminal. 
       FIG. 2  is a view illustrating an equivalent circuit of the metamaterial antenna in accordance with the first embodiment of the present invention. 
     Referring to  FIG. 2 , the metamaterial antenna  100  includes series inductances L R , parallel capacitances C R , and parallel inductances L L . The series inductance L R  represents an inductance component according to a length of the conductor cover  102 , the parallel capacitance C R  represents a capacitance component according to an interval between the conductor cover  102  and the ground  112 , and the parallel inductances L L  represent inductance components according to the feed parallel inductor element  104  and the ground parallel inductor element  106 . 
     The metamaterial antenna  10  has a Right-Handed (RH) property due to the series inductance L R  and the parallel capacitances C R , and has a left-Handed (LH) property due to the parallel inductances L L . The metamaterial antenna  100  has the above-described metamaterial property due to the parallel inductances L L , so that the resonant frequency and the input impedance may be adjusted by inductance values of the parallel inductances L L  without changing the structure of the conductor cover  102 . 
     Meanwhile, although the conductor cover  102  is illustrated as being connected at both ends thereof to the feed parallel inductor element  104  and the ground parallel inductor element  106 , the positions on the conductor cover  102  at which the feed parallel inductor element  104  and the ground parallel inductor element  106  are connected are not limited thereto, and may be variously provided. 
     For example, referring to  FIG. 3 , the feed parallel inductor element  104  may be connected to one end of the conductor cover  102 , and the ground parallel inductor element  106  may be connected to a middle portion of the conductor cover  102 . In this case, the resonant frequency and the input impedance may be adjusted by the positions on the conductor cover  102  at which the feed parallel inductor element  104  and the ground parallel inductor element  106 . 
     That is, the resonant frequency and the input impedance may be adjusted not only by inductance values of the feed parallel inductor element  104  and the ground parallel inductor element  106  but also by the positions on the conductor cover  102  at which the feed parallel inductor element  104  and the ground parallel inductor element  106 . Details thereof will be described with reference to  FIGS. 4 and 5 . 
       FIG. 4  is a graph showing a reflection coefficient of the metamaterial antenna in accordance with the first embodiment of the present invention shown in  FIG. 1 , and  FIG. 5  is a graph showing a reflection coefficient of the metamaterial antenna in accordance with the second embodiment of the present invention shown in  FIG. 3 . 
     Referring to  FIG. 4 , when the feed parallel inductor element  104  and the ground parallel inductor element  106  are connected to both ends of the conductor cover  102 , the metamaterial antenna  100  has reflection coefficients of −3 dB and −14 dB at 1 GHz and 2 GHz. The reflection coefficient at 1 GHz is too great for the metamaterial antenna  100  to serve as an antenna. The reason why a reflection coefficient is great at 1 GHz is that impedance matching is poor due to a large length of the conductor cover  102 . 
     Meanwhile, referring to  FIG. 5 , when the feed parallel inductor element  104  is connected to one end of the conductor cover  102 , and the ground parallel inductor element  106  is connected to a middle portion of the conductor cover  102 , the metamaterial antenna  100  has reflection coefficients of −9.5 dB and −13 dB at 950 MHz and 1.7 GHz. 
     The resonant frequencies are adjusted from 1 GHz and 2 GHz to 950 MHz and 1.7 GHz, and at 950 MHz, improved impedance matching is shown compared to  FIG. 4 . As such, the resonant frequency and the input impedance by changing the connection position of the ground parallel inductor element  106 . 
     According to the embodiment of the present invention, by allowing the conductor cover to serve as an antenna using the ENG construction, the resonant frequency and the input impedance of the metamaterial antenna are easily adjusted through one of inductance values of the parallel inductor elements and the positions of the parallel inductor elements. 
     Although the metamaterial antennas according to the first embodiment and the second embodiment each are illustrated as being formed of a single unit cell, the present invention is not limited thereto. A metamaterial antenna according to another embodiment of the present invention may be formed of a plurality of unit cells. The following description will be made in relation to a metamaterial antenna formed of a plurality of unit cells. 
       FIG. 6  is a view illustrating a metamaterial antenna in accordance with a third embodiment of the present invention. 
     Referring to  FIG. 6 , a metamaterial antenna  200  includes a conductor cover  202 , a feed parallel inductor element  204 , a first ground parallel inductor element  206 , and a second ground parallel inductor element  208 . 
     The feed parallel inductor element  204  is formed to connect one end of the conductor cover  202  to one end of a feed part  216 . The other end of the feed part  216  is spaced at a predetermined interval from a ground  212 . A feeding point  218  is formed at the other end of the feed part  216 . 
     The first ground parallel inductor element  206  is formed to connect a middle portion of the conductor cover  202  to one end of a first ground part  220 . The other end of the first ground part  220  is connected to the ground  212 . Although the first ground parallel inductor element  206  is illustrated as being connected at a middle portion of the conductor cover  202 , the position at which the first ground parallel inductor element  206  is formed is not limited thereto as long as the first ground parallel inductor element  206  is connected to the conductor cover  202  between both ends of the conductor cover  202 . 
     The second ground parallel inductor element  208  is formed to connect the other end of the conductor cover  202  to one end of a second ground part  222 . The other end of the second ground part  222  is connected to the ground  212 . 
     The metamaterial antenna  200  includes a first unit cell  252  and a second unit cell  254 . That is, the first unit cell  252  is formed to include the ground  212 , the second ground part  222 , the second ground parallel inductor element  208 , a portion between the other end of the conductor cover  202  and the middle portion of the conductor cover  202 , the first ground parallel inductor element  206 , and the first ground part  220 , and the second unit cell  254  is formed by the ground  212 , the first ground part  222 , the first ground parallel inductor element  206 , a portion between the middle portion of the conductor cover  202  to the one end of the conductor cover  202 , the feed parallel inductor element  204 , and the feed part  216 . 
     Although the metamaterial antenna  200  is illustrated as being formed of two unit cells  252  and  254 , the present invention is not limited thereto. A metamaterial antenna according to another embodiment of the present invention may include two or more unit cells. The following description will be made in relation that a metamaterial may be formed of two or more unit cells. For example, the metamaterial antenna  200  may be formed of a larger number of unit cells to additionally connect one end of a ground parallel inductor element to the conductor cover  202  between both ends of the conductor cover  202 . In this case, the other end of the added ground parallel inductor element is connected to the ground through a ground part. 
     When the metamaterial antenna  200  is formed of a plurality of unit cells as described above, the input impedance of the metamaterial antenna  200  is changed, thereby the input impedance of the metamaterial antenna  200  is adjusted. In detail, the more unit cells of the metamaterial antenna  200  are, the higher input impedance of the metamaterial antenna  200  is. Accordingly, when the impedance matching is poorly achieved due to a low input impedance of the metamaterial antenna  200 , the number of unit cells of the metamaterial antenna  200  is increased so as to increase the input impedance, thereby smoothly achieving the impedance matching. 
       FIG. 7  is a view illustrating a metamaterial antenna in accordance with a fourth embodiment of the present invention, which is identical to the description of  FIG. 6  except that a conductor cover  302  is provided with a slot  303  having a predetermined length Ls and a predetermined width Ws. 
     In a general antenna, a slot is used to generate another resonant frequency so that the frequency bandwidth is expanded or multiple frequency bands are implemented. However, when the slot  303  is formed at the conductor cover  302 , a capacitance value of the parallel capacitance C R  is changed according to an interval between the conductor cover  302  and a ground  312 , which causes the resonant frequency and the input impedance of the metamaterial antenna  300  to be changed. That is, the capacitance value of the parallel capacitance C R  is changed according to the width Ws and the length Ls of the slot  303 , so that the resonant frequency and the input impedance of the metamaterial antenna  300  are changed. 
       FIG. 8  is a graph showing a change in a resonant frequency according to a width of a slot in the metamaterial antenna in accordance with the fourth embodiment of the present invention, which shows a change in resonant frequency when the width Ws of the slot  303  is increased 1 mm at a time from 1 mm to 5 mm. 
       FIG. 9  is a graph showing a change in a resonant frequency according to a length of a slot in the metamaterial antenna in accordance with a fourth embodiment of the present invention, which shows a change in resonant frequency when the length Ls of the slot  303  is increased 10 mm at a time from 60 mm to 100 mm. 
     As the resonant frequency and the input impedance of the metamaterial antenna  300  are changed by the length Ws and the length Ls of the slot  303 , the resonant frequency and the input impedance of the metamaterial antenna  300  may be adjusted by changing the inductance value of each parallel inductor element. 
       FIG. 10  is a perspective view illustrating a metamaterial antenna in accordance with the fifth embodiment of the present invention, and  FIG. 11  is a plan view illustrating the metamaterial antenna in accordance with the fifth embodiment of the present invention. 
     Referring to  FIGS. 10 and 11 , a metamaterial antenna  400  includes a conductor cover  402 , a first couple patch  404 , a second couple patch  406 , a feed parallel inductor element  408 , and a ground parallel inductor element  410 . The metamaterial antenna  400  exhibits metamaterial properties through the feed parallel inductor element  408  and the ground parallel inductor element  410 . Details thereof will be made described later. 
     The conductor cover  402 , for example, may be fixedly provided at a lateral side of a wireless terminal (not shown) with a predetermined length. The conductor cover  102  may be formed at one side of the wireless terminal (not shown) or both sides of the wireless terminal (not shown). For convenience sake, the following description will be made in relation to the conductor cover  402  formed at a left side of the wireless terminal (not shown), but a metamaterial antenna may be implemented in the same manner by using a conductor cover formed at a right side of the wireless terminal (not shown), and may be implemented using at least one of the conductor covers formed at both sides of the wireless terminal (not shown). Although the conductor cover  402  is illustrated as being formed at a lateral side of the wireless terminal (not shown), the present invention is not limited thereto. For example, the conductor cover  402  may be formed on any of a front side, a rear side, an upper side and a lower side. 
     The first couple patch  404  is fixed to one end of a side of a main board  412  of the wireless terminal. The first couple patch  404  is spaced apart from one end of the conductor cover  402 . For example, the first couple patch  404  may be formed in parallel with the conductor cover  402  while being spaced at a predetermined interval from one end of the conductor cover  402 . 
     Meanwhile, a ground  414  having a predetermined area is formed on the main board  412  of the wireless terminal, and on a region of the main board  412  where the ground  414  is not formed, an internal antenna  416  is provided separately from the metamaterial antenna  400 . For convenience of description, the internal antenna  416  is represented by a dotted line. 
     The second couple patch  406  is fixed to the other end of the side of the main board  412  of the wireless terminal. The second couple patch  406  is spaced apart from the other end of the conductor cover  402 . For example, the second couple patch  406  may be formed in parallel with the conductor cover  402  while being spaced at a predetermined interval from the other end of the conductor cover  402 . 
     The feed parallel inductor element  408  is formed to connect the first couple patch  404  to one end of a feed part  418 . The other end of the feed part  418  is spaced at a predetermined interval from the ground  414 . A feeding point  420  is formed at the other end of the feed part  418 . 
     The ground parallel inductor element  410  is formed to connect the second couple patch  406  to one end of the ground part  422 . The other end of the ground part  422  is connected to the ground  414 . 
     In this case, the one end of the conductor cover  402  is spaced at a predetermined interval from the first couple patch  404  connected to the feed part  418 , and the other end of the conductor cover  402  is spaced at a predetermined interval from the second couple patch  406  connected to the ground part  422 , so that the conductor cover  402  forms an electromagnetic coupling with the first couple patch  404  and the second couple patch  406 , and thus the conductor cover  402  serves as an antenna. 
     Since the conductor cover  402  is not directly connected to the main board  412  of the wireless terminal, the main board  412  of the wireless terminal is prevented from being damaged by an external surge signal, such as static electricity. That is, the conductor cover  402 , which is exposed at a side of the wireless terminal, may come into direct contact with a body of a user in use of the wireless terminal. In this case, an external surge signal, such as static electricity, may be generated at the conductor cover  402 , and if the conductor cover  402  is directly connected to the main board  412  of the wireless terminal, a circuit formed on the main board  412  may be damaged by the external surge signal. However, according to the embodiment of the present invention, the conductor cover  402  is not directly connected to the main board  412  of the wireless terminal, so that the main board  412  of the wireless terminal is prevented from being damaged even if an external surge signal is generated. 
     As described above, the conductor cover  402  is used as an antenna, radiation of the internal antenna  416  formed on the main board  412  of the wireless terminal is prevented from being degraded while maintaining the design of the wireless terminal provided by the conductor cover  401 . In addition, since an antenna is additionally formed without using a separate space in the wireless terminal, multiple antennas may be implemented while maximizing the spatial use of the wireless terminal. Since the conductor cover  402  is not directly connected to the main board  412  of the wireless terminal, the main board  412  of the wireless terminal is prevented from being damaged by an external surge signal. 
       FIG. 12  is a view illustrating an equivalent circuit of the metamaterial antenna in accordance with the fifth embodiment of the present invention. 
     Referring to  FIG. 12 , the metamaterial antenna  400  includes a transmission line TL, additional parallel capacitances C 0 , and parallel inductances L L . The transmission line TL represents the conductor cover  402 , and includes series inductances according to the length of the conductor cover  402  and parallel capacitances according to an interval between the conductor cover  402  and the ground  414 . The additional parallel capacitances C 0  represent parallel capacitance components according to an interval between the first couple patch  404  and the conductor cover  402  and an interval between the second couple patch  406  and the conductor cover  402 , and the parallel inductances L L  represent inductance components according to the feed parallel inductor element  408  and the ground parallel inductor element  410 . 
     The metamaterial antenna  400  has right-hand properties according to the transmission line (TL), that is, the series inductances and the parallel capacitances, and has left-hand properties according to the parallel inductances L L . The metamaterial antenna  100  has the above-described metamaterial properties according to the parallel inductances L L , so that the resonant frequency and the input impedance are adjusted by inductance values of the parallel inductances L L  without changing the structure of the conductor cover  402 . 
     Meanwhile, the metamaterial antenna  400  has the additional parallel capacitances C 0  connected to the parallel inductances L L  in series, thereby forming an LC series resonant circuit. Capacitance values of the additional parallel capacitances C 0  may be changed according to the sizes of the first couple patch  404  and the second couple patch  406  and the intervals between the first couple patch  404  and the second couple patch  406  and the conductor cover  402 . However, the resonant frequency of the metamaterial antenna  400  is not significantly changed even if the capacitance values of the additional parallel capacitances C 0  are changed. Therefore, it is proven that the metamaterial antenna  400  is insensitive to changes in the environments according to the first couple patch  404  and the second couple patch  406 . Details thereof will be described with reference to  FIG. 13 . 
       FIG. 13  is a graph showing a change in resonant frequency according to lengths of the first couple patch and the second couple patch of the metamaterial antenna in accordance with the fifth embodiment of the present invention. 
     A change in resonant frequency of the metamaterial antenna  400  is shown when the lengths L d1  of the first couple patch  404  and the second couple patch  406  are each increased 2 mm at a time from 5 mm to 15 mm. The following experiment is conducted under the condition that the intervals between the first couple patch  404  and the second couple patch  406  and the conductor cover  402  and the widths of the first couple patch  404  and the second couple patch  406  are not changed. In this case, as the lengths of the first couple patch  404  and the second couple patch  406  are increased, the capacitance values of the additional parallel capacitances C 0  are increased, thereby causing the resonant frequency of the metamaterial antenna  400  to be slightly decreased. 
     Referring to  FIG. 13 , when the lengths L d1  of the first couple patch  404  and the second couple patch  406  are changed from 5 mm to 15 mm, the resonant frequency is changed from 1.075 GHz to 0.95 GHz, which corresponds to 10% change of resonant frequency. Therefore, it is proven that the change in a resonant frequency is not significant when the capacitance values of the additional parallel capacitances C 0  are changed, and the metamaterial antenna  400  is insensitive to changes of environments according to the first couple patch  404  and the second couple patch  406 . 
     Although the metamaterial antenna  400  according to the fifth embodiment of the present invention is illustrated as being formed of a single unit cell, the present invention is not limited thereto. For example, a metamaterial antenna according to another embodiment of the present invention may be formed of two or more unit cells. 
     For example, referring to  FIG. 14 , when a third couple patch  424  is additionally formed at a middle portion of a side of the main board  412  of the wireless terminal, the metamaterial antenna  400  includes two unit cells  452  and  454 . In this case, the third couple patch  424  is spaced apart from the conductor cover  402 , and is connected to a ground part  428  through a second ground parallel inductor element  426 . 
     Although the metamaterial antenna  400  in  FIG. 14  is illustrated as being formed of two unit cells  452  and  454 , a metamaterial antenna according to another embodiment may include two or more unit cells. 
     When the metamaterial antenna  400  is formed of a plurality of unit cells as described above, the input impedance of the metamaterial antenna  400  is changed, thereby the input impedance of the metamaterial antenna  400  is adjusted. In detail, the more unit cells of the metamaterial antenna  400  are, the higher input impedance of the metamaterial antenna  400  is. Accordingly, when the impedance matching is poor due to a low input impedance of the metamaterial antenna  400 , the number of unit cells of the metamaterial antenna  400  is increased so as to increase the input impedance, thereby smoothly achieving the impedance matching. 
       FIG. 15  is a perspective view illustrating a metamaterial antenna in accordance with a seventh embodiment of the present invention, and  FIG. 16  is a plan view illustrating the metamaterial antenna in accordance with the seventh embodiment of the present invention. 
     Referring to  FIGS. 15 and 16 , a metamaterial antenna  500  includes a conductor cover  502 , a couple patch  504 , a feed parallel inductor element  508 , and a ground parallel inductor element  510 . 
     The couple patch  504  is provided as an integral body, and is spaced apart from the conductor cover  502  at a side of a main board  512  of a wireless terminal. Both ends of the couple patch  504  are fixed to both ends of the side of the main board  512  of the wireless terminal. For example, the couple patch  504  is formed in a parallel manner while being spaced at a predetermined interval from the conductor cover  502 . 
     The feed parallel inductor element  508  is formed to connect one end of the couple patch  504  to one end of a feed part  518 . The other end of the feed part  518  is spaced at a predetermined interval from a ground  514 . A feeding point  520  is formed at the other end of the feed part  518 . The ground parallel inductor element  510  is formed to connect the other end of the couple patch  504  to one end of a ground part  522 . The other end of the ground part  522  is connected to the ground  514 . 
     According to the embodiment of the present invention, the conductor cover  502  is electromagnetically coupled with the couple patch  504  to operate as an antenna. In this case, the conductor cover  502  is not directly connected to the main board  512  of the wireless terminal, so that even when an external surge signal is generated, the main board  512  of the wireless terminal is prevented from being damaged. 
     Meanwhile, although the metamaterial antenna shown in  FIGS. 15 and 16  is illustrated as being formed of a single unit cell, the present invention is not limited thereto. A metamaterial antenna according to another embodiment of the present invention may be formed of a plurality of unit cells. For example, the metamaterial antenna  500  may include a plurality of unit cells by additionally forming a ground parallel inductor element to connect the couple patch  504  to the ground between both ends of the couple patch  504 . 
       FIG. 17  is a perspective view illustrating an equivalent circuit of the metamaterial antenna in accordance with the seventh embodiment of the present invention. 
     Referring to  FIG. 17 , the metamaterial antenna  500  includes a first transmission line TL 1 , a second transmission line TL 2 , and parallel inductances L L . The first transmission line TL 1  represents the conductor cover  502 , the second transmission line TL 2  represents the couple patch  504 , and the parallel inductances L L  represent inductance components according to the feed parallel inductor element  508  and the ground parallel inductor element  510 . In this case, the first transmission line TL 1  is electromagnetically coupled to the second transmission line TL 2 . 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined in the claims and their equivalents.