Abstract:
The present invention is related to a coplanar antenna unit and a coplanar antenna. By utilizing the unique properties of meta-material to design 1-D balanced CRLH leaky-wave antenna. The antenna can be realized with the coplanar antenna unit consisting of MIM capacitor and grounded inductor. In this invention, all proposed elements are implemented by planar print circuit broad, so the full-space switched beam scanning antenna has shorter length of leaky-wave antenna and good radiation performance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a coplanar antenna unit and a coplanar antenna, in particular to a coplanar antenna unit of a leaky-wave antenna and a coplanar antenna of a leaky-wave antenna. 
     2. Description of the Related Art 
     Leaky-wave antenna has a significant contribution to the development of wireless systems. Antenna capable of integrating a composite right/left-handed (CRLH) material with wireless communications is one of the popular research subjects in recent years. Therefore, a composite right/left-handed (CRLH) structure is one of the best techniques used in these popular applications. At present, methods of designing a leaky-wave antenna are mainly divided into the following types: 
     1. Using a periodic structure: A periodic structure such as a dielectric grid, a metal plate grid and a groove grid of a metal plate is formed by a space harmonic wave produced by a periodical effect. 
     2. Using an opening waveguide: An opening waveguide such as a corrugated waveguide, a non-radiation medium waveguide and a micro strip is operated at a high-order mode to achieve the functions of a leaky-wave antenna. 
     3. Integrating with a meta-material: Both radiation region and waveguide region are provided in a general mode, such that when the operating frequency falls within the radiation region, a leaky-wave antenna is achieved. 
     Although the prior art can achieve the effect of a leaky-wave antenna, most energies in the structure are leaked to the space to achieve high gain and antenna efficiency. To achieve this effect, manufacturers use a leaky-wave antenna made of a periodic structure, an opening waveguide or a meta-material with a sufficient length to transmit a large portion of energies into the space. To cope with the mainstream of miniaturization and meet the requirements of an integrated communication system, it is very important to minimize the volume of the leaky-wave antenna. 
     SUMMARY OF THE INVENTION 
     Therefore, it is a primary objective of the present invention to overcome the shortcomings of the prior art by providing a coplanar antenna unit and a coplanar antenna and designing a miniaturized CLRH leaky-wave antenna. For a communication system, such antenna can be integrated easily with a wireless communication application to improve the development of integrated communication systems in the wireless communication industry and promote the future communication technologies. 
     To achieve the foregoing objective, the present invention provides a coplanar antenna unit, comprising a substrate, a radiation plane, a first ground plane, a second ground plane and a metal plate. The radiation plane is disposed on a first side of the substrate, and includes a feed plane and an output plane. The feed plane is provided for receiving a feed-in signal, and the feed-in signal generates a radiation signal at the feed plane. The output plane has an interval from the feed plane, and the radiation signal is coupled to the output plane, such that the output plane may output a radiation signal, and the output plane includes a first extension portion and a second extension portion. The first ground plane is disposed on the first side of the substrate and situated on a first side of the radiation plane, and the first ground plane has a first hollow portion, and the first extension portion disposed in the first hollow portion, and an end of the first extension portion is coupled to the first ground plane. The second ground plane is disposed on the first side of the substrate and situated on a second side of the radiation plane, and the second ground plane has a second hollow portion at a position corresponding to the first hollow portion, and the second extension portion is disposed in the second hollow portion, and an end of the second extension portion is connected to the second ground plane. The metal plate is disposed on a second side of the substrate and the metal plate is disposed corresponding to the radiation plane. 
     A capacitor structure is formed by the radiation plane and the metal plate, and the capacitor structure defines an equivalent left handed capacitor. 
     A metal-insulator-metal (MIM) capacitor structure is formed by the radiation plane, the substrate and the metal plate. 
     A plurality of coplanar antenna units are coupled in series to form a coplanar antenna. 
     The coplanar antenna unit has a balanced frequency determined by the size of the first extension portion, the second extension portion or the metal plate. 
     The via structures are disposed onto the feed plane and the metal plate respectively, and the positions of the via structures on the feed plane are corresponding to the positions of the via structures of the metal plate. 
     The metal plate is in a square, triangular, circular, pentagonal or hexagonal shape. 
     The first extension portion or second extension portion defines an equivalent circuit of a left handed inductor. 
     Another objective of the present invention is to provide a coplanar antenna, comprising a plurality of coplanar antenna units connected with each other in series, and each of the plurality of coplanar antenna units comprises: a substrate, a radiation plane, a first ground plane, a second ground plane and a metal plate. The radiation plane is disposed on a first side of the substrate, and includes a feed plane and an output plane. The feed plane is provided for receiving a feed-in signal, and the feed-in signal generates a radiation signal at the feed plane. The output plane has an interval from the feed plane, and the radiation signal is coupled to the output plane, such that the output plane may output a radiation signal, and the output plane includes a first extension portion and a second extension portion. The first ground plane is disposed on the first side of the substrate and situated on the first side of the radiation plane, and the first ground plane has a first hollow portion, and the first extension portion is disposed in the first hollow portion, and an end of the first extension portion is coupled to the first ground plane. The second ground plane is disposed on a side of the substrate and situated on another side of the radiation plane, and the second ground plane has a second hollow portion at a position corresponding to the first hollow portion, and the second extension portion is disposed in the second hollow portion, and an end of the second extension portion is connected to the second ground plane. The metal plate is disposed on a second side of the substrate and the metal plate is disposed corresponding to the radiation plane. 
     A capacitor structure is formed by the radiation plane and the metal plate, and the capacitor structure defines an equivalent left handed capacitor. 
     A metal-insulator-metal (MIM) capacitor structure is formed by the radiation plane, the substrate and the metal plate. 
     The plurality of coplanar antenna units are serially coupled. 
     The coplanar antenna unit has a balanced frequency determined by the size of the first extension portion, the second extension portion or the metal plate. 
     The via structures are disposed onto the feed plane and the metal plate respectively, and the positions of the via structures on the feed plane are corresponding to the positions of the via structures of the metal plate 
     The metal plate is in a square, triangular, circular, pentagonal or hexagonal shape. 
     The first extension portion or second extension portion defines the equivalent circuit of a left handed inductor. 
     Four or more coplanar antenna units are serially coupled. 
     The coplanar antenna formed by coupling five coplanar antenna units in series has a leak energy quantity of 90%. 
     The coplanar antenna formed by coupling five coplanar antenna units in series has a balanced frequency of 3.5 GHz. 
     To achieve another objective, the present invention provides a coplanar antenna unit and a coplanar antenna, and designs a miniaturized leaky-wave antenna to achieve an application integrated with the wireless communication easily, so as to overcome the problem of the conventional CRLH leaky-wave antenna having a long structure for the radiation of energy. The concept of miniaturizing the CLRH leaky-wave antenna may be applied to other frequencies in other communication specifications. The miniaturization may meet the requirements of a wireless communication system and an integrated communication system in the future. 
     In summation, the coplanar antenna unit and the coplanar antenna of the present invention have one or more of the following advantages: 
     (1) The coplanar antenna unit and the coplanar antenna have a frequency sweep property, and are capable of performing a continuous scanning from backward to broadside and then to forward, with two main beam directions. 
     (2) The coplanar antenna unit and the coplanar antenna adopt the metal-insulator-metal capacitor made and the grounded inductor made of a meta-material to design serially connected capacitors and parallely connected inductors made of a left handed material respectively. 
     (3) The coplanar antenna unit and the coplanar antenna use the coplanar waveguide and the metal-insulator-metal capacitor having a larger capacitance in the same area to overcome the issue of the prior art having a low leakage constant. 
     (4) The coplanar antenna unit and the coplanar antenna use another layer of a coplanar waveguide to achieve the effect of a metal-insulator-metal capacitor without requiring additional substrate in order to achieve a low profile. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a coplanar antenna unit of the present invention; 
         FIG. 2  is a top view of a coplanar antenna unit of the present invention; 
         FIG. 3  is a bottom view of a coplanar antenna unit of the present invention; 
         FIG. 4  is a top view of a coplanar antenna of the present invention; 
         FIG. 5  is a bottom view of a coplanar antenna of the present invention; 
         FIG. 6  is a graph of leakage constant versus number of coplanar antenna units of the present invention; 
         FIG. 7  is a graph of return loss versus frequency response of the present invention; and 
         FIG. 8  is a schematic view of simulations of far field radiation patterns on the X-Z plane of a coplanar antenna in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The technical characteristics of the present invention will become apparent with the detailed description of the following preferred embodiments and related drawings. 
     With reference to  FIG. 1  for a side view of a coplanar antenna unit of the present invention, the coplanar antenna unit  1  comprises a substrate  11 , a radiation plane  12 , a first ground plane  13 , a second ground plane  14  and a metal plate  15 . The radiation plane  12 , first ground plane  13  and second ground plane  14  are disposed on the first side of the substrate  11 , and the metal plate  15  is disposed on the second side of the substrate and at a position corresponding to the radiation plane  12 . 
     With reference to  FIGS. 2 and 3  for a top view and a bottom view of a coplanar antenna unit of the present invention respectively, the radiation plane  12  includes a feed plane  121  and an output plane  122 . The feed plane  121  is provided for receiving a feed-in signal, and a radiation signal is generated according to the feed-in signal at the feed plane  121 . If the feed-in signal at the feed plane  121  has accumulated radiation energies to a certain extent to generate radiation signals at an edge of the feed plane  121 . The output plane  13  has an interval d from the feed plane  121 , and thus the radiation signal is coupled to the output plane  122 , such that the output plane  122  may output the radiation signals. The output plane  122  includes a first extension portion  1221  and a second extension portion  1222 . The first ground plane  13  is disposed on the first side of the radiation plane  12 , and the first ground plane  13  has a first hollow portion  131 . The second ground plane  14  is disposed on the second side of the radiation plane  12 , and the second ground plane includes a second hollow portion  141  corresponding to the first hollow portion  131 . The first extension portion  1221  is disposed in the first hollow portion  131 , and an end of the first extension portion  1221  is coupled to the first ground plane  13 , and the second extension portion  1222  is disposed in the second hollow portion  141 , and an end of the second extension portion  1222  is coupled to the second ground plane  14 . 
     If the radiation plane  12  is disposed on the first side of the substrate  11 , and the metal plate  15  is disposed on the second side of the substrate  11 , and the substrate  11  is made of a dielectric material, we will find that the structure is a MIM structure, and thus the equivalent circuit is a MIM capacitor. In a preferred embodiment, a RogersRT/Duriod5880 substrate is used, and this substrate has a dielectric constant of 2.2 and a loss tangent of 0.0009, indicating a very low loss. 
     The feed plane  121  and the metal plate  15  have vias  16  disposed thereon and corresponding to each other, and thus the feed-in signals may be transmitted to the metal plate  15  through the vias  16 . In this preferred embodiment, four vias are used, but not limited to such arrangement only, and the balanced frequency will be affected by the number of the vias and size of the vias. The equivalent circuit formed by the radiation plane  12  and the metal plate  15  is a capacitor structure. If both radiation plane  12  and metal plate  15  are made of meta-materials, this capacitor structure has a left handed property, and is called a left handed capacitor. 
     The output plane  122  includes a first extension portion  1221  and a second extension portion  1222 , whose equivalent circuit is an inductor, such that the size of the first extension portion  1221  and the second extension portion  1222  may be adjusted to change the properties of the coplanar antenna unit  1 . If the output plane  122  is made of a meta-material, the inductor of the output plane  22  has the left handed properties, and thus the equivalent circuit has a left handed inductor thereon. 
     The coplanar antenna unit  1  has a balanced frequency determined by the size of the first extension portion  1221 , the second extension portion  1222  or the metal plate  15 . In  FIG. 2 , the width of the first extension portion  1221  and the second extension portion  1222  is Ws. Similarly, the size of the hollow portion is affected by the length and width of the first extension portion  1221  and the second extension portion  1222 , and the balanced frequency may be determined by changing the size of length and width (Ls, Ws) of the hollow portion. The balanced frequency may be changed according to the size of the metal plate, the length and width (such as d 1 , d, Lc) and the geometric shape. The metal plate  15  may be in a square, triangular, circular, pentagonal or hexagonal shape. The invention is not limited to these shapes only, but any other geometric shape can be used. 
     With reference to  FIGS. 4 and 5  for a top view and a bottom view of a coplanar antenna of the present invention respectively, the coplanar antenna  4  includes five coplanar antenna units connected with each other in series, and the coplanar antenna units have been described above, and will not be described here again. The coplanar antenna further includes a sub miniature version A (SMA) connector for connecting the first coplanar antenna unit with a feed-in signal through the SMA connector, and outputting a radiation signal through the last coplanar antenna unit through the SMA connector. 
     With reference to  FIG. 6  for a graph of leakage constant versus number of coplanar antenna units of the present invention, the leakage constant tends to be convergent if the balanced frequency is 3.5 GHz, and the ratio of remaining energy at an end to energy at an output terminal is less than 0.1, and four or more coplanar antenna units are connected with each other in series. Therefore, a coplanar antenna having four or five coplanar antenna units is a better choice. 
     With reference to  FIG. 7  for a graph of return loss versus frequency response of the present invention, two curves are shown in the graph, wherein one curve shows a simulated value, and the other curve is an actual measured value. If the operating frequency is 3.5 GHz, then the return loss is approximately −26 dB. If the operating frequency is 3.9 GHz, then the return loss is approximately −29 dB. 
     With reference to  FIG. 8  for a schematic view of simulations of far field radiation patterns on the X-Z plane of a coplanar antenna in accordance with the present invention, the operating frequencies are 2.75 GHz, 3.5 GHz and 3.9 GHz. If the operating frequency is 2.75 GHz, the antenna is operated within a left handed leak wave region, and main beam directions are 24° and 155°, and gain values are 4.5 dBi and 4.8 dBi respectively. If the operating frequency is 3.5 GHz, the antenna is operated at a leak wave region at a balanced frequency, and the simulated main beam directions are −1° and 179°, and gain values are 4.9 dBi and 5.6 dBi respectively. If the operating frequency is 3.9 GHz, the antenna is operated at a right handed leak wave region, and main beam directions are −39° and −142°, the simulated gain values are 7 dBi and 7.1 dBi respectively. 
     If the operating frequencies of the present invention are 2.75 GHz, 3.5 GHz and 3.9 GHz, the main beam directions under the substrate are 155°, 179° and −142° respectively, and the main beam directions above the substrate are 24°, −1° and −39° respectively, show a frequency sweep property, which is a unique property of a leaky-wave antenna, and thus we are sure that the antenna is a leaky-wave antenna, caused by the radiation property of the CLRH inductor, whose total scanning angle is 63°, and the total scanning angle above the substrate is 63°. 
     The present CRLH leaky-wave antenna and the conventional leaky-wave antenna have a length of 4˜5 wavelength, due to a low leakage constant, and a miniaturized CLRH leaky-wave antenna of the present invention is designed according to such requirement, so as to produce the coplanar antenna of the present invention. The aforementioned applications adopt a micro-strip structure, so that the leakage constant is low, since there is only one radiation plane, and most of the energies are stored in the media and cannot be leaked to the space. Therefore, the coplanar antenna unit (or unit cell) of the present invention has the property of a higher leakage constant. 
     The coplanar antenna unit uses a coplanar waveguide (CPW) inductor structure and the coplanar antenna unit having a shorter overall electric length to achieve the higher leakage constant. Due to the higher leakage constant of the coplanar antenna unit, the serially or parallely connected resistors in the equivalent circuit cannot be ignored. The coplanar antenna of the present invention aims at the objective of achieving a high leakage constant, and overcoming the drawback of having a too-long CRLH leaky-wave antenna by the coplanar waveguide and the MIM capacitor.