Abstract:
A flat cable includes a signal line extending in a longitudinal direction, a thin dielectric sheet with which the signal line is coated and that has plasticity, a pair of spaced apart ground layers extending in the longitudinal direction and sandwiching the dielectric sheet in its thickness direction, and insulators that coat the pair of ground layers so that they are not exposed to the outside. The cross-sectional size of the signal line in a direction orthogonal to the longitudinal direction, the thickness and width of the dielectric sheet, and so forth are selected to obtain a predetermined characteristic impedance for the cable. Each of the pair of ground layers is sized so as to be substantially wider than the signal line.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority from Japanese Application No. 2004-065146 filed Mar. 9, 2004, the disclosure of which is hereby incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a flat cable and a flat cable producing method, in particular, to a flat cable that can be produced at low cost and that can be densely mounted.  
         [0003]     In recent years, as various types of electronic devices that generate radio frequency signals have been developed, their use has become widespread. As a result, many electronic apparatuses are being used in offices and homes. These electronic apparatuses use coaxial cables as radio frequency signal cables.  
         [0004]      FIG. 1  shows the structure of a conventional coaxial cable. Disposed at the center of a coaxial cable  120  is a signal line  121 . Disposed around the signal line  121  is a dielectric substance  122 . Disposed outmost around the dielectric substance  122  is a ground layer  123 . The outermost periphery of the coaxial cable is coated with an insulator  124 . Since the cross-section of the coaxial cable  120  is circular, it cannot be flattened. Consequently, the coaxial cable  120  has a large diameter. Thus, the coaxial cable  120  cannot be densely mounted. In addition, since these layers should be cylindrically formed and deposited, a complicated production process is required. It is therefore difficult to decrease the production cost of the coaxial cable  120 .  
         [0005]     To solve the foregoing problem, Japanese patent laid-open publication No. 2001-135974 and Japanese patent laid-open publication No. HEI 11-162267 propose structures for mounting a plurality of signal lines in flat cables using liquid crystal polymer.  
         [0006]     However, the flat cables disclosed in these Japanese patent publications cannot be suitably used for transmitting radio frequency signals. In this case, the sizes of the cross-sections of the signal lines, the thicknesses of the dielectric substances, and so forth should be adjusted so that a predetermined characteristic impedance can be obtained and insertion loss can be decreased. In addition, the ground layer should be sufficiently wider than the signal line so as to prevent signals from leaking out of the cable.  
         [0007]     In addition, Japanese patent laid-open publication No. 2002-111233 discloses a method for forming a radio frequency transmission line on a printed circuit board. In this method, however, since the transmission lines cannot be freely bent, the method cannot be used for cables.  
       SUMMARY OF THE INVENTION  
       [0008]     Therefore, an object of the present invention is to provide a flat cable that can be flexibly wired. Another object of the present invention is to adjust the size of the cross-section of a signal line for a designated characteristic impedance and to provide a flat cable having a sufficiently wider ground layer than the signal line.  
         [0009]     In addition, a further object of the present invention is to provide a flat cable that can be produced at low cost.  
         [0010]     A first aspect of the present invention is a flat cable, including a signal line extending in a longitudinal direction and having an outer periphery; a dielectric sheet extending in the longitudinal direction to surround the outer periphery of the signal line, the dielectric sheet having a dimension in a width direction orthogonal to the length direction and a dimension in a thickness direction orthogonal to the longitudinal direction; a pair of spaced apart ground layers extending in the longitudinal direction and sandwiching the dielectric sheet in the thickness direction; and a first insulator extending in the longitudinal direction to coat the pair of ground layers so that the pair of ground layers are not exposed to the outside.  
         [0011]     A second aspect of the present invention is a flat cable, including a dielectric sheet extending in a longitudinal direction; a first ground layer formed on the dielectric sheet and extending substantially in the longitudinal direction; a second ground layer formed on the dielectric sheet and extending substantially in the longitudinal direction, the second ground layer being spaced apart from the first ground layer; a signal line formed in the dielectric sheet and extending substantially in the longitudinal direction, the signal line being formed between and spaced apart from the first and second ground layers; a first insulator formed on a first side of the dielectric sheet so as to cover the signal line, the first ground layer and the side of the dielectric sheet opposite the first side.  
         [0012]     A third aspect of the present invention is a flat cable sheet, including a plurality of spaced apart signal lines extending in a longitudinal direction, each signal line having an outer periphery; a dielectric sheet extending in the longitudinal direction to surround the outer periphery of each of the signal lines, the dielectric sheet having a dimension in a width direction orthogonal to the longitudinal direction and a dimension in a thickness direction orthogonal to the longitudinal direction; a pair of spaced apart ground layers extending in the longitudinal direction and sandwiching the dielectric sheet in the thickness direction; and an insulator extending in the longitudinal direction to coat the pair of ground layers so that the pair of ground layers are not exposed to the outside.  
         [0013]     A fourth aspect of the present invention is a flat cable sheet, including a plurality of spaced apart signal lines extending in a longitudinal direction, each signal line having an outer periphery; a dielectric sheet extending in the longitudinal direction to surround the outer periphery of each of the signal lines, the dielectric sheet having a dimension in a width direction orthogonal to the longitudinal direction and a dimension in a thickness direction orthogonal to the longitudinal direction; a pair of spaced apart ground layers extending in the longitudinal direction and sandwiching the dielectric sheet in the thickness direction; a first insulator extending in the longitudinal direction to coat the pair of ground layers so that the pair of ground layers are not exposed to the outside; a pair of spaced apart shield layers extending in the longitudinal direction and sandwiching the first insulator in the thickness direction; and a second insulator extending in the longitudinal direction to coat the pair of shield layers so that the pair of shield layers are not exposed to the outside.  
         [0014]     A fifth aspect of the present invention is a method for producing a flat cable sheet, including providing a first dielectric layer; depositing a first metal film on a first surface of the first dielectric layer; etching the first metal film to define a plurality of signal lines that extend substantially parallel to one another in a longitudinal direction; depositing a second dielectric layer on an exposed surface of the etched metal film; depositing a second metal film over the second dielectric layer; etching the second metal film to define a first plurality of spaced apart ground layers extending in the longitudinal direction, each of the ground layers in the first plurality of ground layers overlying one of the signal lines; depositing a first insulator on an exposed surface of each of the ground layers in the first plurality of ground layers; depositing a third metal film on a second surface of the first dielectric layer opposite the first surface; etching the third metal film to define a second plurality of spaced apart ground layers extending in the longitudinal direction, each of the ground layers in the second plurality of ground layers underlying one of the signal lines; and depositing a second insulator on an exposed surface of each of the ground layers in the second plurality of ground layers.  
         [0015]     According to the present invention, the size of the cross-section of a signal line, the thickness of the dielectric layer and so forth are selected to obtain a predetermined characteristic impedance for the cable. In addition, a flat cable composed of a ground layer that is sufficiently wider than a signal line and a dielectric sheet that has plasticity can be produced at low cost. When such a flat cable is used for an electronic apparatus, it can be miniaturized.  
         [0016]     In a small mobile apparatus that has a radio communication function, for example, a personal digital assistant, an antenna is disposed at an upper portion of a liquid crystal display (inside a liquid crystal panel) so as to increase signal transmission/reception sensitivity against an access point. A radio communication module may be disposed below a keyboard. The flat cable according to the present invention can be used to connect the antenna and the radio communication module. A radio frequency signal as high as 2.4 GHz is transmitted between the antenna and the radio communication module. In recent years, although mobile apparatuses have been miniaturized, with this flat cable, the radio communication function can be mounted in a small space of a mobile apparatus.  
         [0017]     In addition, since the flat cable according to the present invention is a ribbon type cable, it needs a small space to mount. With the flat cable, a liquid crystal panel can be bent. Moreover, the flat cable can be mounted in a very limited space.  
         [0018]     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment of the invention, as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein similar reference numerals denote similar portions, in which:  
         [0020]      FIG. 1  is a perspective view showing the structure of a conventional coaxial cable;  
         [0021]      FIG. 2  is a perspective view showing the structure of a flat cable according to a first embodiment of the present invention;  
         [0022]      FIG. 3  is an exploded perspective view showing the structure of a strip line;  
         [0023]      FIG. 4A ,  FIG. 4B  and  FIG. 4C  are sectional views showing a method for producing the flat cable according to the first embodiment of the present invention;  
         [0024]      FIG. 5 a  perspective view showing the method for producing the flat cable according to the first embodiment of the present invention;  
         [0025]      FIG. 6  is a perspective view showing a method for producing a flat cable according to a second embodiment of the present invention;  
         [0026]      FIG. 7  is a perspective view showing the structure of a flat cable according to a third embodiment of the present invention;  
         [0027]      FIG. 8  is a perspective view showing the structure of a coplanar line;  
         [0028]      FIG. 9  is a perspective exploded view showing the structure of a flat cable according to a fourth embodiment of the present invention;  
         [0029]      FIG. 10  is a sectional view showing the flat cable according to the fourth embodiment of the present invention, viewed from another direction;  
         [0030]      FIG. 11A  and  FIG. 11B  are front and side views, respectively, showing the structure of a flat cable according to a fifth embodiment of the present invention;  
         [0031]      FIG. 12A  and  FIG. 12B  are front and sectional views, respectively, showing the structure of a flat cable according to a sixth embodiment of the present invention;  
         [0032]      FIG. 13A ,  FIG. 13B  and  FIG. 13C  are sectional views showing the structure of the flat cable according to the sixth embodiment of the present invention;  
         [0033]      FIG. 14A  and  FIG. 14B  are front and sectional views, respectively, showing the structure of a flat cable according to a seventh embodiment of the present invention; and  
         [0034]      FIG. 15A ,  FIG. 15B  and  FIG. 15C  are sectional views showing the structure of the flat cable according to the seventh embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0035]     A flat cable according to the present invention is a transmission line that transmits a radio frequency signal and that is produced by forming a signal line in or on the front surface of a bendable (flexible) dielectric substance (sheet), such as a liquid crystal polymer or Teflon (trademark of E.I. Du Pont de Nemours and Company) substrate, and forming a ground layer made of a metal spaced from the signal line by the dielectric substance. Alternatively, two ground layers may be formed on the front surface of the dielectric sheet with a signal line between the two ground layers.  
         [0036]     To transmit a radio frequency signal with a small transmission loss, the characteristic impedance of the signal transmission line needs to be a predetermined value, for example 50 Ω. The characteristic impedance of the transmission line depends on the shape of the signal line, the relative dielectric constant of the dielectric substance, and so forth. To prevent a signal from leaking out of the cable, the ground layer needs to be sufficiently wider than the signal line. To suppress the radiation of a signal from the cable and the influence of external electromagnetic noise against the signal line, it is effective to coat a transmission line in which the signal line and the ground line are paired with a shield layer made of a metal.  
         [0037]     Next, embodiments of the present invention will be described. These embodiments have been made in consideration of the foregoing conditions.  
         [0000]     (First Embodiment)  
         [0038]      FIG. 2  shows the structure of a flat cable according to a first embodiment of the present invention. In  FIG. 2 , a cable  10  is a radio frequency cable that has a strip line structure. Since this cable is flat, it can be more flattened than conventional coaxial cables. In addition, when the dielectric substance is thinned and the ground layer is sufficiently wider than the signal line, the radiation of a signal from the side portion free of the ground layer can be suppressed. The characteristic impedance depends on the size of the cross-section of the signal line, the specific dielectric constant of the dielectric substance, and so forth. In this example, the flat cable is designated to have a characteristic impedance of 50 Ω.  
         [0039]     More particularly, the cable  10  is structured so that a signal line  11  is coated with a thin dielectric sheet  12  and ground layers  13  are formed on an upper surface and a lower surface of the dielectric sheet  12 , the ground layers  13  being sufficiently wider than the signal line  11 . To prevent a current from unnecessarily shortcircuiting through the ground layers  13 , the upper and lower surface of the cable are coated with films of an insulator  14 . The two ground layers are coated with two films of the insulator  14  so that the ground layers are not exposed to the outside. Thus, the side portions of the cable  10  are composed of the dielectric sheet  12  and the insulator  14 .  
         [0040]     The dielectric sheet  12  is made of a material having plasticity. Thus, since the cable  10  can be relatively freely bent, it can be used for a complicated line and an open/close mechanism.  
         [0041]     Next, a method for obtaining the characteristic impedance of a strip line such as the cable  10  according to the first embodiment will be described. As described above, the cable  10  is designed to have a characteristic impedance of, for example, 50 Ω.  FIG. 3  schematically shows the structure of a strip line. A strip line  20  is composed of a signal line  21 , a dielectric sheet  22 , and upper and lower ground layers  23 . The width of each of the ground layers  23  is denoted by w, the height of the dielectric sheet  22  is denoted by h, the width of the cross-section of the signal line  21  is denoted by a, the height thereof is denoted by b, and the relative dielectric constant of the dielectric sheet  22  is denoted by ε r .  
         [0042]     If the width w of the ground layer  23  is sufficiently larger than the width a of the cross-section of the signal line  21 , the characteristic impedance Z 0  can be approximately represented by the following formula 1. 
 
 Z   0 =(60/ε r ) 1/2 )  ln  (4 h /(0.67  πa  (0.8+( b/a ))))   Formula 1 
 
         [0043]      FIG. 4A ,  FIG. 4B ,  FIG. 4C  and  FIG. 5  are sectional views showing a method for producing the flat cable according to the first embodiment of the present invention. In  FIG. 4A , the signal line  11  is accurately formed by an etching process or the like. The upper and lower surfaces of the signal line  11  are coated with the dielectric sheets  12  and metal films. The material of the signal line  11  is, for example, copper.  
         [0044]     Next, as shown in  FIG. 4B , the metal films are processed using an etching process or the like so as to form the ground layers  13 . As described above, the ground layers  13  are processed so that each of them is sufficiently wider than the signal line  11 .  
         [0045]     Finally, as shown in  FIG. 4C , the insulators  14  are formed on the upper and lower ground layers  13 . As a result, a flat cable sheet  30  having a plurality of cables is produced.  
         [0046]     Thereafter, the flat cable sheet  30  produced as shown in  FIG. 4A  to  FIG. 4C  is cut along line A-B shown in  FIG. 5  several times. As a result, a plurality of flat cables  10  are obtained. In this method, radio frequency cables having excellent characteristics can be produced in quantity at low cost. It is preferred that each of the ground layers  13  should be narrower than the cut interval so that the ground layers  13  are not cut.  
         [0000]     (Second Embodiment)  
         [0047]     Next, with reference to  FIG. 6 , a flat cable according to a second embodiment of the present invention will be described. A cable  40  shown in  FIG. 6  contains a signal line  41 , a dielectric sheet  42 , upper and lower ground layers  43 , upper and lower shield layers  44 , and upper and lower insulators  45 . The signal line  41  is coated with the dielectric sheet  42 . The upper and lower ground layers  43  are formed on the upper and lower surfaces of the dielectric sheet  42 , respectively. Each of the ground layers  43  is sufficiently wider than the signal line  41 . The upper and lower ground layers  43  are coated with the upper and lower insulators  45 , respectively. The upper and lower shield layers  44  are formed on the upper and lower insulators  45 , respectively. The upper and lower shield layers  44  are coated with the upper and lower insulators  45 , respectively.  
         [0048]     According to the second embodiment, the shield layers  44  and the insulators  45  are formed on the upper and lower surfaces of the cable  10  of the first embodiment. With the cable  40 , the radiation of a signal is more suppressed than with the cable  10  of the first embodiment. Thus, the influence of external electromagnetic noise against the signal line can be more suppressed than in the first embodiment. In addition, the ground layers  43  and the shield layers  44  are not exposed to the outside. Thus, the side portions of the cable  40  are composed of the dielectric sheet  42  and the insulator  45 .  
         [0049]     The cable  40  is produced in the same method shown in  FIG. 4A  to  FIG. 4C  and  FIG. 5 , except that after the flat cable sheet  30  shown in  FIG. 4A  to  FIG. 4C  is produced, the shield layers  44  are formed and etched and then the outermost insulators  45  are formed. The dielectric sheet  42  is made of a material having, for example, plasticity.  
         [0000]     (Third Embodiment)  
         [0050]     Next, with reference to  FIG. 7 , a flat cable according to a third embodiment of the present invention will be described. A cable  50  shown in  FIG. 7  is a cable having a coplanar structure in which a signal line  51  and two ground layers  53  are formed on the same plane (i.e., the surface of a dielectric sheet  52 ). Since the signal line  51  and the two ground layers  53  are formed on the same plane, namely on the dielectric sheet  52 , the structure of this cable becomes simpler and it can be produced at lower cost than the foregoing cables.  
         [0051]     The cable  50  is composed of a signal line  51 , a dielectric sheet  52 , two ground layers  53 , and upper and lower insulators  54 . As described above, the signal line  51  and the two ground layers  53  are formed almost in parallel in the longitudinal direction of the cable  50  so that the signal line  51  does not contact the two ground layers  53 . In addition, the two ground layers  53  are formed on both sides of the signal line  51 . In the cross-section perpendicular to the longitudinal direction of the cable  50 , each of the ground layers  53  is sufficiently wider than the signal line  51 .  
         [0052]     The upper and lower surfaces of the signal line  51 , the dielectric sheet  52 , and the two ground layers  53  are coated with the upper and lower insulators  54 , respectively.  
         [0053]     The cable  50  can be produced in the same method as the foregoing embodiments shown in  FIG. 4A  to  FIG. 4C , and  FIG. 5 . In this case, the signal line  51  and the two ground layers  53  are formed and etched in the same process as the foregoing embodiments. The dielectric sheet  52  is made of a material having, for example, plasticity.  
         [0054]     The characteristic impedance of a coplanar line (or coplanar waveguide CPW) depends on the relative dielectric constant of the dielectric sheet that is used, the thickness and width of the conductor that is used, and so forth. When a dielectric sheet having a high relative dielectric constant is used, a miniaturized circuit can be accomplished. A coplanar waveguide  60  shown in  FIG. 8  has the same structure as the cable  50  of the third embodiment. The coplanar waveguide  60  is composed of a signal line  61 , a dielectric sheet  62 , two ground layers  63 , and an insulator  64 . The relative dielectric constant of the dielectric sheet  62  is denoted by ε r , the thickness of the dielectric sheet  62  is denoted by h, the width of the cross-section of the signal line  61  (the width of the waveguide) is denoted by s, and the width between the signal line  61  and the ground layers  63  is denoted by w.  
         [0055]     In this case, the characteristic impedance Z 0  can be approximately expressed by a predetermined formula based on these values. Alternatively, the characteristic impedance Z 0  can be calculated using a predetermined simulator.  
         [0000]     (Fourth Embodiment)  
         [0056]     Next, with reference to  FIG. 9 , a flat cable according to a fourth embodiment of the present invention will be described. A cable  70  shown in  FIG. 9  is part of an end portion (terminal portion) of a flat cable. The cable  70  is composed of a signal line  71 , a dielectric sheet  72 , upper and lower ground layers  73 , and upper and lower insulators  74 . The cable  70  has four through-holes  75  and one through-hole  76 . Although the upper and lower ground layers  73  are exposed on the side portions of the cable  70 , one of the flat cables of the first to third embodiments can be used.  
         [0057]     An end portion of the upper ground layer  73  is not coated with the upper insulator  74  so that the end portion of the upper ground layer  73  can be electrically connected to a circuit board. The four through-holes  75  electrically connect the upper and lower ground layers  73 . The through-hole  76  is formed as a terminal with which a signal from the signal line  71  may be connected to the outside. A terminal is disposed above the cable  70  shown in  FIG. 9 . In this example, four through-holes  75  are formed. However, the number of through-holes  75  is not limited to four. The through-holes  75  are formed so that the potentials of the upper and lower ground layers  73  become equal.  
         [0058]     The through-holes can be formed by various methods. In one method, holes are made in two ground layers that sandwich a dielectric sheet having through-holes aligned with the holes in the ground layers. The aligned holes are filled with electro-conductive paste (for example, silver paste or copper paste) so as to electrically connect the two ground layers. In another method, the walls of the aligned holes are plated with an electro-conductive substance so as to electrically connect the two ground layers. In the example shown in  FIG. 9 , the first method is used.  
         [0059]     The cable  70  can be produced in the same method as the first embodiment shown in  FIG. 4A  to  FIG. 4C  and  FIG. 5 . The through-holes  75  and the through-holes  76  are formed by a single process. The dielectric sheet  72  is made of a material having, for example, plasticity.  
         [0060]      FIG. 10  is a sectional view seen in the direction of arrow A shown in  FIG. 9 . The through-holes  75  extend from the upper ground layer  73  to the lower ground layer  73 . The through-holes  75  electrically connect the upper ground layer  73  and the lower ground layer  73 . Although the through-hole  76  extends from the upper ground layer  73  to the lower ground layer, a space portion  80  that is concentrically cut from the upper ground layer  73  around the through-hole  76  keeps it apart from the upper ground layer  73 . A space portion  81  that is concentrically cut from the lower ground layer  73  around the through-hole  76  keeps it apart from the lower ground layer  73 . Alternatively, the space portion  81  may be formed in the same shape as the space portion  80 .  
         [0061]     The through-hole  76  is connected to the signal line  71 . In  FIG. 10 , the signal line  71  extends from the deeper side to the through-hole  76 . With the cable  70  that has such a structure, by connecting a ground of a circuit board to any portion of the upper ground layer  73  external to the space portion  80  and connecting a signal input/output portion of the circuit board to any portion of the space portion  80  of the ground layer  73  interior of the space portion  80 , the circuit board and the cable  70  are electrically connected. These connections are performed by, for example, soldering. Alternatively, the circuit board and the cable  70  can be mechanically contacted or connected by, for example, clamping.  
         [0000]     (Fifth Embodiment)  
         [0062]     Next, with reference to  FIG. 11A  and  FIG. 11B , a flat cable according to a fifth embodiment of the present invention will be described.  FIG. 11A  and  FIG. 11B  show a cable  85  according to the present invention along with a connector  90  electrically connected to the cable  85 .  FIG. 11A  is a front view showing the cable  85  and the connector  90 .  FIG. 11B  is a side view showing the cable  85  and the connector  90 .  
         [0063]     The connector  90  is connected to an end portion of the cable  85  as shown in  FIG. 11A  and  FIG. 11B . A ground terminal  91  of the connector  90  is connected to a ground layer  88  of the cable  85  by, for example, clamping. It is preferred that the ground terminal  91  be connected to two ground layers  88  so that the potentials of the two ground layers  88  become equal. As with the fourth embodiment, through-holes that connect the two ground layers may be formed adjacent to the connector  90 .  
         [0064]     A mating connector that fits the connector  90  is disposed on a circuit board. When these connectors are connected, the cable  85  and the circuit board can be easily connected.  
         [0065]     By inserting the cable  85  into the connector  90  (in the direction of arrow B shown in  FIG. 11A ), the cable  85  and the connector  90  may be electrically connected. In this case, the cable  85  and the connector  90  may be disconnectable.  
         [0000]     (Sixth Embodiment)  
         [0066]     Next, with reference to  FIG. 12A ,  FIG. 12B ,  FIG. 13A ,  FIG. 13B  and  FIG. 13C , a flat cable according to a sixth embodiment of the present invention will be described. This cable is integrated with a dipole antenna.  FIG. 12A  is a front view showing a cable  100 .  FIG. 12B  is a sectional view showing the cable  100  taken along dotted line C of  FIG. 12A . The cable  100  is formed in a T-letter shape. As shown in  FIG. 12B , a forward end of the cable  100  functions as a dipole antenna. Connected to the dipole antenna is the flat cable according to the present invention. In addition, as is clear from  FIG. 12B , the flat cable is composed of a signal line  101 , two dielectric sheets  102 , two ground layers  103 , and two insulators  104 . These structural elements extend to the dipole antenna portion.  
         [0067]      FIG. 13A  to  FIG. 13C  show arrangements of the signal line  101 , the two dielectric sheets  102 , the two ground layers  103 , and two insulators  104 , all of which extend to the dipole antenna portion.  FIG. 13A  is a sectional view showing the flat cable along a layer denoted by arrow a of  FIG. 12B  (namely, the first ground layer  103 ).  FIG. 13B  is a sectional view showing the flat cable along a layer denoted by arrow b of  FIG. 12B  (namely, the signal line  101 ).  FIG. 13C  is a sectional view showing the flat cable along a layer denoted by arrow c shown in  FIG. 12B  (namely, the second ground layer  103 ).  
         [0068]      FIG. 13A  shows that the first ground layer  103  extends from the flat cable to the left of the dipole antenna portion.  FIG. 13B  shows that the signal line that is narrower than each of the ground layers  103  extends from the flat cable to the right of the dipole antenna portion.  FIG. 13C  shows that the second ground layer  103  extends to the dipole antenna portion in the same manner as the first ground layer  103  shown in  FIG. 13A .  
         [0069]     The cable  100  other than the antenna portion is produced in the same manner as the first embodiment shown in  FIG. 4A  to  FIG. 4C  and  FIG. 5 . In addition, the two dielectric sheets  102  are made of a material having, for example, plasticity.  
         [0000]     (Seventh Embodiment)  
         [0070]     Next, with reference to  FIG. 14A ,  FIG. 14B ,  FIG. 15A ,  FIG. 15B  and  FIG. 15C , a flat cable according to a seventh embodiment of the present invention will be described. This cable is integrated with a sleeve antenna.  FIG. 14A  is a front view showing a cable  110 .  FIG. 14B  is a sectional view showing the cable  110  taken along dotted line D of  FIG. 14A . The cable  110  is formed in a strip shape. As shown in  FIG. 14B , a forward end of the cable  110  functions as a sleeve antenna. Connected to the sleeve antenna is the flat cable according to the present invention. In addition, as is clear from  FIG. 14B , the flat cable is composed of a signal line  111 , two dielectric sheets  112 , two ground layers  113 , and two insulators  114 . These structural elements extend to the sleeve antenna portion.  
         [0071]      FIG. 15A  to  FIG. 15C  show arrangements of the signal line  111 , the two dielectric sheets  112 , the two ground layers  113 , and the two insulators  114 , all of which extend to the sleeve antenna portion.  FIG. 15A  is a sectional view showing the flat cable along a layer denoted by arrow d of  FIG. 14B  (namely, the first ground layer  113 ).  FIG. 15B  is a sectional view showing the flat cable along a layer denoted by arrow e of  FIG. 14B  (namely, the signal line  111 ).  FIG. 15C  is a sectional view showing the flat cable along a layer denoted by arrow f of  FIG. 14B  (namely, the second ground layer  113 ).  
         [0072]      FIG. 15A  shows that the first ground layer  113  extends from the flat cable to almost the middle position of the sleeve antenna portion.  FIG. 15B  shows that the signal line  111  that is narrower than each of the ground layers  113  extends from the flat cable to the endmost portion of the sleeve antenna portion. However, from the middle position of the sleeve antenna portion to the endmost portion thereof, the signal line  111  has almost the same width as each of the ground layers  113 .  FIG. 15C  shows that the second ground layer  113  extends from the flat cable to the sleeve antenna portion in the same manner as the first ground layer  113  shown in  FIG. 15A .  
         [0073]     The cable  100  other than the antenna portion is produced in the same manner as the first embodiment shown in  FIGS. 4A  to  4 C and  FIG. 5 . The dielectric sheet  102  is made of a material having, for example, plasticity.  
         [0074]     Although the cables according to the sixth and seventh embodiments are integrated with specific types of antennas, the flat cables according to the present invention can be integrated with various types of antennas. Thus, the present invention is not limited to the foregoing embodiments. These cables and antennas can be simultaneously produced in the same process.  
         [0075]     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.