Patent Publication Number: US-6342856-B1

Title: Method of feeding flat antenna, and flat antenna

Description:
TECHNICAL FIELD 
     The present invention relates to a method of feeding electric power to a plane antenna and to a plane antenna. More particularly, the invention relates a method of feeding electric power to a plane antenna which is designed for decreasing the thickness and for facilitating the assembling, and to a plane antenna capable of transmitting and receiving vertically polarized waves and horizontally polarized waves, or levo-rotary polarized waves and dextro-rotary polarized waves. 
     BACKGROUND ART 
     There has heretofore been known a plane antenna using a loop-type antenna element. In the plane antenna of this kind, both ends of a square or a circular loop-like antenna element are connected to balanced line connection terminals of a balanced-to-unbalanced conversion circuit and of an impedance conversion circuit through balanced feeder lines, and unbalanced line connection terminals of the balanced-to-unbalanced conversion circuit and of the impedance conversion circuit are connected to a receiver or to a transmitter through a coaxial cable. 
     When a feeding point is provided on a horizontal portion of the antenna element, there is obtained a plane antenna for the horizontally polarized waves and when a feeding point is located on a vertical portion, there is obtained a plane antenna for the vertically polarized waves. 
     If the loop-type antenna element is provided with slide element portions, a phase difference of 90 degrees is produced between the horizontally polarized waves and the vertically polarized waves due to the reactance formed by the areas and shape of the slide element portions, whereby the linearly polarized waves are converted into circularly polarized waves, forming a plane antenna for the circularly polarized waves. When the slide elements are provided at positions of about −45 degrees and about +135 degrees from the power-feeding point of the antenna element as viewed from the front, there is obtained a plane antenna for levo-rotary polarized waves. When the slide elements are arranged at positions of about +45 degrees and about −135 degrees from the power-feeding point of the antenna element as viewed from the front, there is obtained a plane antenna for the dextro-rotary polarized waves. 
     Thus, the conventional plane antenna requires a balanced-to-unbalanced conversion circuit and an impedance conversion circuit, i.e., requires an increased number of parts and an increased number of assembling steps, driving up the cost. It has therefore been desired to provide a plane antenna in a small size having a decreased thickness. Besides, a single antenna is not capable of transmitting and receiving horizontally polarized waves and vertically polarized waves, or levo-rotary polarized waves and dextro-rotary polarized waves. To cope with two kinds of polarization modes of the horizontally polarized waves and vertically polarized waves, or the levo-rotary polarized waves and dextro-rotary polarized waves, there must be installed antennas of two systems occupying considerable space and requiring a cost. 
     Thus, there arouses a technical problem that must be solved for providing a plane antenna which is fabricated using a decreased number of parts in a small size, and which is capable of coping with the two kinds of polarization modes of the horizontally polarized waves and vertically polarized waves or the levo-rotary polarized waves and dextro-rotary polarized waves, though the antenna is used in a number of only one. 
     DISCLOSURE OF THE INVENTION 
     The invention was proposed in order to accomplish the above-mentioned object, and provides a method of feeding electric power to a plane antenna in which a plane antenna element is arranged in parallel with a ground plane, a feeder conductor is arranged between the plane antenna element and the ground plane, the feeder conductor being in parallel with the peripheral edges of the plane antenna element, a central conductor of a coaxial line is connected to an end of the feeder conductor, and an external conductor of the coaxial line is connected to the ground plane so that the electric power is fed from the coaxial line to the plane antenna element through the electromagnetic coupling. 
     The invention further provides a plane antenna in which a square or a circular plane antenna element is disposed in parallel with a ground plane, feeding points of a first feeder conductor and of a second feeder conductor are arranged being separated away by 90 degrees from the center of the plane antenna element, the first feeder conductor and the second feeder conductor are provided in parallel with the peripheral edges of the plane antenna element between the plane antenna element and the ground plane, the central conductors of coaxial lines of two systems are separately connected to the feeding points of the first feeder conductor and of the second feeder conductor, and the external conductors of the coaxial lines of the two systems are connected to the ground plane to feed electric power from the first feeder conductor or the second feeder conductor to the plane antenna element through the electromagnetic coupling so as to transmit and receive the horizontally polarized waves and the vertically polarized waves. 
     The invention further provides a plane antenna in which a plane antenna element of the plane antenna having the first and second feeder conductors is further provided with slide elements positioned at an equal distance from the first and second feeding points so as to transmit and receive the levo-rotary polarized waves and the dextro-rotary polarized waves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating a method of feeding electric power to a plane antenna; 
     FIG. 2 is a graph of return loss characteristics of the plane antenna of FIG. 1; 
     FIG. 3 is a graph of a radiation pattern of the plane antenna of FIG. 1 of when θ=0 degrees (front direction of the antenna); 
     FIG. 4 is a graph of axis ratio and gain of the plane antenna of FIG. 1 depending on the frequency; 
     FIG. 5 is a diagram illustrating the plane antenna according to another embodiment; 
     FIG. 6 is a diagram illustrating a plane antenna for the horizontally polarized waves and vertically polarized waves; 
     FIG.  7 ( a ) is a front view of the plane antenna and FIG.  7 ( b ) is a side view thereof; 
     FIG. 8 is a diagram illustrating a plane antenna; 
     FIG.  9 ( a ) is a front view of the plane antenna and FIG.  9 ( b ) is a side view thereof; 
     FIG. 10 is a front view of a plane antenna element; 
     FIG. 11 is a graph showing axis ratio/frequency characteristics of three kinds of plane antennas; and 
     FIG. 12 is a front view of the plane antenna element. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the invention will now be described with reference to the drawings. FIG. 1 is a diagram illustrating the structure of a plane antenna  1  for the dextro-rotary polarized waves, which is constituted by a plane antenna element  2  of the shape of a square loop having a circumferential length C (about one wavelength), an inverse L-shaped feeder conductor  3  having a length LV+LH, slide element portions  4   a  and  4   b  of a length ΔL for generating circularly polarized waves, and a ground plane  5  which is a metal plate having an area larger than that of the plane antenna element  2 . 
     Here, let it now be presumed that the wire radius of the plane antenna element  2  is p, the height of the plane antenna element  2  from the ground plane 5 is h, the circumferential length is C=1.032λ 1.472 , h=0.0491λ 1.472 , ΔL=0.029λ 1.472 , LV=0.014λ 1.472 , and LH=0.236λ 1.472 , where λ 1.472  is a free space wavelength at a design frequency of 1.472 GHz. 
     The plane antenna element  2  is arranged over the ground plane  5  in parallel therewith. The ground plane  5  has a conductor through hole  6  facing a point of the loop of the plane antenna element  2 . The feeder conductor  3  does not come in contact with the ground plane  5 , rises upright through the conductor through hole  6 , is bent in a horizontal direction, and is arranged in parallel with the plane antenna element  2  along the loop of the plane antenna element  2 . 
     An end of the feeder conductor  3  penetrating through to the back surface side of the ground plane  5  is connected to a central conductor  7   a  of a coaxial feeder line, and an outer conductor  7   b  of the coaxial feeder line is connected to the ground plane  5 , to feed electric power from the feeder conductor  3  to the ground plane  5  through the electromagnetic coupling. 
     FIGS. 2 to  4  show characteristics of the plane antenna, wherein FIG. 2 shows return loss of a 50-ohm coaxial line. A band in which the return loss is not larger than −14 dB is 1.5% (from 1.461 GHz to 1.483 GHz). 
     FIG. 3 shows a radiation pattern of when θ=0 degree (direction of front surface of the antenna), wherein the half-power beam width of the dextro-rotary deflected waves (E R ) is about 70 degrees, and the inversely defected wave component (E L ) is −20 dB or smaller. 
     FIG. 4 shows the axis ratio (A.R.) and the gain depending on the frequency, wherein a circularly deflected wave radiation band in which the axis ratio is not larger than 3 dB is about 0.5%. The gain at a center frequency is 9.4 dB, and a change in the gain in this band is about 0.1 dB. 
     Though not diagramed, if the positions of the slide element portions  4   a  and  4   b  are turned by 180 degrees and the direction of the feeder conductor  3  is inverted right side left, then there is obtained a plane antenna for the levo-rotary polarized waves. If the slide element portions  4   a  and  4   b  are removed from the plane antenna element  2 , then, there is obtained a plane antenna for the linearly polarized waves that correspond to the horizontally polarized waves or the vertically polarized waves. 
     FIG. 5 illustrates another embodiment in which the conductor through hole  6  is formed in the ground plane  5  at a position corresponding to a corner of the square plane antenna element  2 , and the feeder conductor  3  is arranged in parallel with a side of the square plane antenna element  2 . 
     FIG. 6 is a diagram illustrating the structure of a plane antenna  11  capable of transmitting and receiving both the horizontally polarized waves and the vertically polarized waves, which is constituted by a plane antenna element of the shape of a square loop having a circumferential length C (about one wavelength), two feeder conductors  13  and  14  having a length LV+LH (about ¼ wavelength), and a ground plane  15 . 
     The plane antenna element  12  is arranged over the ground plane  15  in parallel therewith, and the feeder conductors  13  and  14  of an inverse L-shape rising through the ground plane  15  are arranged between the plane antenna element  12  and the ground plane  15 . The ground plane  15  has conductor through holes  16  and  17  just under an intermediate point of a horizontal element portion  12 H of the plane antenna element  12  and just under an intermediate point of a vertical element portion  12 V. The two feeder conductors  13  and  14  are not brought into contact with the ground plane  15  but are inserted at their ends which are the feeding points in the conductor through holes  16  and  17 . 
     The feeder conductor  13  that vertically rises through the conductor through hole  16  facing the horizontal element portion  12 H, is bent at right angles toward the left as viewed from the front, extends along the horizontal element portion  12  and the left vertical element portion  12 V, and reaches an intermediate point of the left vertical element portion  12 V. The feeder conductor  14  that vertically rises through the conductor through hole  17  facing the right vertical element portion  12 V, is bent at right angles upward in FIG. 6, extends along the vertical element portion  12 V and the upper horizontal element portion  12 H, and reaches an intermediate point of the upper horizontal element portion  12 H. 
     The ends of the feeder conductors  13  and  14  penetrating through to the back surface side of the ground plane  15  are connected to central conductors  18   a  and  19   a  of separate coaxial feeder lines, and external conductors  18   b  and  19   b  of the coaxial feeder lines are connected to the ground plane  5 , to feed electric power from the feeder conductor  13  or  14  to the plane antenna element  12  through the electromagnetic coupling. 
     When the electric power is fed from the horizontal feeder conductor  13 , the horizontally polarized waves are radiated from the plane antenna element  12  and when the electric power is fed from the vertical feeder conductor  14 , the vertically polarized waves are radiated. Upon feeding the electric power by changing over the horizontal feeder conductor  13  and the vertical feeder conductor  14 , the horizontally polarized waves and the vertically polarized waves can be received or transmitted using a single plane antenna  11 . 
     The electric power is fed through the electromagnetic coupling to the plane antenna element  12  from the feeder conductors  13  and  14  arranged close to the plane antenna element  12  of the shape of a loop; i.e., the electric power is fed to the plane antenna element  12  by changing over the feeder lines of two systems to cope with both the horizontally polarized waves and the vertically polarized waves. The feeder lines are not connected to the plane antenna element  12  and have a low impedance, making it possible to decrease the gap between the antenna element and the ground plane and, hence, to constitute an antenna featuring small thickness and high sensitivity. The antenna can be easily assembled since no connection is necessary between the plane antenna element and the feeder lines. 
     Referring to FIG. 7, a plane antenna element  22  of a plane antenna  21  for the horizontally/vertically polarized waves is formed of an electrically conducting metal plate in the shape of a plane square loop having a predetermined width in the radial direction. Feeder conductors  23  and  24  of an inverse L-shape, too, are formed of a metal plate like the plane antenna element  22 , and have a width larger than the feeder conductors made of wires. An annular line passing through midway between the inner circumference and the outer circumference of the plane antenna element  22  has a circumferential length nearly equal to a free space wavelength λ 1.472  at a design frequency of 1.472 GHz. The difference is great between the inner circumferential length and the outer circumferential length, and the frequency band becomes broader than that of the constitution of FIG.  6 . Similarly, the feeder conductors  23  and  24  of the shape of a flat plate offer a wider frequency band than the feeder conductors made of wires. 
     The plane antenna  31  for the circularly polarized waves shown in FIG. 8 comprises a plane antenna element  32  of the shape of a loop provided with two slide element portions  33  having an element length of ΔL=0.029λ 1.472  as slide elements for synthesizing the circularly polarized waves. As shown, the slide element portions  33  are protruding toward the central direction at two places, i.e., at a corner between the lower horizontal element portion  32 H on where a feeding point  34   a  of one feeder conductor  34  is located and a right vertical element portion  32 V on where a feeding point  35   a  of another feeder conductor  35  is located, and at a corner at a symmetrical position turned by 180 degrees from the above corner. 
     When the electric power is fed from the horizontal feeder conductor  34 , the phase of the vertically polarized wave component is delayed by 90 degrees behind the phase of the horizontally polarized wave component due to the reactance of the slide element portions  33 , and the dextro-rotary polarized waves are radiated in the +Z direction (upward in the drawing) from the plane antenna element  32 . When the electric power is fed from the vertical feeder conductor  35 , further, the phase of the horizontally polarized wave component is delayed by 90 degrees behind the vertically polarized wave component, and the levo-rotary polarized waves are radiated in the +Z direction from the plane antenna element  32 . Upon changing over the horizontal feeder conductor  34  and the vertical feeder conductor  35 , the levo-rotary polarized waves and the dextro-rotary polarized waves can be transmitted and received using a single plane antenna  31 . 
     The plane antenna element  41  shown in FIG. 9 comprises a metal plate in which a central hole is formed, and has slide element portions  42  protruding inward from the corners at two places separated by 180 degrees on the inner circumference like the plane antenna element for the circularly polarized waves shown in FIG. 8, to cope with the circularly polarized waves. 
     A plane antenna element  51  shown in FIG. 10 comprises a square metal plate having a hole  52  at the center elongated in the radial direction to increase the width of the plane antenna element of the shape of a loop and to increase the areas of the slide element portions  53 . 
     FIG. 11 shows axis ratio characteristics of the plane antennas using plane antenna elements of FIGS. 8,  9  and  10 , wherein a curve A represents characteristics of a plane antenna element  32  made of a wire of FIG. 8, a curve B represents characteristics of an antenna element  41  made of a metal plate of FIG. 9, and a curve C represents characteristics of an antenna element  51  made of a metal plate of FIG.  10 . The ordinate represents the axis ratio (A.R.), the abscissa represents the frequency (f), and it will be learned that the frequency band increases with an increase in the width of the element, and the antenna element  51  features a broadest frequency band as represented by the curve C. 
     A plane antenna element  61  shown in FIG. 12 has slide element portions  62  protruded in the radial direction from the corners at two opposing places of a square metal plate. This is a modified embodiment from that of FIG.  10 . That is, since the width of the plane antenna element  51  of FIG. 10 is increased as much as possible, the elongated hole at the center is extinguished, and the slide element portions  62  are provided on the outer sides. 
     Though the above-mentioned embodiments have dealt with plane antenna elements of a square shape, the plane antenna element may have a circular shape without being limited to the above-mentioned embodiments. Further, the invention can be modified in a variety of ways within the technical scope of the invention, and the invention encompasses such modified embodiments as a matter of course. 
     Industrial Applicability 
     According to the method of feeding electric power to the plane antenna of the present invention as described above, the feeder line is not connected to the plane antenna element, and the electric power is fed to the plane antenna element from the feeder conductor arranged close to the plane antenna element through the electromagnetic coupling. This makes it possible to decrease the input impedance of the plane antenna, to decrease the gap between the antenna element and the ground plane and, hence, to realize an antenna featuring a decreased thickness and high sensitivity. Further, the antenna can be easily assembled since no connection is necessary between the plane antenna element and the feeder line. 
     Owing to the above-mentioned method, the electric power can be fed to a plane antenna element from the feeder lines of a plurality of systems. Upon feeding the electric power by changing over the feeder lines of the two systems, it is allowed to transmit and receive the electromagnetic waves of two kinds of polarization modes, i.e., the horizontally polarized waves and the vertically polarized waves, or the levo-rotary polarized waves and the dextro-rotary polarized waves, making it possible to realize a plane antenna featuring a decreased thickness and a multiplicity of functions. Upon expanding the width of the loop-shaped plane antenna element, further, the frequency band width is broadened. By employing the feeder conductors of the shape of flat plates, further, the frequency band is more expanded, and the plane antenna features an enhanced practicable performance.