Patent Publication Number: US-7583234-B2

Title: Antenna device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is based on Japanese Priority Patent Application No. 2006-248631, filed on Sep. 13, 2006, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to antenna devices, and more particularly to an antenna device having an element pattern and a microstrip line extending from the feeding point of the element pattern formed on one side (surface) of a dielectric substrate, and having a ground pattern opposing the microstrip line formed on the other side (surface) of the dielectric substrate. 
   2. Description of the Related Art 
   In these years, a radio communications technology using UWB (Ultra Wideband), which enables radar positioning and communications at high data transfer rate, has attracted attention. Since 2002, USB has been approved for use in a frequency band of 3.1 to 10.6 GHz by the U.S. FCC (Federal Communications Commission). 
   UWB is a communication method that communicates pulse signals in an ultra wideband. Accordingly, antennas used for UWB are required to have a configuration that enables transmission and reception in an ultra wideband. 
   As an antenna for use in at least the FCC-approved 3.1-10.6 GHz band, an antenna having a conic or teardrop-shaped feeding body disposed on a flat ground plate has been proposed (Taniguchi, T. and Takehiko Kobayashi (Tokyo Denki University); “An Omnidirectional and Low-VSWR Antenna for the FCC-approved UWB Frequency Band,” Institute of Electronics, Information, and Communications Engineers, B-1-133, B201, Mar. 22, 2003). 
   However, since the conventional antenna device having a conic or teardrop-shaped feeding body disposed on a flat ground plate is large in size, there has been a demand for reduction in the size and thickness of the conventional antenna device. 
   Accordingly, there is proposed a technique that reduces the size and thickness of this type of antenna device by forming an element pattern and a ground pattern of conductive patterns on a dielectric substrate. 
     FIGS. 1A and 1B  are diagrams showing a conventional antenna device  10 . 
   The conventional antenna device  10  includes a dielectric substrate  11 , an element pattern  12 , a microstrip line  13 , and a ground pattern  14 . 
   The dielectric substrate  11  is formed of, for example, an FR-4 substrate having a length L 31  of substantially 40 mm and a width W 31  of substantially 30 mm. The element pattern  12  and the microstrip line  13  are formed on one side (surface) of the dielectric substrate  11 , and the ground pattern  14  is formed on the other side (surface) of the dielectric substrate  11 . 
   The element pattern  12  is shaped like a home plate and has a length L 41  of substantially 15 mm and a width W 41  of substantially 16 mm. The microstrip line  13  extends from a feeding point Ps of the element pattern  12 . 
   The ground pattern  14  is formed on the X 2  side of the feeding point Ps of the element pattern  12  on the other side (surface) of the dielectric substrate  11 . The ground pattern  14  has a length L 42  of substantially 25 mm and a width of substantially 30 mm, which is the same as the width W 31  of the dielectric substrate  11 . 
   Regarding loop antennas used for communications in low-frequency bands, an antenna device having an element formed with a conductive pattern on a flexible substrate has been proposed. (See, for example, Japanese Laid-Open Patent Application No. 2000-196327.) 
   In view of mounting these types of ultra-wide frequency band antenna devices in various communications devices, there is a strong demand for reduction in their sizes without degradation of characteristics such as VSWR. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention may solve or reduce one or more of the above-described problems. 
   According to one embodiment of the present invention, there is provided an antenna device in which one or more of the above-described problems may be solved or reduced. 
   According to one embodiment of the present invention, there is provided an antenna device that can be reduced in size without degradation of its characteristics. 
   According to one embodiment of the present invention, there is provided an antenna device including a dielectric substrate having first and second surfaces facing away from each other, an element pattern formed on the first surface of the dielectric substrate, a conductive pattern formed on the first surface of the dielectric substrate so as to extend from a feeding point of the element pattern, and a ground pattern formed on the second surface of the dielectric substrate so as to form a microstrip line in cooperation with the conductive pattern, wherein the ground pattern has a cutout part formed in a portion thereof opposing the feeding point. 
   According to one aspect of the present invention, by providing a cutout part in a portion of a ground pattern which portion opposes a feeding point of an element pattern in an antenna device having the element pattern formed on one side (surface) of a dielectric substrate, a conductive (line) pattern formed on the one side of the dielectric substrate so as to extend from the element pattern, and the ground pattern formed on the other side (surface) of the dielectric substrate so as to oppose the conductive pattern, it is possible to reduce the size of the antenna device without degradation of its frequency characteristic in a desired frequency band (for example, 3 to 5 GHz). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
       FIGS. 1A and 1B  are diagrams showing a conventional antenna device; 
       FIGS. 2A and 2B  are perspectives views of an antenna device on a first surface and a second surface, respectively, thereof according to a first embodiment of the present invention; 
       FIGS. 3A through 3C  are a top plan view, a side view, and a longitudinal side view, respectively, of the antenna device according to the first embodiment of the present invention; 
       FIG. 4  is a plan view of an element pattern and a line pattern of the antenna device according to the first embodiment of the present invention; 
       FIG. 5  is a plan view of a ground pattern of the antenna device according to the first embodiment of the present invention; 
       FIG. 6  is a graph showing the VSWR characteristic of the antenna device according to the first embodiment of the present invention; 
       FIG. 7  is a Smith chart of the antenna device according to the first embodiment of the present invention; 
       FIG. 8  is a Smith chart of the conventional antenna device shown in  FIGS. 1A and 1B ; 
       FIG. 9  is a Smith chart of the conventional antenna device with reduced width; 
       FIGS. 10A and 10B  are perspective views of an antenna device on a first surface and a second surface, respectively, thereof according to a second embodiment of the present invention; and 
       FIGS. 11A through 11C  are a top plan view, a side view, and a longitudinal side view, respectively, of the antenna device according to the second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description is given below, with reference to the accompanying drawings, of embodiments of the present invention. 
   First Embodiment 
     FIGS. 2A and 2B  are perspectives views of an antenna device  100  on a first side (surface) and a second side (surface), respectively, thereof according to a first embodiment of the present invention.  FIGS. 3A through 3C  are a top plan (Z 1 -side) view, an X 2 -side view, and a longitudinal side view, respectively, of the antenna device  100 . 
   The antenna device  100  includes a dielectric substrate  111 , an element pattern  112 , a line pattern  113 , and a ground pattern  114 . The element pattern  112  and the line pattern  113  are formed on one side (first surface) of the dielectric substrate  111 , and the ground pattern  114  is formed on the other side (second surface) of the dielectric substrate  111 . 
   The dielectric substrate  111  is formed of, for example, an FR-4 substrate, and has a thickness h 0  of substantially 0.8 mm, a width W 0  of substantially 14 mm, and a length L 0  of substantially 40 mm. (See  FIGS. 3A through 3C  for h 0 , W 0 , and L 0 .) The dielectric substrate  111  is not limited to the FR-4 substrate, and may be any substrate as long as it is formed of dielectric such as a ceramic material. Copper foil is formed on each surface of the dielectric substrate  111 . The element pattern  112 , the line pattern  113 , and the ground pattern  114  are formed by etching the copper foil. 
   A description is given first of the element pattern  112  and the line pattern  113  formed on one side (first surface) of the dielectric substrate  111 . 
     FIG. 4  is a plan view of the element pattern  112  and the line pattern  113  according to the first embodiment. 
   The element pattern  112  is formed on the X 1 -directional side (side in the direction indicated by arrow X 1 ) on the Z 1 -side surface (surface in the direction indicated by arrow Z 1 ) (first surface) of the dielectric substrate  111 . The element pattern  112  has a substantially pentagonal shape (home-plate shape) having a width W 11  of substantially 14 mm and a length L 11  of substantially 15 mm. Each of the two sides of the element pattern  112  pinching a feeding point Ps, that is, each of the two sides of the element pattern  112  connected to the line pattern  113 , forms an angle θ ( FIG. 4 ) of substantially 80° to 85° with respect to the directions indicated by arrows X 1  and X 2  in  FIG. 4 , in which the line pattern  113  extends. 
   In the case of  FIG. 4 , θ may be the angle formed between each of the two sides of the element pattern  112  and the center line of the element pattern  112  passing through the feeding point Ps. 
   Here, each of the two sides of the element pattern may be angled, that is, the angle θ may be determined, in accordance with the shape of a cutout part  121  (described below with reference to  FIG. 5  of the ground pattern  114 . 
   Further, the angle of each of the two sides of the element pattern  112  with respect to the ground pattern  114  and the shape of the cutout part  121  of the ground pattern  114  may be determined so that the antenna device  100  has a desired frequency characteristic. 
   The line pattern  113  is formed on the Z 1 -side surface of the dielectric substrate  111  so as to extend in the X 2  direction from the feeding point Ps of the element pattern  112 . The line pattern  113  has a length L 12  of substantially 25 mm and a width W 12  of substantially 1.5 mm. The line pattern  113  forms a microstrip line in cooperation with the ground pattern  114 . 
   Next, a description is given of the ground pattern  114  formed on the other side (second surface) of the dielectric substrate  111 . 
     FIG. 5  is a plan view of the ground pattern  114  according to the first embodiment. 
   The ground pattern  114  is formed on the X 2 -directional side (side in the direction indicated by arrow X 2 ) of the feeding point Ps of the element pattern  112  on the Z 2 -side surface (surface in the direction indicated by arrow Z 2 ) (second surface) of the dielectric substrate  111 . The ground pattern  114  has a width W 2  of substantially 14 mm, which is the same as the width W 11  of the element pattern  112 , and a length L 2  of substantially 24 mm. 
   The cutout part  121  is formed in the center of the X 1 -directional end side of the ground pattern  114 . The cutout part  121  has a substantially rectangular shape. The cutout part  121  is formed at a position opposing the feeding point Ps of the element pattern  112  on the side of the ground pattern  114  opposing the feeding point Ps. In other words, the cutout part  121  may be formed in an end portion of the ground pattern  114 , which end portion is positioned opposite the feeding point Ps in plan view or when viewing the antenna device  100  in a direction of the thickness of the dielectric substrate  111 . 
   The cutout part  121  has a depth B and a width C each of, for example, substantially 5.0 mm. 
   There is a gap A between the feeding point Ps of the element pattern  112  and the X 1 -directional end side of the ground pattern  114 . The gap A is, for example, substantially 1 mm along the X-axis. The length L 2  of the ground pattern  114  may be substantially 24 mm including the gap A. 
   Next, a description is given of a frequency characteristic obtained by the antenna device  100 . 
     FIG. 6  is a graph showing the VSWR characteristic of the antenna device  100 . In  FIG. 6 , a cross indicates the VSWR characteristic of the antenna device  100 , a black square indicates the VSWR characteristic of the conventional antenna device  10 , and a white circle indicates the VSWR of the conventional antenna device  10  with reduced width. 
   As indicated by crosses in  FIG. 6 , the VSWR of the antenna device  100  of this embodiment is less than 2 between 2.5 and 5.5 GHz. VSWR stands for Voltage Standing Wave Ratio, and indicates the peak-to-bottom ratio of a voltage amplitude distribution generated on a transmission line in which reflected waves are generated because of impedance mismatching. 
   In UWB communications, it is desired that VSWR be less than substantially 2 in a wide frequency band. Accordingly, the antenna device  100  of this embodiment, which is reduced in size, sufficiently satisfies the characteristic desired for UWB communications in a frequency band of 2.5 to 5.5 GHz. 
     FIG. 7  is a Smith chart of the antenna device  100 .  FIG. 8  is a Smith chart of the conventional antenna device  10 .  FIG. 9  is a Smith chart of the conventional antenna device  10  with reduced width. 
   The cross indicates impedance at 4.0 GHz in  FIGS. 7 and 9  and indicates impedance at 4.2 GHz in  FIG. 8 . 
   As shown in  FIG. 8 , the impedance is controlled to be substantially 50 Ω around 4.2 GHz in the conventional antenna device  10 . Further, as shown in  FIG. 7 , the impedance is also controlled to be substantially 50 Ω around 4.0 GHz in the antenna device  100  of this embodiment the same as in the conventional antenna device  10 . Thus, the antenna device  100  of this embodiment has substantially the same characteristic as the conventional antenna device  10 . 
   As shown in  FIG. 9 , however, simply making the conventional antenna device  10  as narrow in width as the antenna device  100  of this embodiment results in an impedance greater than 50 Ω at 4.0 GHz, thus preventing the conventional antenna device  10  with reduced width from having the same characteristic as the conventional antenna device  10 . 
   Thus, the same characteristic as the conventional antenna device  10  cannot be obtained if the width of the conventional antenna device  10  is only reduced. However, by providing the cutout part  121  and adjusting the angle of each of the sides of the element pattern  112  between which the feeding point Ps is positioned with respect to the above-described line passing through the feeding point Ps, it is possible to obtain substantially the same frequency characteristic as the conventional antenna device  10  even with reduced width. 
   By shaping the ground pattern  114  as shown in  FIG. 5 , it is possible to improve the VSWR characteristic in a frequency band of 3 to 5 GHz in the antenna device  100 , which is reduced in width, that is, in size, compared with the case of simply narrowing the width of the conventional antenna device  10 . 
   Thus, according to this embodiment, by providing the cutout part  121  in the ground pattern  114  around the feeding point Ps of the element pattern  112 , it is possible to prevent degradation of the VSWR characteristic while reducing the widths of the element pattern  112  and the ground pattern  114 , even when the element pattern  112  and the ground pattern  114  have the same width. Accordingly, it is possible to achieve the downsized antenna device  100  without degradation of the VSWR characteristic. 
   Second Embodiment 
     FIGS. 10A and 10B  are perspectives views of an antenna device  200  on a first side (surface) and a second side (surface), respectively, thereof according to a second embodiment of the present invention.  FIGS. 11A through 11C  are a top plan (Z 1 -side) view, an X 2 -side view, and a longitudinal side view, respectively, of the antenna device  200 . 
   In  FIGS. 10A ,  10 B, and  11 A through  11 C, the same elements as those described above in the first embodiment are referred to by the same reference numerals, and a description thereof is omitted. 
   The antenna device  200  is different from the antenna device  100  of the first embodiment in the shape of the cutout part formed in the ground pattern. 
   The antenna device  200  includes a ground pattern  211  in which a cutout part  221  is formed. The cutout part  221  of this embodiment has a curved shape, for example, a semicircular shape. 
   Since the cutout part  221  is provided with a curved feature, it is possible to moderate the effect of the cutout part  221  on characteristics of the antenna device  200 , thus making it easy to control the characteristics. 
   According to one embodiment of the present invention, there is provided an antenna device including a dielectric substrate having first and second surfaces facing away from each other, an element pattern formed on the first surface of the dielectric substrate, a conductive pattern formed on the first surface of the dielectric substrate so as to extend from the feeding point of the element pattern, and a ground pattern formed on the second surface of the dielectric substrate so as to form a microstrip line in cooperation with the conductive pattern, wherein the ground pattern has a cutout part formed in a portion thereof opposing the feeding point. 
   According to one aspect of the present invention, by providing a cutout part in a portion of a ground pattern which portion opposes a feeding point of an element pattern in an antenna device having the element pattern formed on one side (surface) of a dielectric substrate, a conductive (line) pattern formed on the one side of the dielectric substrate so as to extend from the element pattern, and the ground pattern formed on the other side (surface) of the dielectric element so as to oppose the conductive pattern, it is possible to reduce the size of the antenna device without degradation of its frequency characteristic in a desired frequency band (for example, 3 to 5 GHz). 
   The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.