Patent Publication Number: US-6906678-B2

Title: Multi-frequency printed antenna

Description:
FIELD OF THE INVENTION 
   The present invention relates to a compact printed antenna structure and, more particularly, to an antenna structure capable of producing a multi-frequency resonant mechanism for the application of multi-frequency signal transmission. 
   BACKGROUND OF THE INVENTION 
   With rapid progress of wireless communication technology, mobile communication products have become the mainstream of modern science-and-technology products. These mobile communication products include a notebook computer, a cellular phone, and a personal digital assistant (PDA), etc. After coupling with the wireless communication modules, these products can link to the internet, receive and send electronic mails, and get instant information on news or stocks quotations so as to achieve functions of resource sharing and information transmitting. 
   A conventional “Printed Sleeve Antenna” disclosed by U.S. Pat. No. 5,598,174 relates to formation of a half wavelength resonant mechanism with extension of a ground strip to a quarter wavelength in an “L” shape and extension of a feed strip to a quarter wavelength so as to achieve effects similar to the traditional coaxial sleeve dipole. This conventional antenna design is concerned with single frequency transmission and cannot be applied in multi-frequency signal transmission. Moreover, the planar radiation field pattern is poor in omnidirectional performance due to the asymmetrical structure, and it is difficult to impedance match with a general symmetrical microstrip feeding. Furthermore, a conventional “Printed Antenna” disclosed by U.S. Pat. No. 5,754,145 relates to a printed dipole antenna with three printed strips to form a dipole mechanism so as to achieve effects similar to the traditional sleeve dipole. However, this antenna design is also concerned only with single frequency transmission. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a multi-frequency printed antenna capable of producing multi-frequency resonant mechanisms for the application of multi-frequency signal transmission. 
   Another object of the present invention is to provide a multi-frequency printed antenna which is light and compact, and is easily linked to the feeding signals of a coaxial cable or a printed circuit, and is suitable for a hidden or built-in antenna structure. 
   The multi-frequency printed antenna disclosed in this invention includes an insulating substrate, a feed strip, a ground strip, and a plurality of radiating and grounded conductive strips. The feed strip is formed on the upper surface of the substrate, one end of which is connected to a signal terminal of a RF signal source, and the other end of which is in connection with the plurality of radiating conductive strips. The ground strip is formed on the lower surface of the substrate, one end of which is connected to a ground terminal of the RF signal source, and the other end of which is in connection with the plurality of grounded conductive strips. In this invention, through modification of the lengths and shapes of the radiating and grounded conductive strips, each of the radiating conductive strips together with each of the grounded conductive strips form a dipole resonant mechanism of a certain frequency so as to produce multi-frequency signal transmission. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic exploded diagram illustrating a first embodiment of a multi-frequency printed antenna in accordance with this invention; 
       FIG. 2  is a schematic exploded diagram illustrating a second embodiment of a multi-frequency printed antenna in accordance with this invention; 
       FIG. 3  is a schematic exploded diagram illustrating a third embodiment of a multi-frequency printed antenna in accordance with this invention; 
       FIG. 4  is a measured drawing of the voltage standing wave ratio (VSWR) of the antenna of the third embodiment in accordance with this invention; and 
       FIG. 5  is a measured drawing of the radiation field patterns on the H-plane of the third embodiment in accordance with this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Please refer to  FIG. 1 , which is a schematic exploded diagram illustrating a first embodiment of a multi-frequency printed antenna  11  in accordance with this invention. The antenna  11  includes a substrate  22  with an insulating plate structure, a feed strip  23 , a ground strip  24 , a first radiating conductive strip  25 , a second radiating conductive strip  26 , a first grounded conductive strip  27 , and a second grounded conductive strip  28 . The above-mentioned strips are all formed on two opposite surfaces of the substrate  22  in a manner of circuit printing. The substrate  22  is a circuit board made of an insulating material. 
   The feed strip  23  is formed on the upper surface of the substrate  22  and extends in a first direction. One end of the feed strip  23  is connected to a signal terminal  3  of a RF signal source  1 . The other end of the feed strip  23  is in connection with a connecting portion  251  of the first radiating conductive strip  25  and a connecting portion  261  of the second radiating conductive strip  26 . The first and second radiating conductive strip  25  and  26  are symmetrically disposed on opposite sides with respect to the feed strip  23 . The feed strip  23  and the first radiating conductive strip  25  are disposed on opposite sides with respect to the connecting portion  251 . The feed strip  23  and the second radiating conductive strip  26  are disposed on opposite sides with respect to the connecting portion  261 . The connecting portion  251  may extend in a second direction substantially perpendicular to the first direction. Also, the connecting portion  261  may extend in the second direction. The length of the first radiating conductive strip  25  may be different from that of the second radiating conductive strip  26 . 
   The ground strip  24  is formed on the lower surface of the substrate  22  and extends in the first direction, overlying the feed strip  23 . One end of the ground strip  24  is connected to a ground terminal  4  of the RF signal source  1 . The other end of the ground strip  24  is in connection with a connecting portion  271  of the first grounded conductive strip  27  and a connecting portion  281  of the second grounded conductive strip  28 . The first and second grounded conductive strips  27  and  28  are mutually parallel with and properly spaced from the ground strip  24 , except the connecting portions thereof to the other end of the ground strip  24 . The first and second grounded conductive strips  27  and  28  are symmetrically disposed on opposite sides with respect to the ground strip  24 . The ground strip  24  and the first grounded conductive strip  27  are disposed on the same side with respect to the connecting portion  271 . The ground strip  24  and the second grounded conductive strip  28  are disposed on the same side with respect to the connecting portion  281 . The connecting portion  271  may extend in the second direction substantially perpendicular to the first direction. Also, the connecting portion  281  may extend in the second direction. The length of the first grounded conductive strip  27  may be different from that of the second grounded conductive strip  28 . 
   Depending on desired frequencies, the first radiating conductive strip  25  and the first grounded conductive strip  27  may be designed as a half wavelength dipole antenna of a certain desired frequency through adjustment in length or shape thereof while the second radiating conductive strip  26  and the second grounded conductive strip  28  may be independently designed as a half wavelength dipole antenna of another certain frequency. Furthermore, the first radiating conductive strip  25  and the second grounded conductive strip  28  as well as the second radiating conductive strip  26  and the first grounded conductive strip  27  may also form the other dipole resonant combinations, respectively. Thus, the antenna  11  of this invention can produce multi-frequency resonant mechanisms with dipole-like radiation patterns. 
   Please refer to  FIG. 2 , which is a schematic exploded diagram illustrating a second embodiment of a multi-frequency printed antenna  12  of this invention. The antenna  12  includes a substrate  22 , a feed strip  23 , a ground strip  24 , two radiating conductive strips  37 , and four grounded conductive strips  38 . Similarly to the first embodiment, the feed strip  23  has one end connected to the signal terminal  3  of the RF signal source  1 . The two radiating conductive strips  37  are disposed on opposite surfaces of the substrate  22 , respectively, and mutually connected through a via hole  39  opened in the substrate  22 . One of the two radiating conductive strips  37  is in end-to-end connection with another end of the feed strip. Similarly, the four grounded conductive strips  38  are mutually connected in the same manner as that described in the above through other via holes  39 . In this embodiment, by adjusting the lengths or shapes of the radiating conductive strips  37  and the grounded conductive strips  38 , each of the radiating conductive strips  37  together with each of the grounded conductive strips  38  on the opposite surfaces of the substrate  22  may form a dipole antenna of a different frequency, respectively, so as to produce multi-frequency resonant mechanisms and to be applied in multi-frequency signal transmission. 
   Please refer to  FIG. 3 , which is a schematic exploded diagram illustrating a third embodiment of a multi-frequency printed antenna  13  in accordance with this invention. This embodiment is further designed on the basis of the antenna  11  of the first embodiment. More specifically, the connecting portion  251  of the first radiating conductive strip  25  is connected with one end  321  of a third radiating conductive strip  32  through a via hole  31 . Also, the connecting portion  261  of the second radiating conductive strip  26  is connected with one end  331  of a fourth radiating conductive strip  33  through another via hole  31 . The third and fourth radiating conductive strips  32  and  33  are formed on the lower surface of the substrate  22  in a manner of circuit printing. The third radiating conductive strip  32  extends in the first direction, overlying the first radiating conductive strip  25 . Also, the fourth radiating conductive strip  33  extends in the first direction, overlying the second radiating conductive strip  26 . 
   Furthermore, the connecting portion  271  of the first grounded conductive strip  27  is connected with one end  351  of a third grounded conductive strip  35  through a via hole  34 . Also, the connecting portion  281  of the second grounded conductive strip  28  is connected with one end  361  of a fourth grounded conductive strip  36  through another via hole  34 . The third and fourth grounded conductive strips  35  and  36  are formed on the upper surface of the substrate  22  in a manner of circuit printing. The third grounded conductive strip  35  extends in the first direction, overlying the first grounded conductive strip  27 . Also, the fourth grounded conductive strip  36  extends in the first direction, overlying the second grounded conductive strip  28 . 
   With such a configuration, a plurality of half wavelength dipole antenna structures, each of which is of a certain frequency, may be formed on the surfaces of the substrate  22  by adjusting the lengths and shapes of the radiating conductive strips and the grounded conductive strips such that the length of the electric current path provided by the resonant pair combined by the radiating conductive strip and the grounded conductive strip is the half of an operating wavelength or a multiple of the half operating wavelength. Comparing with the first embodiment, the third embodiment can provide more frequency selections and radiation field patterns without an additional area to the substrate. There are theoretically  16  resonant pairs (4×4) in this embodiment since each of the four radiating conductive strips  25 ,  26 ,  32 , and  33  together with each of the four grounded conductive strips  27 ,  28 ,  35 , and  36  form a resonant pair. FIG.  4  and  FIG. 5  are the measured experimental results of the multi-frequency printed antenna  13  of this embodiment. The antenna is designed to be used in wireless LAN IEEE 802.11b at 2.4 GHz as well as IEEE 802.11a NII at 5.2 GHz and 5.8 GHz for the purpose of three-frequency application. The glass fiber plate FR4 is used as the substrate and the size thereof is 5.6 mm×50 mm×0.8 mm.  FIG. 4  is the measured drawing of the voltage standing wave ratio (VSWR), showing the effects and the characteristics of the multiple frequencies thereof.  FIG. 5  is the measured drawing of radiation field patterns on the H-plane at 2.45 GHz, 5.25 GHz, and 5.8 GHz. As clearly seen from  FIG. 5 , an omnidirectional radiation property is achieved on the horizontal plane for all desired frequency bands. 
   As understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are only illustrated of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.