Abstract:
An ultra-wideband (UWB) antenna is provided. It comprises a dielectric substrate, a ground plate, a metal plate, and a transmission line. The ground plate formed on the dielectric substrate has a first slot thereon. The metal plate formed on the dielectric substrate has a feed-point and a second slot thereon. The total length of the second slot is about a half-wavelength at the desired notched frequency for the UWB antenna. By embedding the second slot of a suitable length on the metal plate resided in the first slot, a band notched characteristic is achieved for the antenna in the 5 GHz band, thereby overcoming the problem of signal interference with the UWB operations. The disclosed antenna and the circuitry for the antenna system are easily integrated. With the simple structure, the fabrication cost for the antenna is also reduced.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to antennas, and more particularly to a band-notched ultra-wideband (UWB) antenna. 
   BACKGROUND OF THE INVENTION 
   In recent years, transmission speed and information capacity of wireless communications are increased in an exponential rate, driven by the increasing demand for short-range wireless communications, wireless local area networks (WLANs), and personal mobile communications devices. For these related applications, Federal Communications Commissions (FCC) specified in February 2002 that ultra-wideband communications technologies are to be used for commercial communications and for high-speed, low-power and short-range communications. In addition, Institute of Electrical and Electronic Engineering (IEEE) also proposed a new standard, IEEE 802.15 WPAN (wireless personal area network), for mobile communications consumer devices to provide high-speed and low-power ultra-wideband communications. However, over the designated UWB frequency band, there are existing WLAN operating bands such as the 5.2 GHz (5150–5350 MHz) and 5.8 GHz (5725–5825 MHz) bands, which may cause interference with the UWB operations. To prevent the interference from the WLAN system, the ultra-wideband communications system conventionally requires that the employed ultra-wideband antenna be connected to an external band-stop filter to block the WLAN signals. This approach, however, increases the production cost and the design complexity of the system circuitry. 
   Schantz et al. disclosed ultra-wideband monopole and dipole antennas in U.S. Pat. No. 6,774,859 issued in 2002. The technique incorporates one or more slits and one or more curved narrow slots on a metal plate of the antenna. An antenna as such exhibits multiple operation bands or a destructive band to cast out the frequency range overlapping with other communications systems. The major disadvantage of the prior art lies in that the antenna requires a very large metal plate and is too difficult to be integrated with the ground plate of the antenna&#39;s RF circuitry. 
   Accordingly, an ultra-wideband planar antenna is provided herein so as to achieve ultra-wideband operation, suppress interference, and be integrated with the antenna system&#39;s ground plate. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to overcome the aforementioned drawback of the conventional ultra-wideband antennas. The primary objective of the present invention is to provide an ultra-wideband antenna that has a band-notched function for suppressing interference. The antenna is also easier to be integrated with the antenna system&#39;s ground plate. 
   Accordingly, the present invention mainly comprises a dielectric substrate, a ground plate, a metal plate, and a transmission line. The dielectric substrate has a first surface and a second surface. The ground plate has a first slot formed on top of the dielectric substrate. The metal plate has a feeding point and a second slot formed on top of the dielectric substrate. The total length of the second slot is about a half-wavelength at the center frequency of the antenna&#39;s notched frequency band. The transmission line has a signal wire and a transmission line ground unit, which are connected to the feeding point and the ground plate, respectively. 
   The major characteristic of the present invention is the configuration of the second slot on the metal plate. The second slot is a curved narrow slot having a U or inverted-U shape positioned symmetrically with respect to the central axis of the metal plate. Around the center frequency of the antenna&#39;s notched frequency band, strong out-of-phase currents surround the outer and inner perimeters of the second slot, causing a destructive interference with the initial current distributions in the metal plate having no second slot. The antenna therefore becomes non-responsive and its radiation efficiency is severely attenuated in the notched frequency band. 
   The ultra-wideband antenna may be excited by a co-planar waveguide feed-line, a microstrip feed-line, or a coaxial feed-line. During the manufacturing process, the formation of the antenna may be integrated with the laminated ceramic co-fire process of the printed circuit board. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is a schematic top view of an ultra-wideband antenna according to the present invention. 
       FIG. 1   b  is a schematic side view of the ultra-wideband antenna of  FIG. 1   a.    
       FIG. 2   a  is a schematic top view of an ultra-wideband antenna according to a first embodiment of the present invention. 
       FIG. 2   b  is a schematic side view of the ultra-wideband antenna of  FIG. 2   a.    
       FIG. 3  shows the experimental results for the voltage standing-wave ratio (VSWR) of an antenna according to the first embodiment of the present invention. 
       FIG. 4  shows the experimental results for the radiation patterns of an antenna according to the first embodiment of the present invention at 4 GHz. 
       FIG. 5  shows the experimental results for the radiation patterns of an antenna according to the first embodiment of the present invention at 8 GHz. 
       FIG. 6  shows the experimental results for the gain of an antenna according to the first embodiment of the present invention within the antenna&#39;s operation frequency band. 
       FIG. 7   a  is a schematic top view of an ultra-wideband antenna according to a second embodiment of the present invention. 
       FIG. 7   b  is a schematic bottom view of the ultra-wideband antenna of  FIG. 7   a.    
       FIG. 7   c  is a schematic side view of the ultra-wideband antenna of  FIG. 7   a.    
       FIG. 8   a  is a schematic top view of an ultra-wideband antenna according to a third embodiment of the present invention. 
       FIG. 8   b  is a schematic side view of the ultra-wideband antenna of  FIG. 8   a.    
       FIGS. 9   a – 9   e  show various shapes adopted by a first slot respectively. 
       FIGS. 10   a – 10   e  shows various shapes adopted by a metal plate respectively. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1   a  is a schematic top view of an ultra-wideband antenna according to the present invention.  FIG. 1   b  is a schematic side view of the ultra-wideband antenna of  FIG. 1   a  As illustrated, the ultra-wideband antenna  100  comprises a dielectric substrate  110 , a ground plate  120 , a metal plate  130 , and a transmission line  140 . The dielectric substrate  110  has a first surface  111  and a second surface  112 . The ground plate  120  has a first slot  121  formed on the first surface  111  of the dielectric substrate  110 . The metal plate  130  has a feed-point  131  and a second slot  132 , formed also on the first surface  111  of the dielectric substrate  110 . The total length of the second slot  132  is about a half-wavelength at the center frequency of the antenna  100 &#39;s notched frequency band. The transmission line  140  comprises a signal wire  141  and a feed-line ground unit  142 , which are connected to the feed-point  131  and the ground plate  120  respectively. The feed-line  140  may be implemented as a co-planar waveguide feed-line, a microstrip feed-line, or a coaxial feed-line, as described in the following embodiments respectively. 
     FIG. 2   a  is a schematic top view of an ultra-wideband antenna according to a first embodiment of the present invention.  FIG. 2   b  is a schematic side view of the ultra-wideband antenna of  FIG. 2   a.    
   As illustrated, the first embodiment adopts a co-planar waveguide feed-line  240  whose signal wire is a central metal wire  241  and whose grounding unit includes a first feed-line ground plate  242   a  and a second feed-line ground plate  242   b . The ultra-wideband antenna  200  according to the present embodiment comprises a dielectric substrate  110 , a ground plate  120 , a metal plate  130 , and the co-planar waveguide feed-line  240 . The dielectric substrate  110  has a first surface  111  and a second surface  112 . Both the ground plate  120  and the metal plate  130  are formed on the first surface  111  of the dielectric substrate  110 . The ground plate  120  has a first slot  121 . The metal plate  130  is located inside the first slot  121 , and has a feed-point  131  and a second slot  132 . The co-planar waveguide feed-line  240  is also formed on the first surface  111  of the dielectric substrate  110 . The central metal wire  241  is connected to the feed-point  131 . The first and second feed-line ground plates  242   a  and  242   b  are located at the two sides of the central metal wire  241 , separated by the central metal wire  241 . Both the first and second feed-line ground plates  242   a  and  242   b  have a matching width as the central metal wire  241 , and are connected to the ground plate  120  respectively. 
   The ultra-wideband antenna  200  according to the present embodiment is a planar print-typed wide slot antenna using a co-planar waveguide feed-line  240 . The advantage of the antenna  200  is that it may be easily integrated with and could be printed on the same dielectric substrate as the antenna  200 &#39;s RF circuitry. In addition, by embedding a second slot having an appropriate length on the metal plate inside the first slot, the ultra-wideband antenna may solve the signal interference problem by having a notched frequency band around the 5 GHz band for wireless LAN within the antenna&#39;s operation bandwidth. 
     FIG. 3  shows the experimental results for the voltage standing-wave ratio (VSWR) of an antenna according to the first embodiment of the present invention. The experiment is performed based on the following parameters. The dielectric substrate  110  is made of fiberglass reinforced epoxy resin having a thickness 0.4 mm and a dielectric constant 4.4. The ground plate  120  has a length about 30 mm and a width about 25 mm. The diameter of the metal plate  130  is about 14 mm. The second slot  132 , having an inverted U shape, is of about 25 mm in length, which is about a half-wavelength at 5.5 GHz. As illustrated in  FIG. 3 , the vertical axis shows the voltage standing-wave ratio and the horizontal axis shows the operation frequency. Based on the measurements shown in  FIG. 3 , the antenna has an ultra-wide frequency band from 3.1 GHz to 10.6 GHz, all satisfying a 2:1 voltage standing-wave ratio and, within this frequency band, there is a notched frequency band  301 , which covers the 5 GHz (5.150–5.825 GHz) band for the wireless LAN. 
     FIGS. 4 and 5  show experimental results for the radiation patterns of an antenna according to the first embodiment of the present invention at 4 GHz and 8 GHz, respectively. As illustrated, the antenna has a bi-directional pattern or a quasi-omnidirectional pattern on the horizontal plane (i.e., x-y plane), both at 4 and 8 GHz. 
     FIG. 6  shows experimental results for the gain of an antenna according to the first embodiment of the present invention within the antenna&#39;s operation frequency band. As illustrated, the vertical axis shows the antenna gain and the horizontal axis shows the operation frequency. Based on the measurements shown in  FIG. 6 , the antenna has a gain about 3.0–5.7 dBi, which satisfies the requirement of ultra-wideband communications, and a notched frequency band having a center frequency at about 5.5 GHz and a minimum gain −6.5 dBi within this notched frequency band. 
     FIG. 7   a  is a schematic top view of an ultra-wideband antenna according to a second embodiment of the present invention.  FIG. 7   b  is a schematic bottom view of the ultra-wideband antenna of  FIG. 7   a.    FIG. 7   c  is a schematic side view of the ultra-wideband antenna of  FIG. 7   a.    
   As illustrated, the second embodiment adopts a microstrip feed-line  740  whose signal wire is a metal wire  741  and whose grounding unit is a feed-line ground plate  742 . The ultra-wideband antenna  700  according to the present embodiment comprises a dielectric substrate  110 , a ground plate  120 , a metal plate  130 , and the microstrip feed-line  740 . The dielectric substrate  110  has a first surface  111  and a second surface  112 . The ground plate  120  having a first slot  121  is formed on the second surface  112  of the dielectric substrate  110 . The metal plate  130  is formed on the first surface  111  of the dielectric substrate  110  and, within a region corresponding the inside of the fist slot  121 , has a feed-point  131  and a U-shaped second slot  132 . The metal wire  741  is on the first surface  111  of the dielectric substrate  110  and connected to the feed-point  131 . The feed-line ground plate  742  is located on the second surface of  112  of the dielectric substrate  110 , within a region correspond to the outside of the first slot  121 , has a matching width as the metal wire  741 &#39;s length, and is electrically connected to the ground plate  120 . In the mean time, a portion of the feed-line ground plate  742  is overlapped with the metal wire  741 . The U-shaped second slot  132 , fed by the microstrip feed-line  740 , has a total length about a half-wavelength at the center frequency of the antenna  700 &#39;s notched frequency band. The rest of the structure of the present embodiment is identical to the first embodiment, and both can provide ultra-wideband operations with a notched frequency band. 
     FIG. 8   a  is a schematic top view of an ultra-wideband antenna according to a third embodiment of the present invention.  FIG. 8   b  is a schematic side view of the ultra-wideband antenna of  FIG. 8   a.    
   As illustrated, the third embodiment adopts a coaxial feed-line  840  whose signal wire is a central wire  841  and whose grounding unit is an external ground element  742 . The ultra-wideband antenna  800  according to the present embodiment comprises a dielectric substrate  110 , a ground plate  120 , a metal plate  130 , and the coaxial feed-line  840 . The present embodiment shares a similar structure with that of the first embodiment except that, besides the difference of the feed-line, the ground plate  120  of the present embodiment further has a ground-point  822 . The central wire  841  is connected to the feed-point  131 . The external ground element  842  is connected to ground-point  822  of the ground plate  120 . In the present embodiment, the second slot  132 , fed by the coaxial feed-line  840 , is a curved one (i.e., an arc shape) and has a total length about a half-wavelength at the center frequency of the antenna  800 &#39;s notched frequency band. The rest of the structure of the present embodiment is identical to the first embodiment, and both can provide ultra-wideband operations with a notched frequency band. 
     FIGS. 9   a – 9   e  show various shapes adopted by a first slot respectively. As illustrated, the shape of the first slot  121  may be a square  121   a  (as in  FIG. 9   a ), a rectangle  121   b  (as in  FIG. 9   b ), an ellipse  121   c  (as in  FIG. 9   c ), a near semi-circle  121   d  (as in  FIG. 9   d ), or a polygon  121   e  (as in  FIG. 9   e ). 
     FIGS. 10   a – 10   e  show various shapes adopted by a metal plate respectively. As illustrated, the shape of the metal plate  130  may be a square  130   a  (as in  FIG. 10   a ), a rectangle  130   b  (as in  FIG. 10   b ), an ellipse  130   c  (as in  FIG. 10   c ), a semi-circle  130   d  (as in  FIG. 10   d ), or a polygon  130   e  (as in  FIG. 10   e ). 
   An ultra-wideband antenna according to the present invention may be fed by a co-planar waveguide feed-line, a microstrip feed-line, or a coaxial feed-line. In terms of the manufacturing process, the present invention may also be integrated, based on different requirements, with the antenna&#39;s RF circuitry in a laminated ceramic co-fire process. All these have contributed to the present invention&#39;s utility and integration capability. 
   According to the present invention, by adjusting the diameter of the ground plate  120 &#39;s first slot  121 , several resonant modes within a large frequency range can be achieved, especially in terms of the control and determination of the higher operation frequency f H . On the other hand, by adjusting the diameter of the metal plate  130 , which is about 0.14 λ L , the lower operation frequency f L  can be controlled and determined, as well as the magnetic flux distribution inside the first slot  121 . Therefore, a better impedance matching can be achieved with an ultra-wide operation frequency band (the frequency ratio is greater than 1:3). Then the U-shaped or inverted U-shaped second slot  132  is embedded on the metal plate  130 , which is substantially symmetrical with respect to the central axis of the metal plate  130  including the feed-point  131 , and which has a total length about a half-wavelength at the center frequency of the notched frequency band (i.e., a half-wavelength at 5.5 GHz within the 5 GHz WLAN band). Around the center frequency of notched frequency band, the stronger currents on the surface of the metal plate  130  are clustered substantially at the inner and outer perimeters of the second slot, forming strong out-of-phase currents on the two sides of the second slot, causing a destructive interference to the initial current distribution in the metal plate with no second slot. The antenna therefore becomes non-responsive and its radiation efficiency is severely attenuated in the notched frequency band. 
   Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.