Abstract:
An antenna structure in the shape of a conducting plate for radiating and receiving electromagnetic signals and having a first opening for producing electromagnetic resonance at a first frequency band and a second opening for producing electromagnetic resonance at a second frequency band. A coaxial cable is used to feed Signals of both frequency bands to the antenna across the first opening. The first opening can be on a deflected part of the conducting plate relative to the second opening.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Reference to Foreign Applications  
           [0002]    The applicant of the present application claims priority date of related Taiwan application No.090111861 filed at 17 th  May, 2001, and entitled “Dual Band Slot Antenna.” 
           [0003]    1. Field of the Invention  
           [0004]    The present invention relates to a dual band antenna, and more particularly, to a dual band slot antenna containing two slots for creating resonance within distinct frequency bands.  
           [0005]    2. Description of the Prior Art  
           [0006]    Wireless transmission has become a required function for today&#39;s mobile devices, such as laptop computers and handhelds. FIG. 1 illustrates a typical planar slot antenna for use in a mobile device for transmitting and receiving wireless signals. Slot antenna  1  includes a conducting plate  10  in which an elongate opening or slot  11  is formed A coaxial cable  14  is connected to the conducting plate  10  by connecting the inner conductor to feed point  12  and connecting the outer conductor to ground  13 . When situated in electromagnetic fields, electric currents will be induced on the conducting plate  10  and resonance is created along the slot  11 . The induced currents carry encoded signals according to the protocol utilized for wireless transmission and are collected and received at feed point  12  by the coaxial cable  14  for further decoding.  
           [0007]    Similarly, when the coaxial cable  14  feed encoded signals to the conducting plate  10  through feed point  12 , electric currents are generated on the conducting plate  10  and resonance is created along the slot  11  so that electromagnetic waves carrying the encoded signals can be radiated away. As a general rule, the longer is the slot  11 , the lower is the resonance frequency, and vice versa. By adjusting the shape and dimension of the slot  11 , one is able to have the slot antenna  1  function within a desired frequency range according to protocol specification.  
           [0008]    Currently there are several protocols available for establishing wireless transmission, each utilizing a particular frequency band. For example, Bluetooth and IEEE 802.11b both operate at 2.4 GHz, while GPRS operates at 900 MHz and 1800 MHz, and IEEE 802.11a at 5.5 GHz. Although the prior art slot antenna  1  illustrated in FIG. 1 can be made to operate at a wide variety of frequency ranges, it can only function for one particular frequency range at one time. That is, the slot antenna  1  is a monoband antenna and is therefore limited in its application. It is needed in this regard to have a dual band slot antenna that can transmit and receive signals of two frequency bands.  
         SUMMARY OF THE INVENTION  
         [0009]    It is therefore a primary objective of the claimed invention to provide a dual band planar slot antenna to overcome the above-mentioned shortcoming of the prior art.  
           [0010]    According to one embodiment of the claimed invention, the antenna comprises a metallic plate having two elongate slots. The first slot and the second slot are longitudinally parallel and close to each other. A coaxial cable feeds signals across the first slot. A securing structure securely and precisely fixes the coaxial cable onto the metallic plate at a desired position. The first slot and the second slot are electrically connected to the coaxial cable so that, by sharing the same feed, the first slot is used to transmit and receive radio signals of a first frequency band and the second slot is used to transmit and receive radio signals of a second frequency band.  
           [0011]    According to another embodiment of the claimed invention, the antenna comprises a metallic plate having two elongate slots formed in oblique surfaces respectively.  
           [0012]    These and other objectives of the claimed invention will no doubt become obvious to any skilled artist in this field after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a frontal view of a planar slot antenna according to the prior art.  
         [0014]    [0014]FIG. 2 is a frontal view of a dual band planar slot antenna according to the first embodiment of the present invention.  
         [0015]    [0015]FIG. 3 is a perspective view of the dual band planar slot antenna in FIG. 2 according to the first embodiment of the present invention.  
         [0016]    [0016]FIG. 4 illustrates one example of installation of the dual band planar slot antenna of the present invention in a laptop computer system.  
         [0017]    [0017]FIG. 5 is a perspective view of a dual band planar slot antenna according to the second embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    [0018]FIG. 2 is a frontal view of a dual band planar slot antenna according to the first embodiment of the present invention. The dual band slot antenna  2  consists of a conducting plate  20  in which a long slot  21  and a short slot  22  are formed. The long slot  21  is endowed with a characteristic length that provides resonance path for electromagnetic fields in a lower frequency band, and similarly the short slot  22  is endowed with a characteristic length that provides resonance path for electromagnetic fields in a higher frequency band. Preferably the conducting plate  20  elongates in a longitudinal direction along which the long slot  21  and the short slot  22  are both aligned. In the present embodiment the conducting plate  20  and the long slot  21  are rectangular, and the short slot  22  is trapezoid. The difference between a rectangular slot and a trapezoid slot is that a rectangular slot provides substantially one resonance length that results in a narrow bandwidth. But a trapezoid slot provides different resonance lengths reflected in its outline so that wider bandwidth can be obtained. A skilled artist in this field should observe that the conducting plate  20 , the long slot  21  and the short slot  22  can have outlines other than those shown in FIG. 2.  
         [0019]    The dual band slot antenna  2  further consists of a coaxial cable  25  for feeding signals. In the present embodiment, the coaxial cable  25  feeds directly across the long slot  21 . This is done by connecting or welding the inner conductor and outer conductor of the coaxial cable  25  to the feed point  23  and the ground  24  of the slot antenna  2 , respectively. When the cable  25  feeds the lower frequency band signals into the conducting plate  20 , lower frequency resonance is created around the long slot  21  and electromagnetic waves carrying wireless signals are radiated away. By the same token, when the cable  25  feeds the higher frequency band signals into the conducting plate  20 , higher frequency resonance is created around the short slot  22  that couples to the cable  25  and electromagnetic waves carrying wireless signals are radiated away.  
         [0020]    [0020]FIG. 3 is a perspective view of the dual band planar slot antenna of FIG. 2 according to the first embodiment of the present invention. For the purpose of securely fixing the coaxial cable  25  onto the conducting plate  20 , two supporting arms  241  and  241  are disposed on opposite sides of the outer conductor of the coaxial cable  25 . The supporting arms  241  and  242  create a recession between them for receiving and precisely positioning the outer conductor on the ground  24 . This is advantageous because a precise positioning of the cable  25  on ground  24  and feed point  23  greatly reduces variations in impedance and other antenna characteristics that may occur during manufacturing process if the cable  25  is displaced out of the desired contact points with the conducting plate  20 . Furthermore, at the corners of the conducting plate  20  are disposed a pair of opening  261  and  262  that are used for mounting the antenna  2  onto mobile devices using fasteners such as screws or bolts.  
         [0021]    [0021]FIG. 4 illustrates one example of installation of the dual band planar slot antenna  2  in a LCD panel of a laptop computer system of which only the display part is shown for simplicity. The liquid crystal display  31  is confined within the covering  33  of the LCD panel. A bracket  32  surrounds the display  31  and buttresses it as a structural support for providing rigidity to the covering  33  and the liquid crystal display  31  as a whole. The dual band slot antenna  2  is mounted on the bracket  32  at the left edge of the display  31  using screws  271  and  272  and therebymakes use of the space available between the covering  33  and the display  31 . Though in this example the dual band slot antenna  2  is embedded in LCD panel, it is general knowledge of a skilled artist that the present invention is installable and applicable to other devices in other settings.  
         [0022]    [0022]FIG. 5 is a perspective view of a dual band planar slot antenna according to the second embodiment of the present invention. The slot antenna  5  consists of conducting surfaces  501  and  502 , one deflected or oblique in relation to the other. The long slot  51  which corresponds to lower frequency band is located in the conducting surface  501 , and the short slot  52  corresponding to higher frequency band is located in the conducting surface  502 . A coaxial cable  55  feeds signals across the long slot  51  into feed point  53  with the help of two supporting arms  541  and  542 . As can be discerned, the slot antenna  5  functions in the same way as to that of the slot antenna  2  of the first embodiment, except that in the second embodiment the long slot  51  and the short slot  52  lie in different surfaces at an angle to each other. The deflection or deformation of the antenna  5  offers possibility and flexibility of placement in limited installation space available in compact mobile devices. In addition to the characteristic lengths of the long slot  51  and the short slot  52 , the deflection angle between surfaces  501  and  502  also counts as a factor determining the resonance frequencies of the dual band antenna  5 . By adjusting the dimensions of the slots and the deflection angle, a skilled artist can construct a dual band slot antenna with a wide variety of frequency combinations.  
         [0023]    A skilled artist will readily observe that numerous modifications and alterations of the embodiments may be made while retaining the teachings of the invention. Accordingly, the above disclosure should not be construed in a limiting sense and the true scope of the invention is determined only by the appended claims.