Abstract:
A multi-branch conductive strip planar antenna is disclosed herein, which is basically a planar antenna with a radiator and a ground plane fed by a transmission line. Specifically, the radiator is composed of a plurality of taper-comb-shaped multi-branch conductive strips. Thus, a broadband antenna can be achieved through a plurality of coupled circuits and a plurality of current paths in the taper-comb-shaped conductive strips.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to a planar antenna, and in particular to a planar monopole antenna that has multi-branch conductive strips. 
         [0003]    2. The Prior Arts 
         [0004]    The traditional thin monopole antenna has a simple structure and a lot of advantages, such as vertical polarization and omnidirectional in a horizontal plane. Therefore it is often used in mobile phones or other mobile devices. The primary disadvantage of the antenna is its narrow bandwidth. In the past, the common way to increase the bandwidth of the thin monopole antenna is to thicken the antenna, such as conical monopole antenna and skeletal conical monopole antenna, and so on. The other means to increase the bandwidth include using load resistance or antenna folding. Compared with the thin monopole antenna, these monopole antennas appear bulky. 
         [0005]    For the past ten more years, a broadband planar monopole antenna is developed to replace the thin monopole antenna. Due to asymmetric structure of the planar monopole antenna, a radiation pattern within a radiation frequency band changes a lot. Especially in a high frequency band, a main beam is unable to keep an omnidirectional characteristic in a horizontal direction and in a vertical direction. These affect practical applications. 
         [0006]    However, the planar antenna has the advantages of lightweight, compact size, easy manufacture, easy attachment and easy integration. Therefore applications are extensive. The planar antennas are suitable for application in wireless communication and wireless broadband system. 
         [0007]    Generally, the methods to increase the bandwidth are using a thick dielectric substrate with a low dielectric constant, piling structure, parasitic component, or passive components such as slot, slit, integrated impedance load, chip resistance or capacitance and so on. The methods to reduce the antenna volume include using a short circuit pin, passive component (such as plate capacitance, chip capacitance or chip resistance), and slot and so on to change the current path or the antenna matching characteristics on the sheet metal. 
         [0008]    In a word, in order to reduce the antenna volume and meet the demand of digital video broadcast and digital audio broadcast (DVB/DAB) reception on UHF band (470-860 MHz) and VHF band (170-240 MHz), after long and painstaking thought, the inventor proposes the present invention, that is a multi-branch conductive strip planar antenna. It is an antenna with a plurality of coupled circuits and a plurality of current paths, which achieves the broadband antenna. 
       SUMMARY OF THE INVENTION 
       [0009]    A primary objective of the present invention is to provide a multi-branch conductive strip planar antenna. Due to a radiator having a plurality of multi-branch conductive strips, the working bandwidth of the antenna can cover VHF and the UHF band, and the volume of the antenna is reduced simultaneously. In addition, a passive component may be provided at the input end of the antenna to improve the efficiency by fine-tuning the digital broadcast frequency according to different nations. 
         [0010]    Based on the objective mentioned above, the multi-branch conductive strip planar antenna according to the present invention uses the radiator on a substrate and a ground plane fed by a microstrip to stimulate. Specifically, the radiator is composed of a plurality of taper-comb-shaped multi-branch conductive strips. Thus, the antenna can achieve the objective of broadband through a plurality of coupled circuits and a plurality of current paths. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will be apparent to those skilled in the art by reading the following detailed description of preferred embodiments thereof, with reference to the attached drawings, in which: 
           [0012]      FIG. 1A  is a schematic view showing a multi-branch conductive strip planar antenna in accordance with a first embodiment of the present invention. 
           [0013]      FIG. 1B  is a graph showing the relation between frequency and return loss of the first embodiment of the present invention. 
           [0014]      FIG. 2A  is a schematic view showing a multi-branch conductive strip planar antenna in accordance with a second embodiment of the present invention. 
           [0015]      FIG. 2B  is a graph showing the relation between frequency and return loss of the second embodiment of the present invention. 
           [0016]      FIG. 3A  is a schematic view showing a multi-branch conductive strip planar antenna in accordance with a third embodiment of the present invention. 
           [0017]      FIG. 3B  is a graph showing the relation between frequency and return loss of the third embodiment of the present invention. 
           [0018]      FIG. 4A  is a schematic view showing a multi-branch conductive strip planar antenna in accordance with a fourth embodiment of the present invention. 
           [0019]      FIG. 4B  is a graph showing the relation between frequency and return loss of the fourth embodiment of the present invention. 
           [0020]      FIG. 5A  is a schematic view showing a multi-branch conductive strip planar antenna in accordance with a fifth embodiment of the present invention. 
           [0021]      FIG. 5B  is a graph showing the relation between frequency and return loss of the fifth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    In the following description, five embodiments are used to explain the spirit of the present invention. What should be noticed is that the strip widths and the spacing of multi-branch conductive strips may be the same as or different from one another. A person skilled in the art may adjust the lengths of the taper-comb-shaped multi-branch conductive strips, the number and location of the multi-branch conductive strips, and the location of a ground plane according to actual needs, such as antenna bandwidth, frequency, and radiation pattern, to achieve a better performance of the antenna. 
         [0023]    Referring to  FIG. 1A , in a multi-branch conductive strip planar antenna  10  constructed in accordance with the present invention, a substrate  16  has a radiator and a ground plane  12 . The radiator includes a plurality of taper-comb-shaped multi-branch conductive strips  10   a ,  10   b , and  10   c . The multi-branch conductive strips  10   a ,  10   b , and  10   c  have different strip lengths and make the impedance bandwidth of the antenna  10  to satisfy UHF and VHF frequency band. 
         [0024]    In addition to the radiator and the ground plane  12 , the multi-branch conductive strip planar antenna  10  according to the present invention further comprises a feed circuit on the substrate  16 . The feed circuit includes a feed section  14   a  and connection strip sections  14   b , and electrically connects with the radiator. Where the feed section  14   a  connects with the connection strip sections  14   b  is usually a right angle. The signals from a microstrip (not shown) are fed into the multi-branch conductive strips  10   a ,  10   b , and  10   c  via a feed point  18  of the feed section  14   a  and the connection strip sections  14   b.    
         [0025]    The ground plane  12  and the multi-branch conductive strips  10   a ,  10   b , and  10   c  may produce a coupling effect to reduce the antenna volume. Therefore the ground plane  12  can be placed not only by the multi-branch conductive strips  10   a ,  10   b , and  10   c  as shown in  FIG. 1A , but also on the side of the substrate  16  without the radiator (i.e. on the reverse side of the radiator). 
         [0026]    In summary, the multi-branch conductive strips  10   a ,  10   b , and  10   c  electrically connected with the connection strip sections  14   b  produce a plurality of current paths of different lengths. It makes the antenna  10  have resonance effects of multi-frequency band and broadband. Specifically speaking, in this kind of current path structure, a current distribution in a short current path resonates at a high frequency band, and a current distribution in a long current path resonates at a low frequency band. The taper-comb-shaped multi-branch conductive strips have different strip lengths. Therefore the antenna  10  has the resonance effects of multi-frequency band and broadband. 
         [0027]      FIG. 1B  is a drawing showing the relation between the return loss of the antenna  10  and the frequency in accordance with the first embodiment, and wherein the ordinate axis stands for return loss (unit is decibel (dB)), and the abscissa axis stands for the frequency (the unit is million hertz, MHz).  FIG. 1B  illustrates that the antenna  10  approximately can cover the UHF (470-860 MHz) band and the VHF (170-240 MHz) band for the reception of Digital Video Broadcast (DVB) and Digital Audio Broadcast (DAB) at −3 dB return loss. 
         [0028]    As shown in  FIG. 2A , tapered shapes of multi-branch conductive strips  20   a ,  20   b ,  20   c , and  20   d  according to a second embodiment of the present invention are shaper than those of the first embodiment. Therefore, the multi-branch conductive strips  20   a ,  20   b ,  20   c , and  20   d  are arranged in shapes of right triangles and have slanted ends. Compared with the first embodiment, the multi-branch conductive strips  10   a ,  10   b , and  10   c  are arranged in shapes of trapezoids and have rectangular ends. An antenna  20  constructed in accordance with the second embodiment also has a ground plane  22 , a feed section  24   a , connection strip sections  24   b , a substrate  26 , and a feed point  28 . Their functions and relations between each other are the same as those of the first embodiment.  FIG. 2B  illustrates that the antenna  20  approximately can cover the UHF band and the VHF band for the reception of Digital Video Broadcast (DVB) and Digital Audio Broadcast (DAB) at −3 dB return loss. 
         [0029]    Compared with the second embodiment, multi-branch conductive strips  30   a ,  30   b ,  30   c ,  30   d , and  30   e  according to a third embodiment of the present invention shown in  FIG. 3A  are also in sharp taper-comb shapes, but unlike the second embodiment, they have fewer branches. Moreover, the arrangement of the taper-comb-shaped multi-branch conductive strips  30   a ,  30   b ,  30   c ,  30   d , and  30   e  on the substrate  36  is different from that arranged in series according to the second embodiment. The taper-comb-shaped multi-branch conductive strips  30   a ,  30   b ,  30   c ,  30   d , and  30   e  are arranged parallel to or perpendicular to each other, and all their sizes are not the same. The antenna  30  according to the third embodiment also has a ground plane  32 , a feed section  34   a , connection strip sections  34   b , a substrate  36 , and a feed point  38 .  FIG. 3B  illustrates that the antenna  30  approximately can cover the UHF band and the VHF band for the reception of Digital Video Broadcast (DVB) and Digital Audio Broadcast (DAB) at −3 dB return loss. 
         [0030]    As shown in  FIG. 4A , tapered shapes of multi-branch conductive strips  40   a ,  40   b ,  40   c , and  40   d  in accordance with a fourth embodiment of the present invention are less sharp than those of the first embodiment. Compared with the first embodiment, the multi-branch conductive strips  40   a ,  40   b ,  40   c , and  40   d  are also arranged in shapes of trapezoids but have fewer branches. The lengths of the multi-branch conductive strips  40   a ,  40   b ,  40   c , and  40   d  are much shorter than those of the connection strip sections  44   b . In addition, an antenna  40  according to the fourth embodiment also has a ground plane  42 , a feed section  44   a , connection strip sections  44   b , a substrate  46 , and a feed point  48 .  FIG. 4B  illustrates that the antenna  40  approximately can cover the UHF band and the VHF band for the reception of Digital Video Broadcast (DVB) and Digital Audio Broadcast (DAB) at −3 dB return loss. 
         [0031]    Compared with the third embodiment, multi-branch conductive strips  50   a ,  50   b ,  50   c ,  50   d , and  50   e  in accordance with a fifth embodiment of the present invention as shown in  FIG. 5A , further comprise conduction portions  51  to electrically connect with the ends thereof. The tapered shapes of the multi-branch conductive strips  50   a ,  50   b ,  50   c ,  50   d , and  50   e  are sharper than those of the third embodiment. Compared with the second embodiment, the multi-branch conductive strips  50   a ,  50   b ,  50   c ,  50   d , and  50   e  have fewer branches and their arrangement on a substrate  56  is different from that arranged in series according to the second embodiment. The multi-branch conductive strips  50   a ,  50   b ,  50   c ,  50   d , and  50   e  are arranged parallel to or perpendicular to each other, and all their sizes are not the same. An antenna  50  comprises a ground plane  52 , a feed section  54   a , connection strip sections  54   b , the substrate  56 , and a feed point  58 .  FIG. 5B  illustrate that the antenna  50  approximately can cover the UHF band and the VHF band for the reception of Digital Video Broadcast (DVB) and Digital Audio Broadcast (DAB) at −3 dB return loss. 
         [0032]    In addition, a π circuit or a T circuit, which is a circuit composed of a capacitance  23   a  and an inductance  23   b , may integrate with an input end of the feed section  14   a ,  24   a ,  34   a ,  44   a , or  54   a  as disclosed in the Taiwan Pat. No. 574769 “multi-frequency resonator antenna device” to achieve the objective of resonating at different frequency bands. 
         [0033]    Although the present invention has been described with reference to the preferred embodiments thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.