Patent Publication Number: US-9843090-B2

Title: Multi-frequency antenna

Description:
TECHNICAL FIELD 
     The present invention relates to antenna technology and more particularly, to a multi-frequency antenna capable of generating a plurality of different resonant frequencies. 
     DESCRIPTION OF THE PRIOR ART 
     With fast progress of wireless communication technology, wireless communication products have been widely used in our daily life. The antenna is one of the most important component parts of any of a variety of wireless communication products. An antenna normally occupies a large installation space in a wireless communication product. How to reduce antenna size so as to reduce electronic device dimension is a very important issue. 
     Compared to other antennas, monopole or Planar-Inverted-F Antennas (PIFA) have a low profile and can easily be integrated into active components or circuit boards for mass production. Due to the aforesaid benefits, monopole or PIFA antennas are intensively used in various wireless transmission devices, such as cell phones, smart phones, tablet computers, notebook computers, navigation devices or RFID (Radio Frequency Identification) devices. However, due to the rapid development of wireless communication industry, most mobile devices are installed with communication modules which need to transmit or receive signals in various frequency bands. Therefore, antennas with multiple resonance frequency are the essential elements for most of mobile devices. In order to design a monopole or PIFA antenna with multiple resonance frequency, large circuit board area or space is needed. In actual application, in order to meet the requirement of at least a quarter of the wavelength, the dimensions of monopole or PIFA antennas cannot be further reduced. Further, due to the complicated surrounding environment, a built-in antenna must be redesigned subject to change of the surroundings, for example, change of housing or circuit board, and will significantly increase the design-in lead time. 
     SUMMARY OF THE PRESENT INVENTION 
     It is, therefore, one object of the present invention to provide a multi-frequency antenna, which comprises a ground layer, at least one antenna unit and at least one antenna network, wherein the antenna unit has its one end electrically connected to the ground layer, and its other end electrically connected to the antenna network for generating at least one first resonance frequency. Each antenna network comprises at least one feeding circuit and at least one resonance unit, wherein each resonance unit comprises at least one resonant segment. Each resonant segment is electromagnetically coupled with the ground layer, the extension unit or the conductive unit to generate at least one respective second resonance frequency. Thus, the multi-frequency antenna of the present invention is capable of generating a plurality of different resonance frequencies, widening the application range of the antenna. 
     It is another object of the present invention to provide a multi-frequency antenna, which enables the antenna network to be electromagnetically coupled with the adjacent ground layer, extension unit or conductive unit subject to the wiring of the antenna network, so that the multi-frequency antenna can generate a plurality of different resonance frequencies without increasing the dimension or manufacturing cost of the antenna unit or the multi-frequency antenna. The occupied circuit board area of the multi-frequency antenna in the present invention is much smaller than circuit board area needed by monopole or PIFA antennas. 
     It is still another object of the present invention to provide a multi-frequency antenna, which comprises a ground layer, at least one antenna unit, and at least one antenna network, wherein the antenna unit has its one end electrically connected to the ground layer via a first adjustment device, and its other end electrically connected to the ground layer via an antenna network and a second adjustment device, and thus, the impedance and resonant frequencies of the multi-frequency antenna can be easily fine-tuned by properly choosing the first adjustment device and the second adjustment device. 
     To achieve these and other objectives of the present invention, the present invention provides a multi-frequency antenna, comprising: a ground layer comprising at least one clearance zone which is the cutout region of the ground layer; at least one antenna unit disposed in the clearance zone and electrically connected to the ground layer for generating at least one first resonance frequency, each the antenna unit comprising a dielectric substrate having a first surface and a second surface, and a plurality of conducting layers located on the surface of the dielectric substrate, the conducting layers comprising at least one first conducting layer and at least one second conducting layer; an antenna network disposed in the clearance zone, the antenna network comprising at least one feeding circuit electrically connected to a signal feed-in point and the ground layer, and at least one resonance unit electrically connected to the antenna unit and the feeding circuit, each the resonance unit comprising at least one resonant segment, each the resonant segment being disposed adjacent to the ground layer and electromagnetically coupled with the ground layer to generate at least one second resonance frequency. 
     The present invention further provides a multi-frequency antenna, comprising: a ground layer comprising at least one clearance zone; at least one antenna unit disposed in the clearance zone and electrically connected to the ground layer for generating at least one first resonance frequency, each the antenna unit comprising a dielectric substrate having a first surface and a second surface, and a plurality of conducting layers located on the surface of the dielectric substrate, the conducting layers comprising at least one first conducting layer and at least one second conducting layer; an antenna network disposed in the clearance zone, the antenna network comprising at least one feeding circuit electrically connected to a signal feed-in point and the ground layer, and at least one resonance unit electrically connected to the antenna unit and the feeding circuit, each the resonance unit comprising at least one resonant segment; and a conductive unit disposed in the clearance zone adjacent to the resonant segment and electromagnetically coupled with the resonant segment for generating at least one second resonance frequency, wherein the conductive unit is an electrically conductive segment disposed within clearance zone without contacting ground layer. 
     The present invention provides a multi-frequency antenna, comprising a ground layer comprising at least one clearance zone; at least one antenna unit disposed in the clearance zone and electrically connected to the ground layer for generating at least one first resonance frequency, each the antenna unit comprising a dielectric substrate having a first surface and a second surface, and a plurality of conducting layers located on the surface of the dielectric substrate and comprising at least one first conducting layer and at least one second conducting layer; first adjustment device set between the ground layer and the antenna unit and electrically connected to the ground layer and the antenna unit for fine-tuning the impedance and resonance frequency of the multi-frequency antenna; an antenna network disposed in the clearance zone, the antenna network comprising at least one feeding circuit electrically connected to a signal feed-in point and the ground layer, and at least one resonance unit electrically connected to the antenna unit and the feeding circuit, each the resonance unit comprising at least one resonant segment disposed adjacent to the ground layer and electromagnetically coupled with the ground layer for generating at least one second resonance frequency; and a second adjustment device set between the feeding circuit and the ground layer and electrically connected to the feeding circuit and the ground layer for fine-tuning the impedance and resonant frequencies of the multi-frequency antenna. 
     In one embodiment of the multi-frequency antenna in the present invention, the first conducting layer of the antenna unit is located on the first surface of the dielectric substrate and electrically connected to the ground layer; the second conducting layer of the antenna unit is located on the second surface of the dielectric substrate and electrically connected to the resonance unit of the antenna network, and a part of the first conducting layer overlaps a part of the second conducting layer. 
     In one embodiment of the multi-frequency antenna in the present invention, the first conducting layer and the second conducting layer are located on the first surface of the dielectric substrate, the first conducting layer and the second conducting layer being respectively electrically connected to the resonance unit and the ground layer, wherein the first conducting layer and the second conducting layer are spaced from each other by a gap. 
     In one embodiment of the multi-frequency antenna in the present invention, the resonant segment of the resonance unit comprises a first resonant segment and a second resonant segment respectively disposed adjacent to a part of the ground layer and respectively electromagnetically coupled with a part of the ground layer to generate one, respectively, the second resonance frequency. 
     In one embodiment of the multi-frequency antenna in the present invention, the spacing between the first resonant segment and the ground layer is in the range of 0.01 mm-3 mm; the spacing between the second resonant segment and the ground layer is in the range of 0.01 mm-3 mm. 
     In one embodiment of the multi-frequency antenna in the present invention, the first surface of the antenna unit has two first conducting layers separately mounted thereon, one of the said first conducting layers being electrically connected to said resonance unit, the other said first conducting layer being electrically connected to another signal feed-in point and said ground layer; at least one second conducting layer is disposed on the said second surface and is electrically connected to said ground layer, a part of each said two first conducting layers overlap respectively a part of said second conducting layer. 
     In one embodiment of the multi-frequency antenna in the present invention, the ground layer comprises at least one extension unit disposed adjacent to the resonant segment of the resonance unit and spaced therefrom by a gap in the range of 0.01 mm-3 mm. 
     In one embodiment of the multi-frequency antenna in the present invention, the first conducting layer of the antenna unit is located on the first surface of the dielectric substrate and electrically connected to the ground layer; the second conducting layer is located on the second surface of the dielectric substrate and electrically connected to the resonance unit, wherein a part of the first conducting layer overlaps a part of the second conducting layer. 
     In one embodiment of the multi-frequency antenna in the present invention, the first conducting layer and the second conducting layer are located on the first surface of the dielectric substrate, the first conducting layer and the second conducting layer being electrically connected respectively to the resonance unit and the ground layer, the first conducting layer being spaced from the second conducting layer by a gap. 
     In one embodiment of the multi-frequency antenna in the present invention, the spacing between the resonant segment and the ground layer is in the range of 0.01 mm-3 mm. 
     In one embodiment of the multi-frequency antenna in the present invention, the first conducting layer of the antenna unit is located on the first surface of the dielectric substrate and electrically connected to the ground layer via the first adjustment device; the second conducting layer is located on the second surface of the dielectric substrate and electrically connected to the ground layer via the resonance unit, the feeding circuit and the second adjustment device, wherein a part of the first conducting layer overlaps a part of the second conducting layer. 
     In one embodiment of the multi-frequency antenna in the present invention, the first conducting layer and the second conducting layer are located on the first surface of the dielectric substrate; the first conducting layer being electrically connected to the ground layer via the first adjustment device, the second conducting layer being electrically connected to the ground layer via the resonance unit, the feeding circuit and the second adjustment device, wherein the first conducting layer being spaced from the second conducting layer by a gap. 
     In one embodiment of the multi-frequency antenna in the present invention further comprises a conductive unit disposed in the clearance zone adjacent to and electromagnetically coupled with one of the resonant segment of the resonance unit. 
     In one embodiment of the multi-frequency antenna in the present invention, the spacing between the resonant segment and the conductive unit is within the range of 0.01 mm-3 mm. 
     In one embodiment of the multi-frequency antenna in the present invention further comprises a third adjustment device electrically connected to the conductive unit and the ground layer for fine-tuning the impedance and resonance frequency of the multi-frequency antenna. 
     In one embodiment of the multi-frequency antenna in the present invention, the first adjustment device, the second adjustment device and the third adjustment device comprise at least one capacitor, at least one inductor or at least one resistor. 
     Other advantages and features of the present invention will be fully understood by referring to the following specification in conjunction with the accompanying drawings, in which like reference signs denote like components of structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top view of a multi-frequency antenna in accordance with one embodiment of the present invention. 
         FIG. 2  is a perspective diagram of an antenna unit of a multi-frequency antenna according to one embodiment of the present invention. 
         FIG. 3  is a perspective diagram of an antenna unit of a multi-frequency antenna according to another embodiment of the present invention. 
         FIG. 4  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 5  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 6  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 7  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 8  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 9  is a perspective diagram of an antenna unit of a multi-frequency antenna according to another embodiment of the present invention. 
         FIG. 10  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 11  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 12  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 13  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 14  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 15  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 16  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
         FIG. 17  is a schematic top view of a multi-frequency antenna in accordance with another embodiment of the present invention. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Please refer to  FIG. 1 , there is shown a schematic top view of a multi-frequency antenna in accordance with one embodiment of the present invention. As illustrated, the multi-frequency antenna  10  comprises an antenna unit  11 , a ground layer  13 , and an antenna network  15 . The ground layer  13  comprises at least one clearance zone  131 . The antenna unit  11  is disposed within the clearance zone  131  and electrically connected to the ground layer  13 . 
     Referring to  FIGS. 2 and 3  and also  FIG. 1 , the antenna unit  11  is adapted for generating at least one first resonance frequency, and comprises a dielectric substrate  12  and a plurality of conducting layers  14  arranged on surfaces of the dielectric substrate  12 . 
     The antenna network  15  is disposed within the clearance zone  131  and electrically connected with the antenna unit  11  and the ground layer  13 , and comprises at least one feeding circuit  151  and at least one resonance unit  153 . The feeding circuit  151  is electrically connected to a signal feed-in point  155  and the ground layer  13 . The resonance unit  153  is electrically connected to the antenna unit  11  and the feeding circuit  151 , enabling the antenna unit  11  to be electrically connected to the signal feed-in point  155  and the ground layer  13  via the resonance unit  153  and the feeding circuit  151 . The resonance unit  153  comprises at least one resonant segment  1531  disposed adjacent to a part of the ground layer  13 , and electromagnetically coupled with a part of the ground layer  13  to generate at least one second resonance frequency. 
     In this embodiment, the resonant segment  1531  is a straight line segment. Preferably, the spacing between the resonant segment  1531  and the ground layer  13  is within the range of 0.01 mm-3 mm. In actual application, the second resonance frequency is adjustable by changing the length, width, area or shape of the resonant segment  1531  and/or the spacing between the resonant segment  1531  and the ground layer  13 . 
     In the embodiment shown in  FIG. 2 , the dielectric substrate  12  of the antenna unit  11  comprises a first surface  121  and a second surface  123 . The first surface  121  and the second surface  123  are disposed opposite to each other, for example, opposing top and bottom surfaces. The conducting layer  14  comprises at least one first conducting layer  141  and at least one second conducting layer  143 . The first conducting layer  141  is located on a part of the first surface  121  of the dielectric substrate  12 , and the second conducting layer  143  is located on a part of the second surface  123  of the dielectric substrate  12 . The first conducting layer  141  is electrically connected to the ground layer  13 . The second conducting layer  143  is connected to the resonance unit  153 , and connected to the ground layer  13  and the signal feed-in point  155  via the resonance unit  153  and the feeding circuit  151 . In another embodiment of the present invention, the first conducting layer  141  can be electrically connected to the resonance unit  153 , and the second conducting layer  143  can be connected to the ground layer  13 . 
     A part of the first conducting layer  141  overlaps a part of the second conducting layer  143 , forming an overlap region  142 . In this overlap region  142 , the first conducting layer  141 , the second conducting layer  143  and the dielectric substrate  12  make up a capacitor, enabling the antenna unit  11  to generate the first resonance frequency. Further, the resonance frequency is adjustable by changing the shape and/or dimensions of the first conducting layer  141  and the second conducting layer  143 , and/or the dimensions of the overlap region  142 , and/or the thickness and/or dielectric constant of the dielectric substrate  12 . 
     In an alternate form of the present invention as shown in  FIG. 3 , the dielectric substrate  12  of the antenna unit  11  comprises a first surface  121  and a second surface  123 . The first surface  121  and the second surface  123  are disposed opposite to each other, for example, opposing top and bottom surfaces. The conducting layer  14  comprises a first conducting layer  141  and a second conducting layer  143 . The first conducting layer  141  and the second conducting layer  143  are located on the first surface  121  of the dielectric substrate  12  with a designated gap  16  left between the first conducting layer  141  and the second conducting layer  143 . The first conducting layer  141  is electrically connected to the resonance unit  153 , and the second conducting layer  143  is electrically connected to the ground layer  13 . In another embodiment, the first conducting layer  141  can be electrically connected to the ground layer  13 , and the second conducting layer  143  can be electrically connected to the resonance unit  153 . 
     The first conducting layer  141 , the second conducting layer  143  and the gap  16  therebetween make up a capacitor, enabling the antenna unit  11  to generate at least one first resonance frequency. Further, the resonance frequency is adjustable by changing the shape and/or dimensions of the first conducting layer  141  and the second conducting layer  143 , and/or the width and/or geometric shape of the gap  146 . 
     In this embodiment, the antenna unit  11  has one end thereof electrically connected to the ground layer  13 , for example, the first conducting layer  141  of the antenna unit  11  is electrically connected to the ground layer  13 , and the other end of the antenna unit  11  is electrically connected to the ground layer  13  and the signal feed-in point  155  via the antenna network  15 , wherein the signal feed-in point  155  is electrically connected to a signal feed-in line (not shown) for transmitting RF signals, for example, the second conducting layer  143  of the antenna unit  11  is electrically connected to the ground layer  13  and the signal feed-in point  155  via the antenna network  15 . 
     Referring to  FIG. 4 , there is shown a schematic top view of another multi-frequency antenna in accordance with the present invention. As illustrated, the multi-frequency antenna  20  comprises an antenna unit  11 , a ground layer  13 , and an antenna network  25 , wherein the ground layer  13  comprises a clearance zone  131 , and the antenna unit  11  is disposed within the clearance zone  131  and electrically connected to the ground layer  13 . 
     The antenna unit  11  in this embodiment can be same as that shown in  FIG. 2  and  FIG. 3 , and adapted for generating at least one first resonance frequency. The antenna network  25  within the clearance zone  131  comprises at least one feeding circuit  251  and at least one resonance unit  253 . The feeding circuit  251  is electrically connected to a signal feed-in point  255  and the ground layer  13 , and the resonance unit  253  is electrically connected to the antenna unit  11  and the feeding circuit  251 , enabling the antenna unit  11  to be electrically connected to the ground layer  13  and the signal feed-in point  255  via the resonance unit  253  and the feeding circuit  251 . The resonance unit  253  comprises at least one resonant segment  2531 . The resonant segment  2531  is disposed adjacent to a part of the ground layer  13 , and electromagnetically coupled with a part of the ground layer  13  for generating at least one second resonance frequency. 
     In this embodiment, the antenna unit  11  has one end thereof electrically connectable to the ground layer  13 , for example, the first conducting layer  141  of the antenna unit  11  is electrically connected to the ground layer  13 , and the other end of the antenna unit  11  is electrically connected to the ground layer  13  and the signal feed-in point  255  via the antenna network  25 , wherein the signal feed-in point  255  is electrically connected to a signal feed-in line (not shown) for transmitting RF signals. For example, the second conducting layer  143  of the antenna unit  11  is electrically connected to the ground layer  13  and the signal feed-in point  255  via the antenna network  25 . 
     In this embodiment, the resonant segment  2531  is a straight line segment. In this embodiment, the spacing between the resonant segment  2531  and the adjacent ground layer  13  is preferably within the range of 0.01 mm-3 mm. In actual applications, the second resonance frequency is adjustable by changing the length, width, area and/or shape of the resonant segment  2531  and/or the spacing between ground layer  13  and the resonant segment  2531 . 
     In still another alternate form of the present invention shown in  FIG. 5 , the resonance unit  353  comprises a resonant segment  3531  and at least one protruding units  3533 , wherein the resonance unit  353  is shaped substantially like an inverted E, and the resonant segment  3531  is a straight line segment. 
     Referring to  FIG. 6 , there is shown a schematic top view of another multi-frequency antenna in accordance with the present invention. As illustrated, the multi-frequency antenna  40  mainly comprises an antenna unit  11 , a ground layer  43  and an antenna network  45 , wherein the ground layer  43  comprises a clearance zone  431  and an extension unit  433 , and the antenna unit  11  is disposed within the clearance zone  431  and electrically connected to the ground layer  43 . 
     The antenna unit  11  in this embodiment can be same as that shown in  FIG. 2  and  FIG. 3 , and adapted to generate at least one first resonance frequency. The antenna network  45  within the clearance zone  431  comprises at least one feeding circuit  451  and at least one resonance unit  453 . The feeding circuit  451  is electrically connected to a signal feed-in point  455  and the ground layer  43 . The resonance unit  453  is electrically connected to the antenna unit  11  and the feeding circuit  451 , enabling the antenna unit  11  to be electrically connected to the signal feed-in point  455  and the ground layer  43  via the resonance unit  453  and the feeding circuit  451 . The resonance unit  453  comprises at least one resonant segment  4531 . The resonant segment  4531  is disposed adjacent to the extension unit  433  of the ground layer  43 , and electrically coupled with the extension unit  433  for generating at least one second resonance frequency. 
     In this embodiment, one end of the antenna unit  11  is electrically connected to the ground layer  43 , for example, the first conducting layer  141  of the antenna unit  11  is electrically connected to the ground layer  43 , and the other end of the antenna unit  11  is electrically connected to the ground layer  43  and the signal feed-in point  455  via the antenna network  45 , wherein the signal feed-in point  455  is electrically connected to a signal feed-in line (not shown) for transmitting RF signals, for example, the second conducting layer  143  of the antenna unit  11  is electrically connected to the ground layer  13  and the signal feed-in point  455  via the antenna network  45 . 
     In this embodiment, the extension unit  433  is electrically connected to the ground layer  43 , therefore the ground layer  43  extends to the inside of the clearance zone  431 . The resonance unit  453  has a zigzag or meandering configuration. The resonant segment  4531  has an L-shaped configuration. In this embodiment, the spacing between the resonant segment  4531  and the adjacent extension unit  433  is preferably within the range of 0.01 mm-3 mm. In actual applications, the second resonance frequency is adjustable by changing the length, width, area and/or shape of the resonant segment  4531  and/or the spacing between the extension unit  433  and the resonant segment  4531 . 
     In still another alternate form of the present invention as shown in  FIG. 7 , the extension unit  433  has a substantially L-shaped configuration, and the resonant segment  4531  of the resonance unit  453  is a straight resonance line segment. 
     Referring to  FIG. 8 , there is shown a schematic top view of another embodiment of the multi-frequency antenna in accordance with the present invention. As illustrated, the multi-frequency antenna  50  comprises an antenna unit  51 , a ground layer  53  and an antenna network  55 , wherein the ground layer  53  comprises a clearance zone  531 , and the antenna unit  51  is disposed within the clearance zone  531 . 
     Referring also to  FIG. 9 , the antenna unit  51  is adapted for generating two different first resonance frequencies, and comprises a dielectric substrate  52  and a plurality of conducting layers  54 , wherein the conducting layers  54  are disposed on the surface of the dielectric substrate  52 . 
     The dielectric substrate  52  of the antenna unit  51  comprises a first surface  521  and a second surface  523 , wherein the first surface  521  and the second surface  523  are disposed opposite to each other, for example, opposing top and bottom surfaces. The conducting layer  54  comprises two first conducting layers  541  and one second conducting layer  543 , wherein the two first conducting layers  541  are located on a part of the first surface  521  of the dielectric substrate  52  with a gap  56  left therebetween, and the second conducting layer  543  is located on a part of the second surface  523  of the dielectric substrate  52 . 
     A part of the two first conducting layers  541  respectively overlap a part of the second conducting layer  543 , forming two overlapping regions  542 . The two first conducting layers  541 , the second conducting layer  543  and the dielectric substrate  52  in the overlapping regions  542  form two capacitors respectively, enabling the antenna unit  51  to generate two same or different first resonance frequencies. Further, the two first resonance frequencies are adjustable by changing the shape and/or dimensions of the first conducting layers  541  and the second conducting layer  543 , the dimensions of the two overlapping regions  542  and/or the thickness and/or dielectric constant of the dielectric substrate  52 . 
     The two first conducting layers  541  are electrically connected to the ground layer  53  and respectively one signal feed-in point  5551  or  5553 . For example, one first conducting layer  541  is directly electrically connected to the first signal feed-in point  5551  and the ground layer  53 , and the other first conducting layer  541  is electrically connected to the second signal feed-in point  5553  and the ground layer  53  via the antenna network  55  (for example, the resonance unit  553  and the feeding circuit  551 ). The second conducting layer  543  is electrically connected to the ground layer  53 . 
     The antenna network  55  is disposed within the clearance zone  531 , and comprises at least one feeding circuit  551  and at least one resonance unit  553 . The feeding circuit  551  is electrically connected to the second signal feed-in point  5553  and the ground layer  53 . The resonance unit  553  is electrically connected to the antenna unit  51  and the feeding circuit  551 , enabling the antenna unit  51  to be electrically connected to the second signal feed-in point  5553  and the ground layer  53  via the resonance unit  553  and the feeding circuit  551 . The resonance unit  553  comprises at least one resonant segment  5531 . The resonant segment  5531  is disposed adjacent to a part of the ground layer  53 , and electromagnetically coupled with a part of the ground layer  53  for generating at least one second resonance frequency. 
     In this embodiment, the spacing between the resonant segment  5531  of the resonance unit  553  and the ground layer  53  is preferably within the range of 0.01 mm-3 mm. In actual application, the second resonance frequency is adjustable by changing the length, width, area and/or shape of the resonant segment  5531  and/or the spacing between the ground layer  53  and the resonant segment  5531 . 
     In still another alternate form of the present invention shown in  FIG. 10 , the ground layer  53  comprises an extension unit  533 . The extension unit  533  is electrically connected to the ground layer  53  and extends to the inside of the clearance zone  531 . The resonance unit  553  has a zigzag or meandering configuration. The resonant segment  5531  has an L-shaped configuration. In this embodiment, the spacing between the resonant segment  5531  and the adjacent extension unit  533  is preferably within the range of 0.01 mm-3 mm. In actual applications, the second resonance frequency is adjustable by changing the length, width, area and/or shape of the resonant segment  5531  and/or the spacing between the extension unit  533  and the resonant segment  5531 . 
     In still another alternate form of the present invention as shown in  FIG. 11 , the resonance unit  573  comprises a first resonant segment  5731  and a second resonant segment  5733 , and the ground layer  53  comprises an extension unit  533 . The first resonant segment  5731  is disposed adjacent to the extension unit  533  of the ground layer  53 , and electromagnetically coupled with the extension unit  533 . The second resonant segment  5733  is disposed adjacent to a part of the ground layer  53 , and electromagnetically coupled with a part of the ground layer  53  for generating two same or different second resonance frequencies. For example, the first resonant segment  5731  and the extension unit  533  can generate a second resonance frequency, and the second resonant segment  5733  is electromagnetically coupled with a part of the ground layer  53  to generate another second resonance frequency. In other words, the multi-frequency antenna  500  in  FIG. 11  is capable of generating four different resonance frequencies, wherein the antenna unit  51  is adapted for generating two different first resonance frequencies, and the resonance unit  573  is adapted for generating two different second resonance frequencies. 
     In this embodiment, the spacing between the first resonant segment  5731  and a part of the ground layer  53 , for example, the extension unit  533  of the ground layer  53  is preferably within the range of 0.01 mm-3 mm. The spacing between the second resonant segment  5733  and the adjacent ground layer  53  is preferably within the range of 0.01 mm-3 mm. In actual application, changing the length, width, area and/or shape of the first resonant segment  5731  and the spacing between the first resonant segment  5731  and the extension unit  533  of the ground layer  53  can adjust the respective second resonance frequency. Changing the length, width, area and/or shape of the second resonant segment  5733  and the spacing between the second resonant segment  5733  and the ground layer  53  can adjust the respective second resonance frequency. 
     Referring to  FIG. 12 , there is shown a schematic top view of another embodiment of the multi-frequency antenna in accordance with the present invention. As illustrated, the multi-frequency antenna  60  mainly comprises an antenna unit  11 , a ground layer  13 , an antenna network  65  and a conductive unit  67 , wherein the ground layer  13  comprises a clearance zone  131 . The antenna unit  11  is disposed in the clearance zone  131 , the conductive unit  67  is a conducting layer disposed in the clearance zone  131 , the antenna unit  11  is electrically connected to the ground layer  13 , and the conductive unit  67  is isolated from the ground layer  13 . 
     Referring also to  FIG. 2  and  FIG. 3  for this embodiment, the antenna unit  11  is adapted for generating at least one first resonance frequency, and comprises a dielectric substrate  12  and a plurality of conducting layers  14 , wherein the conducting layer  14  is located on the surface of the dielectric substrate  12 . 
     The antenna network  65  is disclosed in the clearance zone  131 , and comprises at least one feeding circuit  651  and at least one resonance unit  653 . The feeding circuit  651  is electrically connected to a signal feed-in point  655  and the ground layer  13 . The resonance unit  653  is electrically connected to the antenna unit  11  and the feeding circuit  651  so that the antenna unit  11  is electrically connected to the signal feed-in point  655  and the ground layer  13  via the resonance unit  653  and the feeding circuit  651 . The resonance unit  653  comprises at least one resonant segment  6531  disposed adjacent to the conductive unit  67  and electromagnetically coupled with the conductive unit  67  to generate at least one second resonance frequency. 
     In this embodiment, the resonant segment  6531  is a straight line segment, and the conductive unit  67  has a substantially L-shaped configuration. Further, the spacing between the resonant segment  6531  and the adjacent conductive unit  67  is preferably within the range of 0.01 mm-3 mm. In actual application, the second resonance frequency is adjustable by changing the length, width, area and/or shape of the resonant segment  6531  and/or the spacing between the conductive unit  67  and the resonant segment  6531 . Alternatively, as shown in  FIG. 13 , the resonant segment  6531  can be made having an L-shaped configuration, and the conductive unit  67  can be shaped like C-shaped configuration. 
     Referring to  FIG. 14 , there is shown a schematic top view of another embodiment of the multi-frequency antenna in accordance with the present invention. As illustrated, the multi-frequency antenna  70  mainly comprises an antenna unit  11 , a ground layer  13  and an antenna network  75 , wherein the ground layer  13  comprises a clearance zone  131 , and the antenna unit  11  is disposed in the clearance zone  131  and electrically connected to the ground layer  13 . 
     In this embodiment, referring also to  FIG. 2  and  FIG. 3 , the antenna  11  is adapted for generating at least one first resonance frequency, and comprises a dielectric substrate  12  and a plurality of conducting layers  14 , wherein the conducting layers  14  are located on the surface of the dielectric substrate  12 . 
     The antenna network  75  is disposed in the clearance zone  131  and comprises at least one feeding circuit  751  and at least one resonance unit  753 . The feeding circuit  751  is electrically connected to a signal feed-in point  755  and the ground layer  13 , and the resonance unit  753  is electrically connected to the antenna unit  11  and the feeding circuit  751  so that the antenna unit  11  is electrically connected to the signal feed-in point  755  and ground layer  13  via the resonance unit  753  and the feeding circuit  751 . The resonance unit  753  comprises at least one resonant segment  7531  disposed adjacent to a part of the ground layer  13  and electromagnetically coupled with the ground layer  13  to generate at least one second resonance frequency. In this embodiment the spacing between the resonant segment  7531  and the adjacent ground layer  13  is preferably within the range of 0.01 mm-3 mm. In actual application, the second resonance frequency is adjustable by changing the length, width and/or area of the resonant segment  7531 , and/or the spacing between the resonant segment  7531  and the ground layer  13 . 
     Furthermore, a conductive unit  87  can be provided in the clearance zone  131 . The conductive unit  87  is spaced from the ground layer  13  by a spacing. Further, a part of the conductive unit  87  is disposed adjacent to and electromagnetically coupled with another resonant segment  7533 . The electromagnetic coupling effect between the conductive unit  87  and the resonant segment  7533  interacts with the electromagnetic coupling effect between the resonant segment  7531  and the ground layer  13  to generate another second resonance frequency. The spacing between the resonant segment  7533  and the conductive unit  87  is preferably within the range of 0.01 mm-3 mm. 
     Referring to  FIG. 15 , there is shown a schematic top view of another embodiment of the multi-frequency antenna in accordance with the present invention. As illustrated, the multi-frequency antenna  80  mainly comprises an antenna unit  11 , a ground layer  13 , an antenna network  85  and a conductive unit  87 , wherein the ground layer  13  comprises a clearance zone  131 . The antenna unit  11  and the conductive unit  87  are disposed in the clearance zone  131 . Further, a first adjustment device  871  is set between the antenna unit  11  and the ground layer  13 . The antenna unit  11  is electrically connected to the ground layer  13  via the first adjustment device  871 . Further, a spacing exists between the conductive unit  87  and the ground layer  13 . 
     In this embodiment, referring also to  FIG. 2  and  FIG. 3 , the antenna  11  is adapted for generating at least one first resonance frequency, and comprises a dielectric substrate  12  and a plurality of conducting layers  14 , wherein the conducting layers  14  are located on the surface of the dielectric substrate  12 . 
     The antenna network  85  is disposed in the clearance zone  131  and comprises at least one feeding circuit  851  and at least one resonance unit  853 . The feeding circuit  851  is electrically connected to a signal feed-in point  855 . Further, a second adjustment device  873  is set between the feeding circuit  851  and the ground layer  13 . The feeding circuit  851  is electrically connected to the ground layer  13  via the second adjustment device  873 . The resonance unit  853  is electrically connected to the antenna unit  11  and the feeding circuit  851  so that the antenna unit  11  is electrically connected to the signal feed-in point  855  via the resonance unit  853  and the feeding circuit  851 , and electrically connected to the ground layer  13  via the resonance unit  853 , the feeding circuit  851  and the second adjustment device  873 . The resonance unit  853  comprises at least one resonant segment  8531  that is disposed adjacent to a part of the conductive unit  87 . In this embodiment, a spacing exists between the conductive unit  87  and the ground layer  13 . Further, the resonant segment  8531  and the conductive unit  87  are electromagnetically coupled together to generate at least one second resonance frequency. 
     In this embodiment, the resonant segment  8531  has an L-shaped configuration, and the conductive unit  87  has a substantially C-shaped configuration. In this embodiment, the spacing between at least one resonant segment  8531  and the conductive unit  87  is preferably within the range of 0.01 mm-3 mm. In actual application, the second resonance frequency is adjustable by changing the length, width, area and/or shape of the resonant segment  8531  and/or the conductive unit  87 , and/or the spacing between at least one resonant segment  8531  and the conductive unit  87 . 
     In this embodiment, the first adjustment device  871  and the second adjustment device  873  are adapted for fine-tuning the impedance and resonance frequency of the multi-frequency antenna  80 . The first adjustment device  871  and the second adjustment device  873  can be, for example, capacitor and/or inductor or resistor. Through the use of capacitors of different capacitance values and/or inductors of different inductance values and/or resistors of different resistance values, the impedance and resonance frequency of the multi-frequency antenna  80  are relatively changed. 
     Referring to  FIG. 16 , there is shown a schematic top view of another embodiment of the multi-frequency antenna in accordance with the present invention. As illustrated, this embodiment is substantially similar to the embodiment shown in  FIG. 15  with the exception that this embodiment further comprises a third adjustment device  875  set between the conductive unit  87  and the ground layer  13 . Thus, the conductive unit  87  is electrically connected to the ground layer  13  via the third adjustment device  875 . The third adjustment device  875  can be formed of capacitor and/or inductor and/or resistor. Through the use of capacitor of different capacitance value and/or inductor of different inductance value and/or resistor of different resistance value, the impedance and resonance frequency of the multi-frequency antenna  80  are relatively changed. 
     Referring to  FIG. 17 , there is shown a schematic top view of another embodiment of the multi-frequency antenna in accordance with the present invention. As illustrated, this embodiment is substantially similar to the embodiment shown in  FIG. 14  with the exception that this embodiment further comprises a plurality of adjustment units  771 / 773 / 775 . The first adjustment device  771  is set between the antenna unit  11  and the ground layer  13 , and the antenna unit  11  has one end thereof electrically connected to the ground layer  13  via the first adjustment device  771 . The second adjustment device  773  is set between the feeding circuit  751  and the ground layer  13 , and the antenna unit  11  has an opposite end thereof electrically connected to the ground layer  13  via the antenna network  75  and the second adjustment device  773 . The third adjustment device  775  is set between the conductive unit  87  and the ground layer  13 , and the conductive unit  87  is electrically connected to the ground layer  13  via the third adjustment device  775 . The first adjustment device  771 , the second adjustment device  773  and the third adjustment device  775  are adapted for fine-tuning the impedance and resonance frequency of the multi-frequency antenna  70 . The first adjustment device  771 , the second adjustment device  773  and the third adjustment device  775  can be formed of, for example, capacitors and/or inductors and/or resistors. Through the use of capacitors of different capacitance values and/or inductors of different inductance values and/or resistors of different resistance values, the impedance and resonance frequencies of the multi-frequency antenna  70  are relatively changed. 
     It is to be understood that the invention is not limited to particular systems described which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in the present invention, the singular forms “a”, “an” and “the” include plural referents unless the content clearly indicates otherwise. Thus, for example, reference to “a device” includes a combination of two or more devices and reference to “a material” includes mixtures of materials. 
     Further modifications and alternative embodiments of various aspects of the present invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.