Patent Publication Number: US-11024944-B2

Title: Antenna structure and wireless communication device using same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Chinese Patent Application No. 201710497766.9 filed on Jun. 27, 2017, and claims priority to U.S. Patent Application No. 62/364,303, filed on Jul. 19, 2016, the contents of which are incorporated by reference herein. 
    
    
     FIELD 
     The subject matter herein generally relates to an antenna structure and a wireless communication device using the antenna structure. 
     BACKGROUND 
     Metal housings, for example, metallic backboards, are widely used for wireless communication devices, such as mobile phones or personal digital assistants (PDAs). Antennas are also important components in wireless communication devices for receiving and transmitting wireless signals at different frequencies, such as signals in Long Term Evolution Advanced (LTE-A) frequency bands. However, when the antenna is located in the metal housing, the antenna signals are often shielded by the metal housing. This can degrade the operation of the wireless communication device. Additionally, the metallic backboard generally defines slots or/and gaps thereon, which will affect a structural integrity and an aesthetic quality of the metallic backboard. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present technology will now be described, by way of example only, with reference to the attached figures. 
         FIG. 1  is an isometric view of a first exemplary embodiment of a wireless communication device using a first exemplary antenna structure. 
         FIG. 2  is an assembled, isometric view of the wireless communication device of  FIG. 1 . 
         FIG. 3  is similar to  FIG. 2 , but shown from another angle. 
         FIG. 4  is a circuit diagram of a first switching circuit of the antenna structure of  FIG. 1 . 
         FIG. 5  is a circuit diagram of the first switching circuit of  FIG. 4 , showing the first switching circuit includes a resonance circuit. 
         FIG. 6  is similar to  FIG. 5 , but shown the first switching circuit includes another resonance circuit. 
         FIG. 7  is a schematic diagram of the antenna structure of  FIG. 1 , showing the first switching circuit of  FIG. 5  includes a resonance circuit and generates a resonance mode. 
         FIG. 8  is a schematic diagram of the antenna structure of  FIG. 1 , showing the first switching circuit of  FIG. 6  includes a resonance circuit and generates a resonance mode. 
         FIG. 9  is a current path distribution graph when the antenna structure of  FIG. 1  works at a low frequency operation mode and a Global Positioning System (GPS) operation mode. 
         FIG. 10  is a current path distribution graph when the antenna structure of  FIG. 1  works at a frequency band of about 1710-2690 MHz. 
         FIG. 11  is a scattering parameter graph when the antenna structure of  FIG. 1  works at a low frequency operation mode and a GPS operation mode. 
         FIG. 12  is a radiating efficiency graph when the antenna structure of  FIG. 1  works at a low frequency operation mode. 
         FIG. 13  is a radiating efficiency graph when the antenna structure of  FIG. 1  works at a GPS operation mode. 
         FIG. 14  is a scattering parameter graph when the antenna structure of  FIG. 1  works at a frequency band of about 1710-2690 MHz. 
         FIG. 15  is a radiating efficiency graph when the antenna structure of  FIG. 1  works at a frequency band of about 1710-2690 MHz. 
         FIG. 16  is an isometric view of a second exemplary embodiment of a wireless communication device using a second exemplary antenna structure. 
         FIGS. 17 to 19  are isometric views of the antenna structure of  FIG. 16 , showing a location relationship of an isolating portion. 
         FIG. 20  is a current path distribution graph when the antenna structure of  FIG. 16  works at a high frequency operation mode. 
         FIG. 21  is a current path distribution graph when the antenna structure of  FIG. 16  works at a dual-band WIFI operation mode. 
         FIG. 22  is a scattering parameter graph when the antenna structure of  FIG. 16  works at a middle frequency operation mode and a high frequency operation mode. 
         FIG. 23  is a radiating efficiency graph when the antenna structure of  FIG. 16  works at a middle frequency operation mode and a high frequency operation mode. 
         FIG. 24  is a scattering parameter graph when the antenna structure of  FIG. 16  works at a WIFI 2.4 GHz mode and a WIFI 5 GHz mode. 
         FIG. 25  is a radiating efficiency graph when the antenna structure of  FIG. 16  works at a WIFI 2.4 GHz mode. 
         FIG. 26  is a radiating efficiency graph when the antenna structure of  FIG. 16  works at a WIFI 5 GHz mode. 
         FIG. 27  is an isometric view of a third exemplary embodiment of a wireless communication device using a third exemplary antenna structure. 
         FIG. 28  is an assembled, isometric view of the wireless communication device of  FIG. 27 . 
         FIG. 29  is similar to  FIG. 28 , but shown from another angle. 
         FIG. 30  is a circuit diagram of a first switching circuit of the antenna structure of  FIG. 27 . 
         FIG. 31  is a circuit diagram of a second switching circuit of the antenna structure of  FIG. 27 . 
         FIG. 32  is a current path distribution graph of the antenna structure of  FIG. 27 . 
         FIG. 33  is a circuit diagram of the first switching circuit of  FIG. 30 , showing the first switching circuit includes a resonance circuit. 
         FIG. 34  is similar to  FIG. 33 , but shown the first switching circuit includes another resonance circuit. 
         FIG. 35  is a schematic diagram of the antenna structure of  FIG. 27 , showing the first switching circuit of  FIG. 33  includes a resonance circuit and generates a resonance mode. 
         FIG. 36  is a schematic diagram of the antenna structure of  FIG. 27 , showing the first switching circuit of  FIG. 34  includes a resonance circuit and generates a resonance mode. 
         FIG. 37  is a current path distribution graph when the antenna structure of  FIG. 27  includes a resonance circuit and works at a low frequency operation mode. 
         FIG. 38  is a current path distribution graph when the antenna structure of  FIG. 27  includes a resonance circuit and works at a frequency band of about 1710-2690 MHz. 
         FIG. 39  is a scattering parameter graph when the antenna structure of  FIG. 27  works at a low frequency operation mode. 
         FIG. 40  is a radiating efficiency graph when the antenna structure of  FIG. 27  works at a low frequency operation mode. 
         FIG. 41  is a scattering parameter graph when the antenna structure of  FIG. 27  works at a frequency band of about 1710-2690 MHz. 
         FIG. 42  is a radiating efficiency graph when the antenna structure of  FIG. 27  works at a frequency band of about 1710-2690 MHz. 
         FIG. 43  is an isometric view of a fourth exemplary embodiment of a wireless communication device using a fourth exemplary antenna structure. 
         FIG. 44  is a current path distribution graph when the antenna structure of  FIG. 43  works at a frequency band of about 1710-2400 MHz. 
         FIG. 45  is a current path distribution graph when the antenna structure of  FIG. 43  works at a dual-band WIFI mode. 
         FIG. 46  is a current path distribution graph when the antenna structure of  FIG. 43  works at a frequency band of about 2496-2690 MHz. 
         FIG. 47  is a scattering parameter graph when the antenna structure of  FIG. 43  works at a frequency band of about 1710-2400 MHz. 
         FIG. 48  is a radiating efficiency graph when the antenna structure of  FIG. 43  works at a frequency band of about 1710-2400 MHz. 
         FIG. 49  is a scattering parameter graph when the antenna structure of  FIG. 43  works at a WIFI 2.4 GHz mode and a WIFI 5 GHz mode. 
         FIG. 50  is a radiating efficiency graph when the antenna structure of  FIG. 43  works at a WIFI 2.4 GHz mode and a WIFI 5 GHz mode. 
         FIG. 51  is a scattering parameter graph when the antenna structure of  FIG. 43  works at a frequency band of about 2496-2690 MHz. 
         FIG. 52  is a radiating efficiency graph when the antenna structure of  FIG. 43  works at a frequency band of about 2496-2690 MHz. 
         FIG. 53  is an isometric view of a fifth exemplary embodiment of a wireless communication device using a fifth exemplary antenna structure. 
         FIG. 54  is a current path distribution graph when the antenna structure of  FIG. 53  works at a frequency band of about 1710-2170 MHz. 
         FIG. 55  is a current path distribution graph when the antenna structure of  FIG. 53  works at frequency bands of about 2300-2400 MHz and 2496-2690 MHz. 
         FIG. 56  is a scattering parameter graph when the antenna structure of  FIG. 53  works at a frequency band of about 1710-2170 MHz. 
         FIG. 57  is a radiating efficiency graph when the antenna structure of  FIG. 53  works at a frequency band of about 1710-2170 MHz. 
         FIG. 58  is a scattering parameter graph when the antenna structure of  FIG. 53  works at frequency bands of about 2300-2400 MHz and 2496-2690 MHz. 
         FIG. 59  is a radiating efficiency graph when the antenna structure of  FIG. 53  works at frequency bands of about 2300-2400 MHz and 2496-2690 MHz. 
         FIG. 60  is an isometric view of a sixth exemplary embodiment of a wireless communication device using a sixth exemplary antenna structure. 
         FIG. 61  is an assembled, isometric view of the wireless communication device of  FIG. 60 . 
         FIG. 62  is similar to  FIG. 61 , but shown from another angle. 
         FIG. 63  is a circuit diagram of a first switching circuit of the antenna structure of  FIG. 60 . 
         FIG. 64  is a circuit diagram of a second switching circuit of the antenna structure of  FIG. 60 . 
         FIG. 65  is a circuit diagram of the first switching circuit of  FIG. 63 , showing the first switching circuit includes a resonance circuit. 
         FIG. 66  is similar to  FIG. 65 , but shown the first switching circuit includes another resonance circuit. 
         FIG. 67  is a schematic diagram of the antenna structure of  FIG. 60 , showing the first switching circuit of  FIG. 65  includes a resonance circuit and generates a resonance mode. 
         FIG. 68  is a schematic diagram of the antenna structure of  FIG. 60 , showing the first switching circuit of  FIG. 66  includes a resonance circuit and generates a resonance mode. 
         FIG. 69  is a current path distribution graph when the antenna structure of  FIG. 60  works at a low frequency operation mode. 
         FIG. 70  is a current path distribution graph when the antenna structure of  FIG. 60  works at a middle frequency operation mode. 
         FIG. 71  is a current path distribution graph when the antenna structure of  FIG. 60  works at a high frequency operation mode. 
         FIG. 72  is a scattering parameter graph when the antenna structure of  FIG. 60  works at a low frequency operation mode. 
         FIG. 73  is a radiating efficiency graph when the antenna structure of  FIG. 60  works at a low frequency operation mode. 
         FIG. 74  is a scattering parameter graph when the antenna structure of  FIG. 60  works at a middle frequency operation mode. 
         FIG. 75  is a radiating efficiency graph when the antenna structure of  FIG. 60  works at a middle frequency operation mode. 
         FIG. 76  is a scattering parameter graph when the antenna structure of  FIG. 60  works at a high frequency operation mode. 
         FIG. 77  is a radiating efficiency graph when the antenna structure of  FIG. 60  works at a high frequency operation mode. 
         FIG. 78  is an isometric view of a seventh exemplary embodiment of a wireless communication device using a seventh exemplary antenna structure. 
         FIG. 79  is a current path distribution graph when the antenna structure of  FIG. 78  works at a middle frequency operation mode. 
         FIG. 80  is a scattering parameter graph when the antenna structure of  FIG. 78  works at a low frequency operation mode. 
         FIG. 81  is a radiating efficiency graph when the antenna structure of  FIG. 78  works at a low frequency operation mode. 
         FIG. 82  is a scattering parameter graph when the antenna structure of  FIG. 78  works at a middle frequency operation mode. 
         FIG. 83  is a radiating efficiency graph when the antenna structure of  FIG. 78  works at a middle frequency operation mode. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of the present disclosure. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The term “substantially” is defined to be essentially conforming to the particular dimension, shape, or other feature that the term modifies, such that the component need not be exact. For example, substantially cylindrical means that the object resembles a cylinder, but can have one or more deviations from a true cylinder. The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series and the like. 
     The present disclosure is described in relation to an antenna structure and a wireless communication device using same. 
     Exemplary Embodiment 1-2 
       FIG. 1  illustrates an embodiment of a wireless communication device  400  using a first exemplary antenna structure  100 . The wireless communication device  400  can be a mobile phone or a personal digital assistant, for example. The antenna structure  100  can receive and/or transmit wireless signals. 
     Per  FIG. 2  and  FIG. 3 , the antenna structure  100  includes a metallic member  11 , a first feed source  13 , a second feed source  14 , and a first switching circuit  15 . The metallic member  11  can be a metal housing of the wireless communication device  400 . In this exemplary embodiment, the metallic member  11  is a frame structure and includes a front frame  111 , a backboard  112 , and a side frame  113 . The front frame  111 , the backboard  112 , and the side frame  113  can be integral with each other. The front frame  111 , the backboard  112 , and the side frame  113  cooperatively form the metal housing of the wireless communication device  400 . 
     The front frame  111  defines an opening (not shown). The wireless communication device  400  includes a display  401 . The display  401  is received in the opening. The display  401  has a display surface. The display surface is exposed at the opening and is positioned parallel to the backboard  112 . 
     The backboard  112  is positioned opposite to the front frame  111 . The backboard  112  is an integral and single metallic sheet. The backboard  112  defines holes  404 ,  405  for exposing a camera lens  402  and a flash light  403 . The backboard  112  does not define any slot, break line, and/or gap for dividing the backboard  112 . The backboard  112  serves as a ground of the antenna structure  100 . 
     The side frame  113  is positioned between the front frame  111  and the backboard  112 . The side frame  113  is positioned around a periphery of the front frame  111  and a periphery of the backboard  112 . The side frame  113  forms a receiving space  114  together with the display  401 , the front frame  111 , and the backboard  112 . The receiving space  114  can receive a print circuit board, a processing unit, or other electronic components or modules. 
     The side frame  113  includes a top portion  115 , a first side portion  116 , and a second side portion  117 . The top portion  115  connects the front frame  111  and the backboard  112 . The first side portion  116  is positioned apart from and parallel to the second side portion  117 . The top portion  115  has first and second ends. The first side portion  116  is connected to the first end of the first frame  111  and the second side portion  117  is connected to the second end of the top portion  115 . The first side portion  116  connects the front frame  111  and the backboard  112 . The second side portion  117  also connects the front frame  111  and the backboard  112 . 
     The side frame  113  defines a slot  118 . The front frame  111  defines a gap  119 . In this exemplary embodiment, the slot  118  is defined at the top portion  115  and extends to the first side portion  116  and the second side portion  117 . In other exemplary embodiments, the slot  118  is defined only at the top portion  115  and does not extend to any one of the first side portion  116  and the second side portion  117 . In other exemplary embodiments, the slot  118  can be defined at the top portion  115  and extends to one of the first side portion  116  and the second side portion  117 . The gap  119  communicates with the slot  118  and extends across the front frame  111 . In this exemplary embodiment, the gap  119  is positioned adjacent to the second side portion  117 . The front frame  111  is divided into two portions by the gap  119 , that is, a long portion A 1  and a short portion A 2  (long and short relative to each other). A first portion of the front frame  111  extending from a first side of the gap  119  to a first end E 1  of the slot  118  forms the long portion A 1 . A second portion of the front frame  111  extending from a second side of the gap  119  to a second end E 2  of the slot  118  forms the short portion A 2 . 
     In this exemplary embodiment, the gap  119  is not positioned at a middle portion of the top portion  115 . The long portion A 1  is longer than the short portion A 2 . 
     In this exemplary embodiment, the slot  118  and the gap  119  are both filled with insulating material, for example, plastic, rubber, glass, wood, ceramic, or the like, thereby isolating the long portion A 1 , the short portion A 2 , and the backboard  112 . 
     In this exemplary embodiment, except for the slot  118  and the gap  119 , an upper half portion of the front frame  111  and the side frame  113  does not define any other slot, break line, and/or gap. That is, there is only one gap  119  defined on the upper half portion of the front frame  111 . 
     The first feed source  13  is electrically connected to the end of the long portion A 1  adjacent to the first side portion  116 . The first feed source  13  can feed current to the long portion A 1  and activates the long portion A 1  to a first mode to generate radiation signals in a first frequency band. In this exemplary embodiment, the first mode is a low frequency operation mode. The first frequency band is a frequency band of about 700-900 MHz. 
     The second feed source  14  is electrically connected to the end of the short portion A 2  adjacent to the gap  119 . The second feed source  14  can feed current to the short portion A 2  and activate the short portion A 2  to two modes to generate radiation signals in a wide band mode (1710-2690 MHz). The wide band mode can contain a middle frequency operation mode, a high frequency operation mode, and a WIFI 2.4 GHz band. 
     Per  FIG. 4 , the first switching circuit  15  is electrically connected to the long portion A 1 . The first switching circuit  15  includes a switching unit  151  and a plurality of switching elements  153 . The switching unit  151  is electrically connected to the long portion A 1 . The switching elements  153  can be an inductor, a capacitor, or a combination of the inductor and the capacitor. The switching elements  153  are connected in parallel. One end of each switching element  153  is electrically connected to the switching unit  151 . The other end of each switching element  153  is electrically connected to the backboard  112 . Through controlling the switching unit  151 , the long portion A 1  can be switched to connect with different switching elements  153 . Since each switching element  153  has a different impedance, an operating frequency band of the long portion A 1  can be adjusted through switching the switching unit  151 , for example, the frequency band of the first mode of the long portion A 1  can be offset towards a lower frequency or towards a higher frequency (relative to each other). 
     Per  FIG. 5  and  FIG. 6 , the first switching circuit  15  further includes a resonance circuit  155 . Per  FIG. 5 , in one exemplary embodiment, the first switching circuit  15  includes one resonance circuit  155 . The resonance circuit  155  includes an inductor L and a capacitor C connected in series. The resonance circuit  155  is electrically connected between the long portion A 1  and the backboard  112 . The resonance circuit  155  is connected in parallel to the switching unit  151  and at least one switching element  153 . 
     Per  FIG. 6 , in another exemplary embodiment, the first switching circuit  15  includes a plurality of resonance circuits  155 . The number of the resonance circuits  155  is equal to the number of switching elements  153 . Each resonance circuit  155  includes an inductor L and a capacitor C connected in series. Each resonance circuit  155  is electrically connected in parallel to one of the switching elements  153  between the switching unit  151  and the backboard  112 . 
     Per  FIG. 7 , when the first switching circuit  15  does not include the resonance circuit  155 , the antenna structure  100  works at the first mode (please see the curve S 51 ). When the first switching circuit  15  includes the resonance circuit  155 , the long portion A 1  of the antenna structure  100  can activate an additional resonance mode (that is, the second mode, please see the curve S 52 ) to generate radiation signals in the second frequency band. The second mode can effectively broaden an applied frequency band of the antenna structure  100 . In one exemplary embodiment, the second frequency band is a GPS operation band and the second mode is the GPS resonance mode. 
     Per  FIG. 8 , when the first switching circuit  15  does not include the resonance circuit  155 , the antenna structure  100  works at the first mode (please see the curve S 61 ). When the first switching circuit  15  includes the resonance circuit  155 , the long portion A 1  of the antenna structure  100  can activate the additional resonance mode (please see the curve S 62 ), that is, the GPS resonance mode. The resonance mode can effectively broaden an applied frequency band of the antenna structure  100 . In one exemplary embodiment, an inductance value of the inductor L and a capacitance value of the capacitor C of the resonance circuit  155  can cooperatively decide a frequency band of the resonance mode when the first mode switches. For example, in one exemplary embodiment, as illustrated in  FIG. 8 , when the switching unit  151  switches to different switching elements  153  through setting the inductance value and the capacitance value of the resonance circuit  155 , the resonance mode of the antenna structure  100  can also be switched. For example, the resonance mode of the antenna structure  100  can be moved from f 1  to fn. 
     In other exemplary embodiments, the frequency band of the resonance mode can be fixed through setting the inductance value and the capacitance value of the resonance circuit  155 . Then no matter to which switching element  153  the switching unit  151  is switched, the frequency band of the resonance mode is fixed and keeps unchanged. 
     In other exemplary embodiments, the resonance circuit  155  is not limited to include the inductor L and the capacitor C, and can include other resonance components. 
     Per  FIG. 9 , when the current enters the long portion A 1  from the first feed source  13 , the current flows through the long portion A 1  and towards the gap  119  (please see a path P 1 ) to activate the low frequency operation mode. Since the antenna structure  100  includes the first switching circuit  15 , the low frequency operation mode of the long portion A 1  can be switched through the first switching circuit  15 . Since the first switching circuit  15  includes the resonance circuit  155 , the low frequency operation mode and the GPS operation mode can be active simultaneously. In this exemplary embodiment, a total current of the GPS operation mode is contributed by two current sources. One current source is from the low frequency operation mode (Per the path P 1 ). The other current source is from the inductor L and the capacitor C of the resonance circuit  155  being impedance matched (e.g., path P 2 ). In this exemplary embodiment, a current of the path P 2  flows to one end of the short portion A 2  away from the second feed source  14  from the other end of the short portion A 2  adjacent to the second feed source  14 . 
     Per  FIG. 10 , when the current enters the short portion A 2  from the second feed source  14 , the current flows to the front frame  111 , the second side portion  117 , and the backboard  112  (e.g., path P 3 ) to activate a third mode for generating radiation signals in a third frequency band (1710-2690 MHz) and containing the middle frequency operation mode, the high frequency operation mode, and the WIFI 2.4 GHz band. From  FIG. 4  to  FIG. 10 , the backboard  112  serves as the ground of the antenna structure  100 . 
       FIG. 11  illustrates a scattering parameter graph of the antenna structure  100 , when the antenna structure  100  works at the low frequency operation mode and the GPS operation mode. Curve  91  illustrates a scattering parameter when the antenna structure  100  works at a LTE-A Band 28 (703-803 MHz). Curve  92  illustrates a scattering parameter when the antenna structure  100  works at a LTE-A Band 5 (869-894 MHz). Curve  93  illustrates a scattering parameter when the antenna structure  100  works at a LTE-A Band 8 (925-926 MHz) and the GPS band (1.575 GHz). In this exemplary embodiment, curve  91  and curve  92  respectively correspond to two different frequency bands and respectively correspond to two of the plurality of low frequency bands of the switching circuit  15 . 
       FIG. 12  illustrates a radiating efficiency graph of the antenna structure  100 , when the antenna structure  100  works at the low frequency operation mode. Curve  101  illustrates a radiating efficiency when the antenna structure  100  works at a LTE-A Band 28 (703-803 MHz). Curve  102  illustrates a radiating efficiency when the antenna structure  100  works at a LTE-A Band 5 (869-894 MHz). Curve  103  illustrates a radiating efficiency when the antenna structure  100  works at a LTE-A Band 8 (925-926 MHz). In this exemplary embodiment, curve  101 , curve  102 , and curve  103  respectively correspond to three different frequency bands and respectively correspond to three of the plurality of low frequency bands of the switching circuit  15 . 
       FIG. 13  illustrates a radiating efficiency graph of the antenna structure  100 , when the antenna structure  100  works at the GPS operation mode.  FIG. 14  illustrates a scattering parameter graph of the antenna structure  100 , when the antenna structure  100  works at the frequency band of about 1710-2690 MHz (that is, the middle frequency operation mode, the high frequency operation mode, and the WIFI 2.4 GHz band).  FIG. 15  illustrates a radiating efficiency graph of the antenna structure  100 , when the antenna structure  100  works at the frequency band of about 1710-2690 MHz (that is, the middle frequency band, the high frequency band, and the WIFI 2.4 GHz band). 
     Per  FIGS. 11 to 15 , the antenna structure  100  can work at a low frequency band, for example, LTE-A band 28 (703-803 MHz), LTE-A Band 5 (869-894 MHz), and LTE-A Band 8 (925-926 MHz). The antenna structure  100  can also work at the GPS band (1.575 GHz) and the frequency band of about 1710-2690 MHz. That is, the antenna structure  100  can work at the low frequency band, the middle frequency band, and the high frequency band, and when the antenna structure  100  works at these frequency bands, a working frequency satisfies a design of the antenna and also has a good radiating efficiency. 
       FIG. 16  illustrates a second exemplary embodiment of an antenna structure  200 . The antenna structure  200  includes a metallic member  11 , a first feed source  13 , a second feed source  14 , and a first switching circuit  15 . The metallic member  11  includes a front frame  111 , a backboard  112 , and a side frame  113 . The side frame  113  includes a top portion  115 , a first side portion  116 , and a second side portion  117 . The side frame  113  defines a slot  118 . The front frame  111  defines a gap  119 . The front frame  111  is divided into two portions by the gap  119 , these portions being a long portion A 1  and a short portion A 2  (relative to each other). 
     In this exemplary embodiment, the antenna structure  200  differs from the antenna structure  100  in that the antenna structure  200  further includes a first radiator  26 , a third feed source  27 , an isolating portion  28 , a second switching circuit  29 , a second radiator  30 , and a fourth feed source  31 . 
     The first radiator  26  is positioned in the receiving space  114 . The first radiator  26  is positioned adjacent to the short portion A 2  and is spaced apart from the backboard  112 . In this exemplary embodiment, the first radiator  26  is substantially rectangular and is positioned parallel to the top portion  215 . One end of the first radiator  26  is electrically connected to the isolating portion  28  and the other end of the first radiator  26  extends towards the first side portion  116 . One end of the third feed source  27  is electrically connected to the first radiator  26  through a matching circuit (not shown). Another end of the third feed source  27  is electrically connected to the isolating portion  28  and supplies current to the first radiator  26 . 
     In this exemplary embodiment, since a frequency band of the second feed source  14  approaches a frequency band of the third feed source  27 , there can be interference with each other. The isolating portion  28  can extend a current path of the second feed source  14  and a current path of the third feed source  27 , thereby improving isolation between the short portion A 2  and the first radiator  26 . 
     In this exemplary embodiment, the isolating portion  28  can be any shape and/or size. The isolating portion  28  can also be a planar metallic sheet and only to ensure that the isolating portion  28  can extend a current path of the third feed source  27 , thereby improving isolation between the short portion A 2  and the first radiator  26 . For example, in this exemplary embodiment, the isolating portion  28  can be a block-shaped structure. The isolating portion  28  is positioned on the backboard  112  and extends from the second side portion  117  towards the first side portion  116 . 
     Per  FIG. 17 , in other exemplary embodiments, the antenna structure  200  further includes a metallic frame  32 . The metallic frame  32  is positioned in the receiving space  114  and is connected to the metallic member  11 . The isolating portion  28  is a block-shaped structure. The isolating portion  28  extends from the second side portion  117  towards the first side portion  116  and is connected to the metallic frame  32 . 
     Per  FIG. 18 , in other exemplary embodiments, the antenna structure  200  further includes a metallic frame  32 . The metallic frame  32  is positioned in the receiving space  114  and is connected to the metallic member  11 . The isolating portion  28  is a block-shaped structure. The isolating portion  28  extends from the second side portion  117  towards the first side portion  116  and is spaced apart from the metallic member  11 . 
     Per  FIG. 19 , in other exemplary embodiments, the antenna structure  200  further includes a metallic frame  32 . The metallic frame  32  is positioned in the receiving space  114  and is connected to the metallic member  11 . The isolating portion  28  is still block-shaped, but substantially thinner, thereby approaching a more substantially 2-dimensional rectangular shape. The isolating portion  28  is positioned at one side of the metallic frame  32 . The isolating portion  28  is spaced apart from both the second side portion  117  and the backboard  112 . 
     Per  FIG. 16 , one end of the second switching circuit  29  is electrically connected to the first radiator  26  and another end of the second switching circuit  29  is electrically connected to the backboard  112 . The second switching circuit  29  can adjust the high frequency operation mode of the first radiator  26 . The detail circuit and working principle of the second switching circuit  29  can consult a description of the first switching circuit  15  in  FIG. 4 . 
     The second radiator  30  is positioned in the receiving space  114  and is positioned adjacent to the long portion A 1 . In this exemplary embodiment, the second radiator  30  includes a first radiating portion  301  and a second radiating portion  302 . The first radiating portion  301  is substantially U-shaped and includes a first radiating section  303 , a second radiating section  304 , and a third radiating section  305  connected in that order. The first radiating section  303  is substantially strip-shaped and is parallel to the top portion  215 . The second radiating section  304  is substantially strip-shaped. One end of the second radiating section  304  is perpendicularly connected to one end of the first radiating section  303  adjacent to the second side portion  117 . The other end of the second radiating section  304  extends along a direction parallel to the second side portion  117  towards the top portion  115  to form an L-shaped structure with the first radiating section  303 . The third radiating section  305  is substantially strip-shaped. One end of the third radiating section  305  is connected to one end of the second radiating section  304  away from the first radiating section  303 . The other end of the third radiating section  305  extends along a direction parallel to the first radiating section  303  towards the first side portion  116 . The third radiating section  305  and the first radiating section  303  are positioned at a same side of the second radiating section  304  and are positioned at two ends of the second radiating section  304 . 
     The second radiating portion  302  is substantially T-shaped and includes a first connecting section  306 , a second connecting section  307 , and a third connecting section  308 . The first connecting section  306  is substantially strip-shaped. One end of the first connecting section  306  is electrically connected to one end of the first radiating section  303  away from the second radiating section  304 . The other end of the first connecting section  306  extends a direction parallel to the second radiating section  304  towards the third radiating section  305 . The second connecting section  307  is substantially strip-shaped. One end of the second connecting section  307  is perpendicularly connected to the first connecting section  306  away from the first radiating section  304 . The other end of the second connecting section  307  extends along a direction parallel to the first radiating section  303  towards the second radiating section  304 . The third connecting section  308  is substantially strip-shaped. The third connecting section  308  is connected to a junction of the first connecting section  306  and the second connecting section  307 , extends along a direction parallel to the first radiating section  303  towards the first side portion  116  until the third connecting section  308  is connected to the front frame  111 . The third connecting section  308  is collinear with the second connecting section  307 . 
     The fourth feed source  31  is positioned at the front frame  111  and is electrically connected to a junction of the first radiating section  303  and the first connecting section  306 . The fourth feed source  31  can provide a current to the first radiating portion  301  and the second radiating portion  302  to activate a working mode, for example, the WIFI 2.4 GHz mode and the WIFI 5 GHz mode. 
     In this exemplary embodiment, when the antenna structure  200  works at the low frequency operation mode and the GPS operation mode, a current path distribution graph of the antenna structure  200  is consistent with the current path distribution graph of the antenna structure  100  shown in  FIG. 9 . 
     In this exemplary embodiment, when the antenna structure  200  works at the middle frequency operation mode, a current path distribution graph of the antenna structure  200  is consistent with the current path distribution graph of the antenna structure  100  shown in  FIG. 10 . 
     Per  FIG. 20 , when the current enters the first radiator  26  from the third feed source  27 , the current flows to one end of the first radiator  26  away from the third feed source  27  (e.g., path P 4 ) to activate a fourth mode to generate radiation signals in a fourth frequency band. In this exemplary embodiment, the fourth mode is a high frequency operation mode. Since the antenna structure  200  includes the second switching circuit  29 , the high frequency operation mode can be switched through the second switching circuit  29 , for example, the antenna structure  200  can be switched to an LTE-A Band 40 band (2300-2400 MHz) or LTE-A Band 41 (2496-2690 MHz), and the high frequency operation mode and middle frequency operation mode can be active simultaneously. 
     Per  FIG. 21 , when the current enters the second radiator  30  from the fourth feed source  31 , the current flows to the first radiating section  303 , the second radiating section  304 , and the third radiating section  305  (e.g., path P 5 ) to activate a fifth mode to generate radiation signals in a fifth frequency band. In this exemplary embodiment, the fifth mode is a WIFI 2.4 GHz mode. When the current enters the second radiator  30  from the fourth feed source  31 , the current also flows to the first connecting section  306  and the second connecting section  307  (e.g., path P 6 ) to activate a sixth mode to generate radiation signals in a sixth frequency band. In this exemplary embodiment, the sixth mode is a WIFI 5 GHz mode. 
     In this exemplary embodiment, when the antenna structure  200  works at the low frequency operation mode and the GPS operation mode, a scattering parameter graph and a radiating efficiency graph of the antenna structure  200  are consistent with the scattering parameter graph and a radiating efficiency graph of the antenna structure  100  shown in  FIG. 10 ,  FIG. 11 , and  FIG. 12 . 
       FIG. 22  illustrates a scattering parameter graph of the antenna structure  200 , when the antenna structure  200  works at the middle frequency operation mode and the high frequency operation mode. Curve  201  illustrates a scattering parameter when the inductance value of the switching element  153  of the first switching circuit  15  is about 0.13 pf. Curve  202  illustrates a scattering parameter when the inductance value of the switching element  153  of the first switching circuit  15  is about 0.15 pf. Curve  203  illustrates a scattering parameter when the inductance value of the switching element  153  of the first switching circuit  15  is about 0.2 pf. Curve  204  illustrates a scattering parameter when the first switching circuit  15  is in an open-circuit state (that is, the first switching circuit  15  does not switch to any switching element  153 ). Curve  205  illustrates a scattering parameter when the inductance value of the switching element  153  of the second switching circuit  29  is about 0.13 pf. Curve  206  illustrates a scattering parameter when the inductance value of the switching element  153  of the second switching circuit  29  is about 0.15 pf. Curve  207  illustrates a scattering parameter when the inductance value of the switching element  153  of the second switching circuit  29  is about 0.2 pf. Curve  208  illustrates a scattering parameter when the second switching circuit  29  is in an open-circuit state (that is, the second switching circuit  29  does not switch to any switching element). 
       FIG. 23  illustrates a radiating efficiency graph of the antenna structure  200 , when the antenna structure  200  works at the middle frequency operation mode and the high frequency operation mode. Curve  211  illustrates a radiating efficiency when the inductance value of the switching element  153  of the first switching circuit  15  is about 0.13 pf. Curve  212  illustrates a radiating efficiency when the inductance value of the switching element  153  of the first switching circuit  15  is about 0.15 pf. Curve  213  illustrates a radiating efficiency when the inductance value of the switching element  153  of the first switching circuit  15  is about 0.2 pf. Curve  214  illustrates a radiating efficiency when the first switching circuit  15  is in an open-circuit state (that is, the first switching circuit  15  does not switch to any switching element  153 ). Curve  215  illustrates a radiating efficiency when the inductance value of the switching element  153  of the second switching circuit  29  is about 0.13 pf. Curve  216  illustrates a radiating efficiency when the inductance value of the switching element  153  of the second switching circuit  29  is about 0.15 pf. Curve  217  illustrates a radiating efficiency when the inductance value of the switching element  153  of the second switching circuit  29  is about 0.2 pf. Curve  218  illustrates a radiating efficiency when the second switching circuit  29  is in an open-circuit state (that is, the second switching circuit  29  does not switch to any switching element). 
       FIG. 24  illustrates a scattering parameter graph of the antenna structure  200 , when the antenna structure  200  works at the WIFI 2.4 GHz band and WIFI 5 GHz band.  FIG. 25  illustrates a radiating efficiency graph of the antenna structure  200 , when the antenna structure  200  works at the WIFI 2.4 GHz band.  FIG. 26  illustrates a radiating efficiency graph of the antenna structure  200 , when the antenna structure  200  works at the WIFI 5 GHz band. 
     In view of  FIGS. 11 to 13  and  FIGS. 22 to 26 , the antenna structure  200  can work at a low frequency band, for example, LTE-A band 28 (703-803 MHz), LTE-A Band 5 (869-894 MHz), and LTE-A Band 8 (925-926 MHz). The antenna structure  200  can also work at the GPS band (1.575 GHz), the middle frequency band (1805-2170 MHz), the high frequency band (2300-2400 MHz and 2496-2690 MHz), and the WIFI 2.4/5 GHz dual-frequency bands. That is, the antenna structure  200  can work at the low frequency band, the middle frequency band, the high frequency band, and the WIFI 2.4/5G dual-frequency bands, and when the antenna structure  200  works at these frequency bands, a working frequency satisfies a design of the antenna and also has a good radiating efficiency. 
     As described above, the long portion A 1  can activate a first mode to generate radiation signals in a low frequency band, the short portion A 2  can activate a third mode to generate radiation signals in a middle frequency band and a high frequency band. The first radiator  26  can activate a fourth mode to generate radiation signals in a high frequency band. The wireless communication device  400  can use the first radiator  26 , through carrier aggregation (CA) technology of LTE-A, to receive and/or transmit wireless signals at multiple frequency bands simultaneously. In detail, the wireless communication device  400  can use the CA technology and use at least two of the long portion A 1 , the short portion A 2 , and the first radiator  26  to receive and/or transmit wireless signals at multiple frequency bands simultaneously. 
     In other exemplary embodiments, a location of the first radiator  26  and the second switching circuit  29  can be exchanged with a location of the second radiator  30 . One end of the first radiator is electrically connected to the front frame  111 . The other end of the first radiator  26  extends towards the second side portion  117 . One end of the second switching circuit  29  is electrically connected to the first radiator  26  and the other end of the second switching circuit  29  is electrically connected to the backboard  112 . The third feed source  27  is positioned on the front frame  111  and is electrically connected to the first radiator  26 . The second radiator  30  is positioned in the receiving space  114  and is positioned adjacent to the short portion A 2 . One end of the third connecting section  308  of the second radiator  30  connected to front frame  111  is changed to be electrically connected to the isolating portion  28 . One end of the fourth feed source  31  is electrically connected to a junction of the first radiating section  303  and the first connecting section  306 . The other end of the fourth feed source  31  is electrically connected to the isolating portion  28 . 
     In addition, the antenna structure  100 / 200  includes the housing  11 . The slot  118  and the gap  119  are both defined on the front frame  111  and the side frame  113  instead of the backboard  112 . Then the backboard  112  forms an all-metal structure. That is, the backboard  112  does not define any other slot and/or gap and has a good structural integrity and an aesthetic quality. 
     Exemplary Embodiments 3-5 
       FIG. 27  illustrates an embodiment of a wireless communication device  600  using a third exemplary antenna structure  500 . The wireless communication device  600  can be a mobile phone or a personal digital assistant, for example. The antenna structure  500  can receive and/or transmit wireless signals. 
     Per  FIG. 28  and  FIG. 29 , the antenna structure  500  includes a housing  51 , a first feed source  53 , a second feed source  54 , a first switching circuit  55 , and a second switching circuit  57 . The housing  51  can be a metal housing of the wireless communication device  600 . In this exemplary embodiment, the housing  51  is made of metallic material and includes a front frame  511 , a backboard  512 , and a side frame  513 . The front frame  511 , the backboard  512 , and the side frame  513  can be integral with each other. The front frame  511 , the backboard  512 , and the side frame  513  cooperatively form the metal housing of the wireless communication device  600 . 
     The front frame  511  defines an opening (not shown). The wireless communication device  600  includes a display  601 . The display  601  is received in the opening. The display  601  has a display surface. The display surface is exposed at the opening and is positioned parallel to the backboard  512 . 
     The backboard  512  is positioned opposite to the front frame  511 . The backboard  512  is an integral and single metallic sheet. The backboard  512  defines holes  606 ,  607  for exposing a camera lens  604  and a flash light  605 . The backboard  512  does not define any slot, break line, and/or gap for dividing the backboard  512 . The backboard  512  serves as a ground of the antenna structure  500  and the wireless communication device  600 . 
     In other exemplary embodiments, the wireless communication device  600  further includes a shielding mask or a middle frame (not shown). The shielding mask is positioned at the surface of the display  601  towards the backboard  512  and shields against electromagnetic interference. The middle frame is positioned at the surface of the display  601  towards the backboard  512  and is configured for supporting the display  601 . The shielding mask or the middle frame is made of metallic material. The shielding mask or the middle frame is electrically connected to the backboard  512  and serves as ground of the antenna structure  500  and the wireless communication device  600 . 
     The side frame  513  is positioned between the front frame  511  and the backboard  512 . The side frame  513  is positioned around a periphery of the front frame  511  and a periphery of the backboard  512 . The side frame  513  forms a receiving space  514  together with the display  601 , the front frame  511 , and the backboard  512 . The receiving space  514  can receive a printed circuit board, a processing unit, or other electronic components or modules. 
     The side frame  513  includes an end portion  515 , a first side portion  516 , and a second side portion  517 . In this exemplary embodiment, the end portion  515  is a bottom portion of the wireless communication device  600 . The end portion  515  connects the front frame  511  and the backboard  512 . The first side portion  516  is positioned apart from and parallel to the second side portion  517 . The end portion  515  has first and second ends. The first side portion  516  is connected to the first end of the end portion  515  and the second side portion  517  is connected to the second end of the end portion  515 . The first side portion  516  connects the front frame  511  and the backboard  512 . The second side portion  517  also connects the front frame  511  and the backboard  512 . 
     The side frame  513  defines a through hole  518  and a slot  519 . The front frame  511  defines a gap  520 . In this exemplary embodiment, the through hole  518  is defined at a middle part of the end portion  515  and passes through the end portion  515 . The wireless communication device  600  further includes an electronic element  603 . In this exemplary embodiment, the electronic element  603  is a Universal Serial Bus (USB) module. The electronic element  603  is positioned in the receiving space  514 . The electronic element  603  corresponds to the through hole  518  and is partially exposed from the through hole  518 . A USB device can be inserted in the through hole  518  and be electrically connected to the electronic element  603 . 
     In this exemplary embodiment, the slot  519  is defined at the end portion  515  and communicates with the through hole  518 . The slot  519  further extends to the first side portion  516  and the second side portion  517 . In other exemplary embodiments, the slot  519  can only be defined at the end portion  515  and does not extend to any one of the first side portion  516  and the second side portion  517 . In other exemplary embodiments, the slot  519  can be defined at the end portion  515  and extends to one of the first side portion  516  and the second side portion  517 . 
     The gap  520  communicates with the slot  519  and extends across the front frame  511 . In this exemplary embodiment, the gap  520  is positioned adjacent to the second side portion  517 . The front frame  511  is divided into two portions by the gap  520 , these portions being a long portion T 1  and a short portion T 2  (long and short relative to each other). A first portion of the front frame  511  extending from a first side of the gap  520  to a first end E 1  of the slot  519  forms the long portion T 1 . A second portion of the front frame  511  extending from a second side of the gap  520  to a second end E 2  of the slot  519  forms the short portion T 2 . 
     In this exemplary embodiment, the gap  520  is not positioned at a middle portion of the end portion  515 . The long portion T 1  is longer than the short portion T 2 . 
     In this exemplary embodiment, the slot  519  and the gap  520  are both filled with insulating material, for example, plastic, rubber, glass, wood, ceramic, or the like, thereby isolating the long portion T 1 , the short portion T 2 , and the backboard  512 . 
     In this exemplary embodiment, the slot  519  is defined on the end of the side frame  513  adjacent to the backboard  512  and extends to the front frame  511 . Then the long portion T 1  and the short portion T 2  are fully formed by a portion of the front frame  511 . In other exemplary embodiments, a position of the slot  519  can be adjusted. For example, the slot  519  is defined on the end of the side frame  513  adjacent to the backboard  512  and extends towards the front frame  511 . Then the long portion T 1  and the short portion T 2  are formed by a portion of the front frame  511  and a portion of the side frame  513 . 
     In this exemplary embodiment, except for the through hole  518 , the slot  519 , and the gap  520 , a lower half portion of the front frame  511  and the side frame  513  does not define any other slot, break line, and/or gap. That is, there is only one gap  520  defined on the lower half portion of the front frame  511 . 
     Per  FIG. 27  and  FIG. 31 , through a matching circuit  59 , the first feed source  53  is electrically connected to the end of the long portion T 1  adjacent to the first side portion  516 . The first feed source  53  can feed current to the long portion T 1  and activate the long portion T 1  in a first mode to generate radiation signals in a first frequency band. 
     Through a matching circuit (not shown), the second feed source  54  can be electrically connected to the end of the short portion T 2  adjacent to the gap  520 . The second feed source  54  can feed current to the short portion T 2  and activate the short portion T 2  in a second mode to generate radiation signals in a second frequency band. 
     Per  FIG. 30 , the first switching circuit  55  is electrically connected to a middle portion of the long portion T 1 . The first switching circuit  55  includes a first switching unit  551  and a plurality of first switching elements  553 . The first switching unit  551  is electrically connected to the long portion T 1 . The first switching elements  553  can be an inductor, a capacitor, or a combination of the inductor and the capacitor. The first switching elements  553  are connected in parallel. One end of each first switching element  553  is electrically connected to the first switching unit  551 . The other end of each first switching element  553  is electrically connected to the backboard  512 . 
     Per  FIG. 27  and  FIG. 31 , one end of the matching circuit  59  is electrically connected to the long portion T 1 . Another end of the matching circuit  59  is electrically connected to the first feed source  53 . One end of the second switching circuit  57  is electrically connected to the matching circuit  59 . Another end of the second switching circuit  57  is electrically connected to the backboard  512 . In this exemplary embodiment, the second switching circuit  57  includes a second switching unit  571  and a plurality of second switching elements  573 . The second switching unit  571  is electrically connected to the matching circuit  59  and then is electrically connected to the long portion T 1  through the matching circuit  59 . The second switching elements  573  can be an inductor, a capacitor, or a combination of the inductor and the capacitor. The second switching elements  573  are connected in parallel. One end of each second switching element  573  is electrically connected to the second switching unit  571 . The other end of each second switching element  573  is electrically connected to the backboard  512 . 
     Through controlling the first switching unit  551  and/or the second switching unit  571 , the long portion T 1  can be switched to connect with different first switching elements  553  and/or second switching elements  573 . Since each first switching element  553  and second switching element  573  has a different impedance, a frequency band of the first mode of the long portion T 1  can be adjusted through switching the first switching unit  551  and/or the second switching unit  571 , for example, the frequency band of the first mode of the long portion T 1  can be offset towards a lower frequency or towards a higher frequency (relative to each other). 
     Per  FIG. 32 , when the current enters the long portion T 1  from the first feed source  53 , the current flows through the long portion T 1  and towards the gap  520  (e.g., path I 1 ) to activate the first mode, to generate radiation signals in the first frequency band. When the current enters the short portion T 2  from the second feed source  54 , the current flows through the front frame  511 , the second side portion  517 , and the backboard  512  (e.g., path I 2 ) to activate the second mode, to generate radiation signals in the second frequency band. In this exemplary embodiment, the first mode is a low frequency operation mode. The first frequency band is a frequency band of about 704-960 MHz. The second mode is low to middle frequency operation modes. The second frequency band is a frequency band of about 1710-2690 MHz. 
     Since the antenna structure  500  includes the first switching circuit  55  and the second switching circuit  57 , the low frequency operation mode of the long portion T 1  can be switched through the first switching circuit  55  and the second switching circuit  57  in coordination with each other. The middle frequency operation mode and the high frequency operation mode of the antenna structure  500  are not thereby affected. 
     Per  FIG. 33 , the antenna structure  500  further includes a resonance circuit  58 . In one exemplary embodiment, the antenna structure  500  includes one resonance circuit  58 . The resonance circuit  58  includes an inductor L and a capacitor C connected in series. The resonance circuit  58  is electrically connected between the long portion T 1  and the backboard  512 . The resonance circuit  58  is electrically connected in parallel to the first switching unit  551  and at least one first switching element  553 . 
     Per  FIG. 34 , in another exemplary embodiment, the antenna structure  500  includes a plurality of resonance circuits  58 . The number of the resonance circuits  58  is equal to the number of first switching elements  553 . Each resonance circuit  58  includes inductors L 1 -Ln and capacitors C 1 -Cn connected in series. Each resonance circuit  58  is electrically connected in parallel to one of the first switching elements  553  between the first switching unit  551  and the backboard  512 . 
     Per  FIG. 30 ,  FIG. 31 ,  FIG. 33 , and  FIG. 34 , the backboard  512  can be replaced by the shielding mask or the middle frame for grounding the first switching circuit  55  and/or the second switching circuit  57 . 
     Per  FIG. 35 , when the antenna structure  500  does not include the resonance circuit  58  of  FIG. 33 , the antenna structure  500  works at the first mode (please see the curve S 351 ). When the antenna structure  500  includes the resonance circuit  58 , the long portion T 1  of the antenna structure  500  can activate an additional resonance mode (that is, a third mode, please see the curve S 352 ) to generate radiation signals in a third frequency band. The third mode can effectively broaden an applied frequency band of the antenna structure  500 . 
     Per  FIG. 36 , when the antenna structure  500  does not include the resonance circuit  58  of  FIG. 34 , the antenna structure  500  works at the first mode (please see the curve S 361 ). When the antenna structure  500  includes the resonance circuit  58 , the long portion T 1  of the antenna structure  500  can activate the additional resonance mode (please see the curve S 362 ), that is, the third mode. The third mode can effectively broaden an applied frequency band of the antenna structure  500 . 
     In one exemplary embodiment, inductance values of the inductors L 1 -Ln and capacitance values of the capacitors C 1 -Cn of the resonance circuit  58  can cooperatively decide a frequency band of the resonance mode when the first mode switches. For example, in one exemplary embodiment, as illustrated in  FIG. 36 , when the first switching unit  551  switches to different first switching elements  553  through setting the inductance value and the capacitance value of the resonance circuit  58 , the resonance mode of the antenna structure  500  can also be switched. For example, the resonance mode of the antenna structure  500  can be moved from f 1  to fn. 
     In other exemplary embodiments, the frequency band of the resonance mode can be fixed through setting the inductance value and the capacitance value of the resonance circuit  58 . Then no matter to which first switching element  553  the first switching unit  551  is switched, the frequency band of the resonance mode is fixed and keeps unchanged. 
     In other exemplary embodiments, the resonance circuit  58  is not limited to include the inductor L and the capacitor C, and can include other resonance components. 
     Per  FIG. 37 , when the current enters the long portion T 1  from the first feed source  53 , the current flows through the long portion T 1  and towards the gap  520  (e.g., path I 3 ) to activate the first mode, to generate radiation signals in a first frequency band. Since the antenna structure  500  includes the first switching circuit  55  and the second switching circuit  57 , the low frequency operation mode of the long portion T 1  can be switched through the first switching circuit  55  and the second switching circuit  57  in coordination with each other, and the middle frequency operation mode and the high frequency operation mode of the antenna structure  500  are not affected. In this exemplary embodiment, the first mode is a low frequency operation mode. The first frequency band is a frequency band of about 704-960 MHz. 
     Per  FIG. 38 , when the current enters the short portion T 2  from the second feed source  54 , the current flows through the front frame  511 , the second side portion  517 , and the backboard  512  (e.g., path I 4 ) to activate the second mode, to generate radiation signals in the second frequency band. When the current enters the short portion T 2  from the second feed source  54 , the current is coupled to the long portion T 1  through the gap  520 , flows through the resonance circuit  58  of the first switching circuit  55 , and flows to the backboard  512  (e.g., path I 4 ). Then, through a coupling of the gap  520  and a configuration of the resonance circuit  58 , the short portion T 2  further activates the third mode, to generate radiation signals in the third frequency band. In this exemplary embodiment, the second mode is a middle frequency operation mode. The second frequency band is a frequency band of about 1710-2400 MHz. The third mode is a high frequency operation mode and the third frequency band is about 2400-2690 MHz. 
       FIG. 39  illustrates a scattering parameter graph of the antenna structure  500 , when the antenna structure  500  works at the low frequency operation mode. Curve S 391  illustrates a scattering parameter when the antenna structure  500  works at a frequency band of about 704-746 MHz. Curve S 392  illustrates a scattering parameter when the antenna structure  500  works at a frequency band of about 746-787 MHz. Curve S 393  illustrates a scattering parameter when the antenna structure  500  works at a frequency band of about 824-894 MHz. Curve S 394  illustrates a scattering parameter when the antenna structure  500  works at a frequency band of about 880-960 MHz. Curves S 391 -S 394  respectively correspond to four different frequency bands and respectively correspond to four of the plurality of low frequency operation modes of the first switching circuit  55  and the second switching circuit  57 . 
       FIG. 40  illustrates a radiating efficiency graph of the antenna structure  500 , when the antenna structure  500  works at the low frequency operation mode. Curve S 401  illustrates a radiating efficiency when the antenna structure  500  works at a frequency band of about 704-746 MHz. Curve S 402  illustrates a radiating efficiency when the antenna structure  500  works at a frequency band of about 746-787 MHz. Curve S 403  illustrates a radiating efficiency when the antenna structure  500  works at a frequency band of about 824-894 MHz. Curve S 404  illustrates a radiating efficiency when the antenna structure  500  works at a frequency band of about 880-960 MHz. Curves S 401 -S 404  respectively correspond to four different frequency bands and respectively correspond to four of the plurality of low frequency operation modes of the first switching circuit  55  and the second switching circuit  57 . 
       FIG. 41  illustrates a scattering parameter graph of the antenna structure  500 , when the antenna structure  500  works at the middle, high frequency operation modes (1710-2690 MHz).  FIG. 42  illustrates a radiating efficiency graph of the antenna structure  500 , when the antenna structure  500  works at the middle, high frequency operation modes (1710-2690 MHz). 
     In view of  FIGS. 39 to 42 , the antenna structure  500  can work at a low frequency band, for example, frequency bands of about 704-746 MHz, 746-787 MHz, 824-894 MHz, and 880-960 MHz. The antenna structure  500  can also work at the middle frequency band and the high frequency band (1710-2690 MHz). That is, the antenna structure  500  can work at the low frequency band, the middle frequency band, and the high frequency band, and when the antenna structure  500  works at these frequency bands, a working frequency satisfies a design of the antenna and also has a good radiating efficiency. 
       FIG. 43  illustrates a fourth exemplary antenna structure  500   a . The antenna structure  500   a  includes a housing  51 , a first feed source  53 , a second feed source  54 , a first switching circuit  55 , and a second switching circuit  57 . The housing  51  includes a front frame  511 , a backboard  512 , and a side frame  513 . The side frame  513  includes an end portion  515 , a first side portion  516 , and a second side portion  517 . The side frame  513  defines a slot  519 . The front frame  511  defines a gap  520 . The front frame  511  is divided into two portions by the gap  520 . The two portions include a long portion T 1  and a short portion T 2 . 
     In this exemplary embodiment, the antenna structure  500   a  differs from the antenna structure  500  in that the antenna structure  500   a  further includes a first radiator  61 , a third feed source  62 , an isolating portion  63 , a second radiator  64 , and a fourth feed source  65 . 
     The first radiator  61  is positioned in the receiving space  514 . The first radiator  61  is positioned adjacent to the short portion T 2  and is spaced apart from the backboard  512 . The first radiator  61  includes a first radiating portion  610 , a second radiating portion  611 , and a third radiating portion  612 . The first radiating portion  610  is substantially L-shaped and includes a first radiating arm  613  and a second radiating arm  614 . The first radiating arm  613  is substantially a strip. One end of the first radiating arm  613  is electrically connected to the isolating portion  63  and extends along a direction parallel to the end portion  515  towards the first side portion  516 . The second radiating arm  614  is substantially a strip and is coplanar with the first radiating arm  613 . The second radiating arm  614  is perpendicularly connected to the end of the first radiating arm  613  adjacent to the first side portion  516  and extends along a direction perpendicular to and away from the backboard  512 . 
     The second radiating portion  611  is substantially U-shaped and includes a first radiating section  615 , a second radiating section  616 , and a third radiating section  617 , connected in that order. The first radiating section  615 , the second radiating section  616 , and the third radiating section  617  are coplanar with each other and are positioned at a plane parallel to the plane of the first radiating arm  613 . The first radiating section  615  is substantially rectangular and is positioned parallel to the end portion  515 . One end of the first radiating section  615  is perpendicularly connected to the end of the second radiating arm  614  away from the first radiating arm  613  and extends along a direction towards the first side portion  516 . The second radiating section  616  is substantially a strip. One end of the second radiating section  616  is perpendicularly connected to the end of the first radiating section  615  away from the second radiating arm  614 . Another end of the second radiating section  616  extends along a direction parallel to the second side portion  517  and away from the end portion  515  to form an L-shaped structure with the first radiating section  615 . 
     The third radiating section  617  is substantially rectangular. One end of the third radiating section  617  is connected to the end of the second radiating section  616  away from the first radiating section  615 . Another end of the third radiating section  617  extends along a direction parallel to the first radiating section  615  towards the second side portion  517 . The third radiating section  617  and the first radiating section  615  are positioned at the same side of the second radiating section  616 . The third radiating section  617  and the first radiating section  615  are positioned at two ends of the second radiating section  616 . 
     The third radiating portion  612  is substantially L-shaped and includes a first connecting section  618  and a second connecting section  619 . The first connecting section  618  is substantially rectangular. One end of the first connecting section  618  is electrically connected to a junction of the second radiating arm  614  and the first radiating section  615 . Another end of the first connecting section  618  extends along a direction parallel to the second radiating section  616  towards the third radiating section  617 , until it passes over the third radiating section  617 . The second connecting section  619  is substantially rectangular. One end of the second connecting section  619  is perpendicularly connected to the end of the first connecting section  618  away from the first radiating section  615 . Another end of the second connecting section  619  extends along a direction parallel to the first radiating section  615  towards the second radiating section  616 . The extension continues until the second connecting section  619  is collinear with an end of the third radiating section  617 . 
     One end of the third feed source  62  is electrically connected to the first radiator  61  through a matching circuit (not shown), for example, the first connecting section  618  of the first radiator  61 . Another end of the third feed source  62  is electrically connected to the isolating portion  63  to feed current to the second radiating portion  611  and the third radiating portion  612 , and generates different working modes, for example, a WIFI 2.4 GHz mode and a WIFI 5 GHz mode. 
     In this exemplary embodiment, since a frequency band of the second feed source  54  approaches a frequency band of the third feed source  62 , there can be interference with each other. The isolating portion  63  can extend a current path of the second feed source  54  and a current path of the third feed source  62 , thereby improving isolation between the short portion T 2  and the first radiator  61 . 
     In this exemplary embodiment, the isolating portion  63  can be any shape and/or size. The isolating portion  63  can also be a planar metallic sheet or a metallic housing and only to ensure that the isolating portion  63  can extend a current path of the second feed source  54  and the third feed source  62 , thereby improving isolation between the short portion T 2  and the first radiator  61 . For example, in this exemplary embodiment, the isolating portion  63  can be a block-shaped structure. The isolating portion  63  is positioned on the backboard  512  and extends from the second side portion  517  towards the first side portion  516 . In other exemplary embodiments, the isolating portion  63  can also be positioned on the middle frame. 
     The second radiator  64  is positioned in the receiving space  514  and adjacent to the long portion T 1 . The second radiator  64  is spaced apart from the backboard  512 . In this exemplary embodiment, the second radiator  64  is substantially a strip and is parallel to the end portion  515 . The second radiator  64  is connected to the position of the front frame  511  adjacent to the first feed source  53  and extends along a direction towards the second side portion  517 . The fourth feed source  65  is positioned at the front frame  511 . The fourth feed source  65  is electrically connected to the second radiator  64  and supplies current to the second radiator  64 . 
     In this exemplary embodiment, when the antenna structure  500   a  works at the low frequency operation mode, a current path distribution graph of the antenna structure  500   a  is consistent with the current path distribution graph of the antenna structure  500  shown in  FIG. 37 . 
     Per  FIG. 44 , when the current enters the short portion T 2  from the second feed source  54 , the current flows to the front frame  511 , the second side portion  517 , and the backboard  512  (e.g., path I 6 ) to activate a second mode, to generate radiation signals in a second frequency band. When the current enters the short portion T 2  from the second feed source  54 , the current is coupled to the long portion T 1  through the gap  520 , flows through the resonance circuit  58  of the first switching circuit  55 , and flows to the backboard  512  (e.g., path I 7 ). Then, through a coupling of the gap  520  and a configuration of the resonance circuit  58 , the short portion T 2  further activates a third mode to generate radiation signals in a third frequency band. In this exemplary embodiment, the second mode is a middle frequency operation mode. The second frequency band is a frequency band of about 1710-2170 MHz. The third mode is a high frequency operation mode. The third frequency band is a frequency band of about 2300-2400 MHz (LTE-A band 40). 
     Per  FIG. 45 , when the current enters the first radiator  61  from the third feed source  62 , the current flows to the first radiating section  615 , the second radiating section  616 , and the third radiating section  617  (e.g., path I 8 ) to activate a fourth mode to generate radiation signals in a fourth frequency band. In this exemplary embodiment, the fourth mode is a WIFI 2.4 GHz mode. 
     When the current enters the first radiator  61  from the third feed source  62 , the current flows to the first connecting section  618  and the second connecting section  619  (e.g, path I 9 ) to activate a fifth mode to generate radiation signals in a fifth frequency band. In this exemplary embodiment, the fifth mode is a WIFI 5 GHz mode. 
     Per  FIG. 46 , when the current enters the second radiator  64  from the fourth feed source  65 , the current flows to the end of the second radiator  64  away from the fourth feed source  65  (e.g., path I 10 ) to activate a sixth mode to generate radiation signals in a sixth frequency band. In this exemplary embodiment, the sixth mode is a high frequency operation mode. The sixth frequency band is a frequency band of about 2496-2690 MHz. 
     In this exemplary embodiment, when the antenna structure  500   a  works at the low frequency operation mode, a scattering parameter graph and a radiating efficiency graph of the antenna structure  500   a  are consistent with the scattering parameter graph and a radiating efficiency graph of the antenna structure  500  shown in  FIG. 39  and  FIG. 40 . 
       FIG. 47  illustrates a scattering parameter graph of the antenna structure  500   a , when the antenna structure  500   a  works at frequency bands of about 1710-2170 MHz and 2300-2400 MHz (a LTE-A middle frequency band and LTE-A band 40).  FIG. 48  illustrates a radiating efficiency graph of the antenna structure  500   a , when the antenna structure  500   a  works at frequency bands of about 1710-2170 MHz and 2300-2400 MHz (a LTE-A middle frequency band and LTE-A band 40). 
       FIG. 49  illustrates a scattering parameter graph of the antenna structure  500   a , when the antenna structure  500   a  works at WIFI 2.4 GHz mode and WIFI 5 GHz mode.  FIG. 50  illustrates a radiating efficiency graph of the antenna structure  500   a , when the antenna structure  500   a  works at WIFI 2.4 GHz mode and WIFI 5 GHz mode. 
       FIG. 51  illustrates a scattering parameter graph of the antenna structure  500   a , when the antenna structure  500   a  works at LTE-A Band 41 mode (2496-2690 MHz).  FIG. 52  illustrates a radiating efficiency graph of the antenna structure  500   a , when the antenna structure  500   a  works at LTE-A Band 41 mode (2496-2690 MHz). 
     In view of  FIGS. 39 to 40  and  FIGS. 47 to 52 , the antenna structure  500   a  can work at a low frequency band, for example, frequency bands of about 704-746 MHz, 746-787 MHz, 824-894 MHz, and 880-960 MHz. The antenna structure  500   a  can also work at the middle frequency band (1710-2170 MHz), the high frequency band (2300-2400 MHz and 2496-2690 MHz), and the WIFI 2.4/5G dual-frequency bands. That is, the antenna structure  500   a  can work at the low frequency band, the middle frequency band, the high frequency band, and the WIFI 2.4/5G dual-frequency bands, and when the antenna structure  500   a  works at these frequency bands, a working frequency satisfies a design of the antenna and also has a good radiating efficiency. 
       FIG. 53  illustrates a fifth exemplary antenna structure  500   b . The antenna structure  500   b  includes a housing  51 , a first feed source  53 , a second feed source  54 , a first switching circuit  55 , a second switching circuit  57 , a first radiator  61 , a third feed source  62 , an isolating portion  63 , a second radiator  64 , and a fourth feed source  65 . The housing  51  includes a front frame  511 , a backboard  512 , and a side frame  513 . The side frame  513  includes an end portion  515 , a first side portion  516 , and a second side portion  517 . The side frame  513  defines a slot  519 . The front frame  511  defines a gap  520 . The front frame  511  is divided into two portions by the gap  520 . The two portions include a long portion T 1  and a short portion T 2 . 
     In this exemplary embodiment, the antenna structure  500   b  differs from the antenna structure  500   a  in that the antenna structure  500   b  further includes a third switching circuit  66 . One end of the third switching circuit  66  is electrically connected to the second radiator  64  and another end of the third switching circuit  66  is electrically connected to the backboard  512 . The third switching circuit  66  is configured to adjust a frequency band of the high frequency operation mode of the second radiator  64 . A circuit structure and a working principle of the third switching circuit  66  are consistent with the first switching circuit  55  shown in  FIG. 55 . 
     In this exemplary embodiment, when the antenna structure  500   b  works at the low frequency operation mode, a current path distribution graph of the antenna structure  500   b  is consistent with the current path distribution graph of the antenna structure  500  shown in  FIG. 37 . 
     Per  FIG. 54 , when the current enters the short portion T 2  from the second feed source  54 , the current flows to the front frame  511 , the second side portion  517 , and the backboard  512  (e.g., path I 11 ) to activate a second mode to generate radiation signals in a second frequency band. When the current enters the short portion T 2  from the second feed source  54 , the current is coupled to the long portion T 1  through the gap  520 , flows through the resonance circuit  58  of the first switching circuit  55 , and flows to the backboard  512  (e.g., path I 12 ). Then, through a coupling of the gap  520  and a configuration of the resonance circuit  58 , the short portion T 2  further activate a third mode to generate radiation signals in a third frequency band. In this exemplary embodiment, the second mode is a middle frequency operation mode. The second frequency band is a frequency band of about 1710-1990 MHz. The third mode is a high frequency operation mode. The third frequency band is a frequency band of about 2110-2170 MHz. 
     In this exemplary embodiment, when the antenna structure  500   b  works at the WIFI 2.4 GHz mode and the WIFI 5 GHz mode, a current path distribution graph of the antenna structure  500   b  is consistent with the current path distribution graph of the antenna structure  500   a  shown in  FIG. 45 . 
     Per  FIG. 55 , when the current enters the second radiator  64  from the fourth feed source  65 , the current flows to the end of the second radiator  64  away from the fourth feed source  65  (e.g., path I 13 ) to activate a sixth mode to generate radiation signals in a sixth frequency band. In this exemplary embodiment, the sixth mode is a high frequency operation mode. Since the antenna structure  500   b  includes the third switching circuit  66 , the high frequency operation mode of the antenna structure  500   b  can be switched through the third switching circuit  66 . For example, the antenna structure  500   b  can be switched to a frequency band of about 2300-2400 MHz and/or a frequency band of about 2496-2690 MHz (LTE-A Band 41), and the high frequency operation mode, the middle frequency operation mode, and LTE-A Band 40 mode can be activated and can operate simultaneously. 
     In this exemplary embodiment, when the antenna structure  500   b  works at the low frequency operation mode, a scattering parameter graph and a radiating efficiency graph of the antenna structure  500   b  are consistent with the scattering parameter graph and a radiating efficiency graph of the antenna structure  500  shown in  FIG. 39  and  FIG. 40 . 
       FIG. 56  illustrates a scattering parameter graph of the antenna structure  500   b , when the antenna structure  500   b  works at a frequency band of about 1710-2170 MHz.  FIG. 57  illustrates a radiating efficiency graph of the antenna structure  500   b , when the antenna structure  500   b  works at a frequency band of about 1710-2170 MHz. 
     In this exemplary embodiment, when the antenna structure  500   b  works at the WIFI 2.4 GHz mode and the WIFI 5 GHz mode, a scattering parameter graph and a radiating efficiency graph of the antenna structure  500   b  are consistent with the scattering parameter graph and a radiating efficiency graph of the antenna structure  500   a  shown in  FIG. 49  and  FIG. 50 . 
       FIG. 58  illustrates a scattering parameter graph of the antenna structure  500   b , when the antenna structure  500   b  works at frequency bands of about 2300-2400 MHz and 2496-2690 MHz.  FIG. 59  illustrates a radiating efficiency graph of the antenna structure  500   b , when the antenna structure  500   b  works at frequency bands of about 2300-2400 MHz and 2496-2690 MHz. 
     As described above, the long portion T 1  can activate a first mode to generate radiation signals in a low frequency band, the short portion T 2  can activate a second mode and a third mode to generate radiation signals in a middle frequency band and a high frequency band. The second radiator  64  can activate a sixth mode to generate radiation signals in a high frequency band. The wireless communication device  600  can use carrier aggregation (CA) technology of LTE-A to receive and/or transmit wireless signals at multiple frequency bands simultaneously. In detail, the wireless communication device  600  can use the CA technology and use at least two of the long portion T 1 , the short portion T 2 , and the second radiator  64  to receive and/or transmit wireless signals at multiple frequency bands simultaneously. 
     In other exemplary embodiments, a location of the first radiator  61  can be exchanged with a location of the second radiator  64  and the third switching circuit  66 , and a location of the isolating portion  63  is fixed and keeps unchanged. The first radiator  61  is positioned in the receiving space  514  and is symmetric with the second radiator  30  shown in  FIG. 17 . The first radiator  61  is positioned adjacent to the long portion T 1 . The end of the first radiating arm  613  of the first radiator  61  connecting to the isolating portion  63  is changed to be electrically connected to the front frame  511 . The third feed source  62  is positioned on the front frame  511  and is electrically connected to the first connecting section  618  of the first radiator  61 . 
     The second radiator  61  is connected to the isolating portion  63  and extends towards the first side portion  516 . One end of the fourth feed source  65  is electrically connected to the second radiator  61  through a matching circuit (not shown). Another end of the fourth feed source  65  is electrically connected to the isolating portion  63  to feed current to the second radiator  61 . One end of the third switching circuit  66  is electrically connected to the second radiator  61  and another end of the third switching circuit  66  is connected to the backboard  512 . 
     In addition, the slot  519  and the gap  520  of the housing  51  are both defined on the front frame  511  and the side frame  513  instead of the backboard  512 . Then the backboard  512  forms an all-metal structure. That is, the backboard  512  does not define any other slot and/or gap and has a good structural integrity and an aesthetic quality. 
     Exemplary Embodiments 6-7 
       FIG. 60  illustrates an embodiment of a wireless communication device  800  using a sixth exemplary antenna structure  700 . The wireless communication device  800  can be a mobile phone or a personal digital assistant, for example. The antenna structure  700  can receive and/or transmit wireless signals. 
     Per  FIG. 61  and  FIG. 62 , the antenna structure  700  includes a housing  71 , a first feed source S 1 , a first radiator  73 , a first switching circuit  75 , a second switching circuit  76 , a second radiator  78 , a second feed source S 2 , and a third switching circuit  79 . The housing  71  can be a metal housing of the wireless communication device  800 . In this exemplary embodiment, the housing  71  is made of metallic material and includes a front frame  711 , a backboard  712 , and a side frame  713 . The front frame  711 , the backboard  712 , and the side frame  713  can be integral with each other. The front frame  711 , the backboard  712 , and the side frame  713  cooperatively form the metal housing of the wireless communication device  800 . 
     The front frame  711  defines an opening (not shown). The wireless communication device  800  includes a display  801 . The display  801  is received in the opening. The display  801  has a display surface. The display surface is exposed at the opening and is positioned parallel to the backboard  712 . 
     The backboard  712  is positioned opposite to the front frame  711 . The backboard  712  is directly connected to the side frame  713  and there is no gap between the backboard  712  and the side frame  713 . The backboard  712  is an integral and single metallic sheet. The backboard  712  defines holes  806 ,  807  for exposing a camera lens  804  and a flash light  805 . The backboard  712  does not define any slot, break line, and/or gap for dividing the backboard  712 . The backboard  712  serves as a ground of the antenna structure  700  and the wireless communication device  800 . 
     In other exemplary embodiments, the wireless communication device  800  further includes a shielding mask or a middle frame (not shown). The shielding mask is positioned at the surface of the display  801  towards the backboard  712  and shields against electromagnetic interference. The middle frame is positioned at the surface of the display  801  towards the backboard  712  and is configured for supporting the display  801 . The shielding mask or the middle frame is made of metallic material. The shielding mask or the middle frame can be electrically connected to the backboard  712  and serves as ground of the antenna structure  700  and the wireless communication device  800 . 
     The side frame  713  is positioned between the front frame  711  and the backboard  712 . The side frame  713  is positioned around a periphery of the front frame  711  and a periphery of the backboard  712 . The side frame  713  forms a receiving space  714  together with the display  801 , the front frame  711 , and the backboard  712 . The receiving space  714  can receive a printed circuit board, a processing unit, or other electronic components or modules. 
     The side frame  713  includes an end portion  715 , a first side portion  716 , and a second side portion  717 . In this exemplary embodiment, the end portion  715  is a bottom portion of the wireless communication device  800 . The end portion  715  connects the front frame  711  and the backboard  712 . The first side portion  716  is positioned apart from and parallel to the second side portion  717 . The end portion  715  has first and second ends. The first side portion  716  is connected to the first end of the end portion  715  and the second side portion  717  is connected to the second end of the end portion  715 . The first side portion  716  connects the front frame  711  and the backboard  712 . The second side portion  717  also connects the front frame  711  and the backboard  712 . 
     The side frame  713  defines a through hole  718  and a slot  719 . The front frame  711  defines a gap  720 . In this exemplary embodiment, the through hole  718  is defined at a middle part of the end portion  715  and passes through the end portion  715 . The wireless communication device  800  further includes an electronic element  803 . In this exemplary embodiment, the electronic element  803  is a USB module. The electronic element  803  is positioned in the receiving space  714 . The electronic element  803  corresponds to the through hole  718  and is partially exposed from the through hole  718 . A USB device can be inserted in the through hole  718  and be electrically connected to the electronic element  803 . 
     In this exemplary embodiment, the slot  719  is defined at the end portion  715  and communicates with the through hole  718 . The slot  719  further extends to the first side portion  716  and the second side portion  717 . In other exemplary embodiments, the slot  719  can only be defined at the end portion  715  and does not extend to any one of the first side portion  716  and the second side portion  717 . In other exemplary embodiments, the slot  719  can be defined at the end portion  715  and extends to one of the first side portion  716  and the second side portion  717 . 
     The gap  720  communicates with the slot  719  and extends across the front frame  711 . In this exemplary embodiment, the gap  720  is positioned adjacent to the second side portion  717 . The front frame  711  is divided into two portions by the gap  720 , these portions being a long portion F 1  and a short portion F 2  (long and short relative to each other). A first portion of the front frame  711  extending from a first side of the gap  720  to a first end D 1  of the slot  719  forms the long portion F 1 . A second portion of the front frame  711  extending from a second side of the gap  720  to a second end D 2  of the slot  719  forms the short portion F 2 . 
     In this exemplary embodiment, the gap  720  is not positioned at a middle portion of the end portion  715 . The long portion F 1  is longer than the short portion F 2 . 
     In this exemplary embodiment, the slot  719  and the gap  720  are both filled with insulating material, for example, plastic, rubber, glass, wood, ceramic, or the like, thereby isolating the long portion F 1 , the short portion F 2 , and the backboard  712 . 
     In this exemplary embodiment, the slot  719  is defined on the end of the side frame  713  adjacent to the backboard  712  and extends to the front frame  711 . Then the long portion F 1  and the short portion F 2  are fully formed by a portion of the front frame  711 . In other exemplary embodiments, a position of the slot  719  can be adjusted. For example, the slot  719  is defined on the end of the side frame  713  adjacent to the backboard  712  and extends towards the front frame  711 . Then the long portion F 1  and the short portion F 2  are formed by a portion of the front frame  711  and a portion of the side frame  713 . 
     In this exemplary embodiment, except for the through hole  718 , the slot  719 , and the gap  720 , a lower half portion of the front frame  711  and the side frame  713  does not define any other slot, break line, and/or gap. That is, there is only one gap  720  defined on the lower half portion of the front frame  711 . 
     In this exemplary embodiment, the first feed source S 1  is positioned in the receiving space  714  and is located between the electronic element  803  and the second side portion  717 . The first feed source S 1  is electrically connected to the first radiator  73  to feed current to the first radiator  73 . 
     The first radiator  73  is positioned in the receiving space  714  and is located between the electronic element  803  and the second side portion  717 . The first radiator  73  includes a first radiating portion  731  and a second radiating portion  733 . One end of the first radiating portion  731  is electrically connected to the first feed source S 1  through a matching circuit  81 . Another end of the first radiating portion  731  is spaced apart from the long portion F 1 . When the first feed source S 1  supplies current, the current flows through matching circuit  81  and the first radiating portion  731 , and is coupled to the long portion F 1 . The first radiating portion  731  and the long portion F 1  form a coupling structure to activate a first mode, to generate radiation signals in a first frequency band. In this exemplary embodiment, the first mode is an LTE-A low frequency operation mode. The first frequency band is a frequency band of about 704-960 MHz. 
     In this exemplary embodiment, the first radiating portion  731  includes a first radiating section  734 , a second radiating section  735 , and a third radiating section  736 . The first radiating section  734  is coplanar with the second radiating section  735  and the third radiating section  736 . The first radiating section  734  is substantially rectangular. The first radiating section  734  is electrically connected to the first feed source S 1  through the matching circuit  81 , and extends along a direction parallel to the end portion  715  towards the electronic element  803  until the first radiating section  734  passes over the gap  720 . 
     The second radiating section  735  is substantially rectangular. One end of the second radiating section  735  is perpendicularly connected to the end of the first radiating section  734  away from the first feed source S 1 . Another end of the second radiating section  735  extends along a direction parallel to the second side portion  717  towards the long portion F 1  and forms an L-shaped structure with the first radiating section  734 . The third radiating section  736  is substantially rectangular. The third radiating section  736  is spaced apart from and parallel to the long portion F 1 . The third radiating section  736  is perpendicularly connected to the end of the second radiating section  735  away from the first radiating section  734 . The third radiating section  736  further extends along two directions, that is, towards the first side portion  716  and towards the second side portion  717  respectively, to form a T-shaped structure with the second radiating section  735 . 
     In this exemplary embodiment, the second radiating portion  733  is a capacitor. One end of the second radiating portion  733  is electrically connected to a junction of the matching circuit  81  and the first radiating section  734 . Another end of the second radiating portion  733  is electrically connected to the short portion F 2 . Then, when the first feed source S 1  supplies current, the current flows through the second radiating portion  733 , and flows to the short portion F 2  to activate a second mode to generate radiation signals in a second frequency band. In this exemplary embodiment, the second mode is an LTE-A middle frequency operation mode. The second frequency band is a frequency band of about 1710-1990 MHz. In addition, the current from the second radiating portion  733  and the short portion F 2  is further coupled to the long portion F 1  through the gap  720  to activate a third mode to generate radiation signals in the third frequency band. In this exemplary embodiment, the third mode is also an LTE-A middle frequency operation mode. The third frequency band is a frequency band of about 2110-2170 MHz. Then, the second mode and the third mode cooperatively form a wide band mode (1710-2170 MHz). 
     Per  FIG. 63 , the first switching circuit  75  is electrically connected to a middle portion of the long portion F 1 . The first switching circuit  75  includes a first switching unit  751  and a plurality of first switching elements  753 . The first switching unit  751  is electrically connected to the long portion F 1 . The first switching elements  753  can be an inductor, a capacitor, or a combination of the inductor and the capacitor. The first switching elements  753  are connected in parallel. One end of each first switching element  753  is electrically connected to the first switching unit  751 . The other end of each first switching element  753  is electrically connected to the backboard  712 . 
     Per  FIG. 64 , one end of the matching circuit  81  is electrically connected to the first feed source S 1 . Another end of the matching circuit  81  is electrically connected to the first radiating portion  731 . One end of the second switching circuit  76  is electrically connected to the matching circuit  81 . Another end of the second switching circuit  76  is electrically connected to the backboard  712 . In this exemplary embodiment, the second switching circuit  76  includes a second switching unit  761  and a plurality of second switching elements  763 . The second switching unit  761  is electrically connected to the matching circuit  81  and is electrically connected to the first radiating portion  731  through the matching circuit  81 . The second switching elements  763  can be an inductor, a capacitor, or a combination of the inductor and the capacitor. The second switching elements  763  are connected in parallel. One end of each second switching element  763  is electrically connected to the second switching unit  761 . The other end of each second switching element  763  is electrically connected to the backboard  712 . 
     Through controlling the first switching unit  751  and/or the second switching unit  761 , the long portion F 1  can be switched to connect with different first switching elements  753  and/or second switching elements  763 . Since each first switching elements  753  and second switching element  763  has a different impedance, an operating frequency band of the long portion F 1  can be adjusted through switching the first switching unit  751  and/or the second switching unit  761 , for example, the frequency band of the first mode of the long portion F 1  can be offset towards a lower frequency or towards a higher frequency (relative to each other). In this exemplary embodiment, the first switching circuit  75  and the second switching circuit  76  can be switched independently or together. 
     Per  FIG. 65 , the first switching circuit  75  further includes a resonance circuit  77 . In one exemplary embodiment, the first switching circuit  75  includes one resonance circuit  77 . The resonance circuit  77  includes an inductor L and a capacitor C connected in series. The resonance circuit  77  is electrically connected between the long portion F 1  and the backboard  712 . The resonance circuit  77  is electrically connected in parallel to the first switching unit  751  and at least one first switching element  753 . 
     Per  FIG. 66 , in another exemplary embodiment, the first switching circuit  75  includes a plurality of resonance circuits  77 . The number of the resonance circuits  77  is equal to the number of first switching elements  753 . Each resonance circuit  77  includes inductors L 1 -Ln and capacitors C 1 -Cn connected in series. Each resonance circuit  77  is electrically connected to one of the first switching elements  753  in parallel between the first switching unit  751  and the backboard  712 . 
     Per  FIG. 63 ,  FIG. 64 ,  FIG. 65 , and  FIG. 66 , the backboard  712  can be replaced by the shielding mask or the middle frame for grounding the first switching circuit  75  and/or the second switching circuit  76 . 
     Per  FIG. 67 , when the antenna structure  700  does not include the resonance circuit  77  of  FIG. 65 , the antenna structure  700  works at the first mode (please see the curve S 671 ). When the antenna structure  700  includes the resonance circuit  77 , the long portion F 1  of the antenna structure  700  can activate an additional resonance mode (that is, a third mode, 2110-2170 MHz, please see the curve S 672 ) to generate radiation signals in a third frequency band. The third mode can effectively broaden an applied frequency band of the antenna structure  700 . 
     Per  FIG. 68 , when the antenna structure  700  does not include the resonance circuit  77  of  FIG. 66 , the antenna structure  700  works at the first mode (please see the curve S 681 ). When the antenna structure  700  includes the resonance circuit  77 , the long portion F 1  of the antenna structure  700  can activate the additional resonance mode (please see the curve S 682 ), that is, the third mode. The third mode can effectively broaden an applied frequency band of the antenna structure  700 . 
     In one exemplary embodiment, inductance values of the inductors L 1 -Ln and capacitance values of the capacitors C 1 -Cn of the resonance circuit  77  can cooperatively decide a frequency band of the resonance mode when the first mode switches. For example, in one exemplary embodiment, as illustrated in  FIG. 68 , when the first switching unit  751  switches to different first switching elements  753  through setting the inductance value and the capacitance value of the resonance circuit  77 , the resonance mode of the antenna structure  700  can also be switched. For example, the resonance mode of the antenna structure  700  can be moved from f 1  to fn. 
     In other exemplary embodiments, the frequency band of the resonance mode can be fixed through setting the inductance value and the capacitance value of the resonance circuit  77 . Then no matter to which first switching element  753  the first switching unit  751  is switched, the frequency band of the resonance mode is fixed and keeps unchanged. 
     In other exemplary embodiments, the resonance circuit  77  is not limited to include the inductor L and the capacitor C, and can include other resonance components. 
     In this exemplary embodiment, the second radiator  78  is positioned in the receiving space  714  of the housing  71  and is positioned adjacent to the long portion F 1 . The second radiator  78  is spaced apart from the backboard  712 . In this exemplary embodiment, the second radiator  78  is substantially a strip and is positioned parallel to the end portion  715 . The second radiator  78  is connected to the position of the front frame  711  adjacent to the first end D 1  and extends towards the second side portion  717 . 
     The second feed source S 2  is positioned on the front frame  711  and is electrically connected to the second radiator  78  to feed current to the second radiator  78 . When the second feed source S 2  supplies current, the current flows to the second radiator  78  to activate a fourth mode, to generate radiation signals in a fourth frequency band. In this exemplary embodiment, the fourth mode is an LTE-A high frequency operation mode. The fourth frequency band is a frequency band of about 2300-2400 MHz and 2496-2690 MHz. 
     One end of the third switching circuit  79  is electrically connected to the second radiator  78  and another end of the third switching circuit  79  is electrically connected to the backboard  712 , the shielding mask, or the middle frame to be grounded. The third switching circuit  79  is configured to adjust a frequency band of the high frequency operation mode of the second radiator  78 . A circuit structure and a working principle of the third switching circuit  79  are consistent with the first switching circuit  75  shown in  FIG. 63 . 
     Per  FIG. 69 , when the first feed source S 1  supplies current, the current flows through the first radiating section  734 , the second radiating section  735 , and the third radiating section  736  of the first radiating portion  731 . The current is further coupled to the long portion F 1  through the third radiating section  736 , flows through the first side portion  716  from the long portion F 1 , and then to the backboard  712  (e.g., path J 1 ) to activate the first mode to generate radiation signals in the first frequency band. Since the antenna structure  700  includes the first switching circuit  75  and the second switching circuit  76 , the low frequency operation mode of the long portion F 1  can be switched through the first switching circuit  75  and the second switching circuit  76  in coordination with each other, and the middle frequency operation mode and the high frequency operation mode of the antenna structure  700  are unaffected. 
     Per  FIG. 70 , when the first feed source S 1  supplies current, the current directly flows through the short portion F 2  through the second radiating portion  733 , and flows to the second side portion  717  and the backboard  712  (e.g., path J 2 ) to activate the second mode, to generate radiation signals in the second frequency band. When the first feed source S 1  supplies current, the current flows through the short portion F 2  through the second radiating portion  733 , is coupled to the long portion F 1  through the gap  720 , flows through the resonance circuit  77  of the first switching circuit  75 , and then to the backboard  712  (e.g., path J 3 ). Then, through a coupling of the gap  720  and a configuration of the resonance circuit  77 , the long portion F 1  further activates the third mode to generate radiation signals in the third frequency band. 
     Per  FIG. 71 , when the current enters the second radiator  78  from the second feed source S 2 , the current flows to the end of the second radiator  78  away from the second feed source S 2  (e.g., path J 4 ) to activate the fourth mode, to generate radiation signals in the fourth frequency band. Since the antenna structure  700  includes the third switching circuit  79 , the frequencies of the high frequency operation mode can be effectively switched. 
       FIG. 72  illustrates a scattering parameter graph of the antenna structure  700 , when the antenna structure  700  works at the low frequency operation mode. Curve S 721  illustrates a scattering parameter when the antenna structure  700  works at a frequency band of about 704-746 MHz (LTE-A Band 17). Curve S 722  illustrates a scattering parameter when the antenna structure  700  works at a frequency band of about 746-787 MHz (LTE-A Band 13). Curve S 723  illustrates a scattering parameter when the antenna structure  700  works at a frequency band of about 824-894 MHz (LTE-A Band 5). Curve S 724  illustrates a scattering parameter when the antenna structure  700  works at a frequency band of about 880-960 MHz (LTE-A Band 8). Curves S 721 -S 724  respectively correspond to four different frequency bands and respectively correspond to four of the plurality of low frequency operation modes of the first switching circuit  75  and the second switching circuit  76 . 
       FIG. 73  illustrates a radiating efficiency graph of the antenna structure  700 , when the antenna structure  700  works at the low frequency operation mode. Curve S 731  illustrates a radiating efficiency when the antenna structure  700  works at a frequency band of about 704-746 MHz (LTE-A Band 17). Curve S 732  illustrates a radiating efficiency when the antenna structure  700  works at a frequency band of about 746-787 MHz (LTE-A Band 13). Curve S 733  illustrates a radiating efficiency when the antenna structure  700  works at a frequency band of about 824-894 MHz (LTE-A Band 5). Curve S 734  illustrates a radiating efficiency when the antenna structure  700  works at a frequency band of about 880-960 MHz (LTE-A Band 8). Curves S 731 -S 734  respectively correspond to four different frequency bands and respectively correspond to four of the plurality of low frequency operation modes of the first switching circuit  75  and the second switching circuit  76 . 
       FIG. 74  illustrates a scattering parameter graph of the antenna structure  700 , when the antenna structure  700  works at the middle frequency operation mode (1710-1990 MHz and 2110-2170 MHz).  FIG. 75  illustrates a radiating efficiency graph of the antenna structure  700 , when the antenna structure  700  works at the middle frequency operation mode (1710-1990 MHz and 2110-2170 MHz). 
       FIG. 76  illustrates a scattering parameter graph of the antenna structure  700 , when the antenna structure  700  works at the high frequency operation mode (2300-2400 MHz and 2496-2690 MHz).  FIG. 77  illustrates a radiating efficiency graph of the antenna structure  700 , when the antenna structure  700  works at the high frequency operation mode (2300-2400 MHz and 2496-2690 MHz). When the switching unit of the third switching circuit  79  switches to different switching elements (for example, four different switching elements), each of switching elements has a different impedance, the high frequency band of the antenna structure  700  can be effectively adjusted to obtain a good operating bandwidth. 
     In view of  FIGS. 72 to 77 , the antenna structure  700  can work at a low frequency band, for example, frequency bands of about LTE-A Band 17/13/5/8. The antenna structure  700  can also work at the middle frequency band (1710-1990 MHz and 2110-2170 MHz), and the high frequency band (2300-2400 MHz and 2496-2690 MHz). That is, the antenna structure  700  can work at the low frequency band, the middle frequency band, and the high frequency band, and when the antenna structure  700  works at these frequency bands, a working frequency satisfies a design of the antenna and also has a good radiating efficiency. 
     In this exemplary embodiment, the antenna structure  700  includes the first radiator  73 , the first radiating portion  731  and the long portion F 1  cooperatively a coupling structure, and the second radiating portion  733  is directly connected to the short portion F 2 . That is, the first radiator  73 , the long portion F 1 , and the short portion F 2  cooperatively form a half-coupling feed structure. The long portion F 1  and the short portion F 2  respectively activate a first mode and a second mode. The configuration of the half-coupling feed structure ensures a flexibility for adjusting the antenna structure  700  and can effectively decrease a nonmetallic area of the antenna structure  700 . 
     In addition, the antenna structure  700  includes the first switching circuit  75  and the second switching circuit  76 , the first mode can be effectively adjusted and switched. The antenna structure  700  further includes the resonance circuit  77 , then the long portion F 1  can activate an additional middle frequency operation mode (the third mode). The antenna structure  700  includes the second radiator  78  and the third switching circuit  79 , the antenna structure  700  can activate a high frequency operation mode and the high frequency band of the antenna structure  700  can be effectively adjusted to obtain a good operating bandwidth. 
       FIG. 78  illustrates a seventh exemplary antenna structure  700   a . The antenna structure  700   a  includes a housing  71 , a first feed source S 1 , a first radiator  83 , a first switching circuit  75 , a second switching circuit  76 , a resonance circuit  77 , a second radiator  78 , a second feed source S 2 , and a third switching circuit  79 . The housing  71  includes a front frame  711 , a backboard  712 , and a side frame  713 . The side frame  713  includes an end portion  715 , a first side portion  716 , and a second side portion  717 . The side frame  713  defines a slot  719 . The front frame  711  defines a gap  720 . The front frame  711  is divided into two portions by the gap  720 , these portions being a long portion F 1  and a short portion F 2  (long and short relative to each other). 
     The first radiator  83  includes a first radiating portion  731  and a second radiating portion  831 . The first radiating portion  731  includes a first radiating section  734 , a second radiating section  735 , and a third radiating section  736 . The third radiating section  736  is spaced apart from the long portion F 1 , then the first radiating portion  731  and the long portion F 1  form a coupling structure. 
     In this exemplary embodiment, the antenna structure  700   a  differs from the antenna structure  700  in that a structure of the second radiating portion  831  of the antenna structure  700   a  is different from the second radiating portion  733  of the antenna structure  700 . A connection relationship between the second radiating portion  831  and the short portion F 2  is also different from the connection relationship between the second radiating portion  733  and the short portion F 2 . 
     In this exemplary embodiment, the second radiating portion  831  is symmetrical to the first radiating portion  731  relative to the first feed source S 1 . The second radiating portion  831  includes a first coupling section  832 , a second coupling section  833 , and a third coupling section  834 . The first coupling section  832  is substantially rectangular. The first coupling section  832  is electrically connected to the first radiating section  734  and the matching circuit  81  of the first feed source S 1 , and extends along a direction parallel to the end portion  715  towards the second side portion  717 , so as to be collinear with the first radiating section  734 . 
     The second coupling section  833  is substantially rectangular. One end of the second coupling section  833  is perpendicularly connected to the end of the first coupling section  832  away from the first feed source S 1 . Another end of the second coupling section  833  extends along a direction parallel to the second radiating section  735  towards the end portion  715 . The second coupling section  833 , the first radiating section  734 , the second radiating section  735 , and the first coupling section  832  cooperatively form a U-shaped structure. 
     The third coupling section  834  is substantially rectangular. The third coupling section  834  is spaced apart from and parallel to the short portion F 2 . The third coupling section  834  is electrically connected to the end of the second coupling section  833  away from the first coupling section  832 . The third coupling section  834  further extends along two directions, the two directions being towards the first side portion  716  and towards the second side portion  717  respectively, to form a T-shaped structure with the second coupling section  833 . 
     In this exemplary embodiment, when the antenna structure  700   a  works at the low frequency operation mode, a current path distribution graph of the antenna structure  700   a  is consistent with the current path distribution graph of the antenna structure  700  shown in  FIG. 69 . 
     Per  FIG. 79 , when the first feed source S 1  supplies current, the current directly flows through the first coupling section  832 , the second coupling section  833 , and the third coupling section  834 . The current is further coupled to the short portion F 2  through the third coupling section  834 , and flows to the second side portion  717  and the backboard  712  (e.g., path J 5 ) to activate the second mode, to generate radiation signals in the second frequency band. When the first feed source S 1  supplies current, the current is coupled to the short portion F 2  through the third coupling section  834 , is coupled to the long portion F 1  through the gap  720 , flows through the resonance circuit  77  of the first switching circuit  75 , and flows to the backboard  712  (e.g., path J 6 ). Then, through a coupling of the gap  720  and a configuration of the resonance circuit  77 , the long portion F 1  further activates the third mode to generate radiation signals in the third frequency band. 
     In this exemplary embodiment, when the antenna structure  700   a  works at the high frequency operation mode, a current path distribution graph of the antenna structure  700   a  is consistent with the current path distribution graph of the antenna structure  700  shown in  FIG. 71 . 
       FIG. 80  illustrates a scattering parameter graph of the antenna structure  700   a , when the antenna structure  700   a  works at the low frequency operation mode. Curve S 801  illustrates a scattering parameter when the antenna structure  700   a  works at a frequency band of about 704-746 MHz (LTE-A Band 17). Curve S 802  illustrates a scattering parameter when the antenna structure  700   a  works at a frequency band of about 746-787 MHz (LTE-A Band 13). Curve S 803  illustrates a scattering parameter when the antenna structure  700   a  works at a frequency band of about 824-894 MHz (LTE-A Band 5). Curve S 804  illustrates a scattering parameter when the antenna structure  700   a  works at a frequency band of about 880-960 MHz (LTE-A Band 8). Curves S 801 -S 804  respectively correspond to four different frequency bands and respectively correspond to four of the plurality of low frequency operation modes of the first switching circuit  75  and the second switching circuit  76 . 
       FIG. 81  illustrates a radiating efficiency graph of the antenna structure  700   a , when the antenna structure  700   a  works at the low frequency operation mode. Curve S 811  illustrates a radiating efficiency when the antenna structure  700   a  works at a frequency band of about 704-746 MHz (LTE-A Band 17). Curve S 812  illustrates a radiating efficiency when the antenna structure  700   a  works at a frequency band of about 746-787 MHz (LTE-A Band 13). Curve S 813  illustrates a radiating efficiency when the antenna structure  700   a  works at a frequency band of about 824-894 MHz (LTE-A Band 5). Curve S 814  illustrates a radiating efficiency when the antenna structure  700   a  works at a frequency band of about 880-960 MHz (LTE-A Band 8). Curves S 811 -S 814  respectively correspond to four different frequency bands and respectively correspond to four of the plurality of low frequency operation modes of the first switching circuit  75  and the second switching circuit  76 . 
       FIG. 82  illustrates a scattering parameter graph of the antenna structure  700   a , when the antenna structure  700   a  works at the middle frequency operation mode (1710-1990 MHz and 2110-2170 MHz).  FIG. 83  illustrates a radiating efficiency graph of the antenna structure  700   a , when the antenna structure  700   a  works at the middle frequency operation mode (1710-1990 MHz and 2110-2170 MHz). 
     In this exemplary embodiment, when the antenna structure  700   a  works at the high frequency operation mode, a scattering parameter graph and a radiating efficiency graph of the antenna structure  700   a  are consistent with the scattering parameter graph and a radiating efficiency graph of the antenna structure  700  shown in  FIG. 76  and  FIG. 77 . 
     In this exemplary embodiment, the antenna structure  700   a  includes the first radiator  83 , the first radiating portion  731  of the first radiator  83  and the long portion F 1  cooperatively a coupling structure. The second radiating portion  831  and the short portion F 2  cooperatively a coupling structure. That is, the first radiator  83 , the long portion F 1 , and the short portion F 2  cooperatively form a full-coupling feed structure. The long portion F 1  and the short portion F 2  respectively activate a first mode and a second mode. The configuration of the full-coupling feed structure ensures a flexibility for adjusting the antenna structure  700   a  and can effectively decrease a nonmetallic area of the antenna structure  700   a.    
     In addition, the antenna structure  700   a  includes the first switching circuit  75  and the second switching circuit  76 , the first mode can be effectively adjusted and switched. The antenna structure  700   a  further includes the resonance circuit  77 , then the long portion F 1  can activate an additional middle frequency operation mode (the third mode). The antenna structure  700   a  includes the second radiator  78  and the third switching circuit  79 , the antenna structure  700   a  can activate a high frequency operation mode and the high frequency band of the antenna structure  700   a  can be effectively adjusted to obtain a good operating bandwidth. 
     As described above, the first radiator  73 / 83  is coupled with the long portion F 1 , thus the long portion F 1  can activate a first mode to generate radiation signals in a low frequency band. The first radiator  73 / 83  is directly connected to or coupled to the short portion F 2 , then the short portion F 2  can activate a second mode to generate radiation signals in a middle frequency band. That is, the first radiator  73 / 83  can form a half-coupling feed structure or a full-coupling feed structure with the long portion F 1  and the short portion F 2 , and the long portion F 1  and the short portion F 2  cooperatively activate the first mode and the second mode. The long portion F 1  is coupled with the short portion F 2  through the gap  720 , and through the resonance circuit  77 , the long portion F 1  can activate an additional third mode to generate radiation signals in a middle frequency band. The second radiator  78  can activate a fourth mode to generate radiation signals in a high frequency band. The wireless communication device  800  can use carrier aggregation (CA) technology of LTE-A to receive and/or transmit wireless signals at multiple frequency bands simultaneously. In detail, the wireless communication device  800  can use the CA technology and use at least two of the long portion F 1 , the short portion F 2 , the first radiator  73 / 83 , and the second radiator  78  to receive and/or transmit wireless signals at multiple frequency bands simultaneously. 
     The antenna structure  100  of first exemplary embodiment, the antenna structure  200  of second exemplary embodiment, the antenna structure  500  of third exemplary embodiment, the antenna structure  500   a  of fourth exemplary embodiment, the antenna structure  500   b  of fifth exemplary embodiment, the antenna structure  700  of sixth exemplary embodiment, and the antenna structure  700   a  of seventh exemplary embodiment can be applied to one wireless communication device. For example, the antenna structure  100  or  200  can be positioned at an upper end of the wireless communication device to serve as an auxiliary antenna. The antenna structures  500 ,  500   a ,  500   b ,  700 , or  700   a  can be positioned at a lower end of the wireless communication device to serve as a main antenna. When the wireless communication device transmits wireless signals, the wireless communication device can use the main antenna to transmit wireless signals. When the wireless communication device receives wireless signals, the wireless communication device can use the main antenna and the auxiliary antenna to receive wireless signals. 
     The embodiments shown and described above are only examples. Many details are often found in the art such as the other features of the antenna structure and the wireless communication device. Therefore, many such details are neither shown nor described. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the details, especially in matters of shape, size and arrangement of the parts within the principles of the present disclosure up to, and including the full extent established by the broad general meaning of the terms used in the claims. It will therefore be appreciated that the embodiments described above may be modified within the scope of the claims.