Patent Publication Number: US-2015061951-A1

Title: Communication device and small-size multi-branch multi-band antenna element therein

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No. 102131619 filed on Sep. 3, 2013, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosure generally relates to a communication device, and more particularly, relates to a communication device comprising a small-size multi-branch multi-band antenna element. 
     2. Description of the Related Art 
     Mobile communication technology is progressing fast nowadays and playing a more and more important role in human life. Mobile communication devices need to operate in wider and wider bandwidths since each communication generation has different communication technique and each local telecommunication operator has different operation bands. Furthermore, in order to provide mobility and improve user experience, current mobile communication devices are designed to be thin and light. Hence, there are very limited spaces inside the device to accommodate the antenna elements. A conventional multi-branch multi-band LTE/WWAN (Long Term Evolution/Wireless Wide Area Network) antenna element, for example, has resonant paths as long as about a quarter wavelength of its operation frequency. Therefore, the conventional multi-branch multi-band antenna element occupies more spaces, and it is difficult to apply the conventional design to a variety of small-size mobile communication devices. 
     Furthermore, since the branches of the conventional multi-branch multi-band antenna element have adjacent resonant paths and need similar resonant lengths, the resonant modes excited by the branches tend to affect each other to result in degraded antenna performances. As a result, these resonant modes cannot be combined into a wide band to cover the desired operation bandwidth, or otherwise these resonant modes lead to low radiation efficiency even if appropriate impedance matching is obtained therebetween. 
     Accordingly, it is a critical challenge for antenna designers to design a low-profile, small-size, and wide-band multi-branch antenna element in the limited space of a mobile communication device to cover multiple operation bands (e.g., LTE/WWAN bands). 
     BRIEF SUMMARY OF THE INVENTION 
     To solve the problems in the prior art, the invention provides a communication device comprising a multi-branch multi-band antenna element. This antenna element not only achieves a low-profile and small-size design but also covers LTE/WWAN bands (from about 704 MHz to 960 MHz and from about 1710 MHz to 2690 MHz) and a (Wireless Local Area Network) 2.4 GHz WLAN band. 
     In a preferred embodiment, the invention provides a communication device, comprising: a ground element; and an antenna element, disposed on a dielectric substrate, wherein the dielectric substrate is disposed adjacent to an edge of the ground element, the antenna element has a first connection point, and the antenna element at least comprises: a first branch, having a first length, wherein one end of the first branch is coupled through a first inductive element to the first connection point, the first branch comprises a first segment, and the first segment is substantially parallel to the edge of the ground element; a second branch, having a second length, wherein one end of the second branch is coupled to the first connection point, the second branch comprises a second segment, the second segment is substantially parallel to the first segment, and the second branch is disposed between the first branch and the edge of the ground element; and a third branch, having a third length, wherein one end of the third branch is coupled to a second connection point on the first branch, and the third branch and the first branch substantially extend in opposite directions; wherein the first connection point is further coupled through a high-pass matching circuit to a signal source, and the high-pass matching circuit has a grounding end coupled to the ground element. 
     The antenna element of the invention not only has a unique radiation structure (comprising the first branch, the second branch, and the third branch) but is also integrated with the high-pass matching circuit in such a manner that the antenna element has the advantages of low-profile, small-size, and wide-band characteristics. In some embodiments, the antenna element is configured to cover LTE/WWAN multiple bands. In some embodiments, the antenna element at least operates in a first band and a second band, and frequencies of the first band are lower than frequencies of the second band. Among the multiple branches of the antenna element, the second length may be shorter than the first length, and the third length may be shorter than the second length and is shorter than 0.5 times the first length. When the antenna element is fed by the signal source, the first branch may be excited to generate a first resonant mode in the first band, the second branch may be excited to generate a third resonant mode in the second band, and the third branch may be excited to generate a fourth resonant mode in the second band. The fourth resonant mode is combined with the third resonant mode to significantly increase the bandwidth of the second band. 
     In some embodiments, the high-pass matching circuit comprises at least a second inductive element coupled in parallel and a capacitive element coupled in series. In some embodiments, the high-pass matching circuit is disposed on the dielectric substrate or the ground element. The high-pass matching circuit is used to adjust the impedance matching of the antenna element. Since the second inductive element of the high-pass matching circuit may be further coupled to the ground element, the antenna element may perform like an inverted-F antenna structure and therefore have the advantage of low-profile characteristics. In some embodiments, the high-pass matching circuit causes the antenna element to further generate a second resonant mode in the first band. The second resonant mode is combined with the first resonant mode to significantly increase the bandwidth of the first band. The first inductive element can decrease the resonant lengths of the first branch and the third branch such that the antenna element has the advantage of small-size characteristics. In addition, when the antenna element operates in the second band, the first inductive element can isolate the first branch and reduce the coupling effect of the first branch on the third resonant mode excited by the second branch, such that the third resonant mode can be well excited. On the other hand, since the third branch is coupled to the first branch and the third length is shorter than 0.5 times the first length, the generation of the fourth resonant mode and the generation of the first resonant mode do not affect each other, and therefore both can be well excited. In some embodiments, the antenna element with a small-size planar structure (e.g., 10 mm by 40 mm) generates the wide first and second bands (e.g., from about 704 MHz to 960 MHz and from about 1710 MHz to 2690 MHz). Therefore, the antenna element is at least configured to cover the LTE/WWAN bands and the 2.4 GHz WLAN band. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a diagram for illustrating a communication device according to a first embodiment of the invention; 
         FIG. 2  is a diagram for illustrating a communication device according to a second embodiment of the invention; 
         FIG. 3  is a diagram for illustrating return loss of an antenna element of a communication device according to a first embodiment of the invention; 
         FIG. 4  is a diagram for illustrating antenna efficiency of an antenna element of a communication device according to a first embodiment of the invention; and 
         FIG. 5  is a diagram for illustrating a communication device according to a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a diagram for illustrating a communication device  100  according to a first embodiment of the invention. The communication device  100  may be a smartphone, a tablet computer, or a notebook computer. As shown in  FIG. 1 , the communication device  100  at least comprises a ground element  10  and an antenna element  11 . The ground element  10  may be a metal plane configured to accommodate some electronic components (not shown) of the communication device  100 . In some embodiments, the communication device  100  may further comprise a dielectric substrate  12 , a first inductive element  17 , a high-pass matching circuit  18 , and a signal source  19 . The dielectric substrate  12  may be an FR4 (Flame Retardant 4) substrate. The first inductive element  17  may be a chip inductor. The high-pass matching circuit  18  may comprise one or more capacitors and inductors, such as chip capacitors and chip inductors. The signal source  19  may be an RF (Radio Frequency) module configured to excite the antenna element  11 . The antenna element  11  is disposed on the dielectric substrate  12 . The dielectric substrate  12  is disposed adjacent to an edge  101  of the ground element  10 . The antenna element  11  has a first connection point  16 , and at least comprises a first branch  13 , a second branch  14 , and a third branch  15 . The first branch  13  has a first length. One end of the first branch  13  is coupled through the first inductive element  17  to the first connection point  16 . The first branch  13  comprises a first segment  131 , and the first segment  131  is substantially parallel to the edge  101  of the ground element  10 . In some embodiments, the first branch  13  substantially has an inverted L-shape, and a combination of the first branch  13  and the third branch  15  substantially has an inverted U-shape. The second branch  14  has a second length. In some embodiments, the second length is shorter than the first length. One end of the second branch  14  is coupled to the first connection point  16 . The second branch  14  comprises a second segment  141 , and the second segment  141  is substantially parallel to the first segment  131  of the first branch  13 . The second branch  14  is disposed between the first branch  13  and the edge  101  of the ground element  10 . In some embodiments, the second branch  14  substantially has an inverted N-shape. The third branch  15  has a third length. In some embodiments, the third length is shorter than the second length, and is shorter than  0 . 5  times the first length. One end of the third branch  15  is coupled to a second connection point  132  on the first branch  13 . The third branch  15  and the first branch  13  substantially extend in opposite directions. In other words, an open end of the third branch  15  is away from an open end of the first branch  13 . In some embodiments, the third branch  15  substantially has an inverted L-shape. The first connection point  16  of the antenna element  11  is further coupled through the high-pass matching circuit  18  to the signal source  19 . The high-pass matching circuit  18  has a grounding end  181 , and the grounding end  181  is coupled to the ground element  10 . Note that the communication device  100  may further comprise other components, such as a touch panel, a processor, a speaker, a battery, and a housing (not shown). 
       FIG. 2  is a diagram for illustrating a communication device  200  according to a second embodiment of the invention. The main difference between the second embodiment and the first embodiment is that a high-pass matching circuit  28  of the communication device  200  comprises at least a second inductive element  282  coupled in parallel and a capacitive element  283  coupled in series. More particularly, a first end of the second inductive element  282  is a grounding end  281  coupled to the ground element  10 , and a second end of the second inductive element  282  is coupled to the first connection point  16 . On the other hand, a first end of the capacitive element  283  is coupled to the signal source  19 , and a second end of the capacitive element  283  is coupled to the first connection point  16 . The second inductive element  282  may be a chip inductor, and the capacitive element  283  may be a chip capacitor. Other features of the communication device  200  of the second embodiment are similar to those of the communication device  100  of the first embodiment. Accordingly, the two embodiments can achieve similar performances. 
       FIG. 3  is a diagram for illustrating return loss of the antenna element  11  of the communication device  100  according to the first embodiment of the invention. In some embodiments, the element sizes and element parameters of the communication device  100  are as follows. The ground element  10  has a length of about 200 mm and a width of about 150 mm. The dielectric substrate  12  has a length of about 40 mm and a width of about 10 mm and a thickness of about 0.8 mm. The first branch  13  has a first length of about 44 mm. The second branch  14  has a second length of about 23 mm. The third branch  15  has a third length of about 16 mm (shorter than 0.5 times the first length of the first branch  13 ). The first inductive element  17  is a chip inductor, and the chip inductor has an inductance of about 10 nH. The high-pass matching circuit  18  comprises a chip inductor coupled in parallel and a chip capacitor coupled in series, in which the chip inductor has an inductance of about 10 nH, and the chip capacitor has a capacitance of about 2.7 pF. As shown in  FIG. 3 , the antenna element  11  at least operates in a first band  31  and a second band  32 , and frequencies of the first band  31  are lower than frequencies of the second band  32 . More particularly, the operation principle of the antenna element  11  is described as follows. The first branch  13  of the antenna element  11  is excited to generate a first resonant mode  301  in the first band  31 . The high-pass matching circuit  18  of the antenna element  11  is excited to generate a second resonant mode  302  in the first band  31 . After the first resonant mode  301  is combined with the second resonant mode  302 , the first band  31  substantially covers the LTE700/GSM850/GSM900 bands (from about 704 MHz to 960 MHz). In addition, the second branch  14  of the antenna element  11  is excited to generate a third resonant mode  303  in the second band  32 . The third branch  15  of the antenna element  11  is excited to generate a fourth resonant mode  304  in the second band  32 . After the third resonant mode  303  is combined with the fourth resonant mode  304 , the second band  32  substantially covers the GSM1800/GSM1900/UMTS/LTE2300/LTE2500 bands (from about 1710 MHz to 2690 MHz) and the 2.4 GHz WLAN band. 
       FIG. 4  is a diagram for illustrating the antenna efficiency of the antenna element  11  of the communication device  100  according to the first embodiment of the invention. The element sizes and element parameters of the communication device  100  may be the same as those described in the embodiment of  FIG. 3 . The antenna efficiency curve  41  represents the antenna efficiency (return losses included) of the antenna element  11  operating in the first band  31  (from about 704 MHz to 960 MHz). The antenna efficiency curve  42  represents the antenna efficiency (return losses included) of the antenna element  11  operating in the second band  32  (from about 1710 MHz to 2690 MHz). As shown in  FIG. 4 , the average antenna efficiency of the antenna element  11  is greater than about 55% in the first band  31 , and the average antenna efficiency of the antenna element  11  is greater than about 60% in the second band  32 . Therefore, the antenna efficiency meets the application requirements of mobile communication devices. 
       FIG. 5  is a diagram for illustrating a communication device  500  according to a third embodiment of the invention. The main difference between the third embodiment and the first embodiment is that a high-pass matching circuit  58  of the communication device  500  is disposed on the ground element  10 , rather than the dielectric substrate  12 . Other features of the communication device  500  of the third embodiment are similar to those of the communication device  100  of the first embodiment. Accordingly, the two embodiments can achieve similar performances. 
     Note that the aforementioned element sizes, element shapes, element parameters, and frequency ranges are not limitations of the invention. An antenna designer can change these values according to different requirements. 
     Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the invention. It is intended that the standard and examples be considered as exemplary only, with a true scope of the disclosed embodiments being indicated by the following claims and their equivalents.