Abstract:
A portable electronic device switches between antennas in order to decrease specific absorption rate (SAR). When the portable electronic device performs wireless communication, a sensor detects a distance between an object under test and the portable electronic device. The sensor sends a message to a control circuit, and the control circuit switches to a different antenna to decrease SAR.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims priority of Taiwan Patent Application No. 099129216 filed on Aug. 31, 2010, the entirety of which is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The disclosure relates generally to a portable electronic device, and more particularly, relates to a portable electronic device for switching between different antennas according to the distance between an object and the portable electronic device. 
         [0004]    2. Description of the Related Art 
         [0005]    With the progress of mobile communication technology, portable electronic devices, for example, portable computers, mobile phones, multimedia players, and other hybrid functional portable electronic devices, have become more common. To satisfy the demand of users, portable electronic devices usually can perform wireless communication functions. Some functions cover a large wireless communication area, for example, mobile phones using 2G, 3G, and LTE (Long Term Evolution) systems and using frequency bands of 700 MHz, 850 MHz, 900 MHz, 1800 MHz, 1900 MHz, 2100 MHz, 2300 MHz, and 2500 MHz. Some functions cover a small wireless communication area, for example, using Wi-Fi, Bluetooth, and WiMAX (Worldwide Interoperability for Microwave Access) systems and using frequency bands of 3.5 GHz, 5.2 GHz, and 5.8 GHz. 
         [0006]    However, current portable electronic devices can not prevent users from being exposed to electromagnetic waves. For example, when a wireless communication operation is performed, a mobile phone transmits electromagnetic waves which are near to the head of a user. Therefore, governments have formulated criterion as to how much electromagnetic waves is safe for a human body to absorb, such as specific absorption rate, SAR criterion. SAR is a measure of the rate at which energy is absorbed by the human body when exposed to a radio frequency (RF) electromagnetic field. It is defined as the power absorbed per mass of tissue and has units of watts per kilogram (W/kg). In Taiwan, the criterion of SAR is that it should be smaller than 2 W/kg. 
         [0007]    Now, many famous manufacturers have developed tablet PCs. Because tablet PCs are usually used near a human body, they often fail to pass the SAR criterion. A key factor of SAR is Total Radiated Power, TRP, of antennas, and TRP is determined by antenna efficiency and RF conducting power. Some manufacturers add a detector to tablet PCs, and the detector reduces RF conducting power when a human body approaches thereto. Nevertheless, RF conducting power should be determined based on the distance between a tablet PC and a base station. Thus, the quality of wireless communication degrades, when RF conducting power is reduced due to a human body approaching the detector. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    In one exemplary embodiment, the disclosure is directed to a portable electronic device, comprising: a first antenna; a second antenna; a detector, detecting a distance between the detector and an object, and generating a message according to the distance; and a control circuit, receiving the message, and selecting the first antenna or the second antenna to transmit or receive a signal according to the message. 
         [0009]    In another exemplary embodiment, the disclosure is directed to a method for switching between antennas in a portable electronic device having a first antenna and a second antenna, comprising: detecting a distance from the portable electronic device; generating a message according to the distance; and selecting either the first antenna or the second antenna to transmit or receive a signal according to the message. 
         [0010]    In one exemplary embodiment, the disclosure is directed to a portable electronic device, comprising: an active antenna, comprising a plurality of child antennas to transmit or receive a signal; a detector, detecting a distance between the active antenna and an object, and generating a message according to the distance; and a control circuit, receiving the message, and selecting a first child antenna or the second child antenna among the plurality of child antennas of the active antenna to transmit or receive a signal according to the message. 
         [0011]    In another exemplary embodiment, the disclosure is directed to a portable electronic device, comprising: a plurality of antennas; a plurality of detectors, detecting a distance between each of the plurality of antennas and an object, and generating a message according to the distance; and a control circuit, receiving the message, and selecting one of the plurality of antennas to transmit or receive a signal according to the message. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a diagram illustrating a portable electronic device according to an embodiment of the invention; 
           [0014]      FIG. 2  is a diagram illustrating a portable electronic device according to an embodiment of the invention; 
           [0015]      FIG. 3  is a flowchart illustrating a method for switching between antennas according to an embodiment of the invention; 
           [0016]      FIG. 4  is a diagram illustrating a portable electronic device according to an embodiment of the invention; 
           [0017]      FIG. 5  is a diagram illustrating a portable electronic device according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1  is a diagram illustrating a portable electronic device  10  according to an embodiment of the invention. As shown in  FIG. 1 , the portable electronic device  10  comprises: an antenna  11   a,  an antenna  11   b,  a detector  13 , and a control circuit  14 . The portable electronic device  10  may be a mobile phone, a tablet PC, a personal digital assistant, PDA, a multimedia player, a game player, a GPS (Global Positioning System) navigation system, or other mobile electronic devices having wireless communication. Each of the antennas  11   a  and  11   b  may be a planar inverted F antenna, a monopole antenna, a loop antenna, a helical antenna, a chip antenna, or other antenna that is suitable for the portable electronic device  10 . The detector  13  may be an infrared detector, a light source detector, a pressure detector, a close detector, an ultrasonic detector, or other detectors that can detect the approach of human body. In an embodiment as shown in  FIG. 1 , being close to each other, the antenna  11   a  and the antenna  11   b  may have different antenna efficiencies, or different radiation patterns. Note that,  FIG. 1  only illustrates important components related to the invention, other components like a signal source, or a central processing unit that is well-known for a skilled person in the art, are not discussed or shown. To begin, the control circuit  14  of the portable electronic device  10  uses the antenna  11   a  to wirelessly transmit or receive a signal. During the period, the detector  13  transmits a message A 1  to the control circuit  14  when detecting that the distance between an object  15  under test and the detector  13  is smaller than or equal to a predetermined distance, such as 5 cm. The object  15  under test may be human body or matter. When receiving the message A 1 , the control circuit  14  switches to another antenna not in use, e.g., the control circuit  14  switches from the antenna  11   a  to the antenna  11   b  and uses the antenna  11   b  to transmit or receive a signal. The dashed line arrow and the solid line arrow in  FIG. 1  represent the control circuit  14  switching from one antenna to another antenna. Although  FIG. 1  uses a switch (the dashed and solid line arrows) to represent switching between different antennas, a solid switch is not essential, e.g., the control circuit  14  can use other known technology to transmit a signal and feed in an antenna. In the embodiment, the antenna  11   b  and the antenna  11   a  may have different radiation patterns, and preferably, the antenna  11   b  has a radiation pattern not toward the object  15 . Even more preferable, the SAR of the portable electronic device  10  and the object  15  are able to decrease even if the RF conducting power remains unchanged. In addition, when the antenna  11   b  is in use, the detector  13  may transmit the message A 1  to the control circuit  14  to switch to the antenna  11   a  on the condition that the detector  13  detects the object  15  far away from the detector  13 . 
         [0019]    In another embodiment, the antenna  11   a  has better antenna efficiency than the antenna  11   b.  Therefore, the portable electronic device  10  uses the antenna  11   a  to wirelessly transmit or receive a signal. During the period, the detector  13  transmits a message A 1  to the control circuit  14  when detecting that the distance between an object  15  under test and the detector  13  is smaller than or equal to a predetermined distance. When receiving the message A 1 , the control circuit  14  switches to another antenna not in use, i.e., the control circuit  14  switches from the antenna  11   a  to the antenna  11   b  and uses the antenna  11   b  to transmit or receive a signal. In the embodiment, due to the antenna efficiency of the antenna  11   b  being poorer than the antenna efficiency of the antenna, the SAR of the portable electronic device  10  and the object  15  can decrease even if the RF conducting power remains unchanged. In addition, when the antenna  11   b  is in use, the detector  13  may transmit the message A 1  to the control circuit  14  to switch to the antenna  11   a  on the condition that the detector  13  detects the object  15  far away from the detector  13 . 
         [0020]      FIG. 2  is a diagram illustrating a portable electronic device  20  according to an embodiment of the invention. In the embodiment as shown in  FIG. 2 , an antenna  21   a  and an antenna  21   b  is located in different positions, and the distance between the antenna  21   a  and an object  15  is smaller than the distance between the antenna  21   b  and the object  15 . A detector  23  may be located near the antenna  21   a.  Similarly,  FIG. 2  only illustrates important components related to the invention, and other components are not discussed or shown. To begin, a control circuit  24  of the portable electronic device  20  may switch to the antenna  21   a  to wirelessly transmit or receive a signal. During the period, the detector  23  transmits a message A 2  to the control circuit  24  when detecting that the distance between an object  25  under test and the detector  23  is smaller than or equal to a predetermined distance. When receiving the message A 1 , the control circuit  24  switches to another antenna not in use, e.g., the control circuit  24  switches from the antenna  21   a  to the antenna  21   b  and uses the antenna  21   b  to transmit or receive a signal. The SAR of the portable electronic device  20  and the object  25  can decrease even if the RF conducting power remains unchanged. In addition, when the antenna  21   b  is in use, the detector  23  may transmit the message A 2  to the control circuit  24  to switch to the antenna  21   a  on the condition that the detector  23  detects that the object  25  is far away from the detector  23 . 
         [0021]      FIG. 3  is a flowchart  30  illustrating a method for switching between antennas according to an embodiment of the invention.  FIG. 3  is applicable to a portable electronic device having a first antenna and a second antenna. The portable electronic device can switch to the first antenna or the second antenna to wirelessly transmit or receive a signal. The method comprises: first, detecting a distance between an object under test and the portable electronic device (S 32 ) and generating a message according to the distance; second, if the object is far from the portable electronic device, i.e., the distance is larger than a predetermined distance (S 34 , No), the portable electronic device can switch to the first antenna (S 36 ); if the object is close to the portable electronic device, i.e., the distance is smaller than or equal to the predetermined distance (S 34 , Yes), the portable electronic device can switch to the second antenna that has different antenna efficiency, a different radiation pattern, or different location from the first antenna (S 38 ); and finally, the method ends. 
         [0022]      FIG. 4  is a diagram illustrating a portable electronic device  40  according to an embodiment of the invention. In the embodiment as shown in  FIG. 4 , an active antenna  40  comprises a plurality of child antennas, e.g., child antennas  41   a  and  41   b.  Although only two child antennas are shown in  FIG. 4 , the active antenna  40  may comprise more than 3 child antennas. The plurality of child antennas, e.g., child antennas  41   a  and  41   b,  may have different antenna efficiencies or different radiation patterns. Similarly,  FIG. 4  only illustrates important components related to the invention, and other components are not discussed or shown. To begin, a control circuit  44  of the portable electronic device  40  may switch to the child antenna  41   a  of the active antenna  40  to wirelessly transmit or receive a signal. During the period, a detector  43  transmits a message A 4  to the control circuit  44  when detecting that the distance between an object  45  under test and the detector  43  is smaller than a predetermined distance. When receiving the message A 4 , the control circuit  44  switches to another child antenna not in use, e.g., the control circuit  44  switches from the child antenna  41   a  to the child antenna  41   b  and uses the child antenna  41   b  to wirelessly transmit or receive a signal. The child antenna  41   b  may have antenna efficiency which is poorer than the child antenna  41   a,  or have a radiation pattern not toward the object  45 . The SAR of the portable electronic device  40  and the object  45  can decrease even if the RF conducting power remains unchanged. In addition, when the child antenna  41   b  is in use, the detector  43  may transmit the message A 4  to the control circuit  44  to switch to the child antenna  41   a  on the condition that the detector  43  detects that the object  45  is far away from the detector  43 . 
         [0023]      FIG. 5  is a diagram illustrating a portable electronic device  50  according to an embodiment of the invention. In the embodiment as shown in  FIG. 5 , the portable electronic device  50  comprises a plurality of antennas and a plurality of detectors. Although only two antennas and two detectors, i.e., antennas  51   a,    51   b,  and detectors  53   a,    53   b,  are shown in  FIG. 5 , the portable electronic device  50  may comprise more than  3  antennas and detectors. The antennas  51   a  and  51   b  may be put in different locations. The detectors  53   a,    53   b  may be located near the antennas  51   a,    51   b,  respectively. The antennas  51   a  and  51   b  may have different antenna efficiencies, or have different radiation patterns. Similarly,  FIG. 5  only illustrates important components related to the invention, other components being not discussed or shown. To begin, the plurality of detectors detect the distances, each of which is between each of the plurality of antennas and an object, and then generate messages, e.g., the detector  53   a  detects the distance between the antenna  51   a  and the object  55 , and the detector  53   b  detects the distance between the antenna  51   b  and the object  55 . After then, the detectors  53   a  and  53   b  generate messages A 51  and A 52 , respectively. The control circuit  54  receives the messages A 1  and A 2 , and switches to one of the plurality of antennas which has the longest distance from the object  55 , e.g., the control circuit  54  switches to the antenna  51   a  if the distance between the antenna  51   a  and the object  55  is larger than the distance between the antenna  51   b  and the object  55 . On the contrary, the control circuit  54  switches to the antenna  51   b  if the distance between the antenna  51   a  and the object  55  is smaller than or equal to the distance between the antenna  51   b  and the object  55 . The SAR of the portable electronic device  50  and the object  55  can decrease even if the RF conducting power remains unchanged. 
         [0024]    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.