Patent Document

TECHNICAL FIELD 
       [0001]    The present invention generally relates to antennas and, more particularly, to a semi-planar inverted F-antenna (PIFA) including a switching technology that may switch between, for example, GSM 850 and GSM 900, without affecting the High Band frequencies. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wireless communication equipment, such as cellular and other wireless telephones, wireless network (WiLAN) components, GPS receivers, mobile radios, pagers, etc., use multi-band antennas to transmit and receive wireless signals in multiple wireless communication frequency bands. Consequently, one of the critical components of wireless devices is the antenna which should meet the demands of high performance in terms of high signal transmission strength, good reception of weak signals, increased (or narrowed, if required) bandwidth, and small dimensions. 
         [0003]    Planar inverted F-antennas (PIFAs) have many advantages. They are easily fabricated, have a simple design, and cost little to manufacture. Currently, the PIFA is widely used in small communication devices, such as cellular phones. This is due to the PIFA&#39;s compact size that makes it easy to integrate into a device&#39;s housing, thereby providing a protected antenna. The PIFA also provides an additional advantage over, for example, the popular whip antennas with respect to radiation exposure. A whip antenna has an omnidirectional radiation field, whereas the PIFA has a relatively limited radiation field towards the user. 
         [0004]    The PIFA is generally a λ/4 resonant structure and is implemented by short-circuiting the radiating element to the ground plane using a conductive wall, plate or post. Thus, the conventional PIFA structure consists of a conductive radiator or radiating element disposed parallel to a ground plane and is insulated from the ground plane by a dielectric material, typically air. This radiating element connects to two pins, typically disposed toward one end of the element, giving the appearance of an inverted letter “F” from the side view. The first pin electrically connects the radiating element to the ground plane, and the second pin provides the antenna feed. The frequency bandwidth, gain, and resonant frequency of the PIFA depend on the height, width, and depth of the conductive radiator element, and the distance between the first pin connected to the radiating element and ground, and the second pin connected to the antenna feed. 
         [0005]      FIG. 2  illustrates a conventional PIFA  200  design. The conventional PIFA  200  includes a conductive plate which forms a radiating element  209  of the antenna. Radiating element  209  is disposed about parallel to a ground plane  210  formed on a substrate  211 . This parallel orientation between radiating element  209  and ground plane  210  provides optimal performance, but other orientations are possible. 
         [0006]    Radiating element  209  electrically connects to ground plane  210  via a tuning or shortening element  212 , most often disposed at one side of radiating element  209  and a feed element  213 . Feed element  213  is somewhat electrically insulated from ground plane  210 . When electric current is fed to radiating element  209  mounted above ground plane  210  through feed element  213 , radiating element  209  and ground plane  210  become excited and act as a radiating device. 
         [0007]    The operating frequency or the resonance frequency of PIFA  200  can be modified either by adjusting the dimensions and shape of radiating element  209  or by moving the location of feed element  213  with respect to tuning element  212 . The resonance frequency can also be finely adjusted by changing the height and/or width of tuning element  212 . Thus, in the conventional PIFA, the operating frequency or resonance is fixed by the size, shape, or placement of feed element  213 , tuning element  212 , or radiating elements  209 , respectively. To change the bandwidth of PIFA  200 , the height must be increased which will lead to an undesirable increase in the overall antenna size. 
         [0008]    Currently, various frequency bandwidths are used in different regions of the world. Global system for mobile (GSM) communication networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands, but some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated. 
         [0009]    However, as the PIFA is limited by the space within the mobile communication terminal this results in limited antenna frequency characteristics and therefore the usual PIFA is designed to maximize the frequency for only one of the frequency bandwidths required. 
         [0010]    Embodiments of the present invention provide a PIFA device and a method for controlling the PIFA device that can satisfy the characteristics of various frequencies in a multi-frequency environment in a mobile communication terminal, without compromising performance in terms of high signal transmission strength, good reception of weak signals, and the limited dimensions. 
       SUMMARY OF THE INVENTION 
       [0011]    To cover several transmission frequencies, for example, both GSM 850 and 900 (Bandwidth at −6 dB S11), the resonance of the Low Band can switch between different frequencies, for example, GSM 850 and 900, by changing the length of the ground plane from an antenna point of view with a microstrip having the dimensions, a×b, on the antenna ground clearance area. This may occur without affecting the high frequency bands. 
         [0012]    Embodiments of the invention use an antenna including: a ground plane, a feed element, and a radiating element that couples to the feed element, the radiating element being substantially parallel to and vertically displaced from the ground plane by the feed element and a shortening element. The antenna may also include a conductive portion that may couple to the ground plane by means of a switching element, the conductive portion being configured to alter size of said ground plane. The conductive portion may be, for example, a microstrip. According to one embodiment, the conductive portion may be arranged at a ground clearance area. The conductive portion may be configured to change the resonance frequency of the antenna. In one embodiment, the conductive portion may be configured, when coupled to the ground plane, to shift resonance of the antenna to a lower frequency. 
         [0013]    Embodiments of the invention also relate to a wireless communication device having an antenna that includes: a ground plane, a feed element, and a radiating element coupled to the feed element, the radiating element being substantially parallel to and vertically displaced from the ground plane by the feed element and a shortening element. The antenna may also include a conductive portion that may couple to the ground plane by means of a switching element, the conductive portion being configured to alter size of the ground plane. 
         [0014]    Embodiments of the invention may also relate to a method of controlling an antenna in a wireless communication device. The antenna may include: a ground plane, a feed element, and a radiating element that may couple to the feed element, the radiating element being substantially parallel to and vertically displaced from the ground plane by the feed element, a shortening element, and a conductive portion that may couple to the ground plane by means of a switching element, the conductive portion being configured to alter size of said ground plane. The method may include coupling the conductive portion to the ground plane by the switching element to change the resonance of the ground plane and thereby operation frequency of the antenna. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, explain the invention. In the drawings: 
           [0016]      FIG. 1  illustrates a block diagram of a wireless communication device according to the present invention; 
           [0017]      FIG. 2  illustrates a conventional PIFA design; 
           [0018]      FIG. 3  illustrates a PIFA according to the invention; 
           [0019]      FIG. 4  illustrates a block diagram of a wireless communication device according to the invention; 
           [0020]      FIG. 5  illustrates an operation flowchart for receiving current location information from the user or BS and changing a frequency band based on the location information; 
           [0021]      FIG. 6  illustrates the reflection coefficients of the antenna according to the invention with respect to frequency; and 
           [0022]      FIG. 7  illustrates a cross section through part of PCB and parasitic element according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    Exemplary antenna designs described in the following description may be “planar” antennae. A “planar” antenna may have an extended shape that lies generally along a plane, i.e., the antenna may have three dimensions but one of the dimensions may be an order of a magnitude less than the other two dimensions. 
         [0024]      FIG. 1  illustrates a block diagram of an exemplary wireless communication device  10 . Wireless communication device  10  may include a housing  11 , a controller  101 , a memory  102 , a user interface  103 , a transceiver  104 , a key input unit  105 , a display unit  106 , and a multiband antenna  100 . Transceiver  104  may interface wireless communication device  10  with a wireless network using antenna  100 . It is appreciated that transceiver  104  may transmit or receive signals according to one or more of any known wireless communication standards known to the person skilled in the art. Controller  101  may control the operation of wireless communication device  10  responsive to programs stored in memory  102  and instructions provided by the user via interface  103 . 
         [0025]    Embodiments of the PIFA design according to the present invention allows the antenna to be tuned to the desired operating resonance frequency or resonance frequencies required, while not compromising the antenna size or the operation of the other frequency bands. 
         [0026]    For purposes of illustration, the following describes antenna  100  in terms of a low frequency wireless communication band and a high frequency band, wherein a switch between, for example, 850 MHz and 900 MHz, within the low GSM frequency band, and a switch between, for example, 1800 MHz and 1900 MHz within the GSM high frequency band, will take place. However, it will be appreciated that antenna  100  may be designed to cover additional or alternative wireless communication frequency bands. 
         [0027]      FIG. 3  discloses a PIFA according to the present invention. PIFA  300  may include a ground plane  310  formed on a substrate  311 . In one embodiment, ground plane  310  may be embedded directly on substrate  311  (i.e., a PCB), which also may carry other electrical components of the device. This provides the advantage that the antenna can be mounted relatively close to the PCB, thus saving volume in the wireless device. PIFA  300  may also include a radiating element  309 , which may include a low frequency radiating element and a high frequency radiating element, respectively. Radiating element  309  may comprise any known configuration or pattern and vary in size to optimize the bandwidth, operating frequency, radiation patterns, and the like. Radiating element  309  may electrically connect to ground plane  310  via a tuning or shortening element  312 . Feed element  313  may connect a signal source from a radio or other RF transmitter, receiver, or transceiver (not shown) to radiating element  309 . In one embodiment, feed element  313  may be at least partially electrically insulated from ground plane  310  to prevent grounding therefrom. 
         [0028]    To cover both, for example, GSM 850 and GSM 900 (Bandwidth at −6 dB S11), the resonance of the low band switches between these two bandwidths by changing the size of the ground plane, for example, the length of ground plane  310 , from an antenna point of view, with a microstrip  316  with specific dimensions, a×b, which is arranged on the antenna ground clearance and connected to the ground plane by means of switching element  307 . 
         [0029]    The microstrip antenna according to the invention, which may be a narrowband, wide-beam antenna, may be fabricated by etching the antenna element pattern in metal trace bonded to an insulating dielectric substrate with a continuous metal layer bonded to the substrate which forms a ground plane. Possible microstrip antenna radiator shapes include any regular or irregular shape, such as square, rectangular, circular and elliptical, but any continuous shape is possible. The microstrip antenna may be, for example, a rectangular patch. The rectangular patch antenna may be approximately a one-half wavelength long section of rectangular microstrip transmission line. When air is the antenna substrate, the length of the rectangular microstrip antenna may be approximately one-half of a free-space wavelength. As the antenna is loaded with a dielectric as its substrate, the length of the antenna may decrease as the relative dielectric constant of the substrate increases. 
         [0030]    Because of the orientation and location of microstrip  316  relative to feed element  313  and shortening element  312 , electromagnetic interaction between feed element  313 , shortening element  312 , and microstrip  316  may occur when antenna switching element  307  connects microstrip  316  to ground plane  310 . This electromagnetic interaction may cause microstrip  316  to capacitively couple feed element  313  to shortening element  312 . This coupling may effectively move the feed point between radiating element  309  and ground plane  310  and thereby change the overall electromagnetic impedance of antenna  300 . Microstrip  316  may be configured to improve the impedance of antenna  300  in the first frequency band (e.g. 850 MHz) of the low frequency band, but may not impact the impedance of the antenna in the high frequency band. Thereafter, by disconnecting microstrip  316  from ground plane  310  when the antenna is to operate in the second frequency band (e.g. 900 MHz), antenna switching element  307  may selectively remove the electromagnetic coupling between microstrip  316  and ground plane  310 , and enable normal antenna operation in the second frequency band, also now without affecting the higher frequency band. 
         [0031]    If the size (e.g., length and width) of the microstrip is not sufficient, it is also possible to continue with the microstrip to the other side of the PCB or a suitable direction. This is illustrated in  FIG. 7 , where  316 ′ and  316 ″ denote extension of the parasitic element  316  over the edge and the other side, respectively, of PCB  311 . Parasitic element  316 ′″ may also extend through a via. Additional switches may be arranged to connect several microstrips and alter the total size of the microstrip. 
         [0032]    Antenna switching element  307  may selectively control the electromagnetic coupling by selectively controlling the connection between microstrip  316  and ground plane  310 . This connection may be controlled using any means that creates an impedance connection when the antenna is required to switch between two frequencies within the low frequency band. Antenna switching element  307  may be controlled by a controller  301 . Closing switching element  307  may create an impedance connection. Switching element  307  may be any of a mechanical or electrical element such as a MOS or CMOS transistor, etc. 
         [0033]      FIG. 4  is a block diagram illustrating a structure of a mobile communication terminal  40  in accordance with an embodiment of the present invention. Referring to  FIG. 4 , mobile communication terminal  40  may include a memory  402 , a key input unit  405 , a display unit  406 , a transceiver  404 , a PIFA  400 , an antenna switch element  407 , and a controller  401 . Controller  401  may process voice signals and/or data according to the protocol for a phone call, data communication, or wireless Internet access, and may control the respective components of mobile communication terminal  40 . Controller  401  may also receive key input from key input unit  405 , and control display unit  406  to generate and provide image information in response to the key input. Controller  401  may receive current location information from the user or BS. Through the received location information, controller  401  may identify a frequency band mapped to the current location from a region frequency memory  408  included in memory  402 . Controller  401  may determine if a frequency band change is desired. When the frequency band change is desired, controller  401  may control antenna switching element  407  to selectively connect or disconnect a microstrip  416  from ground plane  410 . 
         [0034]      FIG. 5  is a flowchart illustrating an exemplary operation for receiving current location information from the user or BS and changing a frequency band based on the location information. Referring to the structure in  FIG. 4 , controller  401  of mobile communication terminal  40  proceeds to step  500  to determine if location information has been input from the user. If location information has been input from the user, controller  401  proceeds to step  503 . In step  503 , controller  401  may load information about a frequency band of a region corresponding to the location information input by the user from region frequency memory  408  of memory  402  and determine if a frequency band change is desired. 
         [0035]    If location information is absent, controller  401  proceeds to step  501  to determine if a roaming service is activated. If the roaming service has not been activated, controller  401  may determine that a frequency band change according to the current location is not required. 
         [0036]    However, if the roaming service has been activated as a result of the determination in step  501 , controller  401  proceeds to step  502  to receive location information about the current region from the BS of a cell in which the current roaming service has been activated. Then, controller  401  proceeds to step  504  to control antenna switching element  407  and selectively connect or disconnect microstrip  416  from ground plane  410  according to the located frequency band. 
         [0037]    Curves ( 1 ) and ( 2 ) in  FIG. 6  illustrate the reflection coefficients of antenna  402  with respect to frequency when microstrip  416  is not connected to ground plane  410 . Curve ( 1 ) resonates at frequency 900 MHz and ( 2 ) at 1900 MHz. Curves ( 3 ) and ( 4 ) illustrate the reflection coefficients with respect to frequency when microstrip  409  is connected to ground plane  410 . Here curve ( 3 ) shows the resonation at 850 MHz and ( 604 ) at 1800 MHz frequency. The size of microstrip  416  used in this example is 4×7 mm. As shown by the reflection curves ( 1 ) and ( 3 ), using microstrip  416  to capacitively couple microstrip  416  to ground plane ( 410 ) induces a 40 MHz frequency shift (pointed out with arrow) in the low frequency band from about 900 MHz to about 850 MHz. The curves in the high frequency band are virtually unaffected. 
         [0038]    It should be noted that the word “comprising” does not exclude the presence of other elements or steps than those listed and the words “a” or “an” preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, that the invention may be implemented at least in part by means of both hardware and software, and that several “means”, “units” or “devices” may be represented by the same item of hardware. 
         [0039]    The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.

Technology Category: 5