Abstract:
The method and apparatus described herein improves the bandwidth of a selected frequency band of a multi-band antenna. In particular, a selection circuit selectively applies capacitive coupling to the multi-band antenna to improve the bandwidth of a first frequency band without adversely affecting the bandwidth of a second frequency band. To that end, the multi-band antenna of the present invention comprises a main antenna element and a parasitic element disposed proximate the main antenna element. When the multi-band antenna operates in the first frequency band, the main antenna element capacitively couples to the parasitic element. However, when the multi-band antenna operates in the second frequency band, the selection circuit disables the capacitive coupling. By applying the capacitive coupling only when the multi-band antenna operates in the first frequency band, the present invention increases the bandwidth of the first frequency band without adversely affecting the bandwidth of the second frequency band.

Description:
BACKGROUND  
       [0001]     This invention relates generally to wireless communication antennas, and more particularly to multi-band antennas for wireless communication devices.  
         [0002]     Wireless communication devices typically use multi-band antennas to transmit and receive wireless signals in multiple wireless communication frequency bands, such as Advanced Mobile Phone System (AMPS), Personal Communication Service (PCS), Personal Digital Cellular (PDC), Global System for Mobile communications (GSM), Code Division Multiple Access (CDMA), etc. A bent monopole antenna represents a common multi-band antenna. While bent monopole antennas typically do not have sufficient bandwidth to cover all desired wireless communication frequency bands, the compact size and multi-band design make them ideal for compact wireless communication devices.  
         [0003]     Parasitic elements that improve antenna performance are also known. When applied to multi-band antennas, the parasitic element typically only improves performance in one of the wireless communication frequency bands, but adversely affects the performance of the antenna in the other wireless communication frequency band(s).  
       SUMMARY  
       [0004]     The present invention relates to multi-band antennas for wireless communication devices. The multi-band antenna includes a main antenna element and a parasitic element. When the antenna operates in the first frequency band, a selection circuit connects the parasitic element to ground to capacitively couple the main antenna element to the parasitic element. This capacitive coupling increases the bandwidth of the first frequency band. When the antenna operates in the second frequency band, the selection circuit disables the capacitive coupling. By applying the capacitive coupling only when the antenna operates in the first frequency band, the bandwidth of the first frequency band is increased without adversely affecting the performance of the second frequency band.  
         [0005]     According to the present invention, a low impedance connection between the parasitic element and the antenna ground enables the capacitive coupling between the parasitic element and the main antenna element when the antenna operates in the first frequency band. When the antenna operates in the second frequency band, a high impedance connection between the parasitic element and the antenna ground disables the capacitive coupling. The antenna may use a selection circuit, such as a switch, to generate the desired high and low impedance connections. According to another embodiment, the selection circuit may comprise a filter, where the filter has a low impedance responsive to frequencies in the first frequency band, and has a high impedance responsive to frequencies in the second frequency band. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  illustrates a block diagram of a wireless communication device according to the present invention.  
         [0007]      FIG. 2  illustrates an exemplary antenna according to one embodiment of the present invention.  
         [0008]      FIG. 3  illustrates a block diagram of the exemplary antenna of  FIG. 2 .  
         [0009]      FIG. 4  illustrates an ideal efficiency vs. frequency plot for the antenna of  FIGS. 2 and 3 .  
         [0010]      FIG. 5  illustrates another ideal efficiency vs. frequency plot for the antenna of  FIGS. 2 and 3 .  
         [0011]      FIG. 6  illustrates a block diagram of an exemplary antenna according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  illustrates a block diagram of an exemplary wireless communication device  10 . Wireless communication device  10  comprises a controller  20 , a memory  30 , a user interface  40 , a transceiver  50 , and a multi-band antenna  100 . Controller  20  controls the operation of wireless communication device  10  responsive to programs stored in memory  30  and instructions provided by the user via user interface  40 . Transceiver  50  interfaces the wireless communication device  10  with a wireless network using antenna  100 . It will be appreciated that transceiver  50  may operate according to one or more of any known wireless communication standards, such as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Global System for Mobile communications (GSM), Global Positioning System (GPS), Personal Digital Cellular (PDC), Advanced Mobile Phone System (AMPS), Personal Communication Service (PCS), Wideband CDMA (WCDMA), etc.  
         [0013]     Multi-band antenna  100  transmits and receives signals according to one or more of the above wireless communication standards. For purposes of illustration, the following describes the antenna  100  in terms of a low frequency wireless communication band and a high frequency wireless communication band. An exemplary low frequency wireless communication band includes an AMPS frequency band (850 MHz) and/or a GSM low frequency band (900 MHz). An exemplary high frequency wireless communication band includes a GSM high frequency band (1800 MHz) and/or a PCS frequency band (1900 MHz). However, it will be appreciated that antenna  100  may be designed to cover additional or alternative wireless communication frequency bands.  
         [0014]      FIGS. 2 and 3  illustrate a multi-band antenna  100  according to one exemplary embodiment of the present invention. The exemplary multi-band antenna  100  comprises a bent monopole antenna. However, the present invention also applies to other types of antennas, such as a Planar Inverted F-Antenna (PIFA) as described in the co-pending application filed concurrently with the instant application and entitled “Multi-band PIFA” (Attorney Docket No. 2002-204). This application is hereby incorporated by reference.  
         [0015]     Antenna  100  comprises a main antenna element  110 , a parasitic element  120 , and a selection circuit  140 . Main antenna element  110  transmits and receives wireless communication signals in the low and high wireless communication frequency bands. Selection circuit  140  selectively couples the parasitic element  120  to a ground  132  of a printed circuit board (PCB)  130  to selectively enable capacitive coupling between the parasitic element  120  and the main antenna element  110  when the antenna  100  operates in the low frequency band. In addition, selection circuit  140  selectively disables the capacitive coupling when the antenna  100  operates in the high frequency band. As a result, selection circuit  140  controls the capacitive coupling between parasitic element  120  and main antenna element  110 .  
         [0016]     Main antenna element  110  comprises a radiating element  112  elevated from the antenna ground  132  by RF feed  114 , where RF feed  114  electrically connects the radiating element  112  to transceiver  50 . Radiating element  112  transmits wireless communication signals in one or more frequency bands provided by transceiver  50  via RF feed  114 . Further radiating element  112  receives wireless communication signals transmitted in one or more frequency bands and provides the received signals to the transceiver  50  via RF feed  114 . According to one embodiment of the present invention, radiating element  112  comprises a feed end  116  connected to the RF feed  114  and a terminal end  118 , where the feed end  116  and the terminal end  118  are on opposite ends of the radiating element  112 . As shown in  FIG. 2 , the radiating element  112  is bent along the length of the radiating element  112  to generate the bent monopole shape. According to one exemplary embodiment, radiating element  112  is 40 mm long and 12 mm wide, where the terminal end  116  is 32 mm long, and RF feed  114  positions the radiating element  112  approximately 7 mm from PCB  130 .  
         [0017]     Parasitic element  120  is disposed generally in the same plane as the radiating element  112  and along terminal end  118  so that the parasitic element  120  runs generally parallel to the terminal end  118 . Because of the orientation and location of the parasitic element  120  relative to the terminal end  118 , electromagnetic interaction between the terminal end  118  and the parasitic element  120  occurs when selection circuit  140  connects the parasitic element  120  to ground  132 . This electromagnetic interaction causes the parasitic element  120  to capacitively couple to the radiating element  112 . Generally, this capacitive coupling increases the bandwidth of the low frequency band, but adversely affects operation in the high frequency band. By disconnecting the parasitic element  120  from ground  132  when the antenna  100  operates in the high frequency band, the selection circuit  140  removes the negative effects of the capacitive coupling on the high frequency band.  
         [0018]     Selection circuit  140  controls the capacitive coupling between the parasitic element  120  and the radiating element  112  by controlling the connection between the parasitic element  120  and the antenna ground  132 . Selection circuit  140  may control the connection between the parasitic element  120  and ground  132  using any means that creates a low impedance connection between the parasitic element  120  and ground  132  when the antenna  100  operates in the low frequency band, and that creates a high impedance connection between the parasitic element  120  and ground  132  when the antenna  100  operates in a high frequency band. In one exemplary embodiment, selection circuit  140  may comprise a switch controlled by controller  20 . Closing switch  140  creates a short circuit (low impedance connection) between the parasitic element  120  and the ground  132 , while opening switch  140  creates an open circuit (high impedance connection) between the parasitic element  120  and the ground  132 .  
         [0019]     According to another exemplary embodiment, selection circuit  140  may comprise a frequency dependent lump element circuit, such as a filter  140 . By designing the filter  140  to have a low impedance at low frequencies and a high impedance at high frequencies, the filter  140  selectively connects the parasitic element  120  to ground  132  only when the antenna  100  operates in the low frequency band. According to one exemplary embodiment, the selection circuit  140  may comprises an inductance in series with the parasitic element  120 , where the inductance ranges between 6.8 nH and 22 nH.  
         [0020]      FIGS. 4 and 5  illustrate the efficiency of the antenna  100  as a function of frequency. The efficiency curves illustrated in these figures represent the simulated efficiency as generated by an electromagnetic simulator, such as Zealand IE3D. As such, these efficiency curves represent an ideal efficiency of the antenna and do not consider dielectric/conductor losses or mismatch losses. Regardless, these efficiency curves accurately represent the effect of the capacitive coupling on the antenna&#39;s bandwidth and relative efficiency. Efficiency curve  60  in  FIGS. 4 and 5  illustrate the efficiency response of the antenna  100  when the parasitic element  120  is not capacitively coupled to the radiating element  112 . The efficiency curve  60  shows that the low frequency band has approximately 0.75 GHz of bandwidth with at least 96% efficiency and a peak efficiency of 99%. Further, efficiency curve  60  shows that more than 1.2 GHz of the high frequency band has at least 96% efficiency and a peak efficiency of 99.5%.  
         [0021]     By applying capacitive coupling between the parasitic element  120  and the radiating element  112 , antenna  100  increases the field storage inside the radiating element  112 , which in turn, increases the bandwidth of the low frequency band. Because the bandwidth is inversely proportional to the efficiency, increasing the bandwidth necessarily decreases the efficiency. For frequencies in the low frequency band, this drop in efficiency is minimal relative to the significant bandwidth increase. However, for frequencies in the high frequency band, the efficiency loss can be significant. Efficiency curve  70  in  FIGS. 4 and 5  illustrates these effects. As shown by efficiency curve  70 , capacitively coupling the parasitic element  120  to the radiating element  112  reduces the peak efficiency of the low frequency band to 98.5%, but widens the low frequency bandwidth having at least 96% efficiency to approximately 1.25 GHz. However, efficiency curve  70  also illustrates a significant reduction in the high frequency bandwidth and efficiency.  
         [0022]     The present invention addresses this problem by selectively applying the capacitive coupling only when the antenna  100  operates in the low frequency band; when the antenna  100  operates in the high frequency band, the capacitive coupling is disabled. Efficiency curve  80  in  FIG. 4  illustrates the efficiency of the antenna  100  when the selection circuit  140  comprises a switch  140 , while efficiency curve  90  in  FIG. 5  illustrates the efficiency of the antenna  100  when the selection circuit  140  comprises a filter  140 . In either case, when selection circuit  140  generates a low impedance connection between the parasitic element  120  and the antenna ground  132 , efficiency curves  80  and  90  follow curve  70 . However, when selection circuit  140  generates a high impedance connection between parasitic element  120  and the antenna ground  132 , efficiency curves  80  and  90  follow curve  60 . As a result, the low frequency band has increased the bandwidth having at least 96% efficiency to between 0.8 and 0.9 GHz, while the high frequency band has maintained the bandwidth having at least 96% efficiency at more than 1.2 GHz.  
         [0023]     As shown in  FIG. 4 , switch  140  abruptly disables the capacitive coupling at approximately 1.7 GHz. The filter  140 , in contrast, gradually disables the capacitive coupling as the impedance approaches 1.7 GHz, as shown in  FIG. 5 . While the illustrated examples show a cutoff frequency for the capacitive coupling at 1.7 GHz, those skilled in the art will appreciate that antenna  100  may be designed to cutoff the capacitive coupling at any frequency.  
         [0024]     The capacitive coupling between the parasitic element  120  and the radiating element  112  may cause a slight shift in the low frequency band resonant frequency. To correct for this shift, RF feed  114  may include matching circuitry that tunes the antenna  100  to relocate the resonant frequency to the pre-capacitive coupling resonant frequency. It will be appreciated that the matching circuit may also be modified to shift the resonant frequency to any desired frequency.  
         [0025]     The exemplary embodiment described above increases the bandwidth of the low frequency band without adversely affecting the bandwidth of the high frequency band. However, it will be appreciated that the present invention is not so limited. For example, the parasitic element  120  may be designed to increase the bandwidth of the high frequency band. In this embodiment, selection circuit  140  would be designed and/or controlled to enable capacitive coupling between the parasitic element  120  and the radiating element  112  when the antenna  100  operates in the high frequency band, and to disable the capacitive coupling when the antenna  100  operates in the low frequency band.  
         [0026]     Further, it will be appreciated that antenna  100  may include a low-band parasitic element  120  and a high-band parasitic element  122 , as shown in  FIG. 6 . According to this embodiment, selection circuit  140  enables the low-band capacitive coupling by connecting the low-band parasitic element  120  to ground while selection circuit  142  disconnects the high-band parasitic element  122  from ground during low frequency operation. This increases the low frequency bandwidth when the antenna  100  operates in the low frequency band. When the antenna  100  operates in the high frequency band, selection circuit  142  connects the high-band parasitic element  122  to ground  132  while selection circuit  140  disconnects the low-band parasitic element  120  from ground. This increases the high frequency bandwidth when the antenna  100  operates in the high frequency band.  
         [0027]     The present invention improves the bandwidth of at least one frequency band of a compact multi-band antenna  100  without negatively impacting the bandwidth of the remaining frequency bands. As such, the multi-band antenna  100  of the present invention may be used with a wider range of wireless communication standards and/or in a wider range of wireless communication devices  10 .  
         [0028]     The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.