Patent Publication Number: US-2006001576-A1

Title: Compact, multi-element volume reuse antenna

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to the field of antennas. More specifically, the present invention relates to compact, multi-element antennas.  
     BACKGROUND INFORMATION  
      Many wireless applications require a relatively large bandwidth. In order to achieve this large bandwidth, many wireless devices are required to employ either a large antenna element or multiple antenna elements. This solution is not practical for wireless devices which require the antenna to be accommodated in a relatively small package, thus requiring that the antenna have a low profile.  
      Further, certain wireless communication applications, such as the Global System for Mobile Communication (GSM) and Personal Communications Service (PCS) require that multiple bands be accessible, depending upon the local frequency coverage available from a service provider. Because applications such as GSM and PCS are used in the context of wireless communications devices that have relatively small form-factors, an antenna should generally have a low profile.  
      Embodiments of the present invention address the requirements of certain wireless communication applications by providing low-profile antennas that may provide a larger bandwidth.  
     SUMMARY OF THE INVENTION  
      One embodiment of the invention relates to antennas designed with increased bandwidth and decreased size. One embodiment of an antenna according to the present invention includes a first portion, a second portion, an antenna feed, and a ground. The second portion is configured so that it does not have a direct current conductive path with the first portion. The antenna feed is configured for exciting the first portion and the first portion is not grounded. The ground is connected to the second portion and the second portion is fed through electro-magnetic coupling with the first portion.  
      The first and second portions can be configured to-create substantially linearly independent current distributions. The antenna can be configured to generate a symmetrical current distribution in a first mode and an anti-symmetrical current distribution in a second mode. In some applications, the first mode and the second mode are adjacent in frequency such that the bandwidth of the antenna is increased by the combination of the first mode and the second mode.  
      The antenna feed can be a direct feed coupled to the first portion or an indirect feed coupled to the first portion. Sample indirect feeds can include proximity inductive coupling, proximity capacitive coupling, and proximity slot coupling.  
      The antenna can also include an interstitial portion (or multiple interstitial portions) electromagnetically coupled to the first portion and the second portion. In addition, the first and/or second portions can further comprise a plurality of unconnected portions electromagnetically coupled together such that the plurality of unconnected portions participate in the overall excitation of the respective portion. Parasitic elements can also be included such as for the purpose of impedance matching the antenna.  
      Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The preferred embodiments will hereafter be described with reference to the accompanying drawings, wherein like numerals will denote like elements.  
       FIGS. 1   a - h  are diagrammatical representations of various embodiments of antennas according to the present invention.  
       FIG. 2  is a graphical representation of the frequency response of one embodiment of an antenna according to the present invention.  
       FIG. 3  is a diagrammatical representation of an alternative embodiment of an antenna according to the present invention.  
       FIG. 4  is a graphical representation of the frequency response of another embodiment of an antenna according to the present invention.  
       FIG. 5  is a diagrammatical representation of the vector current density distribution of one embodiment of an antenna according to the present invention at a first mode frequency.  
       FIG. 6  is a diagrammatical representation of the vector current density distribution of one embodiment of an antenna according to the present invention at a second mode frequency. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Antennas according to the present invention can be used to produce larger bandwidths than other antennas of the same size. By reusing volume, antennas according to the present invention can be made smaller than other conventional antennas. Antennas according to the present invention can have multiple modes that exist in separate frequencies arbitrarily near each other. The electromagnetic field distribution in the space near the antennas, corresponding to each mode, can have very different spatial characteristics for each mode. Since the modes can be designed to be very close to each other in frequency, the bandwidth of the antennas can be increased using the same physical volume occupied by conventional antennas having a smaller bandwidth. Within the multiply increased bandwidth, embodiments of the antennas according to the present invention can have excellent radiation efficiency. Thus, antennas according to the present invention can be used to produce smaller size antennas while keeping the bandwidth and radiative efficiency performance of larger conventional antennas.  
      Referring now to  FIG. 1   a , one embodiment of an antenna is generally designated with reference numeral  10 . The antenna  10  comprises two elements  12  and  14  and a ground plane  16 . An antenna feed  18  is connected to element  12  and a ground connection  20  is connected to element  14 . Element  12  is not directly connected to ground and element  14  is not directly connected to a signal feed. In other words, no direct current path exists between elements  12  an  14 . Instead, elements  12  and  14  are electromagnetically coupled to each other through a coupling region  22  by their relative proximity and orientation, but are not directed connected to each other. An optional support element  24  can also be added to either element  12  and/or  14  for providing structural support.  
      Elements  12  and  14  can be formed of and comprise any number of materials such as but not limited to, stamped metal, printed circuit technology, metal tape or paint, or any other metallization or conductive medium method. Furthermore, the present invention is applicable to a variety of antenna  10  and elements  12  and  14  sizes and frequencies. Various geometrical antenna features, such as but not limited to various geometries of radiative slots, edges or stubs, as well as single or multilevel stamped metal, printed metal, and/or metal paint technologies can be used. The elements  12  and  14  can be positioned on the same plane or on different planes and, in fact, various embodiments of the antenna  10  can comprise more than two elements. The elements  12  and  14  can be radiating holes or other openings existing on a metallic or otherwise conductive screen or any other structure that complies with Babinet&#39;s principle. The element design, coupling region design and size of the antenna can be varied in different embodiments of the invention. For example,  FIGS. 1   b - 1   f  illustrate additional examples of embodiments of an antenna  10  according to the present invention.  
      Embodiments of the invention can be fed in many different ways. While the embodiment shown in  FIG. 1  includes a direct feed  18 , other excitation methods can also be used. For example, indirect feeds, such as proximity inductive and/or capacitive coupling or proximity slot coupling, among others, can be used to excite element  12 .  FIG. 1   g  illustrates one embodiment using an alternative excitation method in which element  12  is fed using an indirect feed  18 . In the antenna illustrated in  FIG. 1   g , it can be seen that elements  12  and  14  can include multiple unconnected portions  12   a - 12   d  and  14   a - 14   c , respectively. Some of the unconnected portions of element  12 , such as elements  12   a  and  12   b  may participate in the overall excitation of element  12 , while other portions, such as elements  12   c  and  12   d , may be parasitic elements used, for example for input impedance matching. In addition, element  14  may also include portions that are not grounded and act as parasitic elements, such as element  14   c . Separate grounds can be used for the unconnected portions of element  14 , such as elements  14   a  and  14   b , for impedance matching for example. Further, one or more unconnected portions of element  14  can have multiple connections to ground.  
      Another alternative embodiment of an antenna  10  according to the present invention is illustrated in  FIG. 1   h . The antenna of  Fig. 1   h  includes an interstitial portion  13  between elements  12  and  14 . As shown in this figure, interstitial portion  13  has no direct feed or ground nor a direct current path coupling it to elements  12  or  14 . Instead, element  13  can be feed through electromagnetic coupling with elements  12  and  14 . Additional alternative embodiments of antennas according to the present invention (not shown) may include multiple interstitial portions which electromagnetically couple to elements  12  and  14 .  
      In effect, embodiments of the invention can create multiple modes in adjustably adjacent frequencies. The embodiments can be configured to produce substantially linearly independent current distributions, such as orthogonal or substantially orthogonal. For example, embodiments can comprise symmetric (for the first mode) and anti-symmetric (for the second mode) current density distributions. For example, symmetric and anti-symmetric combinations of current distributions can occur on elements  12  and  14 . A symmetric distribution is one where all antenna parts have the same current distribution as defined by the right-hand rule. An anti-symmetric distribution can be one where some antenna parts have opposite current distributions. These two types of current distributions can create dramatically different electromagnetic field distributions in a space immediately surrounding the antenna  10 . Alternative embodiments of symmetric and anti-symmetric current distributions may involve linear current distributions, rather than the circular ones shown in the example of  FIGS. 5 and 6 . In such a case, the symmetric linear current distribution could be linear currents that flow continuously from one portion (entering the coupling region for example) to another portion (exiting the coupling region for example), while the anti-symmetric current distributions could be linear currents that both enter the coupling region or both exit the coupling region for example in opposite directions. Creating symmetric current distributions in one mode and anti-symmetric current distributions in another mode is one example of creating substantially linearly independent current distributions according to the present invention.  
      Elements  12  and  14  can be excited by feed  18  connected to element  12  to produce two modes that resonate at different frequencies. In one embodiment, the mode frequencies can be designed as close to each other as possible. As the modes become closer in frequency, the antenna  10  can match well over the bandwidth of both modes, thus multiplying the overall bandwidth of the antenna  10 , relative to other antennas of approximately the same size.  FIG. 2  illustrates a comparison of the measured return loss of a conventional embedded antenna (line  26 ) with the measured response of one embodiment of an antenna of relatively the same size according to the present invention (line  28 ).  
      In one embodiment, the frequency separation between the modes can be controlled by the radiating length of each element  12  and  14 , for example, the total length of the slot or spiral of the embodiments shown in  FIGS. 1   a - 1   f . The frequency separation can also be controlled by the amount of coupling between the elements  12  and  14 . In one embodiment, the amount of coupling is controlled by adjusting the width of the coupling region  22  between the elements  12  and  14 . As described herein, various other geometries producing other coupling details may also be used. The elements  12  and  14  can be symmetric or dissimilar and they can be arranged orthogonal or in various other arrangements and configurations.  
      In another embodiment, shown in  FIG. 3 , a ground connection  30  can also be added to element  12 . This ground connection  30  can be used to separate the two modes further apart in frequency, as illustrated in  FIG. 4 . Even though, in such an embodiment, elements  12  and  14  are physically connected through the common ground plane, the two corresponding grounding locations are still not directly connected by a current conducting path, because these locations are at the same (ground) potential and a non-negligible current conducting path needs a non-negligible potential difference in order to be established. When the modes are sufficiently separated they produce multiple distinct bands. The mode of separation in such an embodiment can further be controlled by positioning of ground connection  30  within the first portion of the antenna. In this case, the antenna  10  can be configured to operate as a multi-band antenna.  
      As described herein, embodiments of the invention can include symmetric and anti-symmetric current distributions.  FIG. 5  illustrates one possible current distribution for the embodiment illustrated in  FIG. 1 . The vector current density distribution shown in  FIG. 5  illustrates the current distribution at the frequency of the first mode. As is shown, the current distribution circulates clockwise around the slot (as viewed looking down on the antenna  10  from the top) on both the elements  12  and  14 . This is a symmetric current distribution with respect to elements  12  and  14 . The magnetic field lines thread the volume enclosed by the antenna  10  and the ground plane  16  roughly along the coupling region  22 .  
       FIG. 6  illustrates one possible vector current density distribution for the antenna  10  of  FIG. 1  at the frequency of the second mode. As can be seen, the current distribution circulates clockwise around the slot on element  12  and counter-clockwise around the slot on element  14 . This can be considered an anti-symmetric current distribution. The magnetic field lines thread the volume enclosed by the antenna  10  and the ground plane  16  roughly perpendicular to the coupling region  22 .  
      When the antenna  10  is designed so that the modes are adjacent in frequency, an increase in bandwidth by multiple factors can be achieved. Similar or even better radiation efficiency can also be achieved for the antenna  10  over that broad band. In addition, antennas according to the present invention have incomparably higher efficiency for frequencies that are in-band for antennas  10  in accordance with the present invention but out-of-band for other antennas.  
      It is understood that the invention is not confined to the particular embodiments set forth herein as illustrative, but embraces all such modifications, combinations, and permutations as come within the scope of the appended claims. Thus, the description of the preferred embodiments is for purposes of illustration and not limitation.