Abstract:
The present invention provides an internal antenna for wireless devices comprising a ground plane and a planar loop antenna. The shorted loop antenna is provided with a gap and operates at a quarter wavelength. The compact single feed dual or multi band internal antenna is realized either through composite assembly of an active outer and inner radiating elements or by a selective combination of an active outer radiating element and a parasitic inner radiating element. The inner radiating element of the proposed invention is completely encompassed within the outer radiating element. The resonant tuning of the internal antenna is accomplished by means of the matching stub and capacitive loading plates with the matching stubs being entirely internal to the outer radiating element.

Description:
RELATED APPLICATIONS  
       [0001]    The present invention is related to U.S. patent application Ser. No. 10/314,791, filed Dec. 12, 2002, titled C OMPACT  L OW  P ROFILE  S INGLE  F EED  M ULTI  B AND  P RINTED  A NTENNAS , Kadambi et al., U.S. patent application Ser. No. 10/135,312, filed Apr. 29, 2002 titled S INGLE  F EED  T RI  B AND  PIFA  WITH  P ARASITIC  E LEMENT , Kadambi et al., and U.S. Provisional Patent Application serial No. 60/424,850, filed Nov. 8, 2002, titled O PTIMUM  U TILIZATION OF  S LOT  G AP IN  PIFA D ESIGN , Kadambi et al. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to antenna and, more particularly, to antenna having shorted planar loops.  
         BACKGROUND OF THE INVENTION  
         [0003]    The cellular communication technology has witnessed a gradual and increasing trend of using internal antennas instead of more conventional external antenna. Cellular communication also has experienced an increase and an enhanced emphasis on multi band and multi system capabilities of cellular handsets. These changes have caused a growing demand for single feed single and multi band internal antennas for system applications comprising both the cellular and non-cellular frequency bands, which include GPS and Bluetooth.  
           [0004]    The Planar Inverted F-Antenna (PIFA) has proven to be a versatile choice as an internal antenna for the multi band and multi system antenna. However, the PIFA requires a relatively large volume of space in present compact wireless devices. Despite many improvements in PIFAs, the volume or amount of space the PIFA occupies continues to be a significant determining factor for its desirable performance.  
           [0005]    In view of the emerging constraints on the available volume for internal antennas, there is a need to look for potentially more efficient planar antenna configurations.  
         SUMMARY OF THE INVENTION  
         [0006]    To attain the advantages of and in accordance with the purpose of the present invention, an antenna with shorted active and passive planar loops in provided. The antenna is comprised of a conductive trace forming a first radiating element residing over a ground plane. The radiating element forms a loop antenna having a gap. The loop antenna has a radiating edge opposite a non radiating edge. A shorting element and feed tab are located on the non radiating edge. Multi band operating of the antenna is achieved by placing a second radiating element where at least a portion of the second radiating element is internal to a geometry formed by the first radiating element.  
           [0007]    The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:  
         [0009]    [0009]FIG. 1 is a plan view of an embodiment of an antenna consistent with the present invention;  
         [0010]    [0010]FIG. 1A is an elevation view of the antenna of FIG. 1;  
         [0011]    [0011]FIG. 2 is a plan view of another embodiment of an antenna consistent with the present invention;  
         [0012]    [0012]FIG. 3A is a plan view of another embodiment of an antenna consistent with the present invention;  
         [0013]    [0013]FIG. 3B is a plan view of another embodiment of an antenna consistent with the present invention;  
         [0014]    [0014]FIG. 4 is a plan view of another embodiment of an antenna consistent with the present invention; and  
         [0015]    [0015]FIG. 5 is a plan view of another embodiment of an antenna consistent with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]    [0016]FIGS. 1-5 and the following paragraphs describe some embodiments of the present invention. Like reference characters are used wherever possible to identify like components or blocks to simplify the description of the various subcomponents described herein. More particularly, the present invention is described in relation to particular embodiments thereof; however, one of ordinary skill in the art will understand on reading the following disclosure that other configurations are possible without departing from the spirit and scope of the present invention.  
         [0017]    Conventionally, almost all PIFA designs involve the formation of a slot on the radiating element of the PIFA. The slot forms a quasi partitioning of the radiating element allowing the PIFA to operate in multiple frequency bands. As is well known in the art, the design parameters of interest dictate the position of the slot with respect to a feed post and a shorting post as well as the slot&#39;s contour and length. The slot not only quasi partitions the PIFA to provide multiple band operation, but also is a reactive loading tool to reduce the resonant frequencies of the radiating element. The radiating element of a PIFA also contains capacitive loading elements that are usually bent segments extending from the edges of the radiating plane towards, but not touching, the ground plane.  
         [0018]    While both the slot loading and capacitive loading degrade the gain and bandwidth of the PIFA, they are useful techniques for tuning that does not increase the physical size of the PIFA. However, the overall size of the PIFA does constrain the amount of slot loading and capacitive loading permissible.  
         [0019]    When the volume wireless devices allot for antenna decreases, it decreases the permissible slot length that, in turn, decreases the slot loading. With conventional PIFA designs, the size constraints often make it difficult to realize the single or dual resonance at appropriate frequencies.  
         [0020]    It has been discovered, however, that single or multiple band performance can be achieved using planar loop antennas. The planar loop antennas provide appropriate response using smaller volumes than conventional PIFAs.  
         [0021]    Loop antennas of the present invention can take various configurations including square, rectangular, circular, elliptical, meander, or the like. Conventionally, loop antennas operate at half wavelength for desirable performance. Because conventional loop antennas operate at the half wavelength, they are not associated with shorting strips or vias connecting the radiating element to the ground plane. Further, conventional loop antennas are not usually placed above the ground plane.  
         [0022]    To make the conventional loop antenna operable for internal antennas associated with wireless devices, the loop antenna is oriented above the ground plane and for quarter wavelength operation. These modifications to the conventional loop antenna are due, in part, to the limited volume available for internal antennas in most wireless devices.  
         [0023]    Shorting the radiating element of the loop antenna to the ground plane still allows for operation at the appropriate resonant frequency. Further, shorting the radiating element and the ground plane for quarter wavelength operation results in a desirable reduction in the size of the loop. Of course, placing the radiating element above the ground plane and shorting the radiating element to the ground plane changes the resonance characteristics of the loop antenna.  
         [0024]    A gap or slot provided in loop antenna of the present invention provides additional control of the desired resonance characteristics of the antenna. Multi band operation is achieved by providing two loop antennas coupled through a connecting stub, typically near the feed point of the antenna. Alternatively, multi band operation can be achieved by shorting a combination of active and passive (parasitic) planar loops to the ground plane. It is believed the combination of active and passive loops imparts an easy control of the resonance characteristics of a particular band of operation without significantly influencing another band.  
         [0025]    As one of skill in the art would recognize on reading the disclosure of the present invention, one drawback of conventional loop antennas is the limited ability to tune the resonance frequency of the loop antenna. The shorting of the radiating element to the ground plane, the placement of the gap or slot on the loop and the attachment of capacitive loading plates to the edges of the loop provide increase ability to tune the resonance frequency(ies) of the loop antenna associated with the present invention.  
         [0026]    Referring now to FIG. 1, an embodiment of the present invention is shown. FIG. 1 shows a top or plan view of a loop antenna  100 . Loop antenna  100  has a radiating element  102  residing a distance from a ground plane  104 . Ground plane  104  is shown having a much larger area than radiating element  102  for illustrative purposes only, and ground plane  104  could have other sizes of larger, smaller, or equal area. Optionally, a dielectric carriage  106  can reside between ground plane  104  and radiating element  102  as a matter of design choice. The shape of loop antenna  100  is shown as a conventional rectangular shape, but the shape is largely dictated by the available space associated with a wireless device (not specifically shown). Thus, loop antenna  100  can have the linear configuration as shown or alternative geometric and/or random configurations.  
         [0027]    Loop antenna  100  additionally comprises a slot or gap  108  in radiating element  102 , a shorting element  110  shorting radiating element  102  to ground plane  104 , and a feed tab  112 . Shorting element  110  extends from the edge of radiating element  102  to ground plane  104  while feed tab  112  extends from the edge of radiating element  102  towards ground plane  104 , but does not actually connect to ground plane  104 . Placement of gap  108 , shorting element  110 , and feed tab  112  is largely dependent on the resonant frequency(ies) associated with loop antenna  100 . Tuning characteristics of loop antenna  100  can be further enhanced by the placement of one or more capacitive loading plates (not specifically shown in FIG. 1) along one or more edges of radiating element  102 . The capacitive loading plates, similar to feed tab  112 , would extend from the edge of radiating element  102  towards ground plane  104 , but would not actually connect to ground plane  104 . Antenna  100  has been shown to have improved gain over conventional PIFAs of similar size and decreased volume compared to conventional loop antennas using half wavelength operation.  
         [0028]    Referring now to FIG. 2, another embodiment of the present invention is shown. For convenience, the ground plane and optional dielectric carriage are not specifically shown. Similar to antenna  100 , antenna  200  includes a radiating element  202 , a gap  208 , a shorting element  210 , and a feed tab  212 . Further, antenna  200  could have one or more capacitive loading plates arranged along the edge of radiating element  202 . Unlike antenna  100 , however, antenna  200  includes at least one matching stub  214 . Unlike PIFA matching stubs, matching stub  214  can reside internal to the geometry of radiating element  202 . Placement and size of gap  208 , shorting element  210 , feed tab  212 , capacitive loading plate(s), and matching stub  214  are largely determined by desired resonant frequency characteristics. Without loss of generality, the matching stub  214  also can be attached to that edge of the radiating element that is opposite to the one containing the shorting element  210 .  
         [0029]    Antennas  100  and  200  are generally associated with single band operation. FIGS. 3A and 3B show exemplary embodiments of loop antennas  300 A and  300 B capable of multi band operation. FIGS. 3A and 3B are plan views of antenna  300 A and  300 B, respectfully, and antennas  300 A and  300 B may be arranged above a ground plane and dielectric carriage, similar to antennas  100  and  200 , but not specifically shown in FIGS. 3A and 3B. Referring first to FIG. 3A, antenna  300 A includes an outer boundary radiating element  302  and an inner radiating element  304 . Inner radiating element  304  is connected to outer boundary radiating element  302  at connection  306 . It is believed improved operation of antenna  300 A occurs when inner radiating element  304  is located close to a non radiating edge of outer boundary radiating element  302 . The edge of the outer boundary radiating element  302  containing the shorting post  310  is referred to as the non radiating edge of the element  302 . In this example, a feed tab  308  extends towards a ground plane substantially adjacent connection  306 , although other placements are possible. Connection  306  or auxiliary feed provides power from feed tab  308  to inner radiating element  304  making inner radiating element active. A shorting element  310  exists on outer boundary radiating element  302  extending between the outer boundary radiating element  302  and the ground plane (not specifically shown). In this case, shorting element  310  extends into a gap  312  in outer boundary radiating element  302 . The position and size of inner radiating element  304  helps regulate upper band resonance frequencies of antenna  300 A. Also, while shown with a linear configuration, inner radiating element  304  can have alternative geometries, such as a meanderer geometry, or the like.  
         [0030]    [0030]FIG. 3B shows a top plan view of antenna  300 B. Antenna  300 B is similar to antenna  300 A in that it contains outer boundary radiating element  302 , inner radiating element  304 , feed tab  308 , and short  310 , which as shown is residing in a gap  312 . Instead of connection  306 , however, antenna  300 B has an additional shorted element  314  in gap  312  and the inner radiating element  304  is connected to the shorted element  314 . Because inner radiating element  304  is not connected to a power source, it is passive and therefore the inner radiating element  304  serves as a parasitic element to the outer boundary radiating element  302 .  
         [0031]    For both antenna  300 A and  300 B, additional inner radiating elements  304  can be used to increase the number of operating bands of the antenna. Also, it is possible to combine active and passive inner radiating elements.  
         [0032]    Referring now to FIGS. 4 and 5, plan views of antennas  400  and  500  are shown illustrative of the present invention. Referring first to FIG. 4, antenna  400  includes outer boundary radiating element  402 , and inner radiating element  404 . As shown, outer boundary radiating element  402  can have various dimensions and does not have to be a consistent thickness around the loop. Inner radiating element  404  can similarly vary in size along its length, and can have alternative geometries, such as the meanderer line shown. Similar to the other antennas disclosed above, antenna  400  includes a gap  406 , a feed tab  408 , and a shorting element  410 . As a passive or parasitic element, inner radiating element  404  has a shorted element  412 .  
         [0033]    Strategically arranged along the radiating edge of outer boundary radiating element  402  can reside one or more capacitive loading plates  414 . The size, shape and number of capacitive loading plates  414  depend on antenna  400 &#39;s resonant frequency requirements.  
         [0034]    Antenna  400  is capable of multi band operation. Multi band operation of antenna  400  is achieved by, among other things, changing the geometry of the gap and/or addition of multiple passive inner loops.  
         [0035]    Referring now to FIG. 5, antenna  500  is shown. As can be seen, antenna  500  is mostly identical to antenna  400 , but includes a matching stub  502 . Use of matching stub  502 , as shown, or additional shorting posts and/or strips increases the robustness of the antenna with regard to multi band operation.  
         [0036]    While the invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention.