Abstract:
An antenna structure has the approximate characteristics of a slotted cylinder. Rather than utilizing a complete cylinder, the antenna structure utilizes a pair of spaced-apart patches that are joined together by one or more transmission lines that loop around with the same length as the perimeter of a corresponding cylinder. The antenna structure may be advantageously “wrapped” around an enclosure for an electronic device, such as a cellular telephone or the like.

Description:
RELATED APPLICATIONS 
     This application relates to co-pending U.S. patent application Ser. No. 09/781,770, entitled “Spiral Sheet Antenna Structure and Method” by Eli Yablonovitch, et al., owned by the assignee of this application and incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of wireless communications, and particularly to the design of an antenna. 
     2. Background 
     Small antennas are required for portable wireless communications. To produce a resonant antenna structure at a certain radio frequency and within a certain bandwidth, classical antenna structures need to have a certain volume. This volume becomes fairly large as the bandwidth required is large. 
     H. A. Wheeler, “Small Antennas” IEEE trans. on Antennas and Propagation, Vol. AP-Jul. 23, 1975, provides a formula linking the volume of an antenna and the frequency. It is necessary to have a certain volume in order to obtain a desired bandwidth. In the case of a small portable device, a solution for maximizing the antenna volume, and hence the antenna bandwidth, is to wrap the antenna around the device. 
     Accordingly, the present invention addresses the need for a small compact antenna with wide bandwidth that can be integrated with a mobile device. 
     SUMMARY OF THE INVENTION 
     The present invention provides an antenna structure with the approximate characteristics of a slotted cylinder. Rather than utilizing a complete cylinder, the antenna structure utilizes a pair of spaced-apart patches that are joined together by one or more transmission lines that loop around with the same length as the perimeter of a corresponding cylinder. The antenna structure may be advantageously “wrapped” around an enclosure for an electronic device, such as a cellular telephone or the like. 
     In one form, the present invention provides an antenna comprising first and second conductive patches having respective first edges separated by a gap and having second respective edges; at least one conductor joining the respective second edges of the two patches, the conductor being routed in a loop away from the patches in a direction perpendicular thereto; and an antenna feed coupled to at least one of the patches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a radiating element in the form of a slotted rectangular cylinder. 
     FIG. 2 illustrates a radiating element in the form of a slotted circular or oval cylinder. 
     FIG. 3 is a perspective view of a radiating structure in accordance with the present invention. 
     FIG. 4 illustrates a feed arrangement for the structure of FIG.  3 . 
     FIG. 5 is a perspective view of an alternative feed arrangement. 
     FIG. 6 illustrates an electronic device utilizing an antenna in accordance with the present invention. 
     FIG. 7 illustrates routing of a transmission line in order to avoid an obstruction. 
     FIG. 8 is a perspective view of a radiating structure with filters in the transmission lines. 
     FIG. 9 illustrates a slotted radiating patch. 
     FIG. 10 illustrates an antenna structure utilizing a plurality of electromagnetically coupled elements. 
     FIG. 11 illustrates a multiple element antenna structure with improved bandwidth and polarization diversity. 
     FIG. 12 illustrates another multiple element antenna structure with improved bandwidth and polarization diversity. 
     FIG. 13 illustrates one form of a doubly resonant antenna element. 
     FIG. 14 illustrates another form of a doubly resonant antenna element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the description of the present invention with unnecessary detail. 
     FIG. 1 illustrates a radiating structure  10  in the form of a slotted rectangular cylinder. Energy is radiated principally from the open ends of the cylinder as a result of the electrical charges that accumulate at the ends as described in patent application Ser. No. 09/781,779. The mechanism by which energy is radiated from structure  10  is also explained in detail in the Wheeler reference. The frequency of the radiated energy is a function of the width of gap g, which also defines the capacitance of the structure. 
     With reference to FIG. 2, slotted cylindrical radiating structure  12  may be characterized as an inductor with a magnetic field B propagating from the center of the cylinder, parallel to the slot. The perimeter surface  14  of the cylinder may be analyzed as a transmission line that controls the phase shift appearing across the gap g. In this context, it can be seen that it is not necessary to consider the entire surface of cylinder  12  in order to characterize the radiating properties of the cylinder. 
     FIG. 3 illustrates a modification of the slotted cylinder shown in FIG. 2 wherein substantial portions of the cylindrical surface have been removed. Radiating structure  20  comprises spaced-apart strips or patches  21  and  22  which are connected by a plurality of transmission lines  24 . Provided that the lengths of transmission lines  24  correspond to the perimeter  14  of cylinder  12  (FIG.  2 ), structure  20  and cylinder  12  will have essentially similar radiating characteristics. 
     A feed arrangement for radiating structure  20  is illustrated in FIG.  4 . The center conductor  27  of a coaxial feed  26  is coupled to patch  22 . The ground  28  is coupled to one of the transmission lines  24 . 
     Other feed arrangements may also be used. For example, FIG. 5 illustrates a feed arrangement utilizing a co-planar wave guide  30 . 
     One of the principal advantages of radiating structure  20  is the ability to “wrap” an antenna around an electronic device  40 , such as a cellular telephone as shown in FIG.  6 . The antenna may be wrapped around the surface of the device or may be molded into the plastic casing of the device. 
     FIG. 7 illustrates another advantage of the present invention. Because the radiating patches  21  and  22  are connected by discrete transmission lines  24 , these lines may be conveniently routed around any obstructions, such as component  42 . As long as the lines all have equal lengths, the characteristics of the antenna will not be significantly affected. 
     Referring now to FIG. 8, the characteristics of antenna  20  can be adjusted by inserting filters  50  in some or all of transmission lines  24 . In particular, the lengths of lines  24  and the characteristics of filters  50 , either band stop or band pass, may be set so that the transmission line lengths are different for different frequencies, thereby providing a multifrequency antenna. This approach has particular application to future triple-band mobile phone systems. 
     With reference to FIG. 9, one or both of patches  21  and  22  may include slots  54 . Such slots alter the characteristic impedance of the antenna and may be used as another approach to achieving a multi-frequency antenna. 
     FIG. 10 illustrates another embodiment of the present antenna. Antenna structure  60  utilizes a plurality of elements comprising pairs of radiating patches  61 ,  62 . The individual patches of each such pair are connected by a transmission line  64  as in the previously described embodiments. In this case, only one of the elements may be excited by a direct feed, the other elements being excited through electromagnetic coupling. The various elements of the antenna may be tuned to slightly different frequencies so that the overall bandwidth of the antenna is greatly improved. 
     Variations of the basic radiating structure of the present invention can be used to achieve wide bandwidth and/or diversity. One example is illustrated in FIG. 11 where two radiating elements  71 ,  72  are arranged at right angles to provide polarization diversity. 
     A similar structure is illustrated in FIG.  12 . In this case, elements  81  and  82  are arranged with the radiating patches on the same side of the antenna volume. In this arrangement cross-polarization is reduced. 
     FIG. 13 illustrates an antenna element comprising patches  91  and  92  connected by a transmission line  94 . This structure is doubly resonant at frequencies f 1  and f 2 . At frequency f 1 , only half of the capacitance across gap g is seen. Portion  93  of patch  91  is decoupled. At frequency f 2 , patch  91  appears as if it were a solid conductor and the capacitance between patches  91  and  92  is at a maximum. 
     FIG. 14 illustrates another multi-frequency configuration. Here, patch  101  has two different current repartitions at different frequencies f 1  and f 2 . The capacitance between patches  101  and  102  is therefore different at frequencies f 1  and f 2 . 
     In all of the previously described embodiments, the capacitance of the antenna element(s) may be increased by overlapping the two patches of the element. This allows the overall dimensions of the antenna to be reduced, although with a consequent reduction in the bandwidth of the antenna. 
     It will be recognized that the above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.