Abstract:
A small, printed antenna provides high efficiency, good isolation and a broad working bandwidth. These characteristics are achieved with a patch antenna by placing a shunt to ground connected to the feeding point of the patch. This shunt comprises a line running along one edge of the patch. The patch dimensions can be adjusted, and in particular reduced, by changing the L and C characteristics of the patch. This is accomplished with arrays of slots defining corresponding arrays of fingers along the edges of the patch. Impedance matching is achieved by altering the dimensions of the slots.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to antennas for use with radio transceivers. More particularly, the invention provides a small multiband patch antenna with very high efficiency and high isolation for use in cellular telephones and other personal electronic devices. 
     2. Background 
     Cellular telephones and other wireless electronic devices are widely used. Such devices have steadily grown smaller with advances in miniaturization of electronic components. This has created a challenge for the design of antennas in such devices. At the same time, it is desirable for the antenna to have a broad working bandwidth. 
     Various methods are known in the art to broaden the operating bandwidth of an antenna. Most of these employ parasitic elements that are excited by a driven element. In most cases, the elements are capacitively coupled. In the case of patch elements, the methods often rely on optimization of the coupling between the patches. The modes excited inside the different elements are basically the same. 
     Different methods exist in order to reduce the dimensions of a patch antenna. One such method is described in  Size Reduction of Patch Antenna by Means of Inductive Slits , Reed, S., Desclos, L., Terret, C., Toutain, S., APS/URSI 20000 Utah. This method places a set of slits in the patch that represents an inductive loading. The authors report that a reduction of 50% in the dimensions of the patch antenna was achieved with this approach. Generally speaking, however, as the patch gets smaller, the efficiency decreases and the working bandwidth gets smaller. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a small, printed antenna with high efficiency, good isolation and a broad working bandwidth. These characteristics are achieved with a patch antenna by placing a shunt to ground connected to the feeding point of the patch. This shunt comprises a line running along one edge of the patch. The patch dimensions can be adjusted, and in particular reduced, by changing the L and C characteristics of the patch. This is accomplished with arrays of slots defining corresponding arrays of fingers along the edges of the patch. Impedance matching is achieved by altering the dimensions of the slots. 
     By adding a strip line shunt at the feed point of the antenna, an efficient driving element for exciting the antenna is defined. This strip line at the frequency of use constitutes an inductance. While it helps with broadband matching, it also creates a capacitive coupling with the first neighbor finger. From this strong coupling, it is possible to excite different modes. In fact, the shunt helps to unbalance the antenna, which should not be considered as a patch under a classical mode. The antenna can be considered as a set of fingers that will combine in either an array form or single couple of fingers. 
     The bandwidth of the antenna is increased by adding as many couples of fingers as frequencies needed to form the total bandwidth by the addition of the subside bands. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an equivalent circuit diagram of a simple patch antenna. 
     FIG. 2 is an equivalent circuit diagram of a patch antenna with a shunt coupling the feed point to the ground plane. 
     FIG. 3 is a Smith chart for an antenna having an equivalent circuit diagram as shown in FIG.  2 . 
     FIG. 4 is a plan view of a multi-finger patch antenna in accordance with the present invention. 
     FIG. 5 is a plan view of an alternative embodiment of the present invention. 
     FIG. 6 is a plan view of another alternative embodiment of the present invention. 
     FIG. 7 is a cross-sectional view of the embodiment of FIG.  6 . 
     FIG. 8 is a plan view of another alternative embodiment of the present invention. 
     FIG. 9 is a plan view of a “half” multi-finger patch according to the present invention. 
     FIG. 10 is a perspective view of another alternative embodiment of the present invention. 
     FIG. 11 is a plan view of still another alternative embodiment of the present invention. 
     FIG. 12 is a plan view of yet another alternative embodiment of the present invention. 
     FIG. 13 is a plan view of a further embodiment of the present invention. 
    
    
     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 is an equivalent circuit diagram for a simple patch antenna. The inductance L and capacitance C may be adjusted to control the resonant frequency of the patch. However, adjusting these values are not effective for increasing the bandwidth of the antenna particularly when the physical dimensions of the patch are reduced, nor is it effective for matching the input impedance of the antenna, which, in the most common applications, should be matched to 50 ohms. 
     By introducing an additional inductance at the input to the patch, the input impedance can be easily controlled since it behaves like a matching circuit. The additional inductance also helps to reduce the dimensions of the patch. If we consider a patch fed by a microstrip line, a short to ground at the contact point between the microstrip line and the patch introduces the desired inductance as shown in the equivalent circuit diagram of FIG.  2 . This circuit is resonant at two frequencies. By adjusting the inductance and capacitance characteristics of the patch, the resonant frequencies can be adjusted so that the antenna has a relatively wide operating bandwidth-two to three times that of a singly resonant patch. 
     Referring to FIG. 3, the double resonance of the shorted patch appears on a Smith chart as a large loop l 1  with a smaller loop l 2  that comes closer to the point of matched impedance (typically, but not necessarily, 50 ohms). Without the short, the antenna behaves just like an open circuit. 
     Even with the double resonance achieved with the antenna design of the present invention, the bandwidth may not be large enough for some applications. The bandwidth can be further increased by increasing the thickness of the dielectric substrate. The bandwidth of the antenna is directly proportional to the thickness of the substrate. 
     One method of controlling the inductance and capacitance of the patch is illustrated in FIG. 4. A plurality of slots  16  are cut into opposing edges  12  and  14  of patch  10 . The slots  16  define a corresponding plurality of fingers  18 . The widths of slot  16  and fingers  18  are shown as being approximately equal, but this need not be the case. FIG. 4 also shows strip line feed  20  and shunt  22 . Although feed  20  is illustrated as a microstrip line, patch  10  may also be feed with a coaxial cable from above, from underneath, or from the edge. Feed  20  need not be centered along edge  24  as shown. The placement of the feed gives another degree of freedom for packaging considerations. 
     The characteristics of patch  10  may be tuned by adjusting the depth of slots  16  (dimension d 1 ), the overall length of the patch (dimension d 2 ) and the overall width of the patch (dimension d 3 ). It should be noted that d 1 , d 2  and d 3  need not be uniform across the entire patch. The shape of the patch can be adjusted to fit within packaging constraints. As explained above, shunt  22  is very important for the resonance characteristics of patch  10 , but it does not have a particularly large influence on impedance matching. Shunt  22  may be used to fine-tune the input impedance of patch  10 . 
     Patch  10  is preferably formed of copper cladding using conventional printed circuit techniques on a dielectric substrate. A ground plane of copper cladding is disposed on the surface of the substrate opposite patch  10 . It is desirable for the substrate to have a relatively high dielectric coefficient as this allows the physical dimensions of patch  10  to be made smaller. Suitable materials for the substrate are TMM 6 or TMM 10 available from the Microwave Materials Division of Rogers Corporation, Chandler, Ariz. These materials are thermoset ceramic loaded plastics having dielectric coefficients of approximately 6 and 9.2, respectively. Equivalent materials from other vendors may also be utilized. 
     The effect of dimensions d 1 , d 2  and d 3  on the characteristics of patch  10  may be better understood with reference to the Smith chart shown in FIG.  3 . The effect of changing d 1 , is to rotate the position of the small loop l 2  relative to l 1  on the Smith chart without changing the position of the frequencies relative to the loop. Increasing d 1  causes  1   2  to move clockwise. The effect of d 3  is exactly the opposite of d 1 , i.e., decreasing d 3  causes l 2  to move counterclockwise on the Smith chart, again without affecting the position of the frequencies relative to the loop. The effect of changing d 2  is to rotate the l 2  loop, but with the frequencies rotating in the opposite direction. Reducing d 2  causes the l 2  loop to move clockwise, whereas the frequencies rotate counterclockwise. The distance between shunt  22  and edge  24  controls the diameter of the small loop 1 2 . The closer the shunt is, the larger the diameter of 1 2  is. The dimensions of the ground plane underlying patch  10  also has a large influence on the diameter of the l 2  loop. The smaller the ground plane is, the larger the diameter of the l 2  loop is. In the case of a small ground plane, the increased diameter of the l 2  loop can be compensated for by increasing the distance between the shunt and the patch. 
     The number of slots  16  and fingers  18  does not have a significant effect on impedance matching. As explained above, increasing the length of the slots  16  has the opposite effect of reducing the overall width of the patch. Therefore, impedance matching of the antenna is influenced more by the overall width of the antenna rather than by the number of slots and fingers. However, by reducing the width of the slots and the width of the fingers (as mentioned above, the widths of the slots and fingers need not be equal), it is possible to have better control over the minimum possible width of the antenna. Moreover, due to the current distribution on the antenna, the more fingers the antenna has, the more resonances can be gathered in the same frequency range and the wider the working bandwidth can be. 
     In order to reduce the physical dimensions of the patch, the dielectric coefficient of the substrate may be increased. The overall dimensions of the patch are inversely proportional to the square root of the dielectric coefficient. However, suitable materials with high dielectric coefficients add significantly to the cost. An alternative approach is illustrated in FIG.  5 . Here, the fingers  118  of patch  110  have a zigzag configuration so that, for a given effective width of the fingers, the overall width of the patch may be reduced. 
     The simplest way to further reduce the dimensions of the patch is to increase the capacitance. This can be done directly by adding one or more additional conductive layers as illustrated in FIGS. 6 and 7. Here, a plurality of islands  219  are formed in an additional conductive layer below patch  210 . Each of the islands  219  is positioned below a corresponding slot  216  and is coupled to the ground plane  230 . Alternatively, or in addition, the islands could be above the slots. 
     Another approach for increasing the capacitance is shown in FIG.  8 . Here, parasitic islands  319  are formed within slots  316  in the same layer of conductive material as patch  310 . Again, each of islands  319  is coupled to the underlying ground plane. 
     A straightforward approach for reducing the dimensions of the antenna is illustrated in FIG.  9 . Patch  410  has only a single array of fingers  418 . Although the current distribution with patch  410  is not the same as in patch  10 , the optimization is very similar. In this nonsymmetrical configuration, there are two or more separated frequencies with radiating modes (more widely separated than in a symmetrical configuration), and non-radiating mode(s) in between. 
     Another design employing a “half” multi-finger patch is illustrated in FIG.  10 . Antenna  510  comprises a folded conductor without a separate ground plane. A dielectric substrate is not utilized in this design. Shunt  522  extends from the feed point  520  to a floating ground  530  underlying fingers  518 . 
     FIG. 11 illustrates a patch  610  with a balanced input. Separate feeds  620  and  621  are provided on each side of the antenna with respective shunts  622  and  623 . A slot  640  between the two feeds permits the inputs to be matched so that currents within the patch from the respective feeds are in phase. 
     In order to counteract fading in wireless communications systems, it is desirable to have diversity of antenna characteristics. Once such diversity, for example, is polarization diversity. Polarization diversity can be easily obtained with the finger patch antenna of the present invention by overlapping two patches in orthogonal directions as shown in FIG.  12 . Patches  710  and  711  are each constructed as discussed previously in connection with FIG.  4 . It will be appreciated that these patches can be constructed using any of the various alternative embodiments discussed herein. 
     Another embodiment of the present invention is illustrated in FIG.  13 . Slots  816  are cut into adjoining edges  812  and  814  of patch  810 . Shunts  822  and  823  are provided for each half array of fingers  818 . 
     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.