Abstract:
An antenna comprising a substrate having a pair of oppositely directed surfaces. A source plane conductor is located on one of the surfaces and has a signal line connected thereto. A ground plane conductor is located on another of the surfaces. Each of the conductors has a slot extending therethrough with said slots sized and positioned relative to one another to inhibit the intensity of radiation emanating from the ground plane.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to antennas for wireless communications.  
       BACKGROUND OF THE INVENTION  
       [0002]     Portable devices having wireless communications capabilities are currently available in several different forms, including mobile telephones, personal digital assistants and hand held scanners.  
         [0003]     The demand for wireless connectivity from portable devices is rapidly expanding. As a result, the demand for high performance, low cost, and cosmetically appealing antenna systems for such devices is also increasing.  
         [0004]     One type of antenna commonly used in portable wireless devices is the monopole whip. A monopole whip antenna is essentially a wire that extends along or away from the device and is fed by the printed circuit board (PCB) of the device. One problem of this unbalanced design is that radio frequencies (RF) currents induced on the PCB may cause receiver desensitization, thereby limiting the useful range of the device.  
         [0005]     In a monopole whip design as described above, and other unbalanced designs used in similar applications, the PCB may function as a part of the antenna. As a result, the PCB may also radiate a portion of a signal being transmitted, causing operating characteristics of the antenna such as gain, radiation pattern, and driving point impedance to become dependent on qualities of the PCB such as size, shape, and proximity to other structures (such as a display, a cable, a battery pack, etc.). Therefore, it may become necessary to redesign the antenna to achieve a similar performance with different applications and/or different types of devices.  
         [0006]     Radiation by a PCB due to RF coupling with an unbalanced antenna may also cause efficiency losses. In a mobile phone application, for example, radiation of a PCB that is placed next to the users head may be wasted due to absorption of the radiating fields by the users head and hand. In addition to reducing the efficiency of the device, this effect may also increase the specific absorption rate (SAR) beyond regulatory limits.  
         [0007]     A coaxial sleeve dipole is a balanced antenna that tends to de-couple the antenna system from the PCB or device to which it is connected. Such an antenna is constructed of coaxial cable, where the center conductor extends beyond the outer conductor, and the outer conductor is rolled back to form a jacket. One advantage of this design is that if the jacket has the right length, then current which otherwise might distort the radiation pattern may be impeded from flowing along the outer surface of the feed cable. Unfortunately, coaxial sleeve dipoles are too bulky and heavy to be practical for use in small portable devices and are not compatible with the small, slim profiles of present portable wireless devices. Additionally, coaxial sleeve dipoles are relatively expensive.  
         [0008]     Accordingly, it is an object of the present application to obviate or mitigate the above disadvantages.  
       SUMMARY OF THE INVENTION  
       [0009]     In one aspect, the present invention provides an antenna comprising a substrate having a pair of oppositely directed surfaces. A source plane conductor is located on one of the surfaces having a signal line connected thereto. A ground plane conductor is located on another of the surfaces. Each of the conductors has a slot extending therethrough with the slots sized and positioned relative to one another to inhibit the intensity of radiation emanating from said ground plane. Preferably each of said slots extend from a peripheral edge of said substrate. Preferably also one of said slots is L shaped.  
         [0010]     An embodiment of the invention will now be described by way of example only with reference to the following detailed description in which reference is made to the following appended drawings, in which:  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a perspective view of a hand held scanner,  
         [0012]      FIG. 2  shows a cross-sectional view of an antenna utilized in the scanner of  FIG. 1 .  
         [0013]      FIG. 3A  shows a top view (along axis III-III as shown in  FIG. 2 ) of an antenna utilized in the scanner of  FIG. 1 .  
         [0014]      FIG. 3B  shows a top view (along axis III-III as shown in  FIG. 2 ) of an alternative antenna utilized in the scanner of  FIG. 1 .  
         [0015]      FIG. 3C  shows a top view (along axis III-III as shown in  FIG. 2 ) of an alternative antenna utilized in the scanner of  FIG. 1 .  
         [0016]      FIG. 4A  shows a bottom view (along axis IV-IV as shown in  FIG. 2 ) of the antenna shown in  FIG. 3A .  
         [0017]      FIG. 4B  shows a bottom view (along axis IV-IV as shown in  FIG. 2 ) of the antenna shown in  FIG. 3B .  
         [0018]      FIG. 4C  shows a bottom view (along axis IV-IV as shown in  FIG. 2 ) of the antenna shown in  FIG. 3C .  
         [0019]      FIG. 5  shows a graph of the radiation pattern for the antenna illustrated by  FIGS. 2, 3A ,  4 A,  3 B,  4 B and  3 C,  4 C.  
         [0020]      FIG. 6  shows a Voltage Standing Wave Ratio (VSWR) graph for the antenna illustrated by  FIGS. 2, 3A  and  4 A.  
         [0021]      FIG. 7  shows a Voltage Standing Wave Ratio (VSWR) graph for the antenna illustrated by  FIGS. 2, 3B  and  4 B.  
         [0022]      FIG. 8  shows a Voltage Standing Wave Ratio (VSWR) graph for the antenna illustrated by  FIGS. 2, 3C  and  4 C.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     Referring to  FIG. 1 , there is shown a hand held scanner  2  having a body  4  and a display  14 . The scanner may include an input device, such as keypad  6 , and is used to read and store information from barcodes or the like through a scanner window  8 . The body  4  contains control and data acquisition components as well as a communication module and an internal antenna  100 . The scanner  2  maybe used in a variety of locations in which transfer of data to a central database is desirable.  
         [0024]     Referring therefore to  FIGS. 2, 3A  and  4 A, the antenna  100  comprises a substrate  110  having two oppositely directed conductive planes  120  and  130 . The plane  120  may be referred to as the source plane  120  while the bottom plane  130  may be referred to as the ground plane  130 . Slots  122  and  132  are formed in the planes  120 ,  130  respectively. In a particular embodiment, the substrate  110  may be, for example, the substrate portion of a printed circuit board (PCB). The conductive planes  120 ,  130  are created by covering the substrate  110 , through lamination, roller-cladding or any other such process, with a layer of a conductive material, for example copper. Source slot  122  and ground  132  slot are created by etching, or otherwise removing, conductive material from the conductive planes  120 ,  130  respectively. Each of the slots  120 ,  130  is L shaped with one leg  123 ,  133 , extending parallel to the longitudinal axis of the antenna and the other leg  125 ,  135 , extending normal or transverse to the axis to the periphery of the antenna. The axial legs and transverse legs are juxtaposed on each plans so that the legs are aligned with one another. A signal line (not shown) is connected to the source plane  120  at hole  127 , and the ground plane  130  connected to ground, either by a cable shield or through a mechanical connector with the body  4 .  
         [0025]     Alternatively, substrate  110  may be another non-conductive material such as a silicon wafer or a rigid or flexible plastic material. The substrate  110  may also be formed into a non-flat shape e.g., curved, so has to fit into a specific space within, for example, a scanner body  4 .  
         [0026]     Certain desirable properties such as increased efficiency may be obtained by using a material for substrate  110  that has specific properties, such as a particular permittivity or dielectric constant, at the desired frequency or frequency range of operation. For example, at higher frequencies, such as a frequency of 5 GHz, a higher dielectric constant may be desirable. Preferably, the material used for substrate  110  has uniform thickness and properties.  
         [0027]     In a typical configuration, for the source slot the leg  125  is 0.160 mill and the axial leg  123  is 0.920 mill. The ground slot has a transverse leg  135  of 0.160 mill and an axial leg of 0.580 mill. The axial length of the antenna  100  is 2670 mill and the width 320 mill. The width of the slot is 20 mill.  
         [0028]     It may be desirable to design the contours of the antenna  100  substrate  110  to fit into the available space in a device.  FIG. 3B and 4B  show the top and bottom views respectively of an antenna  100  according to an alternative embodiment of the invention having a substrate  110  that is designed to fit into an irregularly shaped space with a recess  112  to fit around a connector. As will be seen, the source slot  122  is divided into a pair of slots  122   b,    122   c,  extending to either side of the recess  112 . The ground slot is L shaped as with embodiment  3 B for the source slot. The leg  132   b  is aligned with the leg  122   c  on the source plane. In a typical embodiment for an antenna with overall dimensions of 1954×710 mill. The leg  122   b  has a length of 325 mill and  122   c  has a length of 660 mill. On the ground plane the length of transverse leg is 379 mill and the axial leg has a length of 270 mill. In a further embodiment shown in  FIGS. 3C and 4C , the source slot  122  is formed as an H-pattern having an axial bar  122   d  terminating in a pair of transverse legs  122   e.  The bar  122   d  is connected to a intermediate leg  122   f  extending from the bar  122   d  to the periphery. The leg  122   f  is aligned with the transverse leg of slot  132   c  and the axial leg of slot  132   c  aligned with the bar  122   d.  In a typical configuration, the axial length of the bar  122   d  is 1400 mill and each of the transverse legs 415 mill. The intermediate leg is 370 mill and is offset to be 600 mill from one of the legs  122   e.  The ground slot is L shaped with a vertical leg of 0.370 mill and a horizontal leg of 0.370 mill. Again, the width of the slot is 0.020 mill. The overall dimensions of the antenna  100  is 1960×688 mill.  
         [0029]     An antenna  100  described by either  FIGS. 2, 3A  and  4 A,  FIGS. 2, 3B  and  4 B or  FIGS. 2, 3C  and  4 C exhibits a radiation pattern that tends to be directional, as illustrated by  FIG. 5 , which shows a graph of the radiation pattern for such an antenna  100 . It may be observed that the radiation pattern of such an antenna  100  tends to be null along the axis of the antenna  100  and of reduced power when emanating from the ground plane  130  when compared to the source plane  120 . Therefore, it may be desirable to configure a particular application of such an antenna  100  according to an appropriate orientation with respect to a receiver to which the antenna is expected to radiate (or, a transmitter from which the antenna is expected to receive a signal).  
         [0030]     The use of such an antenna  100  may reduce or avoid blockage of the radiated signal by, for example, the users head or hand, in an application such as a cellular telephone, a PDA, a handheld scanner  2  or any other handheld wireless device. A possible benefit is the reduction in measured specific absorption rate (SAR), which is related to the heating of body tissues caused by the radio waves outputted by the wireless device. Another possible benefit is that the ground plane  130  also serves to reduce or block high frequency noise generated by processors used within the wireless device, which clock frequencies may fall within the frequency band of the antenna.  
         [0031]     The relative positioning and sizing of the slots on the source plane and ground plane may be adjusted so as to enhance the radiation intensity in the forward direction and reduce the radiation intensity in the rear direction. This may be accomplished by considering the relative phases of the radiation component from each plane. Similarly, the spacing between the planes may be adjusted to optimize the interaction of the radiation from each plane to attain the desired radiation pattern.  
         [0032]     As know by a person skilled in the art, the voltage standing wave ratio (VSWR) is used as a performance parameter to quantify the percentage of power that will be reflected at the input of the antenna. When VSWR is evaluted, a value closer to 1.00:1 is more desirable than one that is higher. A VSWR of 3.00:1 is considered the maximum acceptable and results in a 25% reduction of power or 1.2 dB loss.  FIGS. 6, 7  and  8  show the VSWR graphs for the antennas  100  described by  FIGS. 2, 3A ,  4 A,  FIGS. 2, 3B ,  4 B and  FIGS. 2, 3C ,  4 C respectively and show band edges (2.40 GHz and 2.50 GHz) having VSWR values between 1.38:1 and 1.74:1 and a center frequency (2.45 GHz) VSWR value between 1.07:1 to 1.22:1, including cable and connector loss.  
         [0033]     Tables 1, 2 and 3 show the effect of the variation in the length of the source slot (S)  122  and the ground slot (G)  132  on the VSWR and bandwidth (BW) values for an application having a center frequency of 2.45 GHz and band edges of 2.40 GHz and 2.50 GHz, such as in the ISM standard, for the antennas  100  described by  FIGS. 2, 3A ,  4 A,  FIGS. 2, 3B ,  4 B and  FIGS. 2, 3C ,  4 C respectively. The lengths of slot S  122  and slot G  132  are expressed in mils (e.g. {fraction (1/1000)} th  of an inch) and represent the total length of the slot including each of the legs in the configurations of  FIGS. 3A, 4A , and  3 B,  4 B. The lengths S and G include axial bar  122   d  and transverse legs  122   e  for the embodiment of  FIG. 3C .  
                                                                           TABLE 1                             FIGS. 2, 3A  and 4A                    VSWR   VSWR   VSWR   VSWR   BW       S   G   2.40 GHz   2.45 GHz   2.50 GHz   Average   VSWR = 2.5                    1040   760   1.67   2.31   2.6   2.19   260       1050   760   1.79   2.25   2.4   2.15   320       1060   760   1.51   2.06   2.28   1.95   330       1070   760   1.41   1.76   2   1.72   340       1080   760   1.21   1.6   2.05   1.62   350       1060   740   1.35   1.56   2.06   1.66   325       1060   750   1.42   1.38   1.76   1.52   320       1060   760   1.51   2.06   2.28   1.95   330       1060   770   1.52   2.22   2.77   2.17   265       1060   780   1.82   2.82   2.97   2.54   230       1080   740   1.74   1.22   1.67   1.54   210                  
 
         [0034]     Changes in the slot length S and G are obtained by varying the length of the axial leg. Thus the ratio of slot length S/G may vary between 1.46 and 1.36.  
                                                                           TABLE 2                             FIGS. 2, 3B  and 4B                    VSWR   VSWR   VSWR   VSWR   BW       S   G   2.40 GHz   2.45 GHz   2.50 GHz   Average   VSWR = 2.5                    975   640   1.86   1.39   1.64   1.63   175       985   640   1.68   1.49   2.28   1.82   175       995   640   1.64   1.85   3.15   2.21   175       1005   640   1.45   2.18   4.17   2.60   175       1015   640   1.57   2.74   6.21   3.51   200       995   620   1.38   1.85   3.47   2.23   190       995   630   1.39   1.64   3.14   2.06   175       995   640   1.64   1.85   3.15   2.21   175       995   650   1.24   1.51   2.88   1.88   200       995   660   1.44   1.52   2.65   1.87   175       985   649   1.38   1.07   1.64   1.36   210                  
 
         [0035]     Changes in the slot length S is obtained by varying the length of the leg  122   c  and the length G by varying the axial leg. The ratio S/G may vary between 1.51 and 1,60.  
                                                                           TABLE 3                             FIGS. 2, 3C  and 4C                    VSWR   VSWR   VSWR   VSWR   BW       S   G   2.40 GHz   2.45 GHz   2.50 GHz   Average   VSWR = 2.5                    2200   740   1.46   1.18   1.9   1.51   260       2210   740   1.42   1.12   1.79   1.44   270       2220   740   1.44   1.18   1.97   1.53   260       2230   740   1.64   1.13   1.71   1.49   280       2240   740   1.54   1.17   1.89   1.53   270       2220   720   1.47   1.14   1.81   1.47   280       2220   730   1.46   1.12   1.79   1.46   270       2220   740   1.64   1.85   3.15   2.21   260       2220   750   1.41   1.18   1.94   1.51   255       2220   760   1.4   1.11   1.84   1.45   260       2230   740   1.64   1.13   1.71   1.49   280                  
 
         [0036]     Variation of the length S is obtained by varying the length of the transverse legs  122   e  by equal amounts. For the slot length G, the horizontal leg  132   c  is varied. The ratio S/G provides values in the range 3.0 to 3.04.  
         [0037]     The preceding values are given as way of example for an application having a center frequency of 2.45 GHz and band edges of 2.40 GHz and 2.50 GHz which represent the ISM standard such as used, for example, by Bluetooth based applications. Antennas  100 , as described by  FIGS. 2, 3A ,  4 A,  FIGS. 2, 3B ,  4 B and  FIGS. 2, 3C ,  4 C, operating in other frequency ranges may be produced as well by varying the length of the source slot  122  and/or the ground slot  132  until the desired VSWR and bandwidth values are attained.