Abstract:
A spatial filter is developed for specific absorption rate (SAR) reduction in a wireless device. A conductive element is designed to modify the near field distribution of an antenna operating in a wireless device. This reduces SAR while minimizing degradation of antenna efficiency at one or several frequency bands that the antenna is designed to operate over. Lumped reactance can be designed into the conductive element to generate low pass, band pass, and/or high pass frequency characteristics. Distributed reactance can be designed into the conductive element to replace or to work in conjunction with the lumped reactance. Active components can be designed into the conductive element to provide dynamic tuning of the frequency response of the conductive element.

Description:
FIELD OF INVENTION 
     The present invention relates generally to the field of wireless communication. In particular, the present invention relates to an antenna system for use within such wireless communication. 
     BACKGROUND OF THE INVENTION 
     A wide range of electrical requirements must be met by antennas in wireless devices. These requirements include TRP (total radiated power), TIS (total isotropic sensitivity), efficiency, and SAR (specific absorption rate). The TRP is a measure of the radiation efficiency of an antenna; the SAR is a measure of the density of the near-field field strength as measured in human tissue adjacent to the antenna enabled device. An improvement in SAR, which is a reduction in SAR value, typically coincides with reduced radiating efficiency. It is highly desirable to develop methods to reduce SAR without impacting antenna radiating efficiency. 
     An antenna positioned on a small to moderate sized wireless device such as a cell phone, laptop, USB dongle, or data card excites the circuit board and other components of the wireless device. The near field electromagnetic field distribution and far field radiation pattern characteristics are affected by the characteristics of the wireless device. 
     In order to achieve good efficiency and SAR from an internal antenna, techniques need to be developed to reduce the amount of near field coupling of the antenna to the user while maintaining good antenna efficiency. This can be achieved by modifying the near field of the combination of the antenna and wireless device by spreading the regions of peak electric and magnetic field strength over a larger volume. This approach reduces the electromagnetic field strength per unit volume in the near field of the wireless device. If the near field distribution can be spread over a larger volume without reducing antenna efficiency then the desired outcome is achieved. 
     SUMMARY OF THE INVENTION 
     A technique has been developed to spread the near field radiated characteristics of an antenna on a small wireless device without significantly altering the far field antenna characteristics such as but not limited to, gain and efficiency. 
     In one aspect of the present invention a conductive element is positioned in close proximity to a wireless device that contains an antenna. The conductive element is dimensioned and shaped to alter the electromagnetic field of the antenna on the wireless device in such a way as to reduce the maxima and/or cause spreading of the near field distribution. The efficiency of the radiated far field of the antenna is monitored and optimized during the design process of the conductive element such that the near field distribution is altered to provide reduced SAR with minimal impact on radiated efficiency. 
     In an embodiment of the invention, distributed reactance can be designed into the conductive element and adjusted to alter the frequency response of the conductive element by spacing slotted portions at variable distances, shaping or otherwise physically altering physical characteristics of the conductive element, and similar design alternatives. The distributed reactance can be implemented in such a way as to reduce the frequency of operation of the conductive element, provide a band-pass response, or to provide low or high pass responses in terms of the frequency response of the conductive element. The distributed reactance can be adjusted to improve SAR performance at a range of frequencies while providing minimal disturbance to antenna efficiency at another range of frequencies. Alternately, lumped reactance components can be designed into the conductive element to provide the reactance to alter the frequency response of the conductive element. Lumped reactance components, or lumped components, include capacitance and inductance features lumped into a functional reactance component for use in electronics, such as an LC lumped component. 
     In another embodiment of the invention, a conductive element is configured to connect various portions of the circuit board of the wireless device. The electrical length of the conductive element can be adjusted to alter the near field distribution of the antenna operating on the wireless device. The conductive element can be separated into two or more portions and reconnected using components to adjust the frequency response. Multiple conductive elements can be connected to various locations on the circuit board of the wireless device to provide additional flexibility in terms of modifying the near field distribution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other attributes of the invention are further described in the following detailed description, particularly when reviewed in conjunction with the drawings, wherein: 
         FIG. 1  illustrates an antenna installed on the circuit board of a wireless device. 
         FIG. 2  illustrates a plot of the TRP (Total Radiated Power) and SAR (Specific Absorption Rate) of an antenna in a wireless device. The arrow illustrates a desired movement of the TRP/SAR metric to the left upper quadrant of the graph. This region maps the high TRP and low SAR region, which is the desired attributes for the antenna. 
         FIG. 3  illustrates a contour plot of the electromagnetic field in the near field of the wireless device. The field maxima is quite often not positioned directly above the antenna, but instead is positioned at other locations above the device, and is dependent on device size, frequency of operation, and other factors. 
         FIG. 4  illustrates a conductive element positioned in close proximity to the wireless device. 
         FIG. 5  illustrates a contour plot of the electromagnetic field in the near field of the wireless device with the conductive element positioned close to the device. The field maxima is reduced in value compared to the contour plot shown in  FIG. 3 . The field distribution represented by the contour plot is spread over a larger volume. 
         FIG. 6  illustrates another contour plot of the electromagnetic field in the near field of the wireless device with a conductive element positioned close to the device. The field distribution is broken into two field maxima separated in distance at different locations of the wireless device. This type of field distribution can be achieved by design of the conductive element. 
         FIG. 7  illustrates the conductive element separated into two portions to adjust the frequency response of the element. 
         FIG. 8  illustrates lumped components used to connect portions of the conductive element. The types and value of components used to connect the portions of the conductive element can be chosen to generate filters to alter the frequency response of the conductive element. 
         FIG. 9  illustrates several types of conductive elements with distributed reactance incorporated into the element. The distributed reactance can be adjusted to alter the frequency response of the conductive element. 
         FIG. 10  illustrates examples of a conductive element with a combination of lumped and distributed reactance incorporated into the element, an active component connecting two portions of the conductive element, and multiple conductive elements stacked to provide additional control of the frequency response. 
         FIG. 11  illustrates an example of a wireless device with a conductive element attached to a host device such as a laptop. 
         FIG. 12  illustrates a conductive element positioned in close proximity to a wireless device, where the conductive element is attached at one or more locations to a shield can, component, or ground layer of the circuit board. 
         FIG. 13  illustrates two conductive elements positioned in proximity to a wireless device. One conductive element is connected to a shield can and the ground layer of the circuit board of the wireless device. The second conductive element is connected to the first conductive element and the ground layer of the wireless device. 
         FIG. 14  illustrates a conductive element attached at two locations of the circuit board of the wireless device. The conductive element is positioned and attached to the circuit board to modify the electromagnetic field distribution in the near field. 
         FIG. 15  illustrates two conductive elements attached at two locations of the circuit board of the wireless device with a lumped component used to connect the conductive elements. The lumped element is used to modify the frequency response of the two conductive elements. 
         FIG. 16  illustrates a conductive element attached to two lumped components, with the lumped elements attached to the circuit board of the wireless device. The lumped elements are used to modify the frequency response of the two conductive elements. 
         FIG. 17  illustrates two conductive elements attached to four locations of the circuit board of the wireless device. The conductive elements are positioned and attached to the circuit board to modify the electromagnetic field distribution in the near field. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for purposes of explanation and not limitation, details and descriptions are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these details and descriptions. 
     Embodiments of the present invention provide for a conductive element that is dimensioned, shaped, and positioned in the vicinity of a wireless device and the antenna on the wireless device. The conductive element is designed to alter the electromagnetic field to reduce the maxima and/or cause a spreading of the field distribution in the near field of the device. The conductive element can be disconnected and then re-joined using lumped components to provide filtering in the frequency domain. Distributed reactance can be designed into the conductive element to provide filtering, and both lumped components and distributed reactance can be incorporated in the same conductive element. Active components can be coupled across portions of the conductive element to provide a dynamically tuned response to adjust the frequency response of the conductive element. Active components include capacitors, switches, varicap or varactor diodes, and the like. 
     A plurality of conductive elements can be used to reduce and/or modify the near field electromagnetic field distribution. This can be achieved by stacking multiple conductive elements or positioning multiple elements in a side by side arrangement. A plurality of conductive elements can be incorporated in a single design by both stacking and by arrangement in a side by side configuration. The conductive elements used in a single design can contain lumped components, distributed reactance, and active components for dynamic frequency tuning. 
       FIG. 1  illustrates an antenna  11  attached to the circuit board  12  of a wireless device. 
       FIG. 2  is a plot of the TRP (Total Radiated Power) and SAR (Specific Absorption Rate) of an antenna in a wireless device. The arrow  21  illustrates a desired movement of the TRP/SAR metric to the left upper quadrant  22  of the graph. This region maps the high TRP and low SAR region, which is the desired attributes for the antenna. 
       FIG. 3  illustrates an antenna  33  attached to the circuit board  31  of a wireless device, and further illustrates a contour plot of the electromagnetic field  32 . 
       FIG. 4  illustrates a conductive element  42  positioned in proximity to a wireless device antenna  43  positioned above a ground plane  41 . 
       FIG. 5  illustrates a contour plot of the electromagnetic field  53  of a wireless device  51  with a conductive element  52  in close proximity. An antenna  54  is located in proximity with the conductive element  52 . The field distribution has spread over a larger volume compared to the field distribution in  FIG. 3 , resulting in reduced field maxima for a set volume. This will result in reduced SAR for the wireless device; and therefore improvements associated with a reduced SAR in the wireless communication device are provided. The conductive element couples to the antenna element for distributing the electromagnetic field over a large volume, i.e. the circuit board and attached electronic components. 
       FIG. 6  illustrates an alternate contour plot of the electromagnetic field  62 ;  64  in the near field of the wireless device  61  with a conductive element  63  positioned close to the device. The field distribution is broken into two field maxima separated in distance at different locations of the wireless device. This type of field distribution can be achieved by design of the conductive element. An antenna  65  is located near the conductive element  63 . 
     The physical design characteristics of the conductive element can be configured to improve the function of the antenna.  FIG. 7  illustrates a conductive element separated into a first portion  72  and a second portion  73  to adjust the frequency response of the element. Second portion  73  is positioned in proximity to an antenna element  74 . The spacing between second portion  73  and the antenna along with the dimensions of second portion  73  can be adjusted to couple more or less between the antenna and second portion  73 , and can be adjusted to couple varying amounts at different frequencies. Similarly, design characteristics of first portion  72 , such as size, shape, thickness, and space between coupling regions, can be configured to vary the attributes of the antenna fields. 
       FIG. 8  illustrates a lumped component  80  used to connect the portions  78  and  79  of the conductive element. In a similar embodiment, two lumped components  81 ;  83  form a resonant circuit and are used to connect two portions  82 ;  84  of a conductive element. In yet another similar embodiment, two sets of lumped components  86  and  88  are used to connect three portions of a conductive element  85 ;  87 ;  89  to provide additional filtering and control of the frequency response. The types and value of components used to connect the portions of the conductive element can be chosen to generate filters to alter the frequency response of the conductive element. 
       FIG. 9  illustrates several types of conductive elements with distributed reactance regions incorporated into the element. The distributed reactance can be adjusted to alter the frequency response of the conductive element. In one embodiment as illustrated in  FIG. 9   a , a distributed LC section  90  is designed into a conductive element.  FIG. 9   b  illustrates two distributed LC sections  91  and  92  are designed into a single conductive element.  FIG. 9   c  illustrates a series of capacitive sections formed by coupling regions  93  designed into a conductive element. In a similar embodiment, a method to reduce the frequency of operation is illustrated in  FIG. 9   d , wherein the design  94  includes a plurality of slots incorporated into a conductive element. The distributed reactance can include a capacitive or inductive reactance generated from the designed structure. In  FIG. 9   e , another method of applying a distributed LC circuit is shown in pattern  95  containing a plurality of coils distributed along a length of the conductive element. A distributed reactance region may include a combination of capacitive sections, and inductive sections. Additionally, the distributed reactance region can be configured to function as a low pass, or high pass component section, or collectively herein referred to as a filter component. 
       FIG. 10   a  illustrates a conductive element with a combination of lumped  104  and distributed reactance  101  incorporated into a conductive element. The conductive element may further include a first portion  100 , a second portion  102 , and a connection therebetween. In  FIG. 10   b , an active component  106  is used to connect first and second portions  105 ;  107  of the conductive element to provide dynamic tuning of the conductive element. In an alternative embodiment as illustrated in  FIG. 10   c , two conductive elements  108  and  109  are stacked to provide additional control of the frequency response. The first portion can be connected to the second portion by at least one of: an inductor, capacitor, resistor, diode, transistor, RF switch, tunable capacitor, and mechanical switch, or the like. 
       FIG. 11  illustrates an example of a wireless device  114  comprising a pair of conductive elements  112 ;  113  in close proximity to the antenna, with the wireless device attached to a host device  111  such as a laptop. A user often couples to the antenna fields when using a radiator with a host device. Using this embodiment, antenna field characteristics can be optimized to overcome coupling from a user.  FIGS. 12(   a - b ) further illustrate examples of the wireless communication device for improving these antenna field parameters. 
       FIG. 12   a  illustrates two conductive elements, a first conductive portion  122  and a second conductive portion  123 , positioned in proximity to a wireless antenna device  121 . Conductive element  123  is connected to a shield can. This connection will provide a ground connection for the conductive element. In a similar embodiment as illustrated in  FIG. 12   b , conductive elements  122  and  123  are shown with multiple connections  125 ;  126 ;  127  to shield cans, components, and the ground layer of the circuit board of the wireless device, respectively. 
       FIG. 13  illustrates two conductive elements  131  and  138  positioned in proximity to a wireless antenna device  130 . Conductive element  131  includes a first portion and second portion  133  connected by a bridge component  132 . The bridge component can be an active component or a lumped component. Similarly, conductive element  138  includes a first conductive portion and a second conductive portion  134  connected by a bridge component. Conductive element  138  is connected to a shield can  136  and the ground layer  137  of the circuit board of the wireless device. Conductive element  131  is connected to conductive element  138  with connection  139  and is connected to the ground layer  137  of the wireless device. 
       FIG. 14  illustrates a conductive element  142  attached to a circuit board of a wireless device at a first portion  141  and a second portion  143  separated by an etched portion  149 . The conductive element is positioned and attached to the circuit board to modify the electromagnetic field distribution in the near field. In this regard, the circuit board can include an etched portion  149 , and the conductive element  142  can be connected across the etched portion  149 . 
       FIG. 15  illustrates two conductive elements  154  and  155  attached to a first portion  151  and a second portion  153  of the circuit board of the wireless device with a lumped component  152  used to connect the conductive elements. The two portions  151  and  153 , respectively, are separated by an etched portion  159  of the circuit board wherein a volume of the circuit board is removed to form the etched portion. The lumped element  152  is used to modify the frequency response of the two conductive elements  154  and  155 . The conductive elements  154  and  155  are positioned and attached to the circuit board to modify the electromagnetic field distribution in the near field. 
       FIG. 16  illustrates a conductive element  162  attached to two lumped components  161  and  163 , with the lumped elements attached to the circuit board of the wireless device. The two lumped elements  161  and  163  are connected to the circuit board at opposing portions of the circuit board separated by an etched portion  159 , wherein a volume of the circuit board is removed to form the etched portion. The lumped elements  161  and  163  are used to modify the frequency response the conductive element  162 . The conductive element  162  is positioned and attached to the lumped elements  161  and  163  to modify the electromagnetic field distribution in the near field. 
       FIG. 17  illustrates two conductive elements  172  and  175  attached to four locations  170 ,  171 ,  173 , and  174  of the circuit board of the wireless device. The conductive elements  172  and  175  are positioned and attached to the circuit board to modify the electromagnetic field distribution in the near field. 
     In certain embodiments, an antenna system for use within a wireless device can comprise a circuit board having a first side and a second side disposed opposite one another about a width of the circuit board. The circuit board may further comprise a first portion extending outwardly along a length of the circuit board at the first side and a second portion extending outwardly along the length of the circuit board at the second side. In this regard, the first and second portions extend from the circuit board and are parallel with respect to one another. An etched portion is formed between the first and second portions, and comprises a volume of the circuit board which has been removed. One or more conductive elements can be coupled to the first and second portions, respectively, such that the conductive elements extend across the etched portion. One or more lumped components can be connected to the one or more conductive elements, respectively. The one or more lumped elements may be coupled to the first and/or second portions of the circuit board separated by the etched portion. An antenna element is positioned above the circuit board. In this regard, the antenna element, conductive elements, lumped components, and etched portion are adapted to modify the electromagnetic field distribution in the near field. The etched portion may also be referred to as a “void”. 
     In the forgoing description of the invention, a number of embodiments are described, each being capable of modifying electromagnetic field characteristics in the antenna near field, without significant effect on far fields. These and similar embodiments can be used to reduce the SAR, and therefore improve antenna quality. 
     The above examples are set forth for illustrative purposes and are not intended to limit the spirit and scope of the invention. One having skill in the art will recognize that deviations from the aforementioned examples can be created which substantially perform the same functions and obtain similar results.