Abstract:
A method of configuring a small wireless communication system is described for body wearable applications where the radiated power from the antenna in the direction of the user is reduced. A method of coupling resonators, which are formed by planar conductors as well as linear conductors, is described where the resonators extend the electrical extent of the ground plane of a small circuit board resulting in improved antenna efficiency and bandwidth characteristics from an antenna coupled or connected to the ground plane. The resonators can be configured to modify the near-field distribution of the antenna radiation pattern of the antenna coupled to the small ground plane which will result in less power radiated in the volume behind the top surface of the ground plane the antenna is coupled to. A technique is also described wherein the antenna element is integrated onto the top surface of a packaged RFIC, with the RFIC containing the transceiver circuit. The technique is optimal for providing efficiency improvements for antennas on electrically small ground planes. A method of forming slots from planar conductors is described where the radiated field from the slot is combined with the radiated field from an antenna coupled to a small ground plane which can result in improved efficiency and/or modified polarization properties from the combination of antenna, resonator, and ground plane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of priority with U.S. Provisional Ser. No. 61/955,050, filed Mar. 18, 2015, titled “RF CIRCUIT AND ANTENNA COMBINATION FOR BODY WEARABLE WIRELESS COMMUNICATION SYSTEMS”; the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to the field of wireless communication. In particular, the invention relates to techniques for improving antenna system performance of antennas coupled to electrically small ground planes when implemented in wearable applications. 
     BACKGROUND OF THE INVENTION 
     Reduced size wireless communication systems designed for wearable or body worn applications are gaining acceptance for a wide variety of applications. Medical devices which monitor a patient&#39;s health and transmit in real-time diagnostic information is one example. Another example is a tracking system comprised of a GPS receiver and a cellular transceiver packaged in a small form factor to allow for convenient and non-intrusive attachment to a person, animal, or small asset. The tracking system uses GPS to determine location which is then sent out over the cellular network to provide real-time or near real-time asset tracking. Advances in transceiver and baseband design along with reduced size RF communication circuits allow for a small communication system to be assembled on a printed circuit board (PCB) and packaged in a plastic housing to provide a small form factor, robust communication device for body worn applications. For thee cellular, WiFi, and GPS transceiver and receiver circuits a small antenna is required to transmit and receive the RF signal. 
     The small PCB dimensions required for a small communication system for body worn devices can cause issues in regards to antenna design, placement, and performance. Internal antennas used for wireless applications are affected by the dimensions of the ground plane that the antenna operates off of. The circuit board of the wireless device is typically used as the ground plane for the antenna. Typically the circuit board dimensions are chosen to accommodate the wireless circuit and cannot be optimized for antenna performance. For common antenna types that are used for internal antennas in wireless mobile communications such as monopoles, IFAs, PIFAs, and isolated magnetic dipoles (IMD) a ground plane that is one quarter wavelength in extent is optimal for antenna performance. Odd multiples of one-quarter wavelength will also provide for optimal impedance and efficiency characteristics for these internal antenna types. As the linear dimensions of a ground plane, the length and width, decrease below a quarter wavelength the bandwidth and the efficiency of an antenna coupled to this ground plane will decrease. When the length or width of the ground plane drops below one-tenth of a wavelength the ground plane can be designated as being electrically small. If an antenna type requires a ground plane for operation, then the bandwidth and efficiency of an antenna coupled to an electrically small ground plane will be reduced due to an inadequate electrical extent presented by the small ground plane. 
     When a multi-frequency band antenna is designed for use on a small ground plane, the lowest frequency band of interest will be the most impacted by the small area and small length and width provided by the ground plane. As the upper frequency bands are considered improved antenna efficiency and bandwidth will be realized compared to the lowest frequency of operation. An antenna coupled to a small ground plane has the potential of radiating a substantial amount of power behind the ground plane, due to the reduced electrical size of the ground plane. If the antenna and ground plane are positioned in close proximity to a user, as in the case of a body worn wireless communication device, a substantial portion of the radiated power can couple to the user&#39;s body. This can result in reduced communication link efficiency between the body worn communication system and the other end of the communication link such as a cellular base station or WLAN access point. This can also result in increased power absorbed by the user, which in turn results in increased Specific Absorption Rate (SAR) levels coupled. 
     SUMMARY 
     An antenna system and related method of configuring a small wireless communication system is described for body wearable applications where the radiated power from the antenna in the direction of the user is reduced. An antenna system and related method of coupling resonators, which are formed by planar conductors as well as linear conductors, is described where the resonators extend the electrical extent of the ground plane of a small circuit board resulting in improved antenna efficiency and bandwidth characteristics from an antenna coupled or connected to the ground plane. The resonators can be configured to modify the near-field distribution of the antenna radiation pattern of the antenna coupled to the small ground plane which will result in less power radiated in the volume behind the top surface of the ground plane the antenna is coupled to. A technique is also described wherein the antenna element is integrated onto the top surface of a packaged RFIC, with the RFIC containing the transceiver circuit. The technique is optimal for providing efficiency improvements for antennas on electrically small ground planes. An antenna system and related method of forming slots from planar conductors is described where the radiated field from the slot is combined with the radiated field from an antenna coupled to a small ground plane which can result in improved efficiency and/or modified polarization properties from the combination of antenna, resonator, and ground plane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a user with multiple body worn communication systems. 
         FIG. 2  illustrates an antenna with a resonator coupled to a ground plane to optimize current flow from the induced fields of the radiating antenna. 
         FIG. 3  illustrates an antenna with a resonator coupled to an antenna feed to contain electric field distribution to the plane of the circuit board. 
         FIG. 4  illustrates an antenna with a resonator coupled to the ground plane to alter current distribution by generating a resonant slot. 
         FIG. 5  illustrates an antenna with an alternate slot placement in relation to the antenna. 
         FIG. 6  illustrates an antenna with a slot created by the planar resonator and ground plane positioned to provide an alternate polarization compared to the antenna element. 
         FIG. 7A  shows the radiation patterns of an antenna without a slot. 
         FIG. 7B  shows the radiation patterns of the antenna as altered by introduction of a slot. 
         FIG. 8  illustrates an antenna with a tuning component used to alter the electrical length or impedance of the resonator. 
         FIG. 9  illustrates an antenna wherein a switch is used to connect the resonator to ground through a lumped reactance. 
         FIGS. 10 (A-B) illustrate a practical implementation of a resonating element coupled to an electrically small ground plane from a top plan view and a perspective view, respectively. 
         FIG. 11  illustrates a practical implementation of a resonating element coupled to an electrically small ground plane. 
         FIG. 12  shows a plot of measured return loss for several configurations of the antenna and planar conductor shown in  FIG. 10 . 
         FIG. 13  shows a plot of measured efficiency for several configurations of the antenna and planar conductor shown in  FIG. 10 . 
         FIG. 14  illustrates a two slot configuration where two planar conductors are used to form two slot regions to shape the radiated field characteristics. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The disclosure concerns an antenna system and method of designing a resonator for use with an antenna coupled to a small ground plane. The resonator can be coupled or connected to either the ground plane or the antenna feed point. Also described is a method of integrating the antenna onto the top surface of a packaged radiofrequency integrated circuit (RFIC), which results in a reduced volume form factor for a wireless communication system comprised of a transceiver in RFIC form, battery, PCB, and antenna. By designing the resonator and antenna as a system or combination of elements, improved antenna efficiency can be achieved. The designing of the antenna and resonator in unison can result in a design that will couple less to the body of a user when the communication system formed by the antenna and resonator along with a transceiver on a small PCB is used in a body worn wireless application. Novel resonator and ground plan configurations are described where improved antenna performance is achieved without increasing the antenna dimensions or the ground plane dimensions. 
     One embodiment is described wherein a planar antenna element is designed and implemented such that it is positioned directly on top of the packaging of an RFIC. The transceiver for a wireless communication system can be designed into the RFIC and the antenna positioned on the top surface of the packaging material can be used as the antenna to connect to the transceiver for use in the wireless communication system. A reduced volume solution is obtained by the combination of the transceiver RFIC and antenna which allows for a smaller PCB. This technique will greatly aid in size reducing the wireless communication system for a body worn communication application. 
     Another embodiment is described wherein one end of a conductor is connected or coupled to a ground plane and the conductor is positioned in close proximity to the ground plane. An antenna is positioned on the top surface of the transceiver RFIC package, with this transceiver/antenna assembly connected or coupled to the ground plane of the PCB. The second end of the conductor is not connected to either the ground plane or the antenna, but is left disconnected. The length of the conductor and the proximity of the conductor in relation to the ground plane is chosen to increase the impedance match and/or bandwidth characteristics of the antenna connected or coupled to the ground plane. Also proper placement and excitation of the conductor can reduce the near-field EM (electromagnetic) distribution behind the ground plane to minimize coupling of power to objects in the region behind the ground plane. This is achieved by forming a slot region between the conductor and an edge or multiple edges of the ground plane. The slot can be enclosed on both edges or can be open on one side. Proper placement of the conductor and dimensioning of the conductor to control the length of the slot will result in a strong electric field distribution in the slot region, with the electric field components predominantly formed in the plane of the ground plane. More power will be coupled to the slot region and radiated and less power will be directly radiated by the antenna element, resulting in less power coupled to a user positioned directly behind the ground plane. 
     Another embodiment is described wherein the conductor is connected or coupled to the ground plane, and a transceiver/antenna assembly is connected or coupled to the same ground plane of the PCB. The conductor is positioned in proximity to the antenna element to allow for the antenna to induce a current directly onto the conductor. The induced current on the conductor alters the current density on the ground plane. Proper selection of the conductor length, position to the antenna, and grounding location on the ground plane results in increased antenna efficiency, impedance match, and/or bandwidth. Again, as previously described, proper placement and excitation of the conductor can reduce the near-field EM (electromagnetic) distribution behind the ground plane to minimize coupling of power to objects in the region behind the ground plane. This is achieved by forming a slot region between the conductor and an edge or multiple edges of the ground plane. 
     In another embodiment, one end of a conductor is connected or coupled to the feed point of an antenna, with the antenna positioned on the top surface of the packaging of the transceiver RFIC and the transceiver/antenna assembly connected or coupled to a ground plane of the PCB. The second end of the conductor is not connected to either the ground plane or antenna. The conductor can be positioned in close proximity to the ground plane to minimize the increase in volume of the ground plane/antenna/conductor assembly. Excitation of the antenna in either the transmit or receive mode results in RF currents flowing on the connected or coupled conductor. The conductor provides the mechanism of increasing the antenna aperture. 
     In another embodiment, the conductor can be configured in a planar fashion to increase the cross sectional area available for current flow. One end of the planar conductor is connected to the ground plane with a major portion of the planar conductor running along one edge of the ground plane, forming a slot region between the planar conductor and the ground plane. The slot formed by the planar conductor and the ground plane is dimensioned and positioned in relation to the antenna that is coupled or connected to the ground plane to support an electric field distribution that radiates. Proper antenna design and slot design can result in improved total radiation performance from the antenna, resonator, ground plane combination. 
     In another embodiment, a planar conductor can be configured to form a slot with the ground plane and an antenna coupled or connected to the ground plane can be positioned to optimally excite an electric field distribution in the slot region. The slot can be positioned along one or multiple sides of a square, rectangular, triangular shaped PCB such that the polarization state of the radiation from the slot is different than the polarization state of the antenna coupled to the ground plane. The resultant composite radiated field will provide far-field components with elliptical or circular polarization. 
     Now turning to the drawings,  FIG. 1  illustrates a user  10  with multiple body worn communication systems  11 . A wireless communication device  20  in the form of a wristwatch is shown. Also show is a transceiver  30  with antenna in module form and in detail. The detailed illustration shows an antenna  33  integrated on the transceiver packaging  32 , an electrically small PCB  31  with ground plane layer, and a resonator  34  which forms a slot region in conjunction with the PCB. An electric field distribution  35  is contained in the slot region. 
       FIG. 2  illustrates an antenna  33  connected to an electrically small circuit board with ground plane  31 . A transceiver chipset  32  is connected to the feed point of the antenna. One end of a conductor  34  is connected to the ground pad  36  of the ground plane and the second end of the conductor is left un-terminated. The conductor is positioned external to one or several edges of the ground plane  31 . The electrically small circuit board with ground plane is disposed on a portion of a user&#39;s body  37  or other object which the antenna system is positioned near. In this regard, the resonator is coupled to the ground plane to optimize current flow from induced fields from the radiating antenna. The antenna is integrated onto the transceiver chipset to reduce overall device size. 
       FIG. 3  illustrates an antenna  33  connected to an electrically small ground plane  31 . A transceiver chipset  32  is connected to the feed point  38  of the antenna. One end of a conductor  34  is connected to the feed point  38  of the antenna and the second end of the conductor is left un-terminated. The conductor  34  is positioned external to one or several edges of the ground plane  31 . The conductor being coupled to the feed point of the antenna can be referred to as a “resonator”, and the resonator is coupled to the antenna feed to contain the electric field distribution to the plane of the circuit board and to increase the electrical length of the antenna. 
       FIG. 4  illustrates an antenna  33  connected to an electrically small ground plane  31 . A transceiver chipset  32  is connected to the feed point of the antenna. One end of a planar conductor  34  is connected to the ground of the ground plane  31  and the second end of the planar conductor is left un-terminated. The planar conductor is positioned parallel to one or several edges of the ground plane, with a slot region being formed by the planar conductor and ground plane. An electric field distribution  35  is generated in the slot region when the antenna is excited. 
       FIG. 5  illustrates an antenna  33  connected to an electrically small ground plane  31 . A transceiver chipset  32  is connected to the feed point of the antenna. One end of a planar conductor  34  also referred to as a “resonator” is connected to the ground of the ground plane  31  and the second end of the planar conductor is left un-terminated. The planar conductor  34  is positioned parallel to one or several edges of the ground plane  31 , with a slot region being formed by the planar conductor  34  and ground plane  31 . The planar conductor  34  is positioned in close proximity to the antenna  33  to provide for improved coupling of the antenna to the planar conductor. An electric field distribution  35   a ;  35   b  is generated in the slot regions when the antenna is excited. The slots are designed to be placed with relation to the antenna. Radiation from the resonant slot regions combine with the radiated fields of the antenna. 
       FIG. 6  illustrates an antenna  33  connected to an electrically small ground plane  31 . A transceiver chipset  32  is connected to the feed point of the antenna. One end of a planar conductor  34  is connected to the ground of the ground plane  31  and the second end of the planar conductor is left un-terminated. The planar conductor is positioned parallel to one edge of the ground plane, with a slot region being formed by the planar conductor  34  and ground plane  31 . An electric field distribution  35  is generated in the slot regions when the antenna is excited. Radiation from the slot region has a polarization state (PVsr) that is different from the polarization state from the antenna (PVae). The slot created by the planar conductor  34  and ground plane  31  is positioned to provide an alternate polarization compared to the polarization of the antenna element. The radiation from the resonant slot combines with the radiated fields of the antenna. 
       FIG. 7A  illustrates the radiation pattern generated from an antenna integrated into the packaging of an RFIC, with RFIC and antenna connected to a small PCB. Also illustrated in  FIG. 7B  is the radiation patterns generated when a resonator is applied to the assembly and a slot is formed and an electric field distribution is contained in the slot region. 
       FIG. 8  illustrates an antenna  33  connected to an electrically small ground plane  31 . A transceiver chipset  32  is connected to the feed point of the antenna. One end of a conductor  34  is connected to the feed point of the antenna and the second end of the conductor is left un-terminated. The conductor is positioned external to one or several edges of the ground plane. A tuning component  39  is connected or coupled to the conductor  34  and is used to alter the electrical length and/or impedance of the conductor. The tuning component is shown connected to the transceiver chipset  32  via control line  40 . Varying the tuning component characteristics results in a frequency shift of the radiated fields from the antenna. 
       FIG. 9  illustrates a tuning configuration applied to a resonator  34  which is integrated into a wireless communication system formed by an antenna  33 /transceiver chipset  32  combination connected to a small PCB  31 . The tuning circuit  39   b  is comprised of a four port switch  41  with reactive loads  42  attached to the four ports and the common port of the switch attached to the resonator  34 . The second end of the reactive loads are each connected to ground  31 . By switching from one reactive load on one port to another reactive load on a second port, the frequency response can be altered as shown in the plot of efficiency as a function of frequency. In this regard, a switch is used to connect the resonator  34  to ground  31  through a lumped reactance. 
       FIGS. 10 (A-B) illustrate a practical implementation of a resonating element coupled to an electrically small ground plane. A triangular ground plane  101  is shown with a battery  102  connected to the circuitry on the ground plane. An antenna  103  is connected to a transceiver  104  and one end of a wire conductor  105  is connected to the ground plane. 
       FIG. 11  illustrates a practical implementation of a resonating element coupled to an electrically small ground plane. A triangular ground plane  101  is shown with a battery  102  connected to the circuitry  106  on the ground plane. An antenna  103  is connected to a transceiver  104  and one end of a planar conductor  105   b  is connected to the ground plane. A slot region  107  is formed between the planar conductor and the ground plane. 
       FIG. 12  shows a plot of measured return loss for several configurations of the antenna and planar conductor shown in  FIG. 10 . As can be seen in the plot, the return loss is substantially improved when the wire resonator is attached to the ground plane. 
       FIG. 13  shows a plot of measured efficiency for several configurations of the antenna and planar conductor shown in  FIG. 10 . As can be seen in the plot, the efficiency is substantially improved when the wire resonator is attached to the ground plane. 
       FIG. 14  illustrates a two slot configuration where two planar conductors are  34   a ;  34   b  used to form two slot regions to shape the radiated field characteristics. Shown is an electrically small ground plane  31 ; a transceiver chipset  32 ; antenna  33 ; first planar conductor  34   a ; first electric field  35   a ; second planar conductor  34   b ; second electric field  35   b ; and a portion of a user&#39;s body  37  or other object the antenna system is positioned near.