Patent Publication Number: US-2009231204-A1

Title: Miniature antenna for wireless communications

Description:
FIELD OF INVENTION 
     The present invention relates to antenna system, and more specifically to antennas for wireless communications, such as hearing aid, wireless implants and on-body based communication 
     BACKGROUND OF THE INVENTION 
     Medical applications having communication capabilities are well known in the art. One of the applications is a hearing aid application. An antenna design is generally an important factor of its performance of the application. In antenna design for the medical applications, especially hearing aid application, it is challenging to design miniaturized and efficient antenna close to a human body. Electrically small antennas generally have high losses and require more powerful transmitters and complex high sensitivity receivers for satisfactory performance. The antennas need to meet the impedance requirements of receiver input and transmitter output. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a system and method that obviates or mitigates at least one of the disadvantages of existing systems. 
     In accordance with an aspect of the present invention, there is provided a method of direct matching an antenna to a transceiver. The method includes designing the antenna to directly match an antenna impedance to at least one of an input impedance of the transceiver and an output impedance of the transceiver. The designing includes modeling the antenna and the transceiver; and implementing an electromagnetic field simulation using a human body phantom model with the antenna model to determine the value of an antenna parameter for the antenna model. 
     In accordance with another aspect of the present invention, there is provided an antenna for a communication device having a transceiver. The antenna includes an antenna element directly coupled with the transceiver having a transmitter and a receiver, an antenna parameter of the antenna element being tuned so that the real part of the impedance of the antenna is maximized: and a plate for optimizing the reactive part of the impedance of the antenna. The impedance of the antenna being directly matched to at least one of an impedance of the transmitter and an impedance of the receiver. 
     In accordance with another aspect of the present invention, there is provided a method for antenna design. The method includes providing estimate of a package, designing possible realization(s) of the antenna given the space limitations of the package to realize maximum power transfer around the head, for a given design of LNA and PA, generating power efficiency maps for all possible bias realizations versus all possible impedance values of the antenna; and modifying the antenna design in order to maximize the overall link efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein: 
         FIG. 1  is a diagram illustrating a human body phantom with an antenna in accordance with an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating the human body phantom with the hearing aid packaged placed in ear; 
         FIG. 3  is a diagram illustrating admittance definitions for an LNA with bias circuits and an antenna; 
         FIG. 4  is a diagram illustrating admittance for the antenna of  FIG. 3  with a matching inductor; 
         FIGS. 5A-5B  are diagrams illustrating a model for transmit and receive sections in accordance with an embodiment of the present invention; 
         FIG. 6  is a diagram illustrating an antenna model with a bias circuit in accordance with an embodiment of the present invention; 
         FIG. 7  is a diagram illustrating a model for the antenna of  FIG. 6  and a LNA in accordance with an embodiment of the present invention; 
         FIG. 8  is a diagram illustrating a reduced circuit model for the antenna of  FIG. 7 ; 
         FIGS. 9A-9F  are graphs illustrating examples of efficiency maps in accordance with an embodiment of the present invention; 
         FIG. 10  is a graph illustrating measured admittance elements for the LNA without an external bias circuit; 
         FIG. 11  is a graph illustrating one example of the admittance parameters of a designed antenna; 
         FIG. 12  is a graph illustrating another example of the admittance parameters of a designed antenna; 
         FIG. 13  is a diagram for calculating the input bandwidth as seen by the antenna in accordance with an embodiment of the present invention; 
         FIG. 14  is a graph illustrating input return loss as seen by the antenna; 
         FIG. 15  is a view illustrating one example of the antenna of  FIG. 1 ; 
         FIG. 16  is a view illustrating another example of the antenna of  FIG. 1 ; 
         FIG. 17  is a view illustrating a further example of the antenna of  FIG. 1 ; 
         FIG. 18  is a view illustrating a further example of the antenna of  FIG. 1 ; 
         FIG. 19A  is a top view illustrating one example of an antenna layout for the antenna of  FIG. 1 ; 
         FIG. 19B  is a cross view for the antenna of  FIG. 19A ; 
         FIG. 20A  is a top view illustrating another example of an antenna layout for the antenna of  FIG. 1 ; 
         FIG. 20B  is a cross view for the antenna of  FIG. 20A ; 
         FIG. 21  is a perspective view of one example of a hearing aid in accordance with an embodiment of the invention; 
         FIG. 22  is an exploded view of the hearing aid of  FIG. 21 ; 
         FIG. 23  is a side view of the hearing aid of  FIG. 21 , with an example of excitation points; 
         FIG. 24  is a side view of the hearing aid of  FIG. 21 , with another example of excitation points; and 
         FIG. 25  is a flow chart showing a method of designing an antenna in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a human body phantom with an antenna in accordance with an embodiment of the present invention. In  FIG. 1 , a hearing aid model  10  having an antenna model  12  and a transceiver model  14  is shown with a human body phantom  2 . In the embodiment, an antenna is designed through an electromagnetic field simulation with the human body phantom  2 . 
     The transceiver  14  includes a transmitter  16  and a receiver  18 . The transmitter  16  includes a power amplifier (PA)  20 . The receiver  18  includes a low noise amplifier (LNA)  22 . The resultant antenna may be detachably connected to the transceiver though a port ( 24 ). The antenna  12  and the transceiver  14  are enclosed in a package  26 . The antenna  12  and the transceiver  14  each may have a package. Each of the LNA and the PA may be on-chip amplifier. 
     In the description, the terms “antenna model” and “antenna” may be used interchangeably. In the description, the terms “hearing aid model” and “hearing aid” may be used interchangeably. In the description, the terms “human body”, “living body”, “body” and “user&#39;s body” are used interchangeably, and indicate a body of a living matter, such as an animal or a human&#39;s body. In the description, the term “body” may indicate a part of the body or a whole body. In the description, the terms “connect (connected)” and “couple (coupled)” may be used interchangeably. In the description below, the terms “antenna” and “antenna device” may be used interchangeably. 
     In one example, the hearing aid  10  may be placed to the back of each ear of the human head. In another example, the hearing aid  10  may be placed in each ear as shown in  FIG. 2 . 
     By using a paired set of hearing aid devices  10 , enabling communication with each other, the set can maintain proper interpretation of the location of various sounds in the environment. The hearing aid devices can then coordinate the action of the directional, noise-reduction, feedback-cancellation, and compression systems to provide the train with a preserved set of pulses enabling it to re-create the asymmetric world of sound around the user of the hearing aid devices, despite his/her hearing loss asymmetry. 
     In the embodiment, the antenna is designed to use the human head as a part of the transmission medium The impedance of the antenna is tuned based on the human head properties. The antenna is first designed to maximize power transfer around the head, thus its impedance is tuned based on the human head properties. The antenna is then modified to realize maximization of a power transfer and matching to active circuitry (PA and LNA). 
     The human body phantom model  2  is used in Finite Element Simulations (FEM) for characterization of the electromagnetic propagation properties around the human head. The model is defined by, for example, an effective dielectric permittivity, permeability, and conductivity. In one example, a six layer head model (brain, cerebro spinal fluid, dura, bone, fat, skin) is used in the electromagnetic field simulations. Table 1 shows one example of the six layer head model. A simple spherical model is used, where the head is modeled as 6 different layers. The outer skin layer was changed in simulations to account for common differences in human heads, and also for different skin conditions, i.e., dry skin, oily skin, etc. The antenna (with package), is then placed around the human head. Simulations for different antennas are done to realize the best possible layout. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Six Layer Head Model 
               
            
           
           
               
               
               
               
            
               
                   
                 Relative 
                 Conductivity 
                 Radius 
               
               
                 Material 
                 Permittivity 
                 s/m 
                 mm 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Brain 
                 49.7 
                 0.59 
                 67.23 
               
               
                 Cerebro Spinal Fluid 
                 71 
                 2.25 
                 68.89 
               
               
                 Dura 
                 46.7 
                 0.83 
                 69.305 
               
               
                 Bone 
                 13.1 
                 0.09 
                 72.708 
               
               
                 Fat 
                 11.6 
                 0.08 
                 73.87 
               
               
                 Skin 
                 46.7 
                 0.69 
                 74.7 
               
               
                   
               
            
           
         
       
     
     In one embodiment, an antenna is designed so as to have no external matching elements added to the network (direct matching). In another embodiment, an antenna is designed so as to have one matching element added (i.e, inductor or capacitor). 
     In the embodiment, the transceiver  14  and the antenna  12  are directly coupled to each other. The antenna is designed by incorporating direct matching technique. The antenna is not designed to be matched to the traditional 50 Ohms impedance. Instead, the antenna is designed to be matched to a driving chip impedance, without adding any matching network. The driving chip impedance may be the output impedance of the transmitter (e.g., the impedance of the PA chip  20 ), the input impedance of the receiver (e.g., the impedance of the LNA chip  22 ) or a combination thereof. 
     The antenna  12  is directly matched to, for example, but not limited to, a chipset designed to operate at the industrial, scientific and medical (ISM) band. However, the direct matching scheme can be used for direct matching of the antenna to the driving circuitry at any other band, extending its applicability to systems such as RFIDs and GPS circuits. 
     A part of the impedance matching is integrated with the antenna structure. This enhances the efficiency of the antenna because of the larger area of such antenna-integrated elements. Given the impedance of LNA or PA, the antenna is designed such that its impedance is matched to the active chipset. Part of the matching is realized using the bias elements as described below. The rest of it is lumped into the antenna inductance/capacitance. 
     The antenna is designed and optimized such that it couples maximum energy to another antenna on a symmetric location around the human head (e.g., behind the ear) as shown in  FIG. 1 . The optimization process includes, for example, incorporating all packaging effects These effects are found by comparing an antenna without package to that with a package. The optimization maximizes the real part of the input impedance of the antenna  12 . The reactive part of the input impedance of the antenna  12  is optimized utilizing a floating sheet metallization for reactance tuning. In one example, the floating sheet metallization is implemented by a shield-like metallic plate so as to meet the values dedicated by efficiency maps. 
     In one example, the floating sheet metallization is implemented by a shield-like metallic plate. The shield-like metallic plate is placed in the antenna and is used in facilitating matching to the given chip impedance (e.g. impedance for LNA chipset, PA chipset or a combination thereof). 
     The efficiency maps are theoretical three dimensional maps (i.e.,  FIGS. 9A-9F ) as described below, where when determining a bias inductor with a Q factor, ranges for the efficiency of the overall system can be directly calculated, dedicating the values of the antenna resistance and reactance corresponding to any efficiency value. These maps are utilized along with maximizing the electromagnetic radiation from one antenna to the other, to maximize the overall system efficiency. The maps are used as described below and illustrated in  FIG. 25 . 
     The efficiency maps were studied for the cases of adding one matching element to the circuitry as described below, and for the cases where direct matching is applied without need for any matching network. As described above, there are two possible scenarios for matching: one is to have no external matching elements added to the network (direct matching), and the other is to have one matching element added (i.e., inductor or capacitor.) Efficiency maps are utilized in both scenarios. 
     Thus, the antenna is designed to maximize both the circuit efficiency and electromagnetic link efficiency with direct matching of the antenna  12  to the circuitry, e.g., active circuitry. 
     The resultant antenna includes a shield-like metallic plate, which is used in facilitating matching to the given chip impedance (e.g. impedance for LNA chipset, PA chipset or a combination thereof). 
     The antenna is designed on three dimensional flexible materials conforming to the hearing aid package  26 . The examples of the packaging are shown in  FIGS. 21-24  and described below. 
     The direct matching technique is described in detail.  FIGS. 3-4  illustrate one approach to match an antenna to a LNA. Referring to  FIGS. 3-4 , one approach to match an antenna  30  to a LNA  32  is to use bias inductors  34  to achieve parallel resonance (anti-resonance) of the input impedance at the terminals of the bias-LNA circuitry for a desired frequency. This is done by Im (Y BC )=0 in  FIG. 3 . Next the antenna is designed such that the real part of its admittance is the same as that of the bias-LNA at resonance. The capacitive part of the antenna admittance is then removed by adding an inductor  36  to resonate the antenna as well at the resonant frequency as shown in  FIG. 4 . It is assumed that when connecting of both of the antenna with its matching inductor  36  and the bias-LNA circuit, that matching between the antenna and the LNA  32  can be achieved. 
     By contrast, in an embodiment of the present invention, instead of using a matching network, the matching is inherently embedded into the antenna  12  of  FIG. 1 , resulting in eliminating the need for an extra matching element (e.g., matching inductor  36  of  FIG. 4 ) on the antenna  12 . 
     In one embodiment, the model  12 A of  FIG. 5  is used for the design of the antenna, in order to assess the communication link featuring direct matching. The antenna model  12 A is connectable to the LNA  22  and PA  20 . In the model  12 A, a bias circuit  40  is on the antenna side. In one simulation, the antenna model  12 A is replaced with its equivalent admittance as shown in  FIG. 6 . In the simulation, the PA  20  and the LNA  22  of  FIG. 5  are replaced with their equivalent admittances. For example, the model of  FIG. 5  is modified as shown in  FIG. 7  using a LNA  22 A.  FIG. 8  illustrates a reduced circuit for the antenna  12 A with the bias circuit  40  for the LNA  22 A. The bias circuit is modeled on the antenna side as a parallel inductor and its associated resistance. 
     For example, in order to match the LNA  22 A to the antenna  12 A for maximum power transfer, the admittance Y AB  for antenna element and the bias circuit  40  meets: 
         Y   AB   =Y*   C   (1) 
     where Yc is the admittance for the LNA  22 A and the “*” denotes a complex conjugate. 
     By investigating the imaginary parts of (1), the following equations are set: 
         jwC   A +1 /jwL′=−jwCc   (2) 
         L′= 1 /{w   2 ( Cc+C   A )}  (3) 
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     where “L′” represents the reactive part of the impedance for the bias circuit  40 , and Q is the quality factor of the bias inductor LB. 
     Hence the following equation is obtained: 
         L   B =*½*1/{(1 /Q ) 2 +1}*1 /w   2 *1/( C   C   +C   A )  (7) 
     This relation is used to find the bias inductance needed as the first step on matching the antenna  12 A to the LNA  22 A. The next step in ensuring matching is to have equal real parts of the admittances. This is done by: 
       1 /R   A +1 /R′= 1 /R   C   (8) 
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     and where “R′” represents the resistive part of the impedance for the bias circuit  40 . 
     Using L′ and R′, the antenna impedance Za can be expressed. The efficiency maps of  FIGS. 9A-9F  show the relationship between the imaginary part of an antenna impedance, im(Za), and the real part of the antenna impedance, re(Za), by changing the values. 
     The efficiency maps coupled with Table 1 of the simulated performance of antennas around the human head serve in predicting the performance of the system in terms of power transmission, sensitivity to variation in circuit elements, and sensitivity to variations in the human head. Higher bias inductor values may degrade the circuit overall power transfer when a small antenna is directly connected to the active circuitry as shown in the efficiency maps in  FIGS. 9A-9F . 
     Antenna design examples are described in detail. Given the measured admittance parameters of the LNA ( FIG. 10 ) between 200 MHz and 600 MHz, a small antenna with a size of about one twentieth of the wavelength covering both the transmit and receive bands of the 400 MHz ISM band (400-410 MHz) was designed for direct connection to the active circuitry. 
     Inspecting the measured results of the LNA chipset at the mid-band (405 MHz), Rc=17045 [Ω] and Cc =−6.779e −13 [F]. The admittance parameters of a designed antenna for a bias of 50 nH and Q=30, yielding a 10% circuit efficiency are R A ′=18036 [Ω] and C A =1.016577e −12 [F], that is the antenna impedance of Za=Ra+jXa=88.973−j402.2[Ω]. Accounting for the circuit efficiency, such antenna is capable of receiving 1.0729e-6[W] for 1 Watt source, if connected directly to the PA and LNA on the transmit and receive sides respectively.  FIG. 1  shows the admittance parameter of a designed antenna with R A =18036 [Ω] and Cc=9.76577e −13 [F]. 
     Assuming a typical conductor quality factor of 50, an inductor of LB is 45.52e −9 [H] to achieve resonance (Im(Y)=0). If the quality factor is taken into consideration, R B  is 2.345[Ω]. The antenna will see a conductance of 1/Rc+1/R′, and thus mismatch will occur at the desired frequency. In particular, if R′=11736[H], the overall resistance of Rc//R′=6950[Ω] instead of 18036. 
     The antenna is first designed to maximize power transfer around the head, given a bias value, and ignoring the quality factor of the bias inductor. Thus, for a realistic system, the antenna may be mismatched due to the effect of the Q factor of the inductor. Thus, an iterative design is applied to match a given antenna to the LNA with real world bias network. 
     If the value of Rc//R′=6950[Ω] is a next iteration design target for R A  and knowing the for small antenna, the value of Cc does not suffer a huge shift, another design of R A =6978.58[Ω] and Cc=1.206e −12 [F] is obtained.  FIG. 12  illustrates admittance plots for another designed antenna. 
     These values require a bias and matching inductor of L B =39.97e −9 [H] with R B =2.0594[Ω], yielding Rc//R′=6422.39[Ω]. It can be seen that this value is sufficient to achieve matching to the re-designed antenna of R A =6978.58[Ω]. 
       FIG. 13  illustrates a schematic for calculating the return loss using the antenna with the LNA. The return loss calculated is defined by: 
         S   11 [dB]=20 log {{(1 /R   A )− Ym )/((1 /R   A )+ Ym )}  (12) 
       FIG. 14  illustrates input return loss as seen by the antenna.  FIG. 14  clearly indicates that matching is achieved, and VSWR less than 2 covers the required 10 MHz bandwidth centered around 405 MHz. Frequency independent R A  and C A  are assumed while the frequency dependent values for the bias and chip admittances are used in the above. Such simplification is justified when noting that the antenna capacitance does not change significantly, (same holds for its resistance) within the desired band of operation, which in turn means that the results achieved above are within a reasonable accuracy. 
     Test setting up for the antenna for the hearing aid may be accomplished by cascading the antenna and a BALUN model to extract the overall impedance and compare it with the measured overall impedance. 
       FIGS. 15-18  illustrate examples of the resultant antenna from the antenna model  12  of  FIG. 1 . The antennas of  FIGS. 15-18  are example only. The configuration of the antenna may vary depending on the design requirements as described herein. 
     The antenna  100  of  FIG. 15  includes a metallic trace  102  that is meandered (i.e., a plurality of turns). The antenna  100  includes port(s)  104  that is coupled to the transceiver ( 14  of  FIG. 1 ). 
     The antenna  110  of  FIG. 16  includes a plurality of metallic strips  112 . The widths of the metallic strips  112  are varied. At least two of the metallic strips  112  have different widths. The metallic strips  112  are connected to port(s)  116  that is connected to the transceiver ( 14  of  FIG. 1 ). The structure of the metallic strips are tuned to optimize the impedance of the antenna. The metallic strips  112  are backed by a large metallic piece (a shield like metallic plate  114 ) to aid in shielding. 
     The antenna  120  of  FIG. 17  includes main meandered metallic traces  122  and metallic strips  124 . The main meandered traces  122  aid in achieving the required input impedance. The metallic strips  124  may be used in fine tunings. The antenna  120  is connected the transceiver ( 14  of  FIG. 1 ) through to port(s)  126 . 
     The antenna  130  of  FIG. 18  is also an example of the antenna obtained from the design process described herein. 
     Referring to  FIGS. 15-18 , the exact length of each component are post tuned based on the results of the simulation. The impedance level is determined by the amount of meandering and the metallic strip used. 
     Based on the sturdy of small antenna around the human head, along with the study seeking maximization of the system power transfer through selecting appropriate values for the antenna impedance, corresponding to a given bias inductance, four antenna layouts were developed. 
       FIG. 19A  is a top view of one prototype for fabrication of the antenna for a hearing aid application.  FIG. 19B  is a cross section view of the antenna of  FIG. 19A . The antenna  150  of  FIG. 19A-18B  includes an antenna top surface  152  and a flexible substrate  154 . The antenna  150  includes a plurality of non-connected arms for post fabrication quick tunings with, for example, copper tapes. 
       FIG. 20A  illustrates another example of a prototype for fabrication of the antenna for a hearing aid application.  FIG. 20B  is a cross section view of the antenna of  FIG. 20A . The antenna  160  of  FIG. 20A-20B  includes an antenna top surface  162 , a flexible substrate  164  and a shield  166  for tuning the reactive part of input impedance. 
     Referring to  FIGS. 19A ,  19 B,  20 A, and  20 B, the antennas  150  and  160  are based on dipoles. Assuming 50 [nH] (Q=30) bias inductors, these antennas can be directly connected to the active circuitry. To operate at 50 [nH], each of the antennas are fabricated in two sets, each fitted on a side of the package, and connected together. 
     The antennas  150  and  160  are capable of realizing a simulated power reception level of around, for example, −69.5 [dB] and −67[dB], when included in the hearing aid package ( 26  of  FIG. 1 ) and placed closed to the human body phantom model ( 2  of  FIG. 1 ). 
       FIGS. 21-24  show some examples of a hearing aid device in accordance with an embodiment of the present invention. The hearing aid device  200  of  FIGS. 21-22  includes an antenna board (antenna)  202 . The hearing aid device  200  of  FIGS. 21-22  has a tone hook  204 , a right shell  206 , a left shell  208 , a battery door compartment  210  with an on/off switch, a volume control bottom  214 . The antenna  202  is enclosed in the shells  206  and  208 . 
     As shown in  FIG. 23 , the hearing aid device may include a plate antenna  220  as shown in  FIG. 23 , and has a plurality of different excitation points  222 . One point connection to the board inside the case (shell) is enough to excite the antenna  220 . 
     As shown in  FIG. 24 , the hearing aid device may include a dipole antenna  230  as shown in  FIG. 24 , and has a plurality of different excitation points  232 . One point connection to the board inside the case (shell) is enough to excite the antenna  230 . 
       FIG. 25  shows one example of a method of designing an antenna in accordance with an embodiment of the invention. One example of designing an antenna is descried, with reference to  FIG. 21 . In the first step ( 250 ), estimate of the package (size/material) is provided. In the second step ( 252 ), possible realization(s) of the antenna is designed, given the space limitations of the package, to realize maximum power transfer around the human head. In the third step ( 254 ), for a given design of LNA and PA, power efficiency maps are generated for all possible bias realizations versus all possible impedance values of the antenna. The efficiency maps will guide as a sensitivity measure of the overall link efficiency. In the third step ( 256 ), the antenna design is modified in order to maximize the overall link efficiency. This is determined by maximizing the combination of the power transfer around the human head, establishing direct matching to the LNA/PA, and reducing the system sensitivity to variations in human head sizes and package tolerances. 
     The embodiments of the present invention are further clarified in “Antenna For AMIL Semiconductors Hearing Aid Devices: Analysis and Design Optimization: Proposed Antenna Solution” as shown below. The contents of “Antenna For AMIL Semiconductors Hearing Aid Devices Analysis and Design Optimization: Proposed Antenna Solution” form a part of the detailed description. 

 
     One of the embodiments is further clarified in “Direct Matching of a Miniaturized Antenna of an On-Chip Low Noise Amplifier” as shown below. The contents of “Direct Matching of a Miniaturized Antenna of an On-Chip Low Noise Amplifier form a part of the detailed description. 

 
     One of the emobodiments is further in “On Design of a Hearing Aid Communication System” as shown below. The contenets of “On Design of a Hearing Aid Communication System” form a part of the detailed description. 

 
     One or more currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.