Patent Publication Number: US-10764695-B2

Title: BTE hearing instrument comprising an open-end transmission line antenna

Description:
The invention relates to a hearing instrument comprising a part to be worn behind the ear of a user (i.e. a Behind-The-Ear (BTE) part) comprising an antenna. 
     In general, different types of antennas may be used with BTE hearing instruments. 
     WO 2012/059302 A2 relates to an antenna known as “inverted-L antenna”, which may be used in e.g. in a BTE hearing aid and which is a vertical antenna having a short vertical element prolonged by a wire parallel to a conductive ground plane. The antenna operates like a monopole folded by 90° and creates a capacitive effect causing the overall length of the antenna to be slightly shorter than λ/4. Typically, such antennas are used on the short wave frequencies, below 10 MHz. 
     EP 2 458 675 A2 relates to an antenna for a BTE hearing aid having a first L-shaped part placed on one side of the hearing aid housing and a second part having the form of a meander line and being placed on the opposite side of the housing, with a conductive part connecting the two parts. The antenna excitation point is between the first part and the conductive part. 
     EP 2 723 101 A2 relates to a BTE hearing aid having a balanced antenna for use at 2.4 GHz, which comprises a first resonant structure located on one side of the housing and a second resonant structure symmetric with regard to the first resonant structure and located on the opposite side of the housing, with a conductive segment providing a current bridge between the two resonant structures, wherein each resonant structure is fed through a transmission line. The resonant structures may have the form of a straight line, a meander line, a sheet or a closed oval line. EP 2 871 860 A1 relates to a variant of such antenna type, wherein the first resonant structure is fed through a transmission line, and the feeding point of the second resonant structure is connected to the ground plane of the electronic module. 
     US 2016/0183015 A1 relates to a BTE hearing aid comprising an antenna having two arms which are separated by a slot and extend in parallel along the length of the upper side of the housing. The arms comprise loading wings angled by about 90° with regard to the arms and extending along the sides of the housing adjacent to the upper side of the housing. 
     WO 2016/130590 A1 relates to a BTE hearing aid comprising an antenna comprising two arms, each of which extends along one of the lateral sides of the housing, with the arms being connected at one end by a conducting bridge. 
     U.S. Pat. No. 9,466,876 B2 relates to an antenna for a BTE hearing aid which comprises two arc-shaped conducting elements extending along the sides of the housing parallel to the user&#39;s skin and being connected by a conducting bridge in a middle portion. 
     WO 2007/112838 A1 relates to an RF receiver device which may be connected to a BTE hearing aid via a three pin plug connector and which comprises a magnetic loop antenna on a flexible printed circuit board (PCB) comprising two parts which are oriented at an angle of about 90° relative to each other. 
     It is an object of the invention to provide for a hearing instrument comprising a part to be worn behind the ear of a user and including an antenna which should be efficient both for wireless communication via a binaural link and for wireless communication with remote devices. 
     According to the invention, this object is achieved by a hearing instrument as defined in claim  1 . 
     The invention is beneficial in that, by providing the antenna a radiating bifilar transmission line having an open connection at one end and comprising two spaced-apart conducting legs parallel to each other and connected at the end opposite to the open end by an impedance matching base portion configured to match the impedance of the antenna to the impedance of a non-radiating transmission line, wherein each leg extends in one of the opposed peripheral regions along the length of a side of the BTE part perpendicular to the user&#39;s skin in such a manner that the open end faces the battery of the BTE part, the antenna enables high radiation efficiency along the head surface. 
     Preferred embodiments of the invention are defined in the dependent claims. 
    
    
     
       Hereinafter, examples of the invention will be illustrated by reference to the attached drawings, wherein: 
         FIGS. 1 and 2  are two different perspective views of an example of components of a BTE part of a hearing instrument according to the invention; 
         FIG. 3  is a schematic circuit diagram of an example of an antenna according to the invention; 
         FIGS. 4 and 5  are circuit diagrams illustrating two different examples of a matching circuit for an antenna according to the invention; 
         FIGS. 6 and 7  are circuit diagrams illustrating two different examples of an antenna according to the invention with serial tuning elements; and 
         FIG. 8  is a representation of an example of the antenna gain in a horizontal plane of the user&#39;s head, wherein a conventional full magnetic loop antenna and an antenna according to the invention are compared. 
     
    
    
       FIGS. 1 and 2  relate to a BTE part  10  of a hearing instrument, which is to be worn behind the ear of a user. The hearing instrument may be, for example, a BTE hearing aid (wherein the speaker is located in the BTE part) or a RIC hearing aid (wherein the speaker is located in the ear canal and is electrically connected to the BTE part). Alternatively, the hearing instrument may be an implantable hearing prosthesis, such as a cochlear implant system, wherein the BTE part  10  then is a BTE sound processor. 
     The BTE part  10  comprises a housing (not shown) and has a first side substantially parallel to the user&#39;s skin when the housing is worn behind the ear, a second side substantially parallel to the first side and a third side connecting the first side and the second side and oriented substantially upwardly when the housing is worn behind the ear; the third side thus is substantially perpendicular to the user&#39;s skin. 
     The BTE part  10  further comprises a radio circuit  12  acting as an RF transmitter or transceiver, a first microphone  14 , a second microphone  16 , a battery  18 , a frame  20  made of plastic material for supporting components of the BTE part, electronic circuitry  22  and an antenna  24  placed on the upper side of the hearing instrument (i.e. the antenna  24  is primarily located at the third side of the BTE part). Typically, the BTE part  10  includes additional components which are not shown in the Figures, such as a user interface with at least one push button, a speaker, etc. In the example of  FIGS. 1 and 2  the BTE part  10  is part of BTE hearing aid of the RIC (receiver in the channel) type, with the BTE part  10  comprising an RIC connector  19  at one end. 
     The transmitter/transceiver  12  is designed for transmission at frequencies from 1 to 6 GHz, preferably from 2.40 to 2.48 GHz. 
     The antenna  24  comprises a radiating bifilar transmission line  26  comprising a conductor  32  having a U-shaped contour comprising a first leg  34  and a second leg  36  which are connected by an impedance matching base portion  38  and which have open ends  28 ,  30 . The conductor  32  is located at the upper side of the BTE part  10 , i.e. it is located at and substantially parallel to the upwardly oriented third side of the housing. 
     The legs  34 ,  36  are parallel to each other and preferably extend over most (typically at least two thirds) of the length of the third side of the housing. The distance between the legs  34 ,  36  typically is at least 2.0 mm and the width of each leg  34 ,  36  typically is from 0.2 to 1.0 mm. 
     The open ends (or antenna tips)  28 ,  30  of the legs  34 ,  36  are located closer to the battery  18  than the base portion  38 , i.e. open ends  28 ,  30  of the legs  34 ,  36  are oriented towards the battery  18 , and typically extend past a boundary of the battery  18  and over at least part of the battery  18 . A plastic frame  43  is provided between the battery  18  and the open ends  28 ,  30  of the legs so as to provide for a minimum spacing of 0.2 to 1 mm between the battery  18  and the legs  34 ,  36 . 
     As illustrated in the example of  FIGS. 1 and 2 , the conductor  32  and the impedance matching base portion  38  may be formed on a flexible PCB  48  which has an opening  50  for a push button of the user interface and an opening  52  for the first microphone  14 . Additional openings may be provided for fixation of the PCB  48 , as indicated by the fixation elements  54  and  56 . 
     According to the example of  FIGS. 1 and 2 , the PCB  48  comprises, in addition to the first portion  58  on which the loop conductor  32  is implemented, a second portion  60  on which the non-radiating transmission line  40  is implemented, with the second portion  60  with the transmission line  40  being folded by about 90° with regard to the first portion  58 , with the second portion  60  being located at substantially parallel to the first or second side of the BTE part  10 . 
     According to one example, the conductor  32  may have a substantially planar configuration (within 5 degrees). However, the legs  34 ,  36  preferably are curved or angled along their length between the open end  28 ,  30  and the end connected to the base portion  38  by more than 5° and less than 20° in order to allow for a curvature of the respective side of the housing. 
     The structure of the antenna  24  is differential, so that it does not require any ground plane to work properly. The antenna  24  is fed by a non-radiating bifilar transmission line  40  which is connected to the conductor  32  through the impedance matching base portion  38 , thereby forming a differential feed structure connected to each of the legs  34 ,  36  at a feed point  44  and  46 , respectively. In the example of  FIGS. 1 and 2  the impedance matching base portion  38  comprises a central shunt matching element  62  in a portion  61  connecting the ends of the legs  34 ,  36  and two lateral serial matching elements  64 , one for each of the legs  34 ,  36 . In the example of  FIGS. 1 and 2  each one of the two strands of the non-radiating transmission line  40  is connected to a different one of the legs  34 ,  36  of the radiating transmission line in such a manner that the respective feed point  44 ,  46  is between the central shunt matching element  62  and the respective lateral serial matching element  64 . Preferably, the feed points  44 ,  46  are arranged mirror-symmetric with regard to each other. Typically, the entire antenna structure is mirror-symmetric with regard to a plane extending in the longitudinal direction of the BTE part  10 . 
     In the example of  FIGS. 1 and 2  each leg  34 ,  36  is provided with a serial tuning element  70  at a position close to the impedance matching base portion  38  for tuning of the antenna resonance frequency, in particular in case that the length of the legs  34 ,  36  does not match with the desired antenna resonance frequency, as will be explained in more detail below. 
       FIG. 3  is a schematic circuit diagram of an example of an antenna according to the invention, wherein the antenna  24  is formed by a radiating transmission line  26  (which is implemented in the example of  FIGS. 1 and 2  by the legs  34 ,  36  formed as a conductor  32  on a PCB  48 ), which has its open end/tip  28 ,  30  located at the battery, wherein the input nodes  74 ,  76  are connected to the output of the impedance matching portion  38 . The input of the impedance matching portion  38  is connected to the output nodes  78 ,  80  of the non-radiating transmission line  40 , the input nodes of which are connected to the radio transceiver  12 . The transmission line  40  is a bifilar transmission line and has a width W 1  which is much smaller than the wavelength of the radio waves supplied by the transceiver  12 . The bifilar radiating transmission line  26  has a relatively large width W 2  (which is at least 2.0 mm) and an electrical length/corresponding to a quarter wavelength of the radio frequency of the signal supplied by the transceiver  12 , so that the transmission line  26  is radiating (the radiation strength increases with increasing width W 2  of the transmission line  26 ). 
     The matching portion  38  is required for matching the output impedance at the output nodes  78 ,  80  of the non-radiating transmission line  40  to the impedance seen at the input nodes  74 ,  76  of the radiating transmission line  26 . In general, the tips  28 ,  30  of the radiating transmission line  26  preferably extend into the region of the battery  18  so as to maximize the length of the radiating transmission line  26  for improving the radiation performance. However, some spacing should be provided between the tips  28 ,  30  and the battery  18  for minimizing the parasitic capacitive coupling; to this end, in the example of  FIGS. 1 and 2  a plastic frame  43  is provided between the battery  18  and the tips  28 ,  30 . 
     Typically, in practice, the impedance of the radiating transmission line  26  between the input nodes  74 ,  76  is smaller than the characteristic impedance at the output nodes  78 ,  80  of the non-radiating transmission line  40 , so that the matching portion  38  has to provide for a transformation from a higher impedance seen between the output nodes  78 ,  80  to a smaller impedance seen between the antenna input nodes  74 ,  76 . 
     In  FIG. 4  a first example of an antenna  24  with a matching portion  38  is shown, wherein the impedance transformation is achieved by serial capacitors C 1 , a shunt inductance L 1  and serial inductances L CON , wherein the serial inductances L CON  are the parasitic inductances of straight wires connecting the output nodes  78 ,  80  of the non-radiating transmission line  40  to the common nodes  44 ,  46  of the inductance L 1  and the respective serial capacitor C 1 . In practice, the values of L CON  are very small, so that their parasitic effects on the impedance transformation may be compensated by small adaptations of the values of L 1  and C 1 . 
     According to a variant of the embodiment of  FIG. 4 , the parallel inductance L 1  may be replaced by a metallic trace having a length providing an inductance value between the nodes  44 ,  46  which is appropriate for the needed impedance transformation. 
     In  FIG. 5  an alternative embodiment for the same impedance transformation as in the example of  FIG. 4  is shown, wherein the central shunt element is a shunt capacitor C 2  and the two lateral serial matching elements are inductances L 2 . The parasitic inductances L CON  are treated in the same manner as in the embodiment of  FIG. 4 . 
       FIG. 6  is a circuit diagram illustrating an example for the serial tuning element  70  in case that the physical length of the radiating transmission line  26  is too short. In this case the serial tuning element  70  is formed by an inductance L 3  which is placed in serial in each of the legs  34 ,  36  so as to provide a λ/4 resonance in case that the length of the radiating transmission line  26 , i.e. the length of the respective leg  34 ,  36  is less than λ/4. Thus, in this case the inductances L 3  serve to increase the electrical length of the radiating transmission line  26  to λ/4. 
       FIG. 7  shows a circuit diagram illustrating an example of the case in which the physical length of the radiating transmission line  26  is too large, i.e. is larger than λ/4. In this case a capacitor C 3  is placed in serial in each leg  34 ,  36  of the radiating transmission line  26  so as to provide for a λ/4 resonance. 
     It has to be noted that, for example, the inductance L 3  of  FIG. 6  may be combined with the capacitor C 1  of  FIG. 4  into a single component having the same serial impedance. Similarly, the inductance L 3  of  FIG. 6  may be combined as well with the inductance L 2  of  FIG. 5  into a single component having the same serial impedance. Similar considerations also apply for the circuit of  FIG. 7  when used with one of the circuits of  FIGS. 4 and 5 , i.e. the serial tuning element  70  may be combined with the lateral serial matching element  64  into a single capacitor or inductance. 
     It is further to be noted that the impedance matching base portion  38  allows for fine tuning of both the resonance frequency and the input impedance of the antenna  24 . However, fine tuning of the resonance frequency may be advantageously realized through the serial tuning elements  70 . 
     The antenna of the invention produces an electromagnetic wave having an electric field component orthogonal to the skin, which is optimal for propagation by diffraction around the head. This is illustrated in  FIG. 8  which compares the simulated radiation pattern of a conventional BTE part with a full size closed magnetic loop antenna and a BTE part provided with an antenna according to the invention, wherein the BTE part is placed at the left side of the head  72  between the skull and the auricle. It can be seen that the conventional magnetic loop antenna (dashed line in  FIG. 8 ) has a radiation maximum in a direction orthogonal to the head  72  (at 180°), while the antenna according to the invention (solid line in  FIG. 8 ) has a radiation maximum that is oriented at about 240°, between the side and the rear of the head  72 , with the gain in the rearward direction (270°) being by 5 dB higher for the antenna according to the invention than for the conventional magnetic loop antenna. 
     The best propagation path for a binaural link is by diffraction around the neck, since this path is shorter than other paths, such as the path around the top of the head or the path around the forehead which is partly obstructed by the auricle. With the antenna of the invention in the example of  FIG. 8  having 5 dB more gain in the direction of the neck than the conventional magnetic loop antenna, using the antenna of the invention in a binaural link between a left ear BTE hearing instrument and a right ear BTE hearing instrument would provide for a 10 dB advantage over a conventional magnetic loop antenna (both the antenna at the left ear and the antenna at the right ear would have 5 dB more gain in the direction of the main propagation path around the neck).