Abstract:
A transmission/reception antenna ( 21 ) comprises a core ( 31 ) and a coil ( 32 ) wound around the core ( 31 ). One of a plurality of core sheets ( 31   a ) constituting the core ( 31 ) is utilized as a sensor electrode ( 23 ). Conductors ( 33, 34 ) extending from the two ends of the coil ( 32 ) are connected to a transmission/reception drive circuit ( 22 ). The conductor ( 35 ) extending from a capacitance detector ( 24 ) is connected to one of the conductors ( 33, 34 ) extending between the coil ( 32 ) and the transmission/reception drive circuit ( 22 ). Consequently, the capacitance detector ( 24 ) is alternately connected to the sensor electrode ( 23 ) through a parasitic capacitance C 1  generated between the coil ( 32 ) and the sensor electrode ( 23 ) (core sheet ( 31   a )).

Description:
FIELD OF THE INVENTION 
   The present invention relates to an antenna device for use in an electronic key system for recognizing the intention of a user to lock or unlock a vehicle door and controlling the locking and unlocking of the vehicle door, and a door handle device including the antenna device. 
   BACKGROUND OF THE INVENTION 
   Patent publication 1 discloses a vehicle door handle used in an electronic key system for locking and unlocking a vehicle door in response to a portable device carried by a user. The vehicle door handle disclosed in patent document 1 accommodates a transmission antenna for communicating with the portable device and a sensor electrode for detecting the user approaching the vehicle door based on a change in capacitance. The transmission antenna includes a core and a coil, which is formed by winding conductive wire around the core. The coil is connected to a transmission drive circuit, which transmits a transmission request signal from the transmission antenna. The sensor electrode is accommodated in the door handle and exposed from the door handle so as to face an outside panel of the vehicle door. The sensor electrode is also connected to a capacitance detector, which detects changes in the capacitance between the sensor electrode and the outside panel. 
   The user approaches the vehicle and first puts his or her hand on the door handle when opening the vehicle door. Therefore, it is desirable that the transmission antenna and the sensor electrode be arranged inside the door handle to achieve the functions of the electronic key system. 
   The door handle does not accommodate only the transmission antenna and the sensor electrode, and the space inside the door handle is not large. In order to accommodate a plurality of components in the door handle, the door handle must be enlarged. Otherwise, limitations are applied to the shape of the door handle in accordance with the shapes of the components accommodated in the door handle. Therefore, it is desirable that the number of components accommodated in the door handle be reduced. 
   One method for reducing the number of components incorporated in the door handle would be to use the core of the antenna as the sensor electrode. In this case, the core used as the sensor electrode must be electrically connected to the capacitance detector by, for example, conductive wire. However, it is difficult to connect the conductive wire, which extends from the capacitance detector, to the core. Furthermore, it is desirable that a waterproof structure be provided for a connection point between the core and the conductive wire. However, such a waterproof structure would increase the manufacturing cost. 
   Patent Publication 1: Japanese Laid-Open Patent Publication No. 2003-13628 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an antenna device and a door handle device in which a core and a capacitance detector are electrically connected to each other without arranging a connection point between the core and the conductive wire, which extends from the capacitance detector. 
   To achieve the above object, the present invention provides an antenna device having an antenna including a core made of a conductive alloy and a coil wound around the core. A sensor electrode is formed by at least part of the core. A capacitance detector is electrically connected to the coil to detect a capacitance change at the sensor electrode. 
   In the present invention, the capacitance detector is electrically connected to the coil. The capacitance detector and the core (sensor electrode) are connected so that alternating current flows therebetween due to parasitic capacitance generated between the core and the coil. As a result, the core and the capacitance detector are electrically connected without an electrical connection point between the core and conductive wire, which extends from the capacitance detector, and at least part of the core functions as the sensor electrode. 
   In a preferred aspect of the present invention, the antenna device further includes a communication drive circuit connected to the coil. The capacitance detector is connected to a conductive wire extending between the coil and the communication drive circuit. 
   Therefore, the capacitance detector only needs to be connected to the conductive wire near the communication drive circuit, and the conductive wire extending from the capacitance detector does not need to be extended to the vicinity of the sensor electrode. 
   In one aspect of the present invention, the antenna device further includes a transmission drive circuit, connected to the coil, for transmitting an electrical signal from the antenna. A switch is arranged between the coil and the transmission drive circuit. The capacitance detector is connected to a conductive wire extending between the coil and the switch. 
   In this case, the capacitance detector is disconnected from the transmission drive circuit by opening the switch. Accordingly, even if, for example, the transmission drive circuit includes a MOSFET having parasitic capacitance and the MOSFET is grounded, the portion of the sensor electrode around which a coil is wound is prevented from becoming a non-detection area regardless of the magnitude of the parasitic capacitance of the MOSFET. The switch is closed when transmitting an electrical signal from the antenna. 
   In another aspect of the present invention, the antenna further includes a transmission drive circuit, connected to the coil, for transmitting an electrical signal from the antenna. A transformer is arranged between the coil and the transmission drive circuit. The capacitance detector is connected to a conductive wire extending between the coil and the transformer. 
   The coil and the transceiver drive circuit are connected by the transformer. Thus, the coil and the transceiver drive circuit are connected so that alternating current flows therebetween and direct current does not flow therebetween. The transformer disconnects the capacitance detector and the transmission drive circuit so that direct current does not flow therebetween. Therefore, even if the transmission drive circuit includes a MOSFET having parasitic capacitance and the MOSFET is grounded, the portion of the sensor electrode around which the coil is wound is prevented from becoming the non-detection area regardless of the magnitude of the parasitic capacitance of the MOSFET. Further, the coil and the transmission drive circuit are connected by the transformer so that alternating current flows therebetween. Thus, the electrical signal output from the transmission drive circuit is transmitted from the antenna through the transformer. Therefore, unlike when the coil and the transmission drive circuit are connected by switches. a control circuit for controlling a switch is not necessary. Thus, the antenna device does not have to be enlarged. 
   In the preferred embodiment, it is preferred that the core be made of amorphous alloy. 
   In the present invention, the core is preferably made of amorphous alloy. Amorphous alloy is conductive and has higher permeance and superior high frequency characteristics in comparison with, for example, steel plates or silicon plates. Accordingly, the amorphous alloy core sheet is optimal for use as a highly sensitive sensor electrode and contributes to enhancing the performance and miniaturization of the transceiver antenna. 
   The present invention also provides a door handle device including the above antenna device and a handle arranged outside a vehicle door. The antenna is accommodated in the handle. 
   In the present invention, at least part of the coil is easily used as the sensor electrode. This reduces the number of components accommodated in the handle. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the electrical configuration of an antenna device according to a first embodiment of the present invention; 
       FIG. 2  is a perspective view showing a vehicle door using the antenna device of  FIG. 1 ; 
       FIG. 3  is a partially cutaway view taken along line A-A in  FIG. 2  and showing an outside door handle for the vehicle door of  FIG. 2 ; 
       FIG. 4  is a block diagram showing the electrical configuration of part of an electronic key system including the antenna device of  FIG. 1 ; 
       FIG. 5  is a block diagram showing the electrical configuration of an antenna device according to a second embodiment of the present invention; 
       FIG. 6  is a block diagram showing the electrical configuration of a transmission drive circuit in the antenna device of  FIG. 5 ; and 
       FIG. 7  is a block diagram showing the electrical configuration of an antenna device according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   A first embodiment of the present invention will now be described with reference to the drawings. 
   As shown in  FIG. 2 , an outside handle  11  is arranged on a vehicle door  10 . The outside handle  11  is attached to a door outer panel  14  of the vehicle door  10  at the rear side of the vehicle door  10 . 
   As shown in  FIG. 3 , the outside handle  11  includes a handle portion (grip)  12 , a handle cap  13  projecting out of the vehicle from the door outer panel  14 , and a handle frame  15  fixed to the door outer panel  14  inside the vehicle door  10 . A user (e.g., owner, driver, passenger of the vehicle) grips and pivots the handle portion  12  when opening and closing the vehicle door  10 . The handle portion  12  is connected to the handle frame  15  in a manner that it is pivotal in a predetermined range and so that the door outer panel  14  is arranged between the handle portion  12  and the handle frame  15 . More specifically, a hinge arm  12   a  and a stroke arm  12   b  are respectively arranged at the two ends of the handle portion  12 . The hinge arm  12   a  is extended through the door outer panel  14  and pivotally connected to the handle frame  15 . The stroke arm  12   b  is extended through the door outer panel  14  and engaged with the handle frame  15  so that it is pivotal in the predetermined range. This enables the handle portion  12  to be pivoted about the hinge arm  12   a  within the range tolerated by the stroke arm  12   b.    
   A key cylinder  16  is attached to the handle frame  15  adjacent to the handle portion  12 . The handle cap  13  is attached to the key cylinder  16  to enclose the key cylinder  16 . That is, the handle cap  13  is fixed to the handle frame  15  by means of the key cylinder  16 . 
   An electronic system for recognizing the intention of the user to lock or unlock the vehicle door  10  and control the locking and unlocking of the vehicle door includes a device (in-vehicle device  20 ) arranged in the vehicle and a portable device (not shown) carried by the user. 
   As shown in  FIG. 4 , the in-vehicle device  20  includes a transceiver antenna  21 , a transceiver drive circuit (communication drive circuit)  22 , a sensor electrode  23 , a capacitance detector  24 , a door lock device  25 , and a controller  26 . The transceiver antenna  21 , the transceiver drive circuit  22 , the sensor electrode  23 , and the capacitance detector  24  form the antenna device  30 . The antenna device  30  and the outside handle  11  form the door handle device. The transceiver drive circuit  22 , the capacitance detector  24 , and the controller  26  are supplied with power from an in-vehicle battery  27 . 
   The transceiver antenna  21  is accommodated in the outside handle  11  (see  FIG. 3 ) and connected to the controller  26  by the transceiver drive circuit  22 . The transceiver antenna  21  receives radio waves (response signal) from outside and inside the passenger compartment. Further, the transceiver antenna  21  transmits radio waves (request signal) as an electrical signal outside the passenger compartment. The transceiver antenna  21  is arranged in the outside handle  11 , which is arranged at the outer side of the vehicle door  10 . Thus, the transceiver antenna  21  optimally radiates radio wave outside of the vehicle with the desired radiation pattern without being shielded by the metal vehicle body. 
   The transceiver drive circuit  22  is connected to the controller  26 . The transceiver drive circuit  22  is controlled by the controller  26  and transmits the request signal via the transceiver antenna  21  to recognize whether the portable device carried by the user is the portable device having identification information (identification code) registered in a memory  26   a  of the controller  26 . The portable device transmits the response signal containing the identification information when receiving the request signal. The transceiver drive circuit  22  transmits the request signal to the controller  26  when receiving the response signal from the portable device via the transceiver antenna  21 . 
   The sensor electrode  23  is connected to the controller  26  by the capacitance detector  24 . The sensor electrode  23  and the capacitance detector  24  form a capacitance sensor system and detect that the user has touched the outside handle  11 . The capacitance detector  24 , which is controlled by the controller  26 , transmits a signal indicating that the user&#39;s hand has touched the handle portion  12  to the controller  26  when detecting a change in capacitance between the sensor electrode  23  and an opposing conductor, that is, a change in capacitance between the sensor electrode  23  and the door outer panel  14  or between the sensor electrode  23  and ground. 
   The controller  26  controls the transceiver drive circuit  22 , the capacitance detector  24 , the door lock device  25 , and the like. The controller  26  drives the door lock device  25  based on the transfer of various signals to lock and unlock the vehicle door  10 . 
   As shown in  FIG. 3 , the handle portion  12  is hollow, and the transceiver antenna  21  is accommodated in the handle portion  12 . The transceiver antenna  21  is mechanically fixed inside the handle portion  12 . 
   As shown in  FIG. 1 , the transceiver antenna  21  has the form of a loop antenna and includes a core  31  and a coil  32 , which is formed by winding a conductive wire around the core  31 . 
   The core  31  is formed by stacking a plurality of core sheets  31   a  made of an amorphous alloy in a manner that the core sheets  31   a  are spaced apart from each other in the thicknesswise direction of the core sheet  31   a . One of the core sheets  31   a  is used as the sensor electrode  23 . For example, the core sheet  31   a  located at the uppermost position in  FIG. 1  is used as the sensor electrode  23  in the first embodiment. 
   Conductive wires  33  and  34  extending from the two ends of the coil  32  are connected to the transceiver drive circuit  22 . A conductive wire  35  extending from the capacitance detector  24  is connected to one of the two conductive wires  33  and  34  extending between the coil  32  and the transceiver drive circuit  22 , in this case, the conductive wire  33 . Accordingly, the capacitance detector  24  is electrically connected to the coil  32  by the conductive wire  35  and the conductive wire  33 . Therefore, the capacitance detector  24  is connected to the sensor electrode  23  so that alternating current flows therebetween due to parasitic capacitance C 1  generated between the coil  32  and the sensor electrode  23 . 
   The operation of the electronic key system including the portable device and the in-vehicle device  20  will now be described. 
   For example, when the vehicle is in a parked state, the controller  26  of the in-vehicle device  20  drives the transceiver drive circuit  22  to transmit radio waves (request signal) outside the vehicle via the transceiver antenna  21 . When a user carrying the portable device approaches the vehicle, the portable device receives the request signal from the vehicle. The portable device then transmits radio waves (response signal) containing the identification information (identification code). 
   In the in-vehicle device  20 , the response signal from the portable device is received by the transceiver antenna  21  and transmitted to the controller  26 . The controller  26  recognizes that the portable device is approaching the vehicle based on the reception of the response signal and verifies the identification information contained in the response signal with the identification information prestored in the memory  26   a.    
   If the identification information contained in the response signal is identical to the identification information stored in the memory  26   a , the controller  26  shifts to a door unlocking request detection mode to check the intention of the user to unlock the vehicle door  10 . In the door unlocking request detection mode, the controller  26  detects capacitance changes in the sensor electrode  23  with the capacitance detector  24 . More specifically, when the user places his or her hand on the handle portion  12 , a capacitance change occurs between the sensor electrode  23  and the door outer panel  14  or between the sensor electrode  23  and ground. The capacitance detector  24  detects such capacitance change and transmits a signal indicating that the user&#39;s hand has touched the handle portion  12 . When receiving the signal, the controller  26  drives the door lock device  25  and unlocks the vehicle door  10 . As a result, the user can open the vehicle door  10  and enter the vehicle. 
   The first embodiment has the advantages described below. 
   (1) The conductive wire  35 , which extends from the capacitance detector  24 , is connected to the conductive wire  33 , which extends from the coil  32 , to connect the coil  32  to the transceiver drive circuit  22 . That is, the capacitance detector  24  is connected to the coil  32 . The capacitance detector  24  and the core sheets  31   a  are connected so that alternating current flows therebetween due to the parasitic capacitance C 1  generated between the core sheets  31   a  and the coil  32 , and a core sheet  31   a  functions as the sensor electrode  23 . More specifically, the core  31  and the capacitance detector  24  are electrically connected without an electrical connection point between the core  31  and the conductive wire  35  extending from the capacitance detector  24 . This enables one of the core sheets  31   a  forming the core  31  to function as the sensor electrode  23 . This reduces the number of components accommodated in the handle portion  12 . Thus, the outside handle  11  does not have to be enlarged, and the degree of freedom in the shape for the outside handle  11  is increased. Furthermore, a waterproof structure does not have to be provided since there is no need for an electrical connection point between the core  31  and the conductive wire  35 . This suppresses manufacturing costs. 
   (2) The core  31  is configured by a plurality of core sheets  31   a  made of an amorphous alloy. Amorphous alloy is conductive and has higher permeance and superior high frequency characteristics in comparison with, for example, steel plates or silicon plates. Therefore, the amorphous alloy core sheet  31   a  is optimal for use as a highly sensitive sensor electrode  23  in the capacitance sensor system. Further, the amorphous alloy core  31  contributes to enhancing the performance and miniaturization of the transceiver antenna  21 . This enables further miniaturization of the handle portion  12 , which accommodates the transceiver antenna  21 , and further increases the degree of freedom of the shape of the outside handle  11 . 
   (3) The conductive wire  35 , which extends from the capacitance detector  24 , is connected to the conductive wire  33 , which extends from the coil  32  to the transceiver drive circuit  22 . The capacitance detector  24  and the sensor electrode  23  (core sheet  31   a ) are thus connected so that alternating current flows therebetween due to the parasitic capacitance C 1  generated between the sensor electrode  23  and the coil  32 . The conductive wire  35  extending from the capacitance detector  24  only needs to be connected to the conductive wire  33  near the transceiver drive circuit  22 , and the conductive wire  35  does not need to be extended to the vicinity of the sensor electrode  23 , that is, to the inside of the handle portion  12 . This minimizes the amount of wires accommodated in the handle portion  12 , enables further miniaturization of the handle portion  12 , and increases the degree of freedom in the shape for the outside handle  11 . 
   Second Embodiment 
   A second embodiment of the present invention will now be described with reference to the drawings. Like or same reference numerals are given to those components that are the same as the corresponding components of the first embodiment, and such components will not be described in detail. 
     FIG. 5  shows an antenna device  40  of the second embodiment. As shown in  FIG. 5 , the coil  32  and the transceiver drive circuit  22  are connected by switches  41  and  42 . The conductive wire  35 , which extends from the capacitance detector  24 , is connected to a conductive wire  33   a , which extends between the switch  41  and the coil  32 , so that a connection point  35   a  is located on the conductive wire  33   a . This electrically connects a capacitance detector  24  to the coil  32 . 
   The switch  41  includes a movable terminal  41   a  connected to the conductive wire  33   a  extending from one end of the coil  32  and a contact point  41   b  connected to a conductive wire  33   b  extending from the transceiver drive circuit  22 . The switch  42  includes a movable terminal  42   a  connected to a conductive wire  34   a  extending from the other end of the coil  32  and a contact point  42   b  connected to a conductive wire  34   b  extending from the transceiver drive circuit  22 . The operation of the movable terminals  41   a  and  42   a  is controlled by the controller  26 . That is, the controller  26  closes (activates) the switches  41  and  42  when driving the transceiver drive circuit  22 . The controller  26  opens (inactivates) the switches  41  and  42  when driving the capacitance detector  24 . 
     FIG. 6  shows the electrical configuration of a transmission drive circuit  50  in the transceiver drive circuit  22 . The transmission drive circuit  50  is driven to transmit the request signal from the transceiver antenna  21 . As shown in  FIG. 6 , the transmission drive circuit  50  includes a carrier oscillation circuit  51  that oscillates a carrier signal (carrier wave), a code generation circuit  52  that generates a code signal for the request signal, an AND circuit  53 , an n-channel MOS (Metal Oxide Semiconductor) FET (Field Effect Transistor)  54 , and a p-channel MOSFET  55 . 
   The carrier oscillation circuit  51  is connected to one input terminal of the AND circuit  53 , and the code generation circuit  52  is connected to the other input terminal of the AND circuit  53 . The output terminal of the AND circuit  53  is connected to the gate of the n-channel MOSFET  54  and the gate of the p-channel MOSFET  55 . 
   The n-channel MOSFET  54  has a source connected to the in-vehicle battery  27  (see  FIG. 4 ) and a drain connected to the coil  32  (see  FIG. 5 ). The p-channel MOSFET  55  has a drain connected to the coil  32  (see  FIG. 5 ) and a source that is grounded. In other words, the n-channel MOSFET  54  and the p-channel MOSFET  55  are combined in a complementary form to configure a C (Complementary)-MOS (Metal Oxide Semiconductor) inverter. 
   The controller  26  (see  FIG. 4 ) drives the transmission drive circuit  50  so that the carrier oscillation circuit  51  oscillates the carrier signal and the code generation circuit  52  generates the code signal for the request signal. The code signal is superimposed on the carrier signal and output to the transceiver antenna  21 , and the request signal is transmitted from the transceiver antenna  21 . 
   In the antenna device  40 , the n-channel MOSFET  54  and the p-channel MOSFET  55  are arranged in the transmission drive circuit  50 , and the source of the p-channel MOSFET  55  is grounded. Parasitic capacitance C 2  is generated between the source and the drain of the n-channel MOSFET  54 , and parasitic capacitance C 3  is generated between the source and the drain of the p-channel MOSFET  55 . 
   It will now be assumed that the transceiver drive circuit  22  of the antenna device  30  in the first embodiment shown in  FIG. 1  includes the transmission drive circuit  50  shown in  FIG. 6 . In the antenna device  30  of  FIG. 1 , the conductive wire  35 , which extends from the capacitance detector  24 , is connected to the conductive wire  33 , which extends from the coil  32  to the transceiver drive circuit  22 . Thus, the capacitance detector  24  is grounded via the parasitic capacitance C 3  generated at the p-channel MOSFET  55 . Generally, the amount of capacitance change detected when the user touches the handle portion  12  is about a few pF. Therefore, if the parasitic capacitance C 3  generated at the p-channel MOSFET  55  is large compared to the capacitance detected at the sensor electrode  23 , the portion of the sensor electrode  23  around which the coil  32  is wound may become a non-detection area incapable of detecting that the user&#39;s hand has been placed on the handle portion  12 . Further, changes in the temperature or the like change the parasitic capacitance C 3  of the p-channel MOSFET  55 . Changes in the parasitic capacitance C 3  is one factor causing variation in the detection sensitivity of the capacitance in the capacitance detector  24 . 
   Comparatively, in the antenna device  40  of the second embodiment shown in  FIG. 5 , the transceiver drive circuit  22  and the coil  32  are connected by the switches  41  and  42 . The controller  26  opens (inactivates) the switches  41  and  42  when driving the capacitance detector  24 . That is, the capacitance detector  24  is mechanically disconnected from the transceiver drive circuit  22  including the transmission drive circuit  50  of  FIG. 6 . Therefore, when the controller  26  drives the capacitance detector  24 , the capacitance detected by the capacitance detector  24  is not affected by the parasitic capacitance C 3  generated in the p-channel MOSFET  55  regardless of the magnitude of the parasitic capacitance C 3 . Therefore, the portion of the sensor electrode  23  around which the coil  32  is wound is prevented from becoming a non-detection area: 
   In addition to the advantages (1) and (2) of the first embodiment, the second embodiment has the advantages described below. 
   (1) The coil  32  is disconnected from the transmission drive circuit  50  by opening the switches  41  and  42 . Therefore, even if the transmission drive circuit  50  includes the p-channel MOSFET  55 , which generates the parasitic capacitance C 3  between the source and the drain and which source is grounded, the portion of the sensor electrode  23  around which the coil  32  is wound is prevented from becoming the non-detection area regardless of the magnitude of the parasitic capacitance C 3 . As a result, this prevents defects such as failure in detection of the user&#39;s hand touching the handle portion  12 . The switches  41  and  42  are closed when the request signal is received from the transceiver antenna  21  or when the response signal is transmitted from the transceiver antenna  21 . 
   Third Embodiment 
   A third embodiment of the present invention will now be described with reference to the drawings. Like or same reference numerals are given to those components that are the same as the corresponding components of the first and second embodiments, and such components will not be described in detail. 
     FIG. 7  shows an antenna device  60  of the third embodiment. The antenna device  60  of the third embodiment includes the transmission drive circuit  50  in the same manner as in the transceiver drive circuit  22  of the second embodiment. As shown in  FIG. 7 , the coil  32  and the transceiver drive circuit  22  are connected by a transformer  61 . The transformer  61  connects the coil  32  and the transceiver drive circuit  22  so that alternating current flows therebetween and direct current does not flow therebetween. The conductive wire  35 , which extends from the capacitance detector  24 , is connected to the conductive wire  33   a  so that the connection point  35   a  is located on the conductive wire  33   a , which extends between the transformer  61  and the coil  32 . This electrically connects the capacitance detector  24  to the coil  32 . 
   In the antenna device  60 , the capacitance detector  24  is disconnected from the transceiver drive circuit  22 , which includes the transmission drive circuit  50 , so that direct current does not flow therebetween. Thus, capacitance detected by the capacitance detector  24  is not affected by the parasitic capacitance C 3  of the p-channel MOSFET  55  regardless of the magnitude of the parasitic capacitance C 3  of the p-channel MOSFET  55 . Accordingly, the portion of the sensor electrode  23  around which the coil  32  is wound is prevented from becoming a non-detection area. 
   In addition to the advantages (1) and (2) of the first embodiment, the third embodiment has the advantages described below. 
   (1) The coil  32  and the transmission drive circuit  50  are connected by the transformer  61 . Thus, the coil  32  and the transceiver drive circuit  22  are connected so that alternating current flows therebetween and direct current does not flow therebetween. The transformer  61  disconnects the capacitance detector  24  from the transmission drive circuit  50  so that direct current does not flow therebetween. Therefore, even if the transmission drive circuit  50  includes the p-channel MOSFET  55 , which generates the parasitic capacitance C 3  between the source and drain and which source is grounded, the portion of the sensor electrode  23  around which the coil  32  is wound is prevented from becoming the non-detection area regardless of the magnitude of the parasitic capacitance C 3 . As a result, this prevents defects such as failure in detection of the user&#39;s hand touching the handle portion  12 . Furthermore, the coil  32  and the transmission drive circuit  50  are connected by the transformer  61  so that alternating current flows therebetween. Thus, the switches  41  and  42  do not need to be controlled as in the second embodiment when receiving the request signal from the transceiver antenna  21  or when transmitting the response signal from the transceiver antenna  21 . Accordingly, the switches  41  and  42  and a control circuit or the like for controlling the switches  41  and  42  are not necessary. Thus, the antenna device  60  does not have to be enlarged. 
   The embodiments of the present invention may be modified as described below. 
   In each of the above embodiments, the uppermost core sheet  31   a  in the plurality of core sheets  31   a  of the core  31  is used as the sensor electrode  23 . However, the present invention is not limited in such a manner, and any one of the core sheets  31   a  other than the uppermost one in the core  31  may be used as the sensor electrode. 
   In each of the above embodiments, the core  31  is formed by stacking the plurality of core sheets  31   a . However, the present invention is not limited in such a manner, and the core may be formed, for example, by compression-molding soft magnetic metal powder. In this case, the entire core  31  is used as the sensor electrode. 
   In each of the above embodiments, the core  31  may be coated by a sealing film. This prevents corrosion of the core  31  and improves the durability of the core  31 . 
   In each of the above embodiments, the core sheet  31   a  is made of amorphous alloy. However, the core sheet  31   a  may also be made of a conductive alloy other than amorphous alloy.