Abstract:
A tire-state detection device including a sensor, an antenna having a predetermined frequency, a detection circuit, a case, and a planar conductor. The sensor detects a predetermined physical state of a tire. The detection circuit transmits information regarding a result of a detection made by the sensor from the antenna as radio waves. The case houses the sensor, the antenna and the detection circuit, and allows radio waves to pass. The case fits on a rim in the tire when the tire-state detection device is to be used. The planar conductor is electrically insulated from the antenna at a position set a predetermined distance away from the antenna so as to form an interface between the antenna and the rim when the case is fitted to the rim, and the planar conductor is set to a potential that is equivalent to a reference potential of the detection circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. National stage application claims priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2009-201574, filed in Japan on Sep. 1, 2009, the entire contents of Japanese Patent Application No. 2009-201574 are hereby incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a tire-state detection device, and more particularly to a tire-state detection device for transmitting detection results using radio signals to an external destination, the device used by being fitted to a rim inside a tire. 
     2. Background Information 
     Generally, a tire-state detection device that is equipped with a wireless device for transmitting air-pressure data is fitted to a rim well in a system for monitoring air pressure inside a tire as described in Japanese Unexamined Patent Publication 2006-327460. Characteristics often greatly fluctuate because an antenna equipped in the tire-state detection device is affected by nearby metallic parts, such as the rim. Therefore, antenna designs have been required for providing tire-state detection devices on metallic parts, such as the rim and the like. However, rim shapes are varied, and the resonance frequency and impedance of the antenna fluctuate dramatically if the distance between the antenna and the metal constituting the rim are different due to the shape of the rim. 
     For example, if a tire-state detection device that is equipped with a transmitter capable of a transmitting frequency of 315 MHz, and an antenna having a resonance frequency of 315 MHz is optimally fitted in the rim, a good impedance characteristic will be obtained, as shown in  FIG. 26 . However, if the tire-state detection device is fitted slightly away from the rim, or is fitted on a rim having a different shape, antenna resonance frequency will shift dramatically, as shown in  FIG. 27 . 
     SUMMARY 
     Because the resonance frequency of an antenna in a tire-state detection device often shifts dramatically if the rim has different shapes where the tire-state detection device is installed, as well as for other reasons, it has been necessary to design antennas for individual shapes in order to maintain good antenna characteristics. 
     Therefore, it has been necessary to create antennas provided with the optimum characteristic for each rim shape. For that reason, manufacturing costs associated with each antenna has increased, and mass production has been difficult. 
     In view of the aforementioned problems, an object of the present invention is to provide a tire-state detection device that can attain good antenna characteristics even if the shape of the rim is different where the device is fitted, that reduces manufacturing costs, and that can be mass produced. 
     In order to attain the aforementioned object, the tire-state detection device of the present invention comprises a sensor for detecting a predetermined physical state of a tire; an antenna having a predetermined resonance frequency; a detection circuit for transmitting information regarding a result of a detection made by the sensor from the antenna as radio waves; and a case for housing the sensor, the antenna and the detection circuit, and for allowing radio waves to pass therethrough, the case being fitted on a rim in the tire when the tire-state detection device is to be used wherein the tire-state detection device is characterized in that there is provided a planar conductor secured in a state of being electrically insulated from the antenna at a position set a predetermined distance away from the antenna so as to form an interface between the antenna and the rim when the case is fitted to the rim, and set to a potential that is equivalent to a reference potential of the detection circuit. 
     According to the present invention, a planar conductor set to a reference potential of the detection circuit is disposed in the device at a position set a predetermined distance from the antenna. When the device is fitted to the rim, the planar conductor becomes an interface between the antenna and the rim. For that reason, it is possible dramatically to reduce any effect that the metal constituting the rim has on the antenna compared to conventional devices 
     According to the tire-state detection device of the present invention, when the tire-state detection device is fitted to a rim, the case is secured to the rim surface so that the bottom surface of the case faces the rim surface. By securing the case to the rim in this way, the planar conductor that is set to a reference potential is disposed between the antenna and rim surface, allowing the planar conductor to be an interface between the antenna and the rim when the tire-state detection device is fitted to the rim. For that reason, it is possible to dramatically reduce the effect that the metal constituting the rim has on the antenna compared to conventional devices. Therefore, it is not necessary to create an antenna that is equipped with optimum characteristics for each rim shape. Furthermore, it is possible to use the same antenna with any rim shape, which greatly reduces the manufacturing cost associated with each tire-state detection device compared to conventional devices, and the tire-state detection device can easily be mass produced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a view of a tire fitted with a tire-state detection device according to a first embodiment of the present invention; 
         FIG. 2  is an external view of the tire-state detection device according to the first embodiment of the present invention; 
         FIG. 3  is an exploded perspective view of the tire-state detection device according to the first embodiment of the present invention; 
         FIG. 4  is a plan view of a printed wiring board according to the first embodiment of the present invention; 
         FIG. 5  is a side view of the printed wiring board according to the first embodiment of the present invention; 
         FIG. 6  is a block diagram of an electrical circuit in the tire-state detection device according to the first embodiment of the present invention; 
         FIG. 7  is a lateral sectional view of the tire-state detection device fitted to a rim, according to the first embodiment of the present invention; 
         FIG. 8  is an exploded perspective view of the tire-state detection device according to a second embodiment of the present invention; 
         FIG. 9  is a plan view of a printed wiring board according to the second embodiment of the present invention; 
         FIG. 10  is a side view of the printed wiring board according to the second embodiment of the present invention; 
         FIG. 11  is a lateral sectional view of the tire-state detection device of the second embodiment of the present invention fitted to a rim; 
         FIG. 12  is an exploded perspective view of the tire-state detection device according to a third embodiment of the present invention; 
         FIG. 13  is a plan view of a printed wiring board according to the third embodiment of the present invention; 
         FIG. 14  is a side view of the printed wiring board according to the third embodiment of the present invention; 
         FIG. 15  is an exploded perspective view of an antenna according to the third embodiment of the present invention; 
         FIG. 16  is a lateral sectional view of the tire-state detection device of the third embodiment of the present invention fitted to a rim; 
         FIG. 17  is an external perspective view of the tire-state detection device according to a fourth embodiment of the present invention; 
         FIG. 18  is a plan view of the tire-state detection device according to the fourth embodiment of the present invention; 
         FIG. 19  is a lateral sectional view of the tire-state detection device according to the fourth embodiment of the present invention; 
         FIG. 20  is an external perspective view of a printed wiring board according to the fourth embodiment of the present invention; 
         FIG. 21  is an external perspective view of the printed wiring board according to the fourth embodiment of the present invention; 
         FIG. 22  is an external perspective view of an essential part of the printed wiring board according to the fourth embodiment of the present invention; 
         FIG. 23  is an external perspective view of a planar conductor plate and holder according to the fourth embodiment of the present invention; 
         FIG. 24  is an external perspective view of the holder according to the fourth embodiment of the present invention; 
         FIG. 25  is a Smith chart to explain characteristics of an antenna according to the fourth embodiment of the present invention; 
         FIG. 26  is a Smith chart to explain characteristics of a normal antenna of the prior art; and 
         FIG. 27  is an explanation of the Smith chart symbols to explain fluctuations of characteristics of the antenna in the prior art. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the drawings provided 
       FIG. 1  is a view of a tire fitted with a tire-state detection device according to a first embodiment of the present invention;  FIG. 2  is an external view of the tire-state detection device according to the first embodiment of the present invention;  FIG. 3  is an exploded perspective view of the tire-state detection device according to the first embodiment of the present invention;  FIG. 4  is a plan view of a printed wiring board according to the first embodiment of the present invention;  FIG. 5  is a side view of the printed wiring board according to the first embodiment of the present invention; and  FIG. 6  is a block diagram of an electrical circuit in the tire-state detection device according to the first embodiment of the present invention. 
     A tire-state detection device  10  is secured at a predetermined position on a rim  3  in an air chamber  2  of a tire  1 . A sensor equipped with a pressure detection element and a temperature detection element, described below, installed in the tire-state detection device  10  detects air pressure and temperature in the air chamber  2  of the tire  1 . The detection results are converted to digital values. The tire-state detection device  10  sends this data by generating digital information that includes these digital values. In addition to the digital values of the detection results, identification information that is unique to the tire-state detection device  10  is included in the digital information. 
     The tire-state detection device  10  is equipped with a case  100 ; a printed wiring board  300  and a battery  420  are housed in the case  100 . A detection circuit  400 , shown in  FIG. 6 , is formed on the printed wiring board  300 . Specifically, the detection circuit  400  is composed of a sensor  410 , a battery  420 , a main controller  430 , a transmitter  440 , and an antenna  450 . 
     The case  100  is composed of a case body  120  formed by a synthetic resin that allows radio waves to pass therethrough, and a top cover  110 . A rectangular opening  113  is formed in a position that faces the sensor  410  position, at a predetermined position in the cover  110 ; the opening  113  is covered by cover body  111  that has a ventilation hole  112 . 
     The sensor  410  is disposed on a top surface of the printed wiring board  300  and is composed of an air pressure detection element  411 , a temperature detection element  412 , and an A/D conversion circuit  413 . The air pressure detection element  411  and the temperature detection element  412  detect the air pressure and temperature inside the air chamber  2  in the tire  1 , respectively. The A/D conversion circuit  413  converts the detection results into digital values and outputs that to the main controller  430 . 
     A battery  420  is connected to the printed wiring board  300  by connecting conductors  421 ,  422 , to supply power to the detection circuit  400  that is formed on the printed wiring board  300 . One connecting conductor  421  is connected to a positive electrode of the battery  420  and to a top surface of the printed wiring board  300 ; the other connecting conductor  422  is connected to a negative electrode of the battery  420 , and to a back surface of the printed wiring board  300 . The potential of the negative electrode on the battery  420  in the detection circuit  400  is a reference potential (=0 V). 
     The main controller  430  is composed of a known CPU and memory; it receives detection results of the sensor  430  as digital values, and outputs them to the transmitter  440  by generating digital information that includes these digital values. In addition to the digital values of the detection results, identification information that is unique to the tire-state detection device  10 , such as its serial number, is included in the digital information. 
     The transmitter  440  uses radio waves of a predetermined frequency, such as 315 MHz, to send digital information input from the main controller  430 . 
     The antenna  450  is a helical antenna whose resonance frequency is set to the transmitting frequency of the transmitter  440 . The antenna  450  is fitted on the top face of the printed wiring board  300 . Furthermore, the antenna  450  is fitted to the top face of the printed wiring board  300  so that an axis of the helical antenna is parallel to the top surface of the printed wiring board  300 . 
     The printed wiring board  300  is composed of a stacked, multi-layer ceramic substrate. A conductor pattern  310  is disposed over substantially the entire back face of the printed wiring board  300 . The conductor pattern  310  is connected to the negative electrode of the battery  420 ; its potential is set to the reference potential (=0 V) of the detection circuit  400 . The back face of the printed wiring board  300  is secured on the case body  120  so that the back face, specifically the conductor pattern  310 , faces the bottom face of the case body  120 . Generally, the thickness of copper foil that forms the conductor pattern  310  is 12, 18, 35, or 70 microns (μm) and the like. However, the thickness of the conductor pattern  310  is preferred to be 18 μm or higher, in consideration of durability (strength to resist peeling). 
       FIG. 7  shows a lateral sectional view of the tire-state detection device  10  configured as described above, fitted to the rim  3 . As shown in the drawing, when the tire-state detection device  10  is fitted to the rim  3 , the case  100  is secured to the rim  3  top face so that the bottom surface of the case body  120  faces the rim  3  surface. By securing the case  100  to the rim  3  in this way, the conductor pattern  310  that is set to a reference potential is disposed between the antenna  450  fitted on the top face of the printed wiring board  300 , and the rim  3 . For that reason, the conductor pattern  310  (planar conductor) becomes an interface between the antenna  450  and the rim  3  when the tire-state detection device  10  is fitted to the rim. For that reason, it is possible to dramatically reduce the effect that the metal constituting the rim  3  has on the antenna  450  compared to conventional devices. Furthermore, a constant distance is maintained between the antenna  450  and conductor pattern  310  because of the thickness of the dielectric body of the printed wiring board  300 ; therefore, good antenna characteristics can be maintained even if the rim  3  shape is changed. 
     Therefore, it is not necessary to create an antenna  450  that is provided with optimum characteristics for each shape of the rim  3 , and it is possible to use the same antenna  450  with any shape of the rim  3 ; therefore, the manufacturing cost associated with each tire-state detection device  10  can be greatly reduced compared to conventional devices, and the tire-state detection device  10  can easily be mass produced. 
     Next, a second embodiment of the present invention will be explained. 
       FIG. 8  is an exploded perspective view of the tire-state detection device  10 A according to the second embodiment of the present invention;  FIG. 9  is a plan view of a printed wiring board  300 A according to the second embodiment of the present invention;  FIG. 10  is a side view of the printed wiring board  300 A according to the second embodiment of the present invention; and  FIG. 11  is a lateral sectional view of the tire-state detection device  10 A according to the second embodiment of the present invention fitted to the rim  3 . The same symbols are used for the same components described in relation to the first embodiment. Therefore, explanations of those symbols will be omitted. Also, points of difference between the first and the second embodiments are that a coil-shaped antenna  460  is disposed instead of the antenna  450  of the first embodiment, that a printed wiring board  300 A is used instead of the printed wiring board  300   a,  and that a conductor film  511  and an insulating film  512  are disposed at a bottom surface inside the case body  120 . 
     This antenna  460  has a spring-coil shape, whose resonance frequency is set to 315 MHz. A power supply point is disposed at a center thereof. This power supply point is connected to the transmitter  440  that is formed on the printed wiring board  300 A. The coil diameter of the antenna  460  is larger than that of the antenna  450 . Therefore, it is disposed outside of the periphery of the printed wiring board  300 A so that the coil axis of the antenna  460  is parallel with a side and the top surface of the nearby printed wiring board  300 A. 
     The conductor pattern  310  disposed on the back face of the printed wiring board  300  in the first embodiment has been eliminated from the printed wiring board  300 A. Therefore, the printed wiring board  300 A is the same as the printed wiring board  300 , except that the conductor pattern  310  has been eliminated. 
     As shown in the drawing, a conductor film  511  is disposed on a bottom face and an electrically insulating film  512  is disposed on a top face of the conductor film  511 , inside the case body  120 . This insulating film  512  prevents a conductive connection between the conductor film  511 , the printed wiring board  300 A and the antenna  460 . Also, the conductor film  511  is electrically connected to the negative electrode on the battery  420 ; the electric potential of the conductor film  511  is set to the reference potential (=0 V) of the detection circuit  400 . Furthermore, the thickness of the conductor film  511  is preferred to be 18 μm or thicker for the same reason described above. 
     As shown in  FIG. 11 , when the tire-state detection device  10 A configured as described above, is fitted to the rim  3 , the case  100  is secured to the top surface of the rim  3  so that the bottom surface of the case body  120  faces the rim  3  surface. By securing the case  100  to the rim  3  in this way, the conductor film  511  that is set to the reference potential is disposed between the antenna  460  and top surface of the rim  3 . For that reason, the conductor film  511  (planar conductor) becomes an interface between the antenna  460  and the rim  3  when the tire-state detection device  10 A is fitted to the rim  3 . Therefore, it is possible dramatically to reduce the effect that the metal constituting the rim  3  has on the antenna  460  compared to conventional devices. Furthermore, a constant distance is maintained between the antenna  460  and conductor pattern  511  because the printed wiring board  300 A is secured to a predetermined position on the case body  120 . Therefore, good antenna characteristics can be maintained even if the rim  3  shape is changed. 
     Therefore, it is not necessary to create an antenna  460  provided with optimum characteristics for each shape of the rim  3 , and it is possible to use the same antenna  460  with any shape of the rim  3 ; therefore, the manufacturing cost associated with each tire-state detection device  10 A can be greatly reduced compared to conventional devices, and the tire-state detection device  10 A can easily be mass produced. 
     Next, a third embodiment of the present invention will now be explained. 
       FIG. 12  is an exploded perspective view of the tire-state detection device  10 B according to the third embodiment of the present invention;  FIG. 13  is a plan view of a printed wiring board  300 B according to the third embodiment of the present invention;  FIG. 14  is a side view of the printed wiring board  300 B according to the third embodiment of the present invention;  FIG. 15  is an exploded, sectional view of an antenna according to the third embodiment of the present invention; and  FIG. 16  is a lateral sectional view of the tire-state detection device  10 B of the third embodiment of the present invention fitted to a rim. The same symbols are used for the same components described in relation to the first embodiment. Therefore, explanations of those symbols will be omitted. Also, points of difference between the first and the third embodiments are that the antenna  470  is disposed instead of the antenna  450  of the first embodiment, and that the printed wiring board  300 B is used instead of the printed wiring board  300 . 
     A widely expanded antenna  470  having a slightly larger shape than the printed wiring board  300 A of the first embodiment is formed on the printed wiring board  300 B. Also, a conductor pattern  320  that is the same as that described in relation to the first embodiment is disposed over substantially the entire back face of the printed wiring board  300 B. The conductor pattern  320  is connected to the negative electrode of the battery  420 ; the potential of the conductor pattern  320  is set to the reference potential (=0 V) of the detection circuit  400 . The thickness of the conductor pattern  320  is preferred to be 18 μm or thicker for the same reason described above. 
     The antenna  470  is composed of the printed wiring board (hereinafter referred to as a wiring pattern) formed on the printed wiring board  300 B, and a printed wiring board  500  connected to the printed wiring board  300 B by a plurality of connection conductors. 
     Specifically, the antenna  470  is composed of printed wiring boards  300 B and  500 , and cylindrically shaped connection conductors  711  to  718 ,  721  to  728  and  731 . Furthermore, the antenna  470  in this embodiment has the resonance frequency as described above (specifically, 315 MHz). 
     The printed wiring board  500  is composed of a dielectric substrate having a rectangular shape with a predetermined surface area, and a predetermined thickness. Pluralities of through-holes  521  to  528 , and  531  to  538  are disposed along both sides in the width direction, at predetermined spaces in a straight line parallel to the long sides. Also, As shown in  FIG. 15 , other through-holes  531  to  538  are disposed at positions to face substantially central positions in the spaces between the through-holes  521  to  528  disposed on the other side edge. 
     Furthermore, a plurality of linear printed wiring patterns (hereinafter referred to as wiring patterns)  511  to  518  is disposed spaced at equal distances between each other on a top surface of the printed wiring board  500 . 
     Also, one end of the wiring pattern  511  is linked to the second through-hole  522  from an end of the row of through-holes  521  to  528 ; the other end is linked to the through-hole  531  at an end of the row of the other through-holes  531  to  538 . One end of the wiring pattern  512  is linked to the third through-hole  523  from the end of the row of through-holes  521  to  528 ; the other end is linked to the through-hole  532  positioned at the second position from the end of the row of the other through-holes  531  to  538 . One end of the wiring pattern  513  is linked to the fourth through-hole  524  from the end of the row of through-holes  521  to  528 ; the other end is linked to the through-hole  533  positioned at the third position from the end of the row of the other through-holes  531  to  538 . One end of the wiring pattern  514  is linked to the fifth through-hole  525  from the end of the row of through-holes  521  to  528 ; the other end is linked to the through-hole  534  positioned at the fourth position from the end of the row of the other through-holes  531  to  538 . One end of the wiring pattern  515  is linked to the sixth through-hole  526  from the end of the row of through-holes  521  to  528 ; the other end is linked to the through-hole  535  positioned at the fifth position from the end of the row of the other through-holes  531  to  538 . One end of the wiring pattern  516  is linked to the seventh through-hole  527  from the end of the row of through-holes  521  to  528 ; the other end is linked to the through-hole  536  positioned at the sixth position from the end of the row of the other through-holes  531  to  538 . One end of the wiring pattern  517  is linked to the eighth through-hole  528  from the end of the row of through-holes  521  to  528 ; the other end is linked to the through-hole  537  positioned at the seventh position from the end of the row of the other through-holes  531  to  538 . An end of the wiring pattern  518  is disposed at a position of the same distance of the through-holes, from an end of the wiring pattern  517 ; the other end is linked to the through-hole  538  positioned at the eighth position from the end of the row of the other through-holes  531  to  538 . 
     Still further, a power supply point  514   a  is set at a predetermined position on the fourth wiring pattern  514  from the end. An end of the power supply wiring pattern  541  is conductively connected to this power supply point. The through-hole  542  disposed on the other end of the power supply wiring pattern  541  is set near a narrow side of one of the printed wiring boards  500 , as shown in the drawing. 
     A plurality of through-holes  621  to  628 , and  631  to  638  is disposed on both sides in the width direction of the printed wiring board  300 B, at predetermined spaces in a straight line parallel to the long sides. The positions of these through-holes  621  to  628 , and  631  to  638  correspond to the through-holes  521  to  528 , and  531  to  538  in the printed wiring board  500 . 
     Furthermore, a plurality of linear printed wiring patterns (hereinafter referred to as wiring patterns)  611  to  618  is disposed spaced at equal distances between each other on a top surface of the printed wiring board  300 B. A width of the length direction of the printed wiring board  300 B of each wiring pattern  611  to  618  is set to the same width as the wiring patterns  511  to  518  on the printed wiring board  500 ; the length of the short side direction is set to the same as the wiring patterns  511  to  518  on the printed wiring board  500 . 
     One end of the wiring pattern  611  is linked to the first through-hole  621  from the end of the row of through-holes  621  to  628 ; the other end is linked to the through-hole  631  positioned at the end of the row of the other through-holes  631  to  638 . One end of the wiring pattern  612  is linked to the second through-hole  622  from the end of the row of through-holes  621  to  628 ; the other end is linked to the through-hole  632  positioned at the second position from the end of the row of the other through-holes  631  to  638 . One end of the wiring pattern  613  is linked to the third through-hole  623  from the end of the row of through-holes  621  to  628 ; the other end is linked to the through-hole  633  positioned at the third position from the end of the row of the other through-holes  631  to  638 . One end of the wiring pattern  614  is linked to the fourth through-hole  624  from the end of the row of through-holes  621  to  628 ; the other end is linked to the through-hole  634  positioned at the fourth position from the end of the row of the other through-holes  631  to  638 . One end of the wiring pattern  615  is linked to the fifth through-hole  625  from the end of the row of through-holes  621  to  628 ; the other end is linked to the through-hole  635  positioned at the fifth position from the end of the row of the other through-holes  631  to  638 . One end of the wiring pattern  616  is linked to the sixth through-hole  626  from the end of the row of through-holes  621  to  628 ; the other end is linked to the through-hole  636  positioned at the sixth position from the end of the row of the other through-holes  631  to  638 . One end of the wiring pattern  617  is linked to the seventh through-hole  627  from the end of the row of through-holes  621  to  628 ; the other end is linked to the through-hole  637  positioned at the seventh position from the end of the row of the other through-holes  631  to  638 . One end of the wiring pattern  618  is linked to the eighth through-hole  628  from the end of the row of through-holes  621  to  628 ; the other end is linked to the through-hole  638  positioned at the eighth position from the end of the row of the other through-holes  631  to  638 . 
     Still further, a power supply point  615   a  is set at a predetermined position on the fifth wiring pattern  615  from the end. An end of the power supply wiring pattern  641  is conductively connected to this power supply point. The position of the power supply point  615   a  is set to a position that faces the power supply point  514   a  on the wiring pattern  514   a  on the printed wiring board  500 . 
     The other end  642  of the power supply wiring pattern  641  is disposed to reach to the transmitter  440  formed on the printed wiring board  300 B, as shown in the drawing. 
     The wiring patterns  511  to  518  on the printed wiring board  500 , and the wiring patterns  611  to  618  on the printed wiring board  300 B are conductively connected so that they form a helical shape in entirety, by the plurality of connection conductors  711  to  718 , and  721  to  728 . With this embodiment, cylindrically shaped conductors having a diameter of 0.75 mm, and a length of 8.0 mm are used as the connection conductors  711  to  718 , and  721  to  728 . 
     Specifically, each end  711   a  to  718   a  on the connection conductors  711  to  718  is inserted into and secured to the through-holes  521  to  528  in the printed wiring board  500 , and each end  712   a  to  718   a  of the connection conductors  712  to  718  is conductively connected to an end of the wiring patterns  511  to  517 . Also, each of the other ends  711   b  to  718   b  of the connection conductors  711  to  718  is inserted into and secured to the through-holes  621  to  628  in the printed wiring board  300 B, and each of the other ends  711   b  to  718   b  on the connection conductors  711  to  718  is conductively connected to an end of the wiring patterns  611  to  617 . Also, each end  721   a  to  728   a  of the connection conductors  721  to  728  is inserted into and secured to the through-holes  531  to  538  in the printed wiring board  500 , and each end  721   a  to  728   a  on the connection conductors  721  to  728  is conductively connected to the other end of the wiring patterns  511  to  518 . Also, each of the other ends  721   b  to  728   b  of the connection conductors  721  to  728  is inserted into and secured to the through-holes  631  to  638  in the printed wiring board  300 B, and each of the other ends  721   b  to  728   b  of the connection conductors  721  to  728  is conductively connected to the other end of the wiring patterns  611  to  618 . 
     Also, the through-hole  651  connected to the wiring pattern  652  is disposed on the printed wiring board  300 B. The through-hole  651  is connected to the through-hole  542  disposed on the printed wiring board  500  by the connection conductor  731 ; an end  731   a  of the connection conductor  731  is conductively connected to the wiring pattern  541 , and the other end  731   b  is conductively connected to the wiring pattern  652 . Furthermore, this wiring pattern  652  is connected to the output of the transmitter  440 . 
     According to the invention described above, an antenna element is formed by conductive wiring patterns  511  to  518 , and  611  to  618  on two printed wiring boards  500  and  300 B. A helically shaped antenna element is composed by these wiring patterns  511  to  518 , and  611  to  618  being sequentially and alternately conductively connected by connection conductors  711  to  718 , and  721  to  728 . With this configuration, it is possible to manufacture an antenna with the dimension precision of the printed wiring patterns  511  to  518 , and  611  to  618 , and the dimension precision of the connection conductors  711  to  718  and  721  to  728  (for example, ±18 μm), and to easily manufacture a high-performance antenna. Furthermore, in addition to making mass production possible, this configuration also achieves a highly reliable electrical connection with printed wiring boards, and antennas that have excellent dimension precisions can be manufactured. 
     As shown in  FIG. 16 , when the tire-state detection device  10 B configured as described above, is fitted to the rim  3 , the case  100  is secured to the rim  3  top surface so that the bottom surface of the case body  120  faces the rim  3  surface. By securing the case  100  to the rim  3  in this way, the conductor pattern  320  that is set to the reference potential is disposed between the antenna  470  and top surface of the rim  3 , so the conductor pattern (planar conductor)  320  becomes an interface between the antenna  470  and the rim  3  when the tire-state detection device  10 B is fitted to the rim  3 . For that reason, it is possible dramatically to reduce the effect that the metal constituting the rim  3  has on the antenna  470  compared to conventional devices. 
     Therefore, it is not necessary to create the antenna  470  that is equipped with optimum characteristics for each shape of the rim  3 , and it is possible to use the same antenna  470  with any shape of the rim  3 , so the manufacturing cost associated with each tire-state detection device  10 B can be greatly reduced compared to conventional devices, and the tire-state detection device  10 B can easily be mass produced. 
     Next, a fourth embodiment of the present invention will be explained. 
       FIG. 17  an external perspective view of the tire-state detection device  10 C according to a fourth embodiment of the present invention;  FIG. 18  is a plan view of the tire-state detection device  10 C according to the fourth embodiment of the present invention;  FIG. 19  is a lateral sectional view of the tire-state detection device  10 C according to the fourth embodiment of the present invention;  FIGS. 20 and 21  are external perspective views of the printed wiring board  300 C according to the fourth embodiment of the present invention; and  FIG. 22  is an external perspective view of an essential part of the printed wiring board  300 C according to the fourth embodiment of the present invention. The same symbols are used for the same components described in relation to the first to the third embodiments. Therefore, explanations of those symbols will be omitted. Also, points of difference between the first to the third embodiments and the fourth embodiment are that instead of the printed wiring board  300 B used in the third embodiment, a printed wiring board  300 C is used and provided with two substrates  351  and  352  disposed to have substantially the same shape, in parallel at a predetermined distance, and that a planar conductor plate  361  is equipped instead of the conductor pattern  320 ; these are housed in the case  130 . 
     As shown in  FIGS. 17 to 19 , the case  130  has a substantially rectangular parallelepiped shape, and has projections used to screw-clamp both ends in the length direction. It is composed of a body  131  and a cover body  132 . As shown in the  FIG. 19 , a storage space  134  is formed in the inside of the body  131  to house the printed wiring board  300 C. An opening of the storage space  134  is closed by securing the cover body  132  to the body  131  using screws  141 . Also, a ventilation hole  133  is formed in the cover body  132 . Air is able to flow through the ventilation hole  133  from outside into the storage space  134  even when the cover body  132  is secured to the body  131 . 
     As shown in  FIGS. 20 to 22 , the two, substantially rectangular printed wiring boards  351  and  352  of the printed wiring board  300 C are disposed substantially parallel at a predetermined distance. These are secured together by the connection conductors  711  to  718  and  721  to  728 , and  731  that constitute the antenna  470  and the printed wiring board  353  for connecting disposed therebetween. The antenna  470  is formed at one end of the length direction of the printed wiring board  300 C; at the other end, are fitted electronic components that constitute an electronic circuit that includes the sensor  410  and the battery  420 . The printed wiring board  353  for connection is soldered to each of the two printed wiring boards  351  and  352 . 
     Also, in the same way as in the third embodiment, a plurality of through-holes  621  to  628 , and  631  to  638  that compose the antenna  470  and a plurality of linear printed wiring patterns  611  to  618  are disposed on the printed wiring board  351 . On the other printed wiring board  352 , in the same way as in the third embodiment, a plurality of through-holes  521  to  528 , and  531  to  538  that compose the antenna  470  and a plurality of linear printed wiring patterns  511  to  518  are disposed. 
     Furthermore, a rectangular, planar conductor plate  361  is secured to the printed wiring board  300 C by four holders  371 . The planar conductor plate  361  is disposed at a position in the antenna  470  to be parallel to the printed wiring board  351  that is positioned at a bottom surface of the case body  131  when the printed wiring board  300 C is housed in the case  130 . The planar conductor plate  361  is secured by the holders  371  to maintain a predetermined space to the printed wiring board  351 . The planar conductor plate  361  is disposed instead of the conductor pattern  320  described in relation to the third embodiment; it is electrically connected to predetermined conductor patterns (a conductor pattern connected to the negative electrode on the battery  420 ) of the printed wiring board  351  and is set to the reference potential. Also, as shown in  FIG. 23  the holders  371  are secured in the four corners of the planar conductor plate  361 . As shown in  FIG. 24 , the holders  371  are equipped at both ends of a cylindrically shaped body  371   a  with cylindrically shaped projections  371   b  having a smaller diameter than the body  371   a.    
     Because the planar conductor plate  361  that is set to the reference potential is disposed at a predetermined distance from the printed wiring board  351  in this way, the planar conductor plate  361  that is set to the reference potential is arranged between the antenna  470  and the rim  3  by securing the case  130  to the rim  3  so that the bottom surface of the case body  131  touches the rim  3 . Therefore, when the tire-state detection device  10 C is fitted to the rim  3 , the planar conductor plate  361  becomes an interface between the antenna  470  and the rim  3 , allowing the the metal constituting the rim  3  to have dramatically less effect on the antenna  470  than experienced in conventional devices. 
     The antenna  470  has a resonance frequency of 315 MHz when the planar conductor plate  361  is fitted to the printed wiring board  351 . The characteristic curve is represented by the curved line A in  FIG. 25  in a Smith chart; the antenna impedance at 315 MHz is 50 ohms. The gap D between the printed wiring board  351  and the planar conductor plate  361  is set to 1.5 mm by the holders  371 . 
     Also, in a test example in  FIG. 25 , characteristic curves B and C are formed by varying the gap D between the printed wiring board  351  and the planar conductor plate  361 . The characteristic curve B in  FIG. 25  is formed with the gap D between the printed wiring board  351  and the planar conductor plate  361  is shifted 0.1 mm from 1.5 mm. At this time, the resonance frequency of the antenna  470  is 321 MHz, and its impedance is 66 ohms. The characteristic curve C in  FIG. 25  is formed with the gap D between the printed wiring board  351  and the planar conductor plate  361  is shifted 0.15 mm from 1.5 mm. At this time, the resonance frequency of the antenna  470  is 310 MHz, and its impedance is 70 ohms. 
     In this way, the antenna  470  characteristics (frequency and impedance) vary according to the distance of the antenna  470  from the planar conductor plate  361 , but no significant gain is obtained. In other words, if the distance between the antenna  470  and the planar conductor plate  361  is shifted even just a little (for example, 50 μm), the characters will vary greatly. This embodiment disposes holders  471  between the antenna  470  and the planar conductor plate  361  to maintain a constant distance D and overcome this issue. It is important that the gap D between the antenna  470  and the planar conductor plate  361  eliminates an effect of the dielectric body (it is best to have air without any loss in dielectric dissipation factor). Therefore, by disposing the planar conductor plate  361  to sandwich space and not to interfere with the antenna  470 , the configuration eliminates loss of the dielectric dissipation factor of the antenna. This makes manufacturing easier, and attains stable antenna characteristics no matter what shape of metal is nearby, so the antenna gain can be improved. 
     Also, as the characteristics in  FIG. 25  show, by varying the gap D between the printed wiring board  351  and the planar conductor plate  361 , the frequency characteristics of the antenna  470  fluctuate, so for the holders  371 , it is preferable to use materials that have low ratios of expansion and contraction caused by humidity and heat. For example, it is preferable to use Invar with a low ratio of expansion and contraction caused by humidity and temperature for the holders  371 .