Abstract:
A motor-driven antenna device in which an antenna element ( 10 ) has a short protruding length so that the outer dimension of the entire device is reduced, and in which damping of an AM-band signal by a signal path is restrained with a small stray capacitance. A rack cord ( 12 ) to be meshed with a pinion gear ( 24 ) is connected to a proximal portion of the antenna element ( 10 ) made of a helical coil over its entire length, and the antenna element ( 10 ) is caused to protrude and move to be housed by rotational driving of a motor drive section. The rack cord ( 12 ), the motor drive section and the pinion gear ( 24 ) are housed in a case ( 14 ), and a guide ( 26 ) to which the rack cord ( 12 ) is abutted and moves while bending is provided in the case ( 14 ). Thus, the rack cord ( 12 ) is abutted and moves to the guide ( 26 ) by the movement of the antenna element ( 10 ). In a protruding state, a linear conductor ( 30 ) is extended from the proximal end of the antenna element ( 10 ) to penetrate an earth conductive fitting ( 16 ) and electrically connected with a connection conductive fitting ( 32 ) at a position not in contact with the earth conductive fitting ( 16 ). A feeder conductive fitting ( 34 ) elastically contacts and is electrically connected with the linear conductor ( 30 ).

Description:
This application is a Continuation of PCT International Application No. PCT/JP98/00168 filed on Jan. 19, 1998, which designated the United States, and on which priority is claimed under 35 U.S.C. §120, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to a motor driven antenna apparatus in which a projection length of an antenna is shortened. Also, the present invention is related to a motor driven antenna apparatus in which a rack cord is moved while this rack cord abuts against a guide provided within a case and is curved. Then, the present invention is related to such a motor driven antenna apparatus that a stray capacitance is reduced, and this stray capacitance is produced in a signal path for transferring an antenna output from a base end of an antenna element. Furthermore, the present invention is related to such a motor driven antenna apparatus that a base end of an antenna element is coupled to a rack cord by employing a small number of structural components, and is electrically connected to a power supply fitting member. Furthermore, the present invention is directed to such a motor driven antenna apparatus that while an antenna element is lifted, a sound is not amplified from a receiver, whereas when the antenna element is reached to the lift-up end, sound is amplified from the receiver. 
     BACKGROUND OF THE INVENTION 
     In a conventional on-vehicle motor driven antenna apparatus for receiving an AM band signal and an FM band signal, a telescopic-shaped multi-stage antenna element is used, and a tip portion of a rack cord is coupled to a base end of the topmost element. Then, a pinion gear coupled to a motor drive unit is geared with the rack of this rack cord, and the rack cord is moved by either the normal rotation or the reverse rotation of the motor drive unit. As a result, the multi-stage antenna element is projected, or moved to be stored. This multi-stage antenna element is so arranged that a project length of this antenna element is set to be approximately 1 m in order that the FM band signal can be resonated, and also the rack cord owns a length of approximately 1 m. Therefore, under storage condition of the multi-stage antenna element, the rack cord is stored in a case such that this rack cord is wound on a winding drum which is rotatably provided by the rotary shaft. 
     There are high risks that since the projection length of this multi-stage antenna element is long, this multi-stage antenna element is broken by receiving unexpected strong force, and furthermore, this broken multi-stage antenna element may scratch other appliances. Also, it is not desirable in view of design aspect that the rod-shaped member having the length of approximately 1 m is projected from the vehicle body. Also, apparently, such a long rack cord owns flexibility so as to be wound/stored. On the other hand, this long rack cord must own the anti-buckling characteristic in order that the multi-stage antenna element is pushed up so as to be brought into the projected condition. As a result, since there are two different natures of the flexibility and the anti-buckling characteristic, the material of the rack cord and the size thereof are considerably restricted. Moreover, even when there is such a rack cord designed to realize a better balance between the flexibility and the anti-buckling characteristic, in the case that excessively large loads are given to this rack cord because of the problem of this multi-stage antenna element, or the characteristic is deteriorated due to lifetime, unbalance conditions occur as to the flexibility and the anti-buckling characteristic. As a result, there is such a problem that malfunction readily occurs. 
     As a consequence, the present invention provides the following motor driven antenna apparatus. That is, although the antenna effective length of the antenna element is equal to about 1 m similar to that of the prior art, the physical length thereof can be shortened, for instance, 15 cm, which is realized by the helical coil. When this very short antenna element is employed, this antenna can be hardly damaged and also can hardly scratch other electronic appliances because of such a short projection length. Also, the rack cord for projecting and storing this short antenna element can be similarly shortened, so that the rack cord can be easily designed. 
     As a consequence, with respect to the above-described antenna element, since the antenna effective length is equal to approximately 1 m, this antenna element can be resonated as to the FM band signal, so that a similar reception characteristic to that of the conventional long antenna element can be obtained. However, since the physical length thereof is shortened, the reception characteristic should be deteriorated as to the AM band signal. Then, an AM band signal having a low level, which is contained in the antenna output, is furthermore attenuated in the signal path defined from the antenna element to the receiver. Thus, the reception sensitivity for the AM receiver would be deteriorated. Then, there is a problem that the structure for coupling the base end portion of the antenna element to the tip portion of the rack cord would become complex, and furthermore, the mechanical strength cannot be sufficiently obtained. In addition, even when it is practically possible to obtain a sufficiently high reception characteristic under such a condition that the antenna element is completely projected, the signal level of the AM band signal would be furthermore low under such a condition that while this antenna element is projected in a half way, the physical length operable as the antenna is further shortened. When such a low level AM band signal is amplified by the receiver, noise is emphasized which may give unpleasant feelings to users. As a consequence, even when such an antenna structure is employed in which when the antenna element reaches the complete projection condition, the base end of the antenna element is elastically made in contact with the power supply member, no signal is applied to the amplification means until the antenna element is brought into the complete projection condition. Therefore, there is such a risk that the noise is produced from this amplification means. 
     SUMMARY OF THE INVENTION 
     The present invention has an object to provide such a motor driven antenna apparatus that a projection length of an antenna element is short, and further, an outer dimension of an entire antenna apparatus can be shortened. Also, the present invention has another object to provide such a motor driven antenna apparatus that while a stray capacitance is reduced and an external load impedance is increased, an attenuation of an AM band signal caused by a signal path through which an antenna output is transferred is suppressed. Then, the present invention has another object to provide such a motor driven antenna apparatus that a base end of an antenna element is firmly coupled to a rack cord by employing a small number of components, and is electrically connected to a power supply fitting member. Furthermore, the present invention has another object to provide a motor driven antenna apparatus in which an antenna output is not amplified by a receiver until an antenna element is brought into a completely projected state. 
     Then, a motor driven antenna apparatus, according to the present invention, is featured by that the motor driven antenna apparatus is arranged in such a manner that a rack cord is coupled in a coaxial direction to a base end portion of an antenna element, the entire portion of which is constructed of a helical coil; a pinion gear coupled to a motor drive unit is geared with a rack of this rack cord; and the antenna element is moved to be projected and stored by way of rotating drive of the motor drive unit. In accordance with the above arrangement, it is possible to obtain such a motor driven antenna apparatus that the projection length of the antenna element is very short. Also, the rack cord is short, and the rack cord can be very easily designed. Moreover, the outer diameter dimension of the entire antenna apparatus can be shortened. 
     Also, the motor driven antenna apparatus is arranged by that the rack cord, the motor drive unit, and the pinion gear are stored into a case; a guide is provided in this case by which the rack cord can be moved while the rack cord abuts against this guide with being curved; the antenna element is moved so as to be projected and stored by rotating drive of the motor drive unit; and the rack cord is moved while the rack cord abuts against the guide. In accordance with the above-described arrangement, since the rack cord is properly curved by the guide, the arrangement of this motor driven antenna apparatus can be made simpler than the conventional antenna apparatus in which the antenna element is wound on the winding drum to be stored thereinto. 
     Furthermore, a motor driven antenna apparatus, according to the present invention, is featured by that the motor driven antenna apparatus is arranged in such a manner that an antenna element is freely projected/stored from/into a cylindrical-shaped base along an axial direction; the cylindrical-shaped base is penetrated through a hole formed in a vehicle body; an earth fitting member is fitted into an outer peripheral portion of the cylindrical-shaped base; a sandwiching member is inserted into the outer peripheral portion of the cylindrical-shaped base and also a top nut is screwed to a tip portion of the cylindrical-shaped base; the vehicle body is sandwiched by the earth fitting member and the sandwiching member by screwing this top nut so as to fix the cylindrical-shaped base to the vehicle body; under such a condition that the antenna element is projected from the vehicle body, a straight-line-shaped conductive line is extended from a base end of the antenna element while having a length defined when the straight-line-shaped conductive line is penetrated through the earth fitting member; a connection fitting member movable along the axial direction is fixed on the cylindrical-shaped base and further such a position where the earth fitting member is not faced to this straight-line-shaped conductive line and is electrically connected thereto; and a power supply fitting member made of a conductive material and positioned in correspondence with the connection fitting member present at this position is arranged on an inner peripheral wall of the cylindrical-shaped base, so that the connection fitting member is elastically made in contact with the power supply fitting member so as to be electrically connected thereto. In accordance with this arrangement, the stray capacitance produced between the earth fitting member and the connection fitting member can be reduced, and the attenuation of the AM band signal caused by the signal path can be suppressed. 
     Furthermore, the motor driven antenna apparatus is arranged by that under such a condition that the antenna element is projected from the vehicle body, a base end of the antenna element is projected from either the vehicle body or an electric conductive member at the same potential as that of this vehicle body by a predetermined distance. In accordance with this arrangement, the stray capacitance produced between the antenna element and the vehicle body can be reduced, and the attenuation of the AM band signal caused by the signal path can be suppressed. 
     Then, alternatively, the motor driven antenna apparatus may be arranged by that a hot tube made of an electric conductive material and being long along an axis direction is arranged on an inner peripheral wall of the cylindrical-shaped base; a connection fitting member movable within the hot tube along the axis direction is arranged on the side of the base end of the antenna element and is electrically connected thereto; and under such a condition that the antenna element is projected from the vehicle body, the connection fitting member is electrically made in contact with the hot tube so as to be elastically connected thereto. In accordance with this arrangement, the earth fitting member is loosely fitted to the cylindrical-shaped base, the inner diameter of the earth fitting member can be increased, and the distance between the inner diameter of the earth fitting member and the outer diameter of the hot tube provided on the inner peripheral wall of the cylindrical-shaped base can be easily increased. The stray capacitance between these members can be decreased, and the attenuation of the AM band signal caused by the signal path can be suppressed. 
     Then, in addition, a motor driven antenna apparatus, according to the present invention, is featured by that the motor driven antenna apparatus is arranged in such a manner that an insulating resin covering member is provided while surrounding an outer peripheral portion of an antenna element; a connection conductive member made of an electric conductive member and for coupling one end of a rack cord is arranged on a lower end of this insulating resin covering member; a base end of said antenna element is electrically connected to this connection conductive member; a joint pipe made of an electric conductive material is engaged with this connection conductive member while bridging both the lower end portion of said insulating resin covering member and the connection conductive member and caulked so as to couple/fix the insulating resin covering member and the connection conductive member, and further to electrically connect the joint pipe to the connection conductive member; both the coupled insulating resin covering member and the rack cord are movably inserted into the cylindrical-shaped base along the axis direction; under projection condition of the antenna element, a power supply fitting member made of an electric conductive material is arranged on the cylindrical-shaped base in correspondence with the joint pipe; and the joint pipe is electrically connected to the power supply fitting member by a conductive elastic tongue. In accordance with the above-described arrangement, since the joint pipe is caulked, the antenna element can be firmly coupled to the rack cord by employing the small number of structural components. Moreover, the antenna element can be surely, electrically connected via this joint pipe to the power supply fitting member. 
     Also, a motor driven antenna apparatus, according to the present invention, is featured by such a motor driven antenna apparatus in which a rack cord is coupled to a base end portion of an antenna element in a coaxial direction, the entire portion of the antenna element being constructed of a helical coil; a pinion gear coupled to a motor drive unit is geared with a rack of this rack cord; the antenna element is lifted and lowered by a rotary drive of the motor drive unit; and an antenna output of the antenna element is amplified by amplifying means to output an amplified antenna output, wherein: the motor driven antenna apparatus is arranged by comprising a control means for causing the amplifying means to be set to an operating condition when the control means detects that the antenna element is located at a lift-up end. In accordance with this arrangement, since the amplifying means is brought into the operating condition under such a state that the antenna element is projected up to the lift-up end and thus the antenna output having the sufficiently high level can be obtained, there is no risk that the noise caused by the insufficient antenna output is amplified from the receiver. 
     Alternatively, the motor driven antenna apparatus may be arranged by employing a muting means for muting a signal path used to output the amplified signal of the amplifying means; and a control means for releasing the muting operation by the muting means when the control means detects that the antenna element is located at a lift-up end. In accordance with this structure, since the muting operation is released under such a condition that the sufficient antenna output is obtained, there is no risk that the noise is amplified, and this noise is caused by that no signal is entered into the receiver while the antenna element is being projected. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 shows an entire structural diagram of a motor driven antenna apparatus according to an embodiment of the present invention. 
     FIG. 2 is an enlarged cross-sectional view for showing a guide provided in a case indicated in FIG.  1 . 
     FIG. 3 is a diagram for representing a structure of a motor driven antenna apparatus according to another embodiment of the present invention. 
     FIG. 4 is a cross-sectional view for indicating a structure of a base end portion of an antenna element of the motor driven antenna apparatus shown in FIG.  1 . 
     FIG. 5 is an enlarged cross-sectional view for indicating an antenna element and a connection fitting member. 
     FIG. 6 indicates a joint pipe on which an electroconductive elastic tongue is provided; 
     FIG.  6 ( a ) is a front view; and 
     FIG.  6 ( b ) is a sectional view, taken along an arrow “A” of FIG.  6 ( a ). 
     FIG. 7 indicates one holder for sandwiching and also gearing with one end of a rack cord; 
     FIG.  7 ( a ) is a front view; 
     FIG.  7 ( b ) is a sectional view, taken along an arrow “B—B” of FIG.  7 ( a ); and 
     FIG.  7 ( c ) is a sectional view, taken along an arrow “C” of FIG.  7 ( a ). 
     FIG. 8 indicates the other holder for sandwiching one end of a rack cord; 
     FIG.  8 ( a ) is a front view; 
     FIG.  8 ( b ) is a sectional view; taken along an arrow “D—D” of FIG.  8 ( a ); and 
     FIG.  8 ( c ) is a sectional view, taken along an arrow “E” of FIG.  8 ( a ). 
     FIG. 9 is a cross-sectional view for showing an insulating resin rod in which an antenna element is formed with a connection base in an integral form by insert molding. 
     FIG. 10 is a circuit diagram of a motor driven antenna apparatus according to an embodiment of the present invention. 
     FIG. 11 shows a structure of a motor driven antenna apparatus according to a second embodiment of the present invention, namely mainly indicates a structure portion of this second embodiment different from the embodiment shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to accompanying drawings, the present invention will be described in detail. 
     In FIG.  1  and FIG. 2, an antenna element  10  whose entire portion is constructed of a helical coil is covered by an insulating resin in an integral body, and a tip portion of a rack cord  12  is properly coupled to a base end portion of this antenna element  10  (will be explained later). An antenna effective length of this antenna element  10  is approximately 1 m, and a physical length thereof is selected to be, for example, approximately 15 cm. Then, a rack is formed on one side surface of the rack cord  12 , and this rack cord  12  has a properly-selected anti-buckling characteristic and properly-selected flexibility. Furthermore, a projection length of the antenna element  10  under projected state may be made substantially same length as the physical length, namely approximately 15 cm. Accordingly, the length of the rack cord  12  may be made slightly longer than approximately 15 cm. 
     Also, antenna output power may be properly supplied to the base end portion of the antenna element  10  by way of such a member for coupling a tip portion of the rack cord  12  to the base end portion of the antenna element  10  (will be discussed later). Then, both the antenna element  10  and the rack cord  12  are constructed in such a manner that these antenna element and rack cord are freely movable in a cylindrical-shaped base  15  formed on a case  14  along an axial direction, and furthermore, movement of these antenna element and rack cord along an extract direction is restricted with a predetermined projection length. Moreover, the cylindrical-shaped base  15  is fixed on a vehicle body  22  by employing an earth fitting member  16  and a top nut  20 , or the like. This earth fitting member  16  is inserted into an outer peripheral portion of the cylindrical-shaped base  15 . This top nut  20  is engaged with a sandwiching member  18  and a tip portion of the cylindrical-shaped base  15 . It should be understood that the earth fitting member  16  is inserted into the cylindrical-shaped base  15  from the tip direction, and the insertable dimension of this earth fitting member  16  is restricted by a properly-designed structure. 
     Within the case  14 , a motor drive unit (not shown) is stored/arranged, to which a pinion gear  24  geared with the rack of the rack cord  12  is coupled. Furthermore, a guide  26  of the rack cord  12  is formed in the case  14  on the down stream side from the pinion gear  24  when this pinion gear  24  is driven along the antenna storage direction, and is furthermore, formed with a pipe-shaped case member  14   a  in an integral form. This pipe-shaped case member  14   a  is formed on the case  14 . This guide  26  is formed having a large curvature in such a way that the rack cord  12  abuts against this guide  26  within the case  14 , so that this rack cord  12  is curved in a U-shaped range, or at an angle larger than, or equal to 180 degrees. 
     With employment of the above-described structure, the projection length of the antenna element  10  can be made very short, and moreover, the length of the rack cord  12  can be similarly shortened. Then, when the pinion gear  24  is driven along the antenna storage direction, the rack cord  12  is moved with being guided by the guide  26  while this rack cord  12  abuts against this side wall thereof and is curved. In this case, since the rack cord  12  is short, the anti-buckling performance required for projecting the antenna may be decreased, as compared with the anti-buckling performance required for projecting the conventional antenna by approximately 1 m. This implies that the rack cord  12  may have the higher flexibility. As a result, the rack cord  12  may be moved in a smooth manner while being guided by the guide  26 . As previously described, since the rack cord  12  is curved to be stored into the case  14  and the case member  14   a , the outer dimension of the overall apparatus can be shortened. In addition, the structure of this antenna apparatus becomes simple, as compared with such a conventional antenna apparatus that the freely rotating wind-up drum is used. In response to the shape of the guide  26 , the transverse width of the case  14  may be properly designed, and this transverse width of the case  14  may be decreased. This guide  26  owns such a curvature required to curve the rack cord  12  in a substantially U shape. Also, since there is no serious limitation in the flexibility and the anti-buckling characteristic, the rack cord  12  can be very easily designed. 
     Another embodiment shown in FIG. 3 owns the following different point from the embodiment shown in FIG.  1 . That is, the guide  28  is formed in such a manner that this guide  28  is directed along the substantially transverse direction on the down stream side of the pinion gear  24  with respect to the projection direction of the antenna element  10  and the storage/move direction thereof. The rack cord  12  is stored into this case  14  while being curved in a substantially L-shape in such a way that this rack cord  12  is guided by the guide  28  formed on the case member  14   b  formed on the case  14 . In accordance with another embodiment with employment of the above structure shown in FIG. 3, although the rack cord  12  is curved at an angle of about 90 degrees in the vicinity of the pinion gear  24 , this rack cord  12  may be set in a substantially straight-line fashion on the down stream side thereof, so that there is a small curve. Accordingly, although the transverse width of the case  14  is increased, the friction resistance force caused by the abutment/movement between the guide  28  and the rack cord  12  may be small, and furthermore, torque required to rotary-drive the pinion gear  24  may be decreased. 
     It should be understood that the present invention is not limited only to the above-explained motor driven antenna apparatus according to the above-described embodiment, but may be applied to any other motor driven antenna apparatuses such that the rack cord  12  is moved to be stored while being guided/curved along the guides  26  and  28  without employing the rotating wind-up drum. Furthermore, the rack cord  12  may be alternatively moved in a straight line fashion without being curved. 
     Next, the structure of the base end portion of the antenna element  10  employed in the motor driven antenna apparatus will now be explained with reference to FIG. 4 to FIG.  9 . First, in FIG. 4, the cylindrical-shaped base  15  is formed from the case  14  (not shown), and this cylindrical-shaped base  15  is fixed in a hole  22   a  of the vehicle body  22  by being screwed by the top nut  20  by employing the earth fitting member  16  and the sandwiching member  18 . Under the projection condition of the antenna element  10 , such a length is defined by that a straight-line-shaped conductive line  30  extended from the base end of the antenna element  10  penetrates through the earth fitting member  16 , and a connection fitting member  32  is electrically connected to the earth fitting member  16  at an edge portion thereof at a position not opposed to the earth fitting member  16 , which is formed with the antenna element  10  in an integral body. Apparently, this connection fitting member  32  may be freely moved within the cylindrical-shaped base  15  along the axial direction in connection with the movement of the antenna element  10  along the axial direction. Then, a power supply fitting member  34  is arranged on an inner peripheral wall of the cylindrical-shaped base  15  in correspondence with the position of the connection fitting member  32 , and then, this power supply fitting member  34  is elastically made in contact with conductive/elastic tongues  36 ,  36 , - - - , so as to be electrically connected. The conductive/elastic tongues  36 ,  36 , - - - , have electric conductive characteristics, and are formed on the outer peripheral portion of the connection fitting member  32 . An electronic circuit storage housing unit  38  is provided in the vicinity of this power supply fitting member  34  independent from the cylindrical-shaped base  15 . Then, such an electronic circuit  40  as an amplifying circuit and a matching circuit is stored into this electronic circuit storage housing unit  38 . Furthermore, the electronic circuit  40  is electrically connected to the power supply fitting member  34  by way of a conductive line path  42  having a short pre-selected dimension as a signal path. 
     Then, the antenna element  10  is arranged in such a manner that while the base end of this antenna element  10  is projected upwardly from the sandwiching member  18  under projection condition of this antenna element  10 , this base end is separated by a pre-selected distance. In this case, the sandwich member  18  is an electric conductive member, the potential of which is equal to that of the vehicle body  22 . If this sandwich member  18  is not such an electric conductive member, then the base end of the antenna element  10  may be alternatively projected in such a manner that this base end is separated upwardly from the vehicle body  22  by a pre-selected distance. In other words, the base end of the antenna element  10  may be separated from either the vehicle body  22  or the conductive member whose potential is equal to that of this vehicle body  22  by a pre-selected distance. 
     Moreover, referring now to FIG. 5 to FIG. 9, a detailed description is made of such a structure that the antenna element  10  is formed with the connection fitting member  32  in an integral body. That is, the straight-line-shaped conductive line  30  is extended from the base end of the antenna element  10  constructed of the helical coil, and the tip portion of this straight-line-shaped conductive line  30  is electrically connected to a connection base  44  made of an electric conductive member by way of the soldering manner. Then, both the outer diameter of the antenna element  10  and the outer peripheral portion of the connection base  44  are fixed by the molding. Then, as indicated in FIG. 9, an insulating resin rod  46  is formed by the insert molding, and these structural elements are formed in an integral body. This integral-formed antenna element  10  is entirely covered with an antenna cover  48  made of an insulating resin. A larger-diameter portion is provided on an outer peripheral portion of an intermediate portion of this antenna cover  48  along the axial direction so as to set such a condition that the top nut  20  cannot pass through this larger-diameter portion, namely preventing pass-through along the projection direction. A hole  44   a  having a bottom is formed in a lower end surface of the connection base  44 , and while the tip portion of the rack cord  12  is sandwiched and also geared, a first holder  50  and a second holder  52  are inserted into this hole  44   a  having the bottom. 
     As illustrated in FIG.  7  and FIG. 8, a concave/convex portion  50   a  which is geared with the rack of the rack cord  12  is provided on an inner peripheral wall of the first holder  50 . Also, projections  50   b ,  50   b , - - - , and engaging holes  52   b ,  52   b , - - - , are provided opposite to each other on an abutting plane between the first holder  50  and the second holder  52 . Furthermore, larger-diameter portions  50   c  and  52   c  are formed on the first and second holders  50  and  52  in such a way that these larger-diameter portions  50   c  and  52   c  are continued around an axis. Both the first holder  50  and the second holder  52  which are combined with each other by sandwiching the tip portion of the rack cord  12  are inserted into the hole  44   a  having the bottom of the connection base  44 , and are rotatable around the axis. 
     Furthermore, a fixing nut member  54  made of an conductive material is screwed with the lower end portion of the connection base  44  in order that both the first and second holders  50  and  52  are not extracted from the hole  44   a  having bottom. A hole is formed in this fixing nut member  54 , and this hole has such a diameter through which both the larger-diameter portions  50   c  and  52   c  of the first and second holders  50  and  52  are not extracted. Thus, the rack cord  12  can be freely rotated around the axis with respect to the connection base  44 , and moreover, can be prevented to be extracted therefrom, and is coupled to the connection base  44 . An outer diameter of this fixing nut member  54  is substantially equal to that of the antenna cover  48 . In addition, grooves around the axis are properly formed in the lower end portions of the fixing nut member  54  and of the antenna cover  48 . 
     Furthermore, as shown in FIG. 6, a joint pipe  56  made of a conductive material is fitted, while bridging over the lower end portion of the antenna cover  48  and the outer peripheral portion of the fixing nut member  54 , and this joint pipe  56  is caulked, so that the antenna cover  48  is coupled to be fixed with the fixing nut member  54 . A plate member is provided to wind the outer peripheral portion of this joint pipe  56  around the axis, and is fixed thereon by way of the spot welding manner and further is electrically connected thereto. A plurality of conductive elastic tongues  36 ,  36 , - - - , made of conductive materials and also having elastic characteristics are formed on this plate member while being expanded along the outer peripheral direction. In this case, the connection base  44 , the fixing nut member  54 , and the joint pipe  56  are electrically connected to each other, which may form the connection fitting member  32 . Also, the connection conductive member is formed by the connection base  44  and the fixing nut member  54 . 
     With employment of the above-described structure, since the base end of the antenna element  10  is separated from either the vehicle body  22  or the conductive member having the same potential as that of this vehicle body  22  by a predetermined distance , so that this base end is projected from the vehicle body  22 , the stray capacitance produced between the base end portion of the antenna element  10  and the vehicle body  22  can be suppressed to become small. Also, under such a projection state of the antenna element  10 , both the connection fitting member  32  and the power supply fitting member  34  are not located at the positions corresponding to the earth fitting member  16 . Thus, the stray capacitance produced among the earth fitting member  16 , the connection fitting member  32 , and also the power supply fitting member  34  can be considerably reduced, as compared with the conventional case that these connection fitting member, power supply fitting member, and earth fitting member are located at the positions corresponding to the earth fitting member  16 . Then, since the diameter of the straight-line-shaped conductive line  30  which passes through the earth fitting member  16  is narrow, the distance between this straight-line-shaped conductive line  30  and the earth fitting member  16  is extended and further the opposite area is reduced, so that the stray capacitance produced in the straight-line-shaped conductive line  30  is very small. In addition, the stray capacitance may be furthermore decreased by such a fact that the conductive line  42  defined from the power supply fitting member  34  to the electronic circuit  40  becomes short. As a consequence, a total capacitance value of stray capacity produced in the signal paths defined from the base end of the antenna element  10  to the electronic circuit  40  is small. As a result, a large external load impedance can be obtained, and thus, the attenuation of the AM band signal is small, so that the AM band signal having the large level is transferred to the electronic circuit  40 . 
     Also, since the first holder  50  and the second holder  52  which are inserted into the hole  44   a  having the bottom of the connection base  44  can be rotated around the axis within the hole  44   a  having the bottom, the antenna cover  48  can be relatively rotated around the axis with respect to the rack cord  12 . As a consequence, even when the force along the twist direction is given to the antenna cover  48 , there is no risk that the rack cord  12  is twisted. Moreover, since the joint pipe  56  for coupling the rack cord  12  to the lower end portion of the insulating resin covering member for surrounding the antenna element  10  is commonly used as the structural member for electrically connecting the base end of the antenna element  10  to the power supply fitting member  34 , a total number of components required to construct the motor driven apparatus can be reduced. Then, the connection fitting member  32  for coupling the rack cord  12  is formed in such a manner that both the first holder  50  and the second holder  52  for sandwiching the rack cord  12  and also geared with the rack are inserted into the hole  44   a  having the bottom of the connection base  44 , and furthermore the fixing nut member  54  is screwed with the connection base  44 , which can prevent the first and second holders  50  and  52  from being extracted from the hole  44   a  having the bottom. As a consequence, since the joint pipe  56  is caulked, the fixing nut member  54  is fixed, so that extraction of the rack cord  12  can be firmly blocked. 
     Also, since the plate member on which a plurality of conductive elastic tongues  36 ,  36 , - - - , are formed is wound on the joint pipe  56  to be welded, the strength of the joint pipe  56  is high, as compared with such a case that the joint pipe  56  itself is cut/raised and then, the conductive elastic tongues  36 ,  36 , - - - , are formed. In this case, since the joint pipe  56  itself is fixed by the caulking manner, it is not preferable to employ a thick plate member, or an excessively hard material. As a consequence, since the plate member on which the conductive elastic tongues  36 ,  36 , - - - , are formed is wound on the joint pipe to be fixed, this joint pipe can be properly caulked and furthermore, the structure having the high strength can be made. Furthermore, since a plurality of conductive elastic tongues  36 ,  36 , - - - , formed on the joint pipe  56  are elastically made in contact with the power supply fitting member  34 , the electric connection can be firmly established. 
     Furthermore, since both the antenna element  10  and the connection base  44  electrically connected to this antenna element  10  are embedded into the insulating resin rod  46  by the insert molding in an integral molding, a total number of structural components can be reduced, and the antenna element can be easily assembled with respect to the cylindrical-shaped base  15 . Moreover, there is no fluctuation in the dimensions between the antenna element  10  and the connection base  44 , so that a constant antenna characteristic can be obtained. 
     In the above-described embodiment, the connection fitting member  32  is arranged by a plurality of members, the present invention is not limited thereto. Alternatively, the present invention may be applied to any structures manufactured by such that the rack cord  12  maybe coupled to this connection fitting member  32  and the base end of the antenna element  10  may be electrically connected to the joint pipe  56 . Also, in the embodiment, the electronic circuit storage housing unit  38  is provided independent from the case  14  in order to arrange the electronic circuit  40  in the vicinity of the power supply fitting member  34 . Alternatively, the electronic circuit  40  may be arranged in the case  14 , and may be connected to the power supply fitting member  34  by employing such a coaxial cable having a small attenuation. Furthermore, in the embodiment, the line material for constituting the antenna element  10  is extended at the base end along the axis so as to be used as the straight-line-shaped conductive line  30 . Alternatively, any electrical conductive lines independent from the antenna element  10 , and rod-shaped members made of conductive materials may be used to constitute the straight-line-shaped conductive line  30 . Furthermore, the above-described embodiment has explained such an assumption that the present invention is embodied in the on-vehicle motor driven antenna apparatus. Alternatively, the present invention may be similarly applied to other types of antenna apparatuses which have the cylindrical-shaped base  15  fixed to the vehicle body  22  by employing the earth fitting member  16 , and capable of freely projecting/extracting the antenna elements. For example, it is alternatively possible to realize an antenna apparatus in which an antenna element is manually extracted and/or drawn. 
     Moreover, in the above-described embodiment, the antenna cover  48  is employed to cover the insulating resin rod  46  which is insert-molded by embedding the antenna element  10 . Alternatively, both the insulating resin rod  46  and the antenna cover  48  may be formed in an integral form. In other words, if the antenna element  10  is embedded into the resin and at least the outer peripheral portion thereof are not exposed, then the insulating resin rod  46  may also function as the antenna cover  48 . As a result, the lower end portion of the insulating resin rod  46  functioning as the insulating resin covering member may be coupled/fixed with the fixing nut member  54  by the joint pipe  56 . It is of course possible to arrange the antenna apparatus such that this insulating resin rod  46  is formed in a pipe shape, and no “drop” is produced in the insulating resin when the insert molding is carried out. 
     Also, the conductive elastic tongues  36 ,  36 , - - - , are formed on the joint pipe  56  by way of the welding manner. The present invention is not limited to this structure. For example, the joint pipe  56  itself may be cut/raised to form these conductive elastic tongues. In addition, such conductive elastic tongues  36 ,  36 , - - - , may be provided which are elastically made in contact with the joint pipe  56  on the side of the power supply fitting member  34  so as to be electrically connected thereto. 
     Referring now to FIG. 10, a description will be made of a circuit for controlling the motor driven antenna apparatus according to the present invention. A power supply terminal “+B” is connected via a forward direction diode and a smoothing circuit to a drain of a P-channel type MOS field-effect transistor FET 1 . A source of this field-effect transistor FET 1  is connected to the ground via a series circuit of an N-channel type MOS field-effect transistor FET 2  and another P-channel type MOS field-effect transistor FET 3 . A source of the field-effect transistor FET 1  is connected to the ground via a series circuit of another N-channel type MOS field-effect transistor FET 4  and another P-channel type MOS field-effect transistor FET 5 . Then, a drive motor M is inserted between a joint point between the field-effect transistor FET 2  and the field-effect transistor FET 3 , and another joint point between the field-effect transistor FET 4  and the field-effect transistor FET 5 . Furthermore, the source of the field-effect transistor FET 1  is connected to a collector of a transistor Tr 1 , and further, is connected via a resistor R 1  to a collector of a transistor Tr 2  and also the gate of the field-effect transistor FET 2 . Also, the source of the field-effect transistor FET 1  is connected via another resistor R 2  to a collector of a transistor Tr 4  and the gate of the field-effect transistor FET 4 . Then, an UP/DOWN terminal is connected to both the emitter of the transistor Tr 3  and an input terminal of an inverter INV. Also, this UP/DOWN terminal is connected via a smoothing circuit to both the base of the transistor Tr 1 , and an UP timer/DOWN timer of a fail-safe timer. Either an UP signal for lifting the antenna element  10  or a DOWN signal for lowering the antenna element  10  is selectively supplied to the UP/DOWN terminal from a receiver. The output terminal of the UP timer and the output terminal of the DOWN timer are connected to the gate of the field-effect transistor FET 1 . When the UP signal corresponding to the “H” signal is supplied to the UP/DOWN terminal of the UP timer, this UP timer starts the time counting operation, and outputs the “H” signal while executing the time counting operation. Also, when the DOWN signal corresponding to the “L” signal is supplied to the UP/DOWN terminal of the DOWN timer, this DOWN timer starts the time counting operation, and outputs the “H” signal while executing the time counting operation. These time measuring time is set to a time duration during which the antenna element  10  can be sufficiently lifted, or lowered. Furthermore, the emitter of the transistor Tr 1  is connected to the base of the transistor Tr 2 , and via a series circuit of a resistor R 3  and a delay circuit DT 1  to the gate of the field-effect transistor FET 3 , and also via a series circuit of a resistor R 4  and a delay circuit DT 2  to the gate of the field-effect transistor FET 5 . Then, the joint point between the resistor R 3  and the delay circuit DT 1  is connected via a resistor R 5  to the collector of the transistor Tr 2 , and also via a DOWN switch to the ground. Also, the joint point between the resistor R 4  and the delay circuit DT 2  is connected via the UP switch to the ground, and also is connected via the delay circuit DT 3  to the base of the transistor Tr 3 . It should be understood that when the antenna element  10  is lowered up to the lower end, the DOWN switch is turned ON, whereas when the antenna element  10  is lifted up to the lift-up end, the UP switch is turned ON. Furthermore, the collector of the transistor Tr 3  is connected to a power supply terminal of an amplifying circuit AMP functioning as an amplifying means for amplifying an antenna output. Furthermore, the power supply terminal of this amplifying circuit AMP is grounded via a solenoid coil “S” of a relay. Then, the output terminal of the amplifying circuit AMP is connected to a normally-open contact “b” of the relay. A normally-close contact “a” of this relay is grounded via a resistor R 6  having 75 ohms, and a common contact “c” thereof is connected to a signal path to the receiver. A muting means is constituted by these contacts of the relay. Also, the output terminal of the inverter INV is connected to the base of the transistor Tr 4 , and also to the collector of the transistor Tr 2 . Both the emitter of the transistor Tr 2  and the emitter of the transistor Tr 4  are grounded. The resistance value of the resistor R 5  is set to be very small, as compared with the resistance value of the resistor R 3 . 
     With employment of the above-described circuit arrangement, under such a condition that the antenna element  10  is lowered so that the DOWN switch is turned ON and the UP switch is turned OFF, when the UP signal is supplied to the UP/DOWN terminal, the transistor Tr 1  is first turned ON, and the UP timer commences the time counting operation, and further, the field-effect transistor FET 1  is turned ON. Since this transistor Tr 1  is turned ON, the transistor Tr 2  is similarly turned ON and the field-effect transistor FET 2  is turned ON. Also, the field-effect transistor FET 5  is turned ON. In this case, both the field-effect transistors FET 3  and FET 4  are turned OFF. As a result, the drive motor M is rotatably driven, so that the antenna element  10  starts to be lifted. Since the antenna element  10  is lifted, the DOWN switch is turned OFF. However, since the value of the resistor R 5  is very small, the field-effect transistor FET 3  is continued to be turned OFF. Then, when the antenna element  10  is moved up to the lift-up end to cause the UP switch to be turned ON, the field-effect transistor FET 5  is turned OFF after the delay time defined by the delay circuit DT 2 . Also, after the delay time defined by the delay circuit DT 3 , the transistor Tr 3  is turned ON, so that the operating voltage is applied to the amplifying circuit AMP so as to be brought into the operating condition, and also the relay is brought into the energizing state. Therefore, both the normally-open contact “b” and the common contact “c” are become conductive, so that the muting operation is released. Then, furthermore, the UP timer completes the time counting operation, and the field-effect transistor FET 2  is also turned OFF. 
     Also, when the DOWN signal is supplied to the UP/DOWN terminal under this condition, applying of the operating voltage to the emitter of the transistor Tr 3  is interrupted, so that the amplification operation of the amplifying circuit AMP is stopped, the relay is brought into the non-energizing state, and both the normally-close contact “a” and the common contact “c” are become conductive. Thus, the signal path is grounded via the resistor R 6  having the resistance value of 75 ohms, so that the muting operation is carried out. Also, the DOWN timer starts the time counting operation thereof and the output of this DOWN timer becomes the “H” signal, so that the field-effect transistor FET 1  is turned ON. Then, since the output of the inverter INV is the “H” signal, this “H” signal is applied via the resistor R 5  to the gate of the field-effect transistor FET 3  so as to turn ON the field-effect transistor FET 3 , and the transistor Tr 4  is also turned ON, so that the field-effect transistor FET 4  is turned ON. In this case, both the field-effect transistors FET 2  and FET 5  are turned OFF. As a result, the drive motor M is rotatably driven along the reverse direction, so that the antenna element  10  starts to be lowered. Then, when the antenna element  10  is reached to the lower end, the DOWN switch is turned ON and the field-effect transistor FET 3  is turned OFF, so that the drive motor M is stopped. Furthermore, when the DOWN timer completes the time counting operation, the field-effect transistor FET 1  is turned OFF. It should be understood that the relay is brought into the non-energizing state and the muting operation is carried out in a time period other than such a time period during which the amplifying circuit AMP is brought into the operation condition. 
     As previously explained, in the embodiment shown in FIG. 10, when the antenna element  10  is reached to the lift-up end, the amplifying circuit AMP is brought into the operation condition. As a consequence, while the antenna element  10  is being lifted up and the signal level of the antenna out is still low, this low-leveled antenna output is not amplified. Also, under such a condition that the connection fitting-member  32  to which the base end of the antenna element  10  is connected is not yet elastically made in contact with the power supply member  34 , the signal path is grounded via the resistor having the resistance value of 75 ohms, so that the muting operation is carried out. As a consequence, the sounds are not implied in the receiver, and there is no risk that unpleasant feeling caused by the noise is given to the users. 
     Furthermore, a motor driven antenna apparatus according to another embodiment of the present invention will now be made with reference to FIG.  11 . In FIG. 11, this embodiment owns the following different structure from that of the above embodiment shown in FIG.  4 . That is, a hot tube  58  made of a conductive material, which is long along an axis direction, is arranged on an inner peripheral wall of the cylindrical-shaped base  15 , and a base end portion thereof is electrically connected via the electric conductive line path  42  to the electronic circuit  40 . Under such a condition that the antenna element  10  is projected from the vehicle body  22 , the connection fitting member  32  is arranged on the side of the base end thereof at a position where this connection fitting member  32  is faced with the earth fitting member  16 , and also is electrically connected to the base end of the antenna element  10 . This connection fitting member  32  is moved within the hot tube  58  along the axis direction while the antenna element  10  is moved along the axis direction. At least under the projection condition of the antenna element  10 , this connection fitting member  32  is elastically made in contact with the hot tube  58  by employing the conductive elastic tongues  36 ,  36 , - - - , so as to be electrically connected thereto. 
     With employment of the above-described arrangement, the dimension defined from the tip portion of the antenna element  10  up to the connection fitting member  32  can be made shorter than that of FIG. 4, which may suitably shorten the overall dimension of the motor driven antenna apparatus. In this case, the stray capacitance produced between the hot tube  58  and the earth fitting member  16  can be sufficiently reduced by setting the distance between the outer diameter of the hot tube  58  and the inner diameter of the earth fitting member  16  to be a long distance. Then, the earth fitting member  16  is loosely fitted to the cylindrical-shaped base  15 , so that the inner diameter of this earth fitting member  16  can be readily set to the large diameter. In accordance with the experiments executed by the Inventors, since the inner diameter of the earth fitting member  16  was made more than 1.2 times larger than the outer diameter of the hot tube  58 , the stray capacitance between these members could be sufficiently reduced. Thus, the AM band signal could be applied to the electronic circuit  40  without any practical problems. 
     INDUSTRIAL APPLICABILITY 
     As previously described, in accordance with the motor driven antenna apparatus of the present invention, since the length of this antenna element is shortened, the our dimension of the entire antenna apparatus can be shortened. As a result, this antenna apparatus is suitable for the on-vehicle antenna arranged on the vehicle body. Moreover, the length of the rack cord for projecting and storing the antenna element can be made short, the structure of this rack cord can be made simple, and no malfunction thereof occurs. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.