Patent Publication Number: US-7586461-B2

Title: Antenna unit having improved antenna radiation characteristics

Description:
This application claims priority to prior Japanese patent applications JP 2005-219018 and 2005-369430, the disclosures of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   This invention relates to a pole-type antenna unit and, in particular, to a pole-type and personal-type miniature antenna unit for a digital radio receiver for receiving an electric wave from an artificial satellite (that may be called a “satellite wave”) or an electric wave on the ground (that may be called a “terrestrial wave”) to listen in a digital radio broadcasting. 
   In recent years, a digital radio receiver, which receives the satellite wave or the terrestrial wave to listen the digital radio broadcasting, has been developed and is put to practical use in the United States of America. The digital radio receiver is generally mounted on a mobile station, such as an automobile, and can receive an electric wave having a frequency of about 2.3 gigahertz (GHz) to listen in a radio broadcasting. That is, the digital radio receiver is a radio receiver which can listen in a mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance frequency) λ thereof is equal to about 128.3 mm. In addition, the terrestrial wave is an electric wave in which a signal where the satellite wave is received in an earth station is frequency shifted a little and is retransmitted in a linearly polarized wave. That is, the satellite wave is a circularly polarized wave, while the terrestrial wave is the linearly polarized wave. 
   As described above, since the electric wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting, an antenna unit for receiving such an electric wave should be installed outdoors. 
   As digital radio receivers, there are a type adapted to be mounted in an automobile, a type adapted to be installed in a house or the like, and a type that is portable using a battery as a power source. 
   As a specific example of the portable digital radio receiver, there is available a portable electronic device such as a portable sound device. This portable electronic device comprises, in addition to a digital tuner for listening to the digital radio broadcasting, for example, an optical disk drive for reproducing an optical disk such as a compact disk (CD), an amplifier, and a speaker, which are integrally incorporated in a case. 
   On the other hand, there have been proposed antennas with various structures that are adapted to receive the electric wave having the frequency of about 2.3 GHz. Based on the shapes, they are roughly classified into a planar type (plate type) such as a patch antenna and a cylindrical type such as a loop antenna or a helical antenna. Such an antenna of the planar or cylindrical type is prepared as a separate member from the case of the foregoing portable electronic device and is connected to the digital radio tuner incorporated in the case through a cable and a connector so as to be used. 
   Generally, the antennas of the cylindrical type are more used than the antennas of the planar type because a wider directivity can be achieved by making the shape of the antenna cylindrical. As described above, the antennas of the cylindrical type are roughly classified into the loop antenna and the helical antenna. 
   As the loop antenna, there is known an electromagnetic coupling type four-point feeding loop antenna (see, e.g. Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2003-298335). The electromagnetic coupling type four-point feeding loop antenna disclosed in Patent Document 1 comprises a hollow cylindrical member formed by rolling a flexible insulating film member into a hollow cylinder about a center axis, a loop portion formed into a loop about the center axis on the hollow cylindrical member along its peripheral surface, and four feed lines for power feeding to the loop portion formed on the peripheral surface of the hollow cylindrical member. Four electromagnetic coupling lines are connected to the loop antenna such that each of them extends along the corresponding one of the four feed lines from the loop portion with a gap defined therebetween, thereby carrying out the power feeding through electromagnetic coupling. This loop antenna has a ground conductor pattern formed on the back of a circuit board extending in a direction perpendicular to the center axis. 
   On the other hand, the helical antenna is also known (see, e.g. Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2003-37430). Patent Document 2 proposes to produce a flexible insulating film member having one surface printed with an antenna pattern composed of four helical conductors (hereinafter referred to as an “antenna pattern printed insulating film member”) and then roll the antenna pattern printed insulating film member into a hollow cylinder about a center axis such that the foregoing one surface becomes an outer peripheral surface, thereby manufacturing a helical antenna. This helical antenna also has a ground conductor pattern formed on the back of a circuit board extending in a direction perpendicular to the center axis. 
   In the case of each of the foregoing antennas of the cylindrical type, after a satellite wave (circularly polarized wave) is received from the loop portion through the four electromagnetic coupling lines or by the helical conductors as a plurality of received waves, the received waves are phase-shifted by a phase shifter so as to be matched (adjusted) in phase, thereby obtaining a combined wave, and then the combined wave is amplified by a low-noise amplifier (LNA) and sent to a receiver body. Herein, a combination of the helical antenna, the phase shifter, and the LNA is called an antenna unit. 
   On the other hand, there has also been proposed an antenna unit comprising a helical antenna in the form of an antenna pattern formed on an outer peripheral surface of a cylindrical member, and a phase shifter in the form of a phase shifter pattern formed on the outer peripheral surface of the cylindrical member so as to be continuous with (connected to) the antenna pattern (see, e.g. Japanese Unexamined Patent Application Publication (JP-A) No. 2001-339228). 
   Such an antenna unit is placed in a topped hollow cylindrical cover case (cylinder) in order to prevent water invasion. Accordingly, the external appearance of the overall antenna unit exhibits a pole shape. In view of this, the antenna unit having such external appearance is called a pole-type antenna unit. Since the pole-type antenna unit is used while being carried, i.e. clipped to a pocket or the like, it is disposed in close proximity to the human body. 
   In the case where the foregoing electromagnetic coupling type four-point feeding loop antenna is used as the antenna of the cylindrical type, electric wave radiation of the same intensity occurs in upward and downward directions if there is no ground conductor pattern with a certain size extending in the direction perpendicular to the center axis. Specifically, with respect to cross polarization, assuming that there is left-hand circular polarization in the upward direction, there is, in the downward direction, right-hand circular polarization whose intensity is equal to that of the left-hand circular polarization. However, in the pole-type antenna unit, there is no space for providing such a ground conductor pattern extending in the direction perpendicular to the center axis. 
   Further, as described above, in the electromagnetic coupling type four-point feeding loop antenna, the power feeding to the loop portion is electromagnetically carried out from the four electromagnetic coupling lines through the four feed lines with the gaps defined therebetween. Therefore, the gaps should be accurately set and, thus, there is a problem that it is complicated to achieve impedance matching. 
   Moreover, when applied to any of mobile, vehicular, and portable receivers, the pole-type antenna unit is required to have as small a size as possible particularly in the center axis direction of the pole. 
   In addition, when applied to the portable receiver, the influence of a human body, for example, a shift in resonant frequency, tends to occur while the receiver is held in hand. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide a pole-type antenna unit that can improve antenna radiation characteristics even if there is no ground conductor pattern extending in a direction perpendicular to a center axis. 
   It is another object of this invention to provide a pole-type antenna unit that can easily achieve impedance matching. 
   It is still another object of this invention to provide a pole-type antenna unit that can reduce the size, particularly the size in a center axis direction of a pole, as much as possible. 
   It is yet still another object of this invention to provide a pole-type antenna unit that is hardly affected by a human body even when applied to a portable receiver. 
   An antenna unit according to this invention comprises a pole-shaped member and an antenna pattern composed of a plurality of conductors formed on a peripheral surface of the pole-shaped member. 
   According to an aspect of this invention, the antenna pattern comprises a helical pattern extending helically in a direction of the center axis and a loop pattern connected to an end portion of the helical pattern at an upper end portion of the pole-shaped member. 
   In the antenna unit, it is preferable that the pole-shaped member is a hollow cylindrical member obtained by forming a flexible insulating film member into a hollow cylinder about a center axis. It is preferable that helical pattern comprises a bent portion that is bent at least once in an opposite direction in the direction of the center axis. Also, it is preferable that antenna unit further comprises a phase shifter pattern formed on the peripheral surface of the pole-shaped member and electrically connected to the helical pattern. It is preferable that the antenna pattern and the phase shifter pattern are formed on an inner peripheral surface of the pole-shaped member. In this case, the antenna unit may further comprise a ground pattern formed on an outer peripheral surface of the pole-shaped member at a portion corresponding to a portion where the phase shifter pattern is formed. The antenna unit may further comprise a hollow cylindrical cover case covering the pole-shaped member. At least part of a helically extending portion of the helical pattern may be formed in a meander shape. 
   According to the antenna unit of this invention, since the antenna pattern is composed of the combination of the helical pattern and the loop pattern, even if there is no ground conductor pattern extending in a direction perpendicular to the center axis, the antenna radiation characteristics can be improved and the impedance matching can be easily achieved. 
   In the helical pattern, the portion extending helically has the bent portion that is bent at least once in the opposite direction in the center axis direction and at least part of the helically extending portion is formed in the meander shape, so that the size in the center axis direction can be reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic exploded view showing a pole-type antenna unit according to a first embodiment of this invention; 
       FIGS. 2A and 2B  are developed views each of an antenna pattern portion and a phase shifter portion used in the pole-type antenna unit illustrated in  FIG. 1 , wherein  FIG. 2A  is a plan view showing a first surface (inner peripheral surface) and  FIG. 2B  is a plan view showing a second surface (outer peripheral surface); 
       FIG. 3  is an exploded rear view showing the pole-type antenna unit illustrated in  FIG. 1  with a cover case removed; 
       FIG. 4  is an exploded rear view showing the pole-type antenna unit illustrated in  FIG. 3  with a hollow cylindrical member removed; 
       FIG. 5  is an exploded side view of the pole-type antenna unit illustrated in  FIG. 4 ; 
       FIG. 6  is a sectional view of an undercap used in the pole-type antenna unit illustrated in  FIG. 1 ; 
       FIGS. 7A ,  7 B, and  7 C are diagrams showing a packing used in the pole-type antenna unit illustrated in  FIG. 1 , wherein  FIG. 7A  is a front view,  FIG. 7B  is a plan view, and  FIG. 7C  is a sectional view taken along line B-B in  FIG. 7B ; 
       FIG. 8  is an exploded front sectional view of the pole-type antenna unit illustrated in  FIG. 1 ; 
       FIG. 9  is a front view showing the external appearance of the pole-type antenna unit illustrated in  FIG. 1 ; 
       FIG. 10  is a front sectional view of the pole-type antenna unit illustrated in  FIG. 1 ; 
       FIG. 11  is an exploded side view for explaining a positional relationship between a board and the hollow cylindrical member used in the pole-type antenna unit illustrated in  FIG. 1 ; 
       FIG. 12  is an exploded rear view for explaining the positional relationship between the board and the hollow cylindrical member illustrated in  FIG. 11 ; 
       FIG. 13  is a rear view showing the state where the board and the hollow cylindrical member illustrated in  FIG. 11  are assembled together; 
       FIG. 14  is an enlarged view of an encircled portion in  FIG. 13 ; 
       FIGS. 15A and 15B  are diagrams for explaining cross polarization characteristics (radiation pattern) of the antenna unit according to the first embodiment of this invention, wherein  FIG. 15A  is a transparent perspective view of the antenna unit according to this invention and  FIG. 15B  is a diagram showing a radiation pattern of the antenna unit according to this invention; 
       FIGS. 16A and 16B  are diagrams for explaining cross polarization characteristics (radiation pattern) of a conventional antenna unit, wherein  FIG. 16A  is a transparent perspective view of the conventional antenna unit and  FIG. 16B  is a diagram showing a radiation pattern of the conventional antenna unit; 
       FIGS. 17A and 17B  are developed views each of an antenna pattern portion and a phase shifter portion used in a pole-type antenna unit according to a second embodiment of this invention, wherein  FIG. 17A  is a plan view showing a first surface (inner peripheral surface) and  FIG. 17B  is a plan view showing a second surface (outer peripheral surface); and 
       FIG. 18  is a diagram showing the external appearance when an insulating film member having a conductor pattern shown in  FIGS. 17A and 17B  is rolled into a hollow cylinder. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Now, an embodiment of this invention will be described in detail with reference to the drawings. 
   First Embodiment 
   Referring to  FIG. 1  and  FIGS. 2A and 2B , description will be made about a pole-type antenna unit  10  according to the embodiment of this invention. The pole-type antenna unit  10  is an antenna unit for a digital radio receiver and is connected to a digital radio tuner (not shown) incorporated in a housing of a portable electronic device (not shown) through a cable  31  and a connector (not shown) so as to be used. 
   The pole-type antenna unit  10  comprises a hollow cylindrical member  11  formed by rolling a flexible insulating film member  20  about a center axis as shown in  FIGS. 2A and 2B  into a hollow cylinder.  FIG. 2A  shows a first surface  20 - 1  of the insulating film member  20  while  FIG. 2B  shows a second surface  20 - 2  of the insulating film member  20 . The insulating film member  20  is composed of an antenna pattern portion  20 A and a phase shifter portion  20 P. The antenna pattern portion  20 A has a substantially parallelogram shape while the phase shifter portion  20 P has a substantially rectangular shape. 
   The antenna pattern  20 A comprises a helical pattern portion  20 H formed so as to extend helically in a longitudinal direction (direction of the center axis) of the pole-type antenna unit  10  and a loop pattern portion  20 L connected to an end portion of the helical pattern portion  20 H at an upper end portion of the hollow cylindrical member  11 . 
   By connecting together a pair of lateral sides SL 1  and SL 2  of the insulating film member  20  so that the first surface  20 - 1  becomes an inner peripheral surface, the hollow cylindrical member  11  as shown in  FIG. 1  is formed. The connection between the pair of lateral sides is carried out, for example, by the use of double-sided adhesive tape, an adhesive agent, or soldering. 
   A first antenna pattern comprising first to fourth helical conductors  21 ,  22 ,  23 , and  24  is formed on the first surface  20 - 1  of the helical antenna portion  20 H. Each of the first to fourth helical conductors  21  to  24  is formed so as to extend in parallel to the lateral sides of the helical antenna portion  20 H in the state where each conductor is bent twice in opposite directions in the longitudinal direction (direction of the center axis) of the pole-type antenna unit  10 . Therefore, when the insulating film member  20  is rolled so that the hollow cylindrical member  11  is formed as described above, each of the first to fourth helical conductors  21  to  24  extends in a helical fashion on the inner peripheral surface of the hollow cylindrical member  11  in the state where each conductor is bent twice in the opposite directions in the longitudinal direction (direction of the center axis) of the pole-type antenna unit  10 . The first antenna pattern composed of the first to fourth helical conductors  21  to  24  functions as a helical antenna. 
   As described above, in this first embodiment, the first to fourth helical conductors  21  to  24  are each bent in the longitudinal direction of the pole-type antenna unit  10  and, therefore, the height of the pole-type antenna unit  10  can be reduced as compared with the case where the helical conductors are not bent. 
   A second antenna pattern comprising a loop conductor  28  connected to end portions (upper end portions) of the first to fourth helical conductors  21  to  24  is formed on the first surface  20 - 1  of the loop antenna portion  20 L. The second antenna pattern comprising the loop conductor  28  functions as a loop antenna. 
   A phase shifter pattern  25  electrically connected to the foregoing first antenna pattern is formed on the first surface  20 - 1  of the phase shifter portion  20 P. Therefore, when the insulating film member  20  is rolled so that the hollow cylindrical member  11  is formed as described above, the phase shifter pattern  25  is formed on the inner peripheral surface of the hollow cylindrical member  11 . This phase shifter pattern  25  functions as a phase shifter. 
   A ground pattern  27  is formed on the second surface  20 - 2  of the phase shifter portion  20 P. That is, the ground pattern  27  is formed on the surface of the phase shifter portion  20 P on the opposite side with respect to the surface thereof where the phase shifter pattern  25  is formed. Therefore, when the insulating film member  20  is rolled so that the hollow cylindrical member  11  is formed as described above, the ground pattern  27  is formed on the outer peripheral surface of the hollow cylindrical member  11  on the opposite side with respect to the surface thereof where the phase shifter pattern  25  is formed. The ground pattern  27  functions as a shield member provided so as to cover the phase shifter pattern  25 . 
   The pole-type antenna unit  10  further comprises a topped hollow cylindrical cover case (cylinder)  40  covering the hollow cylindrical member  11 . The inner diameter of the cover case  40  is greater than the diameter of the hollow cylindrical member  11 . 
   As described above, in this first embodiment, since the first antenna pattern comprising the first to fourth helical conductors  21  to  24  and forming the helical antenna portion  20 H and the second antenna pattern comprising the loop conductor  28  and forming the loop antenna portion  20 L are formed on the inner peripheral surface  20 - 1  of the hollow cylindrical member  11 , there is no direct contact between the first and second antenna patterns and an inner wall of the cover case  40 . Therefore, antenna characteristics of the pole-type antenna unit  10  can be prevented from being affected by the cover case  40 . Further, since the ground pattern  27  serving as the shield member is disposed on the outer side of the phase shifter pattern  25 , the antenna characteristics of the pole-type antenna unit  10  can be prevented from being affected by the human body. As a result, the pole-type antenna unit  10  according to this first embodiment can achieve desired antenna characteristics even during use. 
   In the illustrated embodiment, a first annular cushion member  51  is wound around the antenna pattern portion  20 A at its tip end as shown in  FIG. 3 . Further, just below the first annular cushion member  51 , a second annular cushion member  52  is wound around the antenna pattern portion  20 A. The thickness of the second annular cushion member  52  is slightly greater than a clearance between the hollow cylindrical member  11  and the cover case  40 . The first and second annular cushion members  51  and  52  are made of, for example, urethane foam. 
   By winding the first annular cushion member  51  around the antenna pattern portion  20 A at its tip end as described above, it is possible to change permittivity of the antenna pattern portion  20 A at its tip end, thereby enabling adjustment of antenna frequency characteristics of the pole-type antenna unit  10 . Therefore, by changing the thickness or width of the first annular cushion member  51 , it is possible to change the antenna frequency characteristics of the pole-type antenna unit  10 . 
   On the other hand, the second annular cushion member  52  serves as a cushion between the inner wall of the cover case  40  and the antenna pattern portion  20 A so that the clearance between the inner wall of the cover case  40  and the antenna pattern portion  20 A can be maintained constant. Accordingly, since it is possible to prevent an extreme inclination of the antenna pattern portion  20 A with respect to the cover case  40 , variation in directivity of the pole-type antenna unit  10  can be suppressed. As described above, since the thickness of the second annular cushion member  52  is slightly greater than the clearance between the antenna pattern portion  20 A and the inner wall of the cover case  40 , the second annular cushion member  52  is press-fitted into the cover case  40 . As a result, the distance between the inner wall of the cover case  40  and the antenna pattern portion  20 A can be held constant. 
   The pole-type antenna unit  10  comprises a board  32 , such as a printed circuit board. An electronic component, such as a low-noise amplifier (LNA), is mounted on the board  32 . The low-noise amplifier is connected to an output terminal  25   a  of the phase shifter pattern  25  and one end of the cable  31 . 
   A satellite wave (circularly polarized wave) is received by the loop conductor  28  of the loop antenna portion  20 L and the four conductors  21  to  24  of the helical antenna portion  20 H as a plurality of received waves. The received waves are phase-shifted by the phase shifter pattern  25  so as to be matched (adjusted) in phase, thereby obtaining a combined wave. Then, the combined wave is amplified by the low-noise amplifier and sent to a receiver unit (not shown) through the cable  31 . 
   Referring also to  FIGS. 3 to 5  in addition to  FIG. 1 , the pole-type antenna unit  10  further comprises a boot  33  slidably attached to the cable  31 , an undercap (bottom cover)  34  that is attached to a lower end of the cover case  40  as will be described later, and a waterproof packing  35 . The boot  33  is made of polyurethane. 
   By disposing the boot  33  and the packing  35  in the undercap  34  and inserting the board  32  therein, there are provided a waterproof function on the cable  31  and a board fixing function. 
     FIG. 6  is a sectional view of the undercap  34 . As shown in  FIG. 6 , the undercap  34  is formed with a pair of cutouts  341  on its upper end side for receiving therein both side end portions  321  ( FIG. 5 ) of the board  32 . The undercap  34  is provided with a pawl  342  at each of the cutouts  341  in order to prevent the board  32  from returning back upon press-fitting thereof. Further, the undercap  34  is formed at its lower end with an opening  343  in which the boot  33  is inserted. 
   As described above, the board  32  has the side end portions  321  projecting laterally from its both side surfaces. As shown in  FIG. 3 , each side end portion  321  of the board  32  is formed with a cutout  321   a  for engagement with the corresponding pawl  342  of the undercap  34 . 
     FIGS. 7A ,  7 B, and  7 C are diagrams showing the packing  35 , wherein  FIG. 7A  is a front view,  FIG. 7B  is a plan view, and  FIG. 7C  is a sectional view taken along line B-B in  FIG. 7B . As shown in  FIGS. 6 and 7A , an outer diameter D 2  of the packing  35  is slightly greater than an inner diameter D 1  of the undercap  34 . The packing  35  is formed with a cutout  351  in which a lower end portion  322  ( FIG. 5 ) of the board  32  is inserted. 
   By press-fitting the packing  35  into the undercap  34  and fixing such a press-fitted state by the board  32 , the waterproof function on the cable  31  is realized. In this event, since the board  32  is also fixed in the undercap  34 , positioning of the board  32  can also be carried out. 
   Referring to  FIG. 8 , the cover case  40  comprises a cylinder portion  41  and a top cover  42 . The cylinder portion  41  is formed on its inner wall with a pair of grooves  411  for receiving therein the side end portions  321  of the board  32 . 
     FIG. 9  is a front view showing the external appearance of the pole-type antenna unit  10  and  FIG. 10  is a sectional view of the pole-type antenna unit  10 . The top cover  42  is bonded to an upper end of the cylinder portion  41  by ultrasonic bonding. The undercap (bottom cover)  34  is bonded to a lower end of the cylinder portion  41  by ultrasonic bonding. Since, as described above, the pole-type antenna unit  10  has the structure using no screws, it is possible to reduce the number of components. 
   Referring to  FIGS. 11 to 14 , description will be made about a positional relationship between the board  32  and the hollow cylindrical member  11 . The hollow cylindrical member  11  has a pair of cutouts  11   a  for receiving therein the side end portions  321  of the board  32 . 
   As shown in  FIG. 13 , part of the board  32  mounted with a low-noise amplifier (LNA)  61  ( FIG. 11 ) is inserted into the inside of the hollow cylindrical member  11 . As shown in  FIG. 14 , the output terminal  25   a  of the hollow cylindrical member  11  is connected to the board  32  (low-noise amplifier  61 ) by solder  62 . 
   Since the part of the board  32  is inserted into the inside of the hollow cylindrical member  11  as described above, it is possible to reduce the size of the pole-type antenna unit  10  in its longitudinal direction. Further, since the connection between the hollow cylindrical member  11  and the board  32  (low-noise amplifier  61 ) is carried out by the use of the output terminal  25   a  formed at the flexible insulating film member  20 , the particular or dedicated terminal component required in the conventional pole-type antenna unit becomes unnecessary and, therefore, it is possible to reduce the number of components. 
     FIGS. 15A and 15B  show cross polarization characteristics (radiation pattern) of the antenna unit  10  according to the first embodiment.  FIG. 15A  is a transparent perspective view of the antenna unit  10  according to the first embodiment and  FIG. 15B  is a diagram showing a radiation pattern of the antenna unit  10 . As shown in  FIG. 15B , the radiation pattern of the antenna unit  10  is composed of a radiation pattern of left-hand circular polarization E L  and a radiation pattern of right-hand circular polarization E R . 
   As a reference example,  FIGS. 16A and 16B  show cross polarization characteristics (radiation pattern) of a conventional antenna unit having only a loop antenna.  FIG. 16A  is a transparent perspective view of the conventional antenna unit and  FIG. 16B  is a diagram showing a radiation pattern of the conventional antenna unit. As shown in  FIG. 16B , the radiation pattern of the conventional antenna unit is also composed of a radiation pattern of left-hand circular polarization E L  and a radiation pattern of right-hand circular polarization E R . 
   As shown in  FIG. 16B , it is understood that, in the conventional antenna unit, the left-hand circular polarization E L  is radiated upward and the right-hand circular polarization E R  having an intensity substantially equal to that of the left-hand circular polarization E L  is radiated downward. 
   In contrast, as shown in  FIG. 15B , it is understood that, in the antenna unit  10  according to the first embodiment, the radiation intensity of the left-hand circular polarization E L  radiated upward is increased while the radiation intensity of the right-hand circular polarization E R  radiated downward is reduced. That is, the electric wave is mainly radiated upward in the antenna unit  10  according to the first embodiment. Therefore, even if there is no ground conductor pattern extending in the direction perpendicular to the center axis, the antenna radiation characteristics of the antenna unit  10  can be improved. In other words, by combining the loop antenna and the helical antenna, it is possible to improve the antenna radiation characteristics when there is no ground conductor pattern. 
   Further, in the conventional antenna unit, four electromagnetic coupling lines are coupled to a loop conductor  28  through gaps defined therebetween as shown in  FIG. 16A . In contrast, in the antenna unit  10  according to the first embodiment, the first to fourth helical conductors  21  to  24  are directly connected to the loop conductor  28  as shown in  FIG. 15A  and, therefore, the impedance matching can be easily achieved. 
   The antenna radiation characteristics of the antenna unit  10  can be changed to some degree by changing, on a design basis, a diameter of the loop conductor  28  and/or an angle of the first to fourth helical conductors  21  to  24 . 
   Second Embodiment 
   Referring now to  FIGS. 17A ,  17 B, and  18 , a second embodiment of this invention will be described. 
   A pole-type antenna unit according to the second embodiment has the same external shape as that shown in  FIG. 1  and differs only in the shape of a conductor pattern formed on an insulating film member. Accordingly, the same reference symbols are assigned to portions that are the same as those of the pole-type antenna unit according to the first embodiment, thereby omitting detailed explanation thereof. 
   The pole-type antenna unit  10  according to the second embodiment also comprises a hollow cylindrical member  11  formed by rolling a flexible insulating film member  20  as shown in  FIGS. 17A and 17B  into a hollow cylinder about a center axis.  FIG. 17A  shows a first surface  20 - 1  of the insulating film member  20  while  FIG. 17B  shows a second surface  20 - 2  of the insulating film member  20 . The insulating film member  20  is produced by the use of a film made of a low-loss dielectric material, for example, a Teflon (registered trademark)-based material and has an antenna pattern portion  20 A and a phase shifter portion  20 P on the first surface  20 - 1  side. The antenna pattern portion  20 A has a substantially parallelogram shape while the phase shifter portion  20 P has a substantially rectangular shape. 
   The antenna pattern  20 A comprises a helical pattern portion  20 H formed so as to extend helically in a longitudinal direction (direction of the center axis) of the pole-type antenna unit  10  and a loop pattern portion  20 L connected to an end portion of the helical pattern portion  20 H at an upper end portion of the hollow cylindrical member  11 . 
   By rolling the insulating film member  20  and connecting together a pair of lateral sides of the insulating film member  20  so that the first surface  20 - 1  becomes an inner peripheral surface, the hollow cylindrical member  11  as shown in  FIG. 1  is formed. The connection between the pair of lateral sides is carried out, for example, by the use of double-sided adhesive tape, an adhesive agent, or soldering. 
   A first antenna pattern comprising first to fourth helical conductors  21 ′,  22 ′,  23 ′, and  24 ′ is formed on the first surface  20 - 1  of the helical antenna portion  20 H. Each of the first to fourth helical conductors  21 ′ to  24 ′ is formed so as to extend in parallel to the lateral sides of the helical antenna portion  20 H in the state where each conductor is bent four times in opposite directions in the longitudinal direction (direction of the center axis) of the pole-type antenna unit  10 . Particularly, it is configured that, in each of the first to fourth helical conductors  21 ′ to  24 ′, at least one of five conductor patterns each extending in parallel to the lateral sides, i.e. the conductor pattern connected to a phase shifter pattern  25 ′ in  FIG. 17A , is formed in a meander shape, i.e. lying in a zigzag line. 
   As described above, when the insulating film member  20  is rolled so that the hollow cylindrical member  11  is formed as described above, each of the first to fourth helical conductors  21 ′ to  24 ′ extends in a helical fashion on the inner peripheral surface of the hollow cylindrical member  11  in the state where each conductor is bent four times in the opposite directions in the longitudinal direction of the pole-type antenna unit  10 . The first antenna pattern composed of the first to fourth conductors  21 ′ to  24 ′ functions as a helical antenna. 
   As described above, in this second embodiment, since the first to fourth helical conductors  21 ′ to  24 ′ are each bent in the longitudinal direction of the pole-type antenna unit  10  and part of each of the helical conductors are formed in a meander shape, it is possible to lengthen the conductors. The height of the pole-type antenna unit  10  can be reduced as compared with the case where the helical conductors are not bent and further as compared with the first embodiment. 
   On the first surface of the loop antenna portion  20 L, a second antenna pattern comprising a loop conductor  28  connected to end portions (upper end portions) of the first to fourth helical conductors  21 ′ to  24 ′ is formed. The second antenna pattern comprising the loop conductor  28  functions as a loop antenna. 
   The phase shifter pattern  25 ′ electrically connected to the foregoing first antenna pattern is formed on the first surface  20 - 1  of the phase shifter portion  20 P. Therefore, when the insulating film member  20  is rolled so that the hollow cylindrical member  11  is formed as described above, the phase shifter pattern  25 ′ is formed on the inner peripheral surface of the hollow cylindrical member  11 . This phase shifter pattern  25 ′ functions as a phase shifter. 
   A ground pattern  27  is formed on the second surface  20 - 2  of the phase shifter portion  20 P. That is, the ground pattern  27  is formed on the surface of the phase shifter portion  20 P on the opposite side with respect to the surface thereof where the phase shifter pattern  25 ′ is formed. Therefore, when the insulating film member  20  is rolled so that the hollow cylindrical member  11  is formed as described above, the ground pattern  27  is formed on the outer peripheral surface of the hollow cylindrical member  11  on the opposite side with respect to the surface thereof where the phase shifter pattern  25 ′ is formed. The ground pattern  27  functions as a shield member provided so as to cover the phase shifter pattern  25 ′. 
   Each part except for parts illustrated in  FIG. 17  is the same as that of the first embodiment. 
   As described above, in this second embodiment, since the first antenna pattern comprising the first to fourth helical conductors  21 ′ to  24 ′ and forming the helical antenna portion  20 H and the second antenna pattern comprising the loop conductor  28  and forming the loop antenna portion  20 L are formed on the inner peripheral surface  20 - 1  of the hollow cylindrical member  11 , there is no direct contact between the first and second antenna patterns and an inner wall of the cover case  40 . Therefore, antenna characteristics of the pole-type antenna unit  10  can be prevented from being affected by the cover case  40 . Further, since the ground pattern  27  serving as the shield member is disposed on the outer side of the phase shifter pattern  25 ′, the antenna characteristics of the pole-type antenna unit  10  can be prevented from being affected by the human body (hand, fingers, or the like). As a result, the pole-type antenna unit  10  according to this second embodiment can achieve desired antenna characteristics even during use. Moreover, since the pole-type antenna unit  10  no use high permittivity ceramics, it is possible to reduce the dielectric loss and is possible to provide the pole-type antenna unit  10  having high gain in low price. 
   Further, in the conventional antenna unit, four electromagnetic coupling lines are coupled to a loop conductor  28  through gaps defined therebetween as shown in  FIG. 16A . In contrast, in the antenna unit  10  according to the second embodiment, the first to fourth helical conductors  21 ′ to  24 ′ are directly connected to the loop conductor  28  as shown in  FIG. 17A  and, therefore, the impedance matching can be easily achieved. 
   In  FIGS. 17A and 17B , as described in detail in the first embodiment, part of the lower side of the phase shifter portion  20 P in the insulating film member  20  is projected downward for forming cutouts  11   a  adapted to receive therein side end portions  321  of a board  32 . However, such cutouts  11   a  are not necessarily required. That is, the lower side of the phase shifter portion  20 P may be formed as shown by a chain line in  FIGS. 17A and 17B . In this case, an output terminal for connection to the board  32  (low-noise amplifier  61 ) is formed at a portion identified by symbol  25   a ′. That is, part of the lower side of the phase shifter portion  20 P is slightly projected downward and an end portion of the phase shifter pattern  25 ′ extending to this projected portion is connected, as the output terminal  25   a ′, to the low-noise amplifier  61  on the board  32 . 
   Also in the antenna unit  10  according to the second embodiment, its antenna radiation characteristics can be changed to some degree by changing, on a design basis, a diameter of the loop conductor  28  and/or an angle of the first to fourth helical conductors  21 ′ to  24 ′. 
   While this invention has been described in terms of the two embodiments, the invention is of course not limited to those embodiments. For example, although the four helical conductors formed on the inner peripheral surface of the hollow cylindrical member are used as the first antenna pattern in each of the foregoing embodiments, the first antenna pattern may be composed of at least one helical conductor. In the case of the single helical conductor, the phase shifter (phase shifter portion) is not required. In each of the foregoing embodiments, each of the helical conductors forming the first antenna pattern is bent twice or four times in the opposite directions in the longitudinal direction (center axis direction) of the pole-type antenna unit. However, each helical conductor may be bent at least once in the opposite direction. In each of the foregoing embodiments, the ground pattern formed on the outer peripheral surface of the hollow cylindrical member is used as a shield member. However, the shield member is not limited thereto, but may be another as long as it is provided so as to cover the phase shifter pattern. For example, the shield member may be a conductor pattern formed on the inner wall of the cover case at a portion corresponding to the portion where the phase shifter pattern is formed or a tape with a shielding effect stuck to the outer wall of the cover case at a portion corresponding to the portion where the phase shifter pattern is formed. The hollow cylindrical member  11  may be a pole-shaped member. 
   The pole-type antenna unit described in the embodiment is suitable as a personal-type miniature antenna unit for a digital radio receiver, but not limited thereto, and is also applicable as an antenna unit for a GPS receiver or an antenna unit for mobile communication adapted to receive other satellite waves or ground waves.