This invention relates to 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 and, in particular, to an antenna for use in the digital radio receiver.
In recent years, a digital radio receiver, which receives the satellite wave or the terrestrial wave to listen in the digital radio broadcasting, has been developed and is put to practical use in the United States of America. The digital radio receiver is mounted on a mobile station such as an automobile and can receive an electric wave having a frequency of about 2.338 gigahelts (GHz) to listen in a radio broadcasting. Inasmuch as a received electric wave has the frequency of about 2.338 GHz, the received electric wave has a reception wavelength (resonance wavelength) λ of 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 retransmitted at a linear polarization.
In order to receive such an electric wave having the frequency of about 2.338 GHz, it is necessary to set up an antenna outside the automobile. Although such antennas have been proposed those having various structures, the antennas of cylindrical-type are generally used rather than those of planer-type (plane-type). It is possible to obtain a wider directivity by making a shape of the antenna cylindrical.
In the manner which is well known in the art, an electromagnetic wave emitted into a free space is a transversal wave having an electric field and a magnetic field vibrating in a plane perpendicular to a propagating direction of the wave. The electric field and the magnetic field are variable in intensity within the above-mentioned plane. Such electromagnetic wave in which the direction of the electric field is not random but constant or varied in some regular way is referred to as a polarized wave. The satellite wave is a circular polarized wave exhibiting circular polarization while the terrestrial wave is a linear polarized wave exhibiting linear polarization.
Now, the description will be mainly made as regards the antennas for receiving the satellite wave. A helical or helix antenna is known in the art as one of the antennas of the cylindrical-type. The helical antenna has structure where at least one antenna lead is wound around an outer peripheral surface of a hollow or solid cylindrical (which is collectively called “cylindrical”) member in a helix fashion (spiral fashion). The helical antenna can effectively receive the above-mentioned circular polarized wave. Accordingly, the helical antenna is exclusively for use in receiving the satellite wave. The cylindrical member is made of an insulation material such as plastics. In addition, antenna leads are equal, for example, in number to four. On the other hand, it is remarkably difficult to really wind the plurality of antenna leads around the outer peripheral surface of the cylindrical member. Accordingly, alternatively, another helical antenna is proposed in which an antenna pattern film where a plurality of conductive patterns are printed or formed an insulation flexible sheet is wound around the outer peripheral surface of the cylindrical member.
Various helical antennas of the type are already proposed. By way of example, Japanese Unexamined Patent Publication Tokkai No. 2001-326523 or JP-A 2001-326523 discloses a helical antenna structure which improves a strength of structure by altering the cylindrical member of the helical antenna. More specifically, in order to resolve a problem in a conventional helical antenna having a week strength when the cylindrical member has a hollow cylindrical shape, the helical antenna structure disclosed in JP-A 2001-326523 disposes, between a center axis and an inner peripheral surface of the hollow cylindrical member, at least three ribs symmetrically extending in a radial manner at equal angular intervals.
In addition, Japanese Unexamined Patent Publication Tokkai No. 2001-339227 or JP-A 2001-339227 discloses a helical antenna which is capable of easily adjusting a resonance frequency of the helical antenna to a desired resonance frequency. More specifically, in JP-A 2001-339227, a hollow cylindrical member has a female threaded screw hole where an upper end portion of the cylindrical member is threaded in an inner peripheral wall of the hollow cylindrical member. A ceramic bolt having a relative permittivity of 10-100 is threaded in the female threaded screw. When the ceramic bolt is inserted in the female threaded screw of the hollow cylindrical member, it is possible to equivalently shorten a length of the hollow cylindrical member due to a wavelength shortening effect.
Furthermore, Japanese Unexamined Patent Publication Tokkai No. 2003-37430 or JP-A 2003-37430 discloses a helical antenna uses a cylindrical body formed an insulating film member which is rolled into a cylindrical shape without using the cylindrical member. The cylindrical body is fixedly disposed on a circuit board at an end in an axial direction.
Attention will be directed to a four-phase feed helical antenna which has four antenna leads wound around the outer peripheral surface of the cylindrical member. After the satellite wave (the circular polarized wave) is received by the four antenna leads as four received waves, the four received waves are phase shifted and combined a phase shifter (a phase converting circuit) to as to match phases of the four received waves to obtain a combined wave, and then the combined wave is amplified by a low-noise amplifier (LNA) to obtain an amplified wave which is delivered to a receiver body. A combination of the four-phase feed helical antenna, the phase shifter (the phase converting circuit), and the low-noise amplifier is called an antenna unit.
In other words, it is necessary for the helical antenna comprising a plurality of antenna leads to feed to each antenna lead and to combine circular polarized waved received by means of the phase shifter (the phase converting circuit) in order to drive the helical antenna. In the helical antenna, each antenna lead has a length which is selected from a range between 0.8-1.3λ.
On the other hand, as antenna for receiving the linear polarized wave, a monopole antenna is known which has an antenna length of λ/4 and which has an end grounded to an ground plate. The monopole antenna requires no phase converting circuit. However, the monopole antenna is unsuitable to receive the circular polarized wave because the monopole antenna is an antenna for exclusively receiving the linear polarized wave.
In the manner which is described above, the conventional helical antenna, which is an antenna for receiving the circular polarized wave, comprising a plurality of antenna leads is disadvantageous in that it is complicated in structure because the conventional helical antenna requires the phase converting circuit (the phase shifter) and so on in order to drive it.
Accordingly, an antenna enable to receive the circular polarized wave without using the phase converting circuit (the phase shifter) is desired.