Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2002-10-23
2004-10-26
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S702000, C343S846000
Reexamination Certificate
active
06809687
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a monopole antenna that is used for transmission and reception of satellite communication and satellite broadcast. In particular, the invention relates to a monopole antenna that is suitable for vehicular and portable use.
2. Description of the Related Art
A rod-shaped radiation conductor that erects from a ground surface of a metal plate or the like and that has an overall length of &lgr;/4 (&lgr;: the free space wavelength of radio waves) is widely employed as a monopole antenna that is used in mobile communication equipment etc. for transmission and reception of radio waves in a frequency range of 800-2,000 MHz. In such a monopole antenna, a feeder line such as a coaxial cable is connected to the bottom end of the radiation conductor that extends in the vertical direction. The length of the radiation conductor is so set that the radiation conductor resonates with radio waves having a desired frequency.
In vehicular telephones etc., a dual-band monopole antenna that can be used for transmission and reception of both of radio waves of an 800-MHz frequency band and radio waves of 1.9-GHz frequency band, for example, is required. Conventionally, two rod-shaped radiation conductors that erect from a ground surface of a metal plate or the like and that extend in the vertical direction are widely employed in this kind of dual-band monopole antenna. Since the overall length of each of the two radiation conductors is set to &lgr;/4 (&lgr;: the free space wavelength of corresponding radio waves), one rod-shaped radiation conductor for transmission and reception of radio waves of a lower frequency band is long and the other rod-shaped radiation conductor for transmission and reception of radio waves of a higher frequency band is short. Feeder lines such as coaxial cables are connected to the bottom ends of the two rod-shaped radiation conductors, respectively, whereby signals having different frequencies are supplied to the respective radiation conductors.
However, in the above-described conventional monopole antenna, the overall length of the rod-shaped radiation conductor is equal to &lgr;/4. Therefore, to transmit and receive radio waves of the 800-MHz band which is frequently used for cellular phones, for example, a radiation conductor whose overall length almost amounts to 10 cm is necessary. This means a problem that the height dimension is too large for use as a vehicular monopole antenna. In addition, this kind of monopole antenna has a narrow resonance frequency band, that is, it resonates with only radio waves whose frequency is close to a particular frequency. This raises fear that the sensitivity may decrease extremely when ratio waves to be received are deviated in frequency.
In view of the above, recently, a monopole antenna has been proposed that is reduced in height dimension by forming a band-shaped radiation conductor having a constant width on the surface of a dielectric substrate made of ceramics or the like by printing, etching, or a like method. According to this conventional technique, the overall length of the radiation conductor can be reduced by about 20% by virtue of the wavelength shortening by the dielectric. However, where the height dimension is restricted severely as in the case of monopole antennas for vehicular use, it is desired that the radiation conductor be shortened further.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances in the art, and an object of the invention is therefore to provide a monopole antenna that can easily be reduced in height dimension and hence can accelerate miniaturization.
To attain the above object, a first aspect of the invention provides a monopole antenna comprising a dielectric substrate that erects from a ground surface; and a radiation conductor that is provided on a surface of the dielectric substrate so as to extend in the vertical direction, a bottom end of the radiation conductor being connected to a feeder line, the radiation conductor having a bottom portion and a top portion that is distant from the ground surface and is wider than the bottom portion.
In the monopole antenna having the above configuration, since the top portion (capacitive region), having a large voltage variation, of the radiation conductor is wide, the capacitance is increased. In general, the resonance frequency of a resonance circuit lowers as its capacitance increases. Therefore, if the length of the radiation conductor is equivalent, the resonance frequency of this monopole antenna is lower than that of a monopole antenna in which a band-shaped radiation conductor not having a wide top portion is formed on the surface of a dielectric substrate. That is, providing the wide portion as the top portion makes it possible to set short a length of the radiation conductor that is necessary to attain resonance at a desired frequency and hence to reduce the height dimension of the entire monopole antenna easily.
In the above configuration, the dielectric substrate may be formed with through-holes or thin portions in a bottom region. In this case, the inductance increases because the dielectric constant decreases around the bottom portion (inductive region) of the radiation conductor. In general, the resonance frequency of a resonance circuit lowers as its inductance increases. Therefore, in this case, a length of the radiation conductor that is necessary to attain resonance at a desired frequency can be set shorter and hence the height dimension of the entire monopole antenna can further be reduced. A ground electrode may be provided on the dielectric substrate in a bottom end region and soldered to the ground surface. This makes it unnecessary to screw the dielectric substrate to the ground surface, which makes work of connecting the monopole antenna to a feeder line such as a coaxial cable easier.
A second aspect of the invention provides a monopole antenna comprising a dielectric substrate that erects from a ground surface; a first radiation conductor that is provided on a surface of the dielectric substrate so as to extend in the vertical direction, a bottom end of the first radiation conductor being connected to a feeder line; and a second radiation conductor that extends parallel with a plane that is approximately perpendicular to the dielectric substrate, the second radiation conductor being connected to a top end of the first radiation conductor.
In the monopole antenna having the above configuration, a maximum voltage variation occurs in the second radiation conductor which is connected to the top end of the first radiation conductor. Since the second radiation conductor is extended to a plane that is approximately perpendicular to the dielectric substrate, the capacitance is large there. In general, the resonance frequency of a resonance circuit lowers as its capacitance increases. Therefore, if the overall height dimension is equivalent, the resonance frequency of this monopole antenna is lower than that of a conventional one in which only a band-shaped radiation conductor having a constant width is formed on the surface of a dielectric substrate. Therefore, in this monopole antenna, an overall height dimension for attaining resonance at a desired frequency can be made shorter than in such a conventional monopole antenna.
In the above configuration, the second radiation conductor may be a metal plate. Alternatively, a small dielectric substrate may be provided on the dielectric substrate so as to be approximately perpendicular to the dielectric substrate, and the second radiation conductor may be provided on one or both surfaces of the small dielectric substrate. In this case, in manufacture, the first and second radiation conductors can be formed together on the surfaces of the dielectric substrate and the small dielectric substrate that are formed from a common substrate, which is suitable for mass-production. In addition, the resonance frequency can further be lowered by utilizing the wavelength shorte
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Ho Tan
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