Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2001-08-09
2002-09-17
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S770000, C343S767000
Reexamination Certificate
active
06452552
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a microstrip antenna used as a built-in antenna of a portable telephone or mobile terminal for example.
DESCRIPTION OF THE RELATED ART
A &lgr;/2 patch antenna is a typical microstrip antenna to be built in a portable telephone or a mobile terminal such as a GPS. In this case, &lgr; denotes a wavelength in a frequency used.
This antenna is mainly constituted of a dielectric substrate having a rectangular or circular radiating conductor (patch conductor) with a side length or a diameter of approximately &lgr;/2 on one face and having a ground plate conductor on the other face.
It has been recently requested to further downsize the portable telephone and mobile terminal and thereby, it is requested to further downsize a built-in type patch antenna. A dielectric substrate with a high dielectric constant is typically used to physically downsize the patch antenna with the above-mentioned patch conductor dimension of approximately &lgr;/2.
However, the relative dielectric constant of a dielectric material having a low temperature coefficient suitable for a high frequency is up to &egr;
r
of approximately 110 and therefore, it is limited to downsize an antenna by raising the dielectric constant of the dielectric material. Since a dielectric material becomes more expensive by raising its dielectric constant, the cost for fabricating a microstrip antenna will increase if such raised dielectric constant material is used.
Japanese patent publication No. 05152830 A (U.S. Pat. No. 2,826,224) discloses, as a known art for downsizing a microstrip antenna without raising the dielectric constant of the dielectric material, to produce two resonant modes orthogonal to each other and having phases different from each other by forming a degenerate separation element, to form a power-supply point in a straight-line direction orthogonal to the direction of the resonant mode at ±45°, and to form notches at the both ends in the straight-line direction of the radiating conductor. By forming such notches, it is possible to equivalently increase electric lengths of two resonant modes, and lower a resonance frequency. Therefore, it is possible to downsize the antenna element to a certain extent.
Japanese patent publication No. 06276015 A discloses, as a known art of a microstrip antenna, that two crossing slots with different lengths from each other are formed as a degenerate separation element in a radiating conductor and that notches or stubs are formed at the outer edge of the radiating conductor in order to adjust the inductance component of the radiating conductor.
Japanese patent publication No. 09326628 A discloses, as another known art of a microstrip antenna, that two resonance characteristics for generating two modes with different route lengths from each other are obtained by forming a crossed cutout with two arm lengths different from each other on a square radiating plate so that these symmetry axes coincide with two diagonal lines of the plate, respectively.
However, according to the known art disclosed in Japanese patent publication No. 05152830 A (U.S. Pat. No. 2,826,224), because the notches are formed only the both ends of the radiating conductor in the direction coinciding with the power-supply point of the conductor and a current-route width is not changed at the central portion of the radiating conductor corresponding to an antinode of current flowing under resonance, it cannot be expected to greatly reduce a resonance frequency. Furthermore, since a capacitance with respect to ground is reduced by forming the notches at the both ends of the radiating conductor corresponding to antinodes of voltage under resonance, it cannot be also expected to greatly reduce the resonance frequency. Therefore, it is difficult to extremely downsize the microstrip antenna.
Although Japanese patent publication No. 06276015 A discloses to form two crossing slots having different lengths from each other as a degenerate separation element, it is silent for downsizing an antenna element. In this disclosed art furthermore, since notches or stubs are formed at the outer edge of the radiating conductor, it is impossible to effectively use the limited surface area of a dielectric substrate for improving the radiation efficiency.
In addition, although Japanese patent publication No. 09326628 A discloses that two resonance characteristics are obtained by forming a crossed cutout with two arm lengths different from each other so that symmetry axes coincide with diagonal lines of a radiation plate, it is silent for downsizing an antenna element at all. Moreover, because the position of the power-supply point is present on a vertical line passing through the center of a side, it is very difficult to mount an antenna element when it is downsized and its terminal interval is decreased.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a microstrip antenna, whereby further downsizing can be expected.
Another object of the present invention is to provide a microstrip antenna, whereby its radiation efficiency can be improved by effectively using the limited surface area of a dielectric substrate.
A further object of the present invention is to provide a microstrip antenna, whereby a power-supply point is located at an easily-mounting position.
According to the present invention, a microstrip antenna includes a rectangular dielectric substrate, a ground plate conductor formed on one surface of the dielectric substrate, a rectangular radiating conductor formed on the other surface of the dielectric substrate, a crossed slot formed in the radiating conductor and provided with two arms extended along orthogonal sides of the radiating conductor, the two arms having lengths different from each other, and at least one power-supply point formed on a diagonal line of the radiating conductor or an extension line of the diagonal line but different from a center of the radiating conductor. The length of at least one of the arms is equal to or more than a value obtained by subtracting a four times value of a thickness of the dielectric substrate from a length of a side of the radiating conductor along the arm.
Thus, according to the present invention, the length of at least one of the two arms of the crossed slot, parallel with orthogonal sides of the radiating conductor is set so as to be equal to or more than a value obtained by subtracting a four times value of the thickness of the dielectric substrate from the length of the side of the radiating conductor in that direction. That is, if it is assumed that a central point of each arm is located at the center of the radiating conductor, the distance between the top end of at least one arm of the slot and outer edge of the radiating conductor is set so that the distance becomes equal to or less than a double value of the thickness of the dielectric substrate. Each region between the top end of the arm or slot and the outer edge of the radiating conductor locates at the antinode of current in a current route under resonance. Therefore, by decreasing the width of the region of the current route, magnetic field is concentrated on the region to increase the inductance at that region, and the area of the region decreases to lower the capacitance at the region. Thus, by making a region with a low potential more inductive, the resonance frequency lowers resulting that dimensions of a microstrip antenna are further decreased.
Particularly, according to the present invention, the distance between the top end of at least one arm of the slot and the outer edge of the radiating conductor, in other words, the width of a current route serving as an antinode of current in the current route under resonance is set so as to be equal to or less than a double value of the thickness of the dielectric substrate. Therefore, a resonance frequency is greatly lowered and as a result, it is possible to further downsize an antenna.
Furthermore, since at least one power-supply point is located on a diagonal line
Ishitobi Norimasa
Misawa Nobutaka
Clinger James
TDK Corporation
Wong Don
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