Monopole antenna

Details Monopole antenna Monopole antenna

1999-06-02

2001-02-13

Ho, Tan (Department: 2821)

Communications: radio wave antennas

Antennas

With lumped reactance filter in active antenna

C343S752000

Reexamination Certificate

active

06188366

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a monopole antenna mainly used for mobile communications, and in particular, to a monopole antenna that is suitable as an antenna for a base station.
BACKGROUND OF THE INVENTION
FIG. 20
shows a conventional monopole antenna comprising one antenna element, which excites electromagnetic waves at two frequencies. In
FIG. 20
, numeral
91
denotes a disk-shaped earth conductor, numeral
92
denotes a current supply portion located at the center of the earth conductor
91
, and numeral
93
is an antenna element made of a linear conductor. The antenna element
93
has a coil
93
a
at its center, and one end of the antenna element
93
is electrically connected to the current supply portion
92
located at the center of the earth conductor
91
so that it stands perpendicularly on the earth conductor
91
.
In the antenna element
93
, electromagnetic waves with lower frequency are excited in the entire antenna element, and due to the central coil
93
a,
electromagnetic waves with the same phase but higher frequency are excited above and below the coil
93
a.
Thus, a two-frequency monopole antenna oscillating at different frequencies can be obtained.
However, in this conventional monopole antenna, the height of the antenna element
93
has to be at least ¼ of the wavelength at the lower excitation frequency or at least ¾ of the wavelength at the higher excitation frequency, so that the antenna element
93
becomes relatively tall and miniaturization becomes difficult. Moreover, it is structurally impossible to excite electromagnetic waves whose frequencies are close to each other, so that the frequencies that can be excited are limited. In practice, operation of up to two frequencies is possible.
Furthermore, if this conventional monopole antenna is installed, for example, at the ceiling of a room, it is preferable that the antenna is installed head-down facing the floor, so that the antenna element
93
faces the space into which the electromagnetic waves are being radiated, in order to improve the radiation efficiency of the antenna. In this case, it is preferable that there are no objects hindering the transmission between the antenna element
93
and the radiation space, and that visual contact can be established between the antenna element
93
and the entire radiation space. Moreover, there is a need for monopole antennas that can be installed in a manner so that they can hardly be noticed, but if a conventional monopole antenna with an antenna element
93
protruding from the ceiling is used, its optical appearance is unpleasant, because the antenna element
93
is relatively tall.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve these problems of the prior art and provide a monopole antenna that has a simple configuration, can be operated at a plurality of frequencies, and is small.
A monopole antenna in accordance with the present invention comprises an earth conductor, a current supply portion located on a surface of the earth conductor, a linear conductor having a first end connected to the current supply portion and a second end, a planar conductor that is connected to the second end of the linear conductor, and a ring-shaped conductor whose inner edge is connected to an outer edge of the planar conductor via an anti-resonance circuit. According to this first configuration of a monopole antenna, by setting the resonance frequency of the anti-resonance circuit to f
1
, the impedance of the anti-resonance circuit at the frequency f
1
becomes high, and the planar conductor and the ring-shaped conductor are electrically blocked from each other, so that the system comprising the linear conductor and the planar conductor can be excited at the first frequency f
1
, and the system extending from the linear conductor to the ring-shaped conductor can be excited at a second frequency f
2
. If the planar conductor is connected perpendicularly to the linear conductor, and the ring-shaped conductor is arranged in the same plane as the planar conductor, the height of the portion of the antenna that includes the linear conductor, the planar conductor and the ring-shaped conductor can be reduced. Consequently, with this first configuration of a monopole antenna, a compact monopole antenna with simple configuration that can be operated at a plurality of frequencies can be obtained.
It is preferable that the monopole antenna according to this first configuration further comprises an earth wire that connects at least one of the planar conductor and the ring-shaped conductor to the earth conductor. With this configuration, the input impedance of the antenna can be raised at each operating frequency. As a result, the impedance matching between the antenna input impedance and the current supply portion can be improved for every operation frequency, which improves the characteristics of the antenna.
In the monopole antenna according to this first configuration, the planar conductor and the ring-shape conductor can be arranged in one plane, or in different planes.
It is preferable that in the monopole antenna according to this first configuration, the ring-shape conductor comprises a plurality of ring-shaped conductors, and that opposing inner edges and outer edges of adjacent ring-shaped conductors are connected via an anti-resonance circuit. With this configuration, it is possible to obtain a monopole antenna that can be operated at three or more operating frequencies. In this case, it is preferable that the monopole antenna further comprises an earth wire that connects at least one of the planar conductor and the plurality of ring-shaped conductors to the earth conductor. In some cases, the impedance matching with a ring-shaped conductor is sufficient, and in these cases, it is not necessary to match it with a earth wire. Especially, the impedance matching of the innermost planar conductor or ring-shaped conductor is sometimes sufficient. Also in this case, the planar conductor and the plurality of ring-shape conductors can be arranged in one plane, or in different planes.
It is preferable that in the monopole antenna according to this first configuration, the planar conductor is a disk-shaped conductor. In this case, it is preferable that the current supply portion is arranged at the center of the surface of the earth conductor, and the first end of the linear conductor is connected to the current supply portion so that the linear conductor is perpendicular to the earth conductor, the second end of the linear conductor is connected to the center of the planar conductor so that the linear conductor is perpendicular to the planar conductor, and the ring-shape conductor is arranged concentrically around the planar conductor.
It is preferable that in the monopole antenna according to this first configuration, the anti-resonance circuit is a parallel circuit comprising a coil and a capacitor.
It is also preferable that in the monopole antenna according to this first configuration, the anti-resonance circuit consists only of a coil. With this configuration, the number of parts can be reduced.
It is preferable that in the monopole antenna according to this first configuration, the planar conductor, the anti-resonance circuit, and the ring-shaped conductor are patterned on a dielectric substrate. With this configuration, electrical blocking at the desired frequencies is possible by adjusting the pattern of the anti-resonance circuit.
It is preferable that the monopole antenna according to this first configuration further comprises a reflection conductor arranged on a side of the earth conductor opposite the side on which the planar conductor is arranged, in a manner that the reflection conductor is electrically coupled to the earth conductor through a space between the two. With this configuration, the following effects can be achieved. Because an electric current flows also in the reflection conductor due to the electrical coupling through space, an electromagnetic wave is radiated also from the edge of t

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