Communications: radio wave antennas – Antennas – With radio cabinet
Reexamination Certificate
2003-04-03
2004-10-26
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With radio cabinet
C343S789000
Reexamination Certificate
active
06809691
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an antenna and an antenna unit both for use mainly in mobile communications. More specifically, the present invention relates to an antenna for a base station in mobile communications, and an antenna unit using the antenna.
2. Description of the Background Art
With reference to
FIGS. 36 through 40
, one example of a conventional antenna is described below.
FIG. 36
is an illustration showing the configuration of a monopole antenna described in Japanese Patent Laid-Open Publication No. 2001-308630. The antenna includes a top conductor
111
, a ground conductor
112
, side conductors
113
, an antenna element
114
, and a power supply point
115
. The antenna has a feature of having bi-directional directivity on a horizontal plane. The top conductor
111
has there parts
111
a,
111
b,
and
111
c,
with the part
111
b
located at the center of the conductor
111
being implemented by a linear conductor. The top conductor
111
, the ground conductor
112
, and the side conductors
113
form an antenna box having the shape of a rectangular parallelepiped. The antenna box has two openings
116
and
117
on the top. The power supply point
115
is located at the center of the ground conductor
112
. The antenna element
114
is connected at one end to the power supply point
115
. Furthermore, the antenna element
114
is mechanically or electrically connected at the other end to the center point of the ground conductor
111
b
by, for example, soldering. When a coordinate system is set as illustrated in
FIG. 36
by taking the power supply point
115
as an origin, the antenna has a symmetric structure with respect to both of a Z-Y plane and a Z-X plane.
With reference to
FIG. 37
, the operation of the antenna illustrated in
FIG. 36
is described below. Excitation of electric waves occurs at the antenna element
114
, from which an electric wave having a frequency of f
0
is emitted. The electric waves are emitted through two openings
116
and
117
to the outside of the antenna box. These two openings
116
and
117
are located symmetrically to the antenna element
114
, which is an emitting source. Therefore, the distance from the antenna element
114
to the opening
116
is equal to the distance from the antenna element
114
to the opening
117
. Also, as illustrated in
FIG. 37A
, the direction of the electric field excited at the opening
116
is opposed to the direction thereof excited at the opening
117
. Here, for the sake of convenience in description, consider a case in which the electric fields excited at these openings
116
and
117
are replaced by magnetic flows. In this case, as illustrated in
FIG. 37B
, it can be assumed that two linear magnetic flow sources B
1
and B
2
are located at the openings
116
and
117
, respectively, in parallel to the Y axis. These linear magnetic flow sources B
1
and B
2
have the same amplitude, but are oriented to opposite directions. Here, the electric waves emitted from the antenna can be considered as being emitted from these two magnetic flow sources B
1
and B
2
. That is, emission from the antenna can be regarded as emission from an array of the magnetic flow sources B
1
and B
2
.
As illustrated in
FIG. 37B
, the magnetic flow sources B
1
and B
2
are located symmetrically with respect to the Z-Y plane. For this reason, the electric waves emitted from the magnetic flow sources B
1
and B
2
are equal in amplitude and opposite in phase to each other on the Z-Y plane, and therefore are cancelled by each other. With this, no electric wave is emitted on the Z-Y plane. On the Z-X plane, the electric waves emitted from the magnetic flow sources B
1
and B
2
are equal in phase to each other in one direction. In that direction, the electric wave from the antenna is intensified. For example, when an interval between the magnetic flow sources B
1
and B
2
is ½ a wavelength in free space, two electric waves are equal in phase to each other at an arbitrary point on the X axis. Therefore, the electric wave from the antenna is intensified in both of the +X direction and the −X direction. That is, the antenna illustrated in
FIG. 36
has a bi-directional directivity in the X direction.
As such, according to the antenna illustrated in
FIG. 36
, an effect of an antenna array can be achieved only by a single antenna element, and a directivity can be provided to the antenna. Furthermore, when the openings
116
and
117
are made longer in the Y direction, for example, the magnetic flow sources also become longer. Therefore, emission in the X direction is reduced, thereby producing larger gain. As such, gain can be adjusted depending on the length of the openings.
In general, the size of conductive members that construct the antenna is finite. Therefore, the electric wave is diffracted at the end portion of each conductive member. Therefore, precisely speaking, the electric wave emitted from the antenna is a sum of an electric wave emitted from the antenna element and diffracted waves at the end portions of the respective conductive members. The same goes for the antenna illustrated in FIG.
36
. That is, the electric wave is diffracted at every end portion and every refraction point of the conductors
111
,
112
, and
113
. Particularly, since the top conductor
111
and the openings
116
and
117
are located on a same plane, the electric wave emitted from the antenna is greatly influenced by a diffracted wave at the end of the top conductor
111
. Thus, the directivity of the antenna illustrated in
FIG. 36
is varied by the number or locations of the openings
116
and
117
as well as the size and shape of the conductors
111
,
112
, and
113
.
By way of example only, the characteristics of a prototype antenna illustrated in
FIG. 38
are described. In
FIG. 38
, when a free space wavelength of &lgr;
0
is taken as a reference, the ground conductor
112
is shaped like a square whose side is 0.836×&lgr;
0
each, and the height of each side conductor
113
is 0.0836×&lgr;
0
. The top conductor
111
b
at the center is a linear conductor being located in parallel to the Y axis and having a length of 0.836×&lgr;
0
. Both ends of the top conductor
111
b
are electrically connected to the side conductors
113
. The top conductors
111
a
and
111
c
are each shaped like a rectangle having two sides each being parallel to the X axis and having a length of 0.209×&lgr;
0
and the other two sides each being parallel to the Y axis and having a length of 0.836×&lgr;
0
. These top conductors
111
a
and
111
c
are connected to the side conductors
113
. The two openings
116
and
117
are also each shaped like a rectangle having two sides each being parallel to the X axis and having a length of 0.209×&lgr;
0
and the other two sides each being parallel to the Y axis and having a length of 0.836×&lgr;
0
. These openings
116
and
117
are located adjacently to each other so as to sandwich the conductor
111
b
therebetween. Therefore, the antenna box has a symmetric structure with respect to both of a Z-Y plane and a Z-X plane. The antenna element
114
is a linear conductor having the element length of 0.0835×&lgr;
0
. One end of the antenna element
114
is electrically connected to the midpoint of the top conductor
111
b.
FIG. 39
is a graph showing VSWR (Voltage Standing Wave Ratio) characteristics with respect to a power supply line of 50&OHgr; as input impedance characteristics of the prototype antenna illustrated in FIG.
38
. In
FIG. 39
, the horizontal axis represents frequencies standardized with a center frequency of f0. Frequencies of f
1
and f
2
are a minimum frequency and a maximum frequency, respectively, both of whose VSWR is 2 or less. According to
FIG. 39
, a frequency band whose VSWR is 2 or less is ((f
2
−f
1
)/f
0
)=18.2%. Therefore, the prototype antenna illustrated in
FIG. 38
has improved impedance characteristics with less reflec
Iwai Hiroshi
Ogawa Koichi
Yamamoto Atsushi
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