Antenna and wireless device incorporating the same

Details Antenna and wireless device incorporating the same Antenna and wireless device incorporating the same

2001-11-20

2003-10-14

Ho, Tan (Department: 2821)

Communications: radio wave antennas

Antennas

Microstrip

C343S846000, C343S860000

Reexamination Certificate

active

06633261

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an antenna and a wireless device incorporating the antenna. More particularly, the present invention relates to an antenna for mobile wireless communications which is especially useful in wireless devices such as mobile phone terminals, and a wireless device incorporating such an antenna.
2. Description of the Background Art
In recent years, technologies related to mobile communications, e.g., mobile phones, have seen a rapid development. In a mobile phone terminal, the antenna is a particularly important component. The trend for downsizing mobile phone terminals has required antennas to be downsized and also to become internalized elements.
Hereinafter, a conventional example of an antenna for mobile wireless communications, which may be used for a mobile phone terminal, will be described.
FIG. 16
schematically illustrates the structure of a conventional antenna for mobile wireless communications. As shown in
FIG. 16
, the conventional antenna for mobile wireless communications includes a conductive base plate
101
, a conductive plate
102
of a planar configuration, and two metal leads
103
and
104
. A predetermined voltage is supplied from a supply point
105
to the conductive plate
102
via the metal lead
103
. Moreover, the conductive plate
102
is coupled to the conductive base plate
101
, which provides as a ground (GND) level, via the metal lead
104
.
An antenna of the above-described structure, commonly referred to as a PIFA (Planar Inverted F Antenna), is employed usually as a low-profile and small antenna device in a mobile phone terminal. The PIFA is a &lgr;/4 resonator, which is equivalent to a &lgr;/2 micro-strip antenna being short-circuited in a middle portion thereof to have its volume halved.
FIGS. 17A and 17B
show current paths which emerge when a voltage is applied from the supply point
105
of the conventional antenna for mobile wireless communications shown in FIG.
16
.
FIG. 17A
shows a current path in an opposite phase mode. As shown by the arrows therein, the current path in the opposite phase mode begins at the supply point
105
, extends through the metal lead
103
and along the lower surface of the conductive plate
102
, and further extends through the metal lead
104
so as to be short-circuited to the conductive base plate
101
. In the opposite phase mode, a current flowing through the metal lead
103
and a current flowing through the metal lead
104
do not contribute to the resonance of antenna because they have opposite phases and therefore cancel each other.
FIG. 17B
shows a current path in an in-phase mode. As shown by the arrows therein, the current path in the in-phase mode begins at the supply point
105
, extends through the metal lead
103
and along the lower surface of the conductive plate
102
so as to turn around at the open end, and further extends along the upper surface of the conductive plate
102
and through the metal lead
104
, so as to be short-circuited to the conductive base plate
101
. In the in-phase mode, a current flowing through the metal lead
103
and a current flowing through the metal lead
104
have the same phase at a frequency at which the length of the current path equals a ½ wavelength. Therefore, the antenna resonates at this frequency (referred to as the “resonance frequency”).
FIG. 18
illustrates a detailed structure of the conventional antenna for mobile wireless communications shown in FIG.
16
. As shown in
FIG. 18
, the conductive base plate
101
has a rectangular shape with a width of 40 mm and a length of 125 mm. The conductive plate
102
has a rectangular shape with a width of 40 mm and a length of 30 mm. The metal leads
103
and
104
are each 7 mm long. The volume occupied by the antenna (hereinafter referred to as the “occupied volume” of the antenna), which is defined within a region enclosed by an orthogonal projection of the conductive plate
102
on the conductive base plate
101
, is equal to a product of the area of the conductive plate
102
and the lengths of the metal leads
103
and
104
, i.e., 8.4 cc (=3≧4≧0.7), in this example.
In
FIG. 18
, the metal lead
103
functioning as a supply pin and the metal lead
104
functioning as a short-circuiting pin are shown with an interval of d therebetween. If the interval d is 3 mm, then the antenna shown in
FIG. 18
will have a central frequency of 1266 MHz in the case of a 50&OHgr; system. Since the bandwidth (i.e., frequency bandwidth which has a voltage-standing wave ratio (VSWR) equal to or less than 2) under these conditions is 93 MHz, a band ratio of this antenna is calculated to be 7.3% (≈93/1266).
In the above-described conventional antenna for mobile wireless communications (PIFA), the resonance frequency and the length of the antenna element are generally in inverse proportion. Therefore, there is a problem in that the resonance frequency is increased if the length of the antenna element (i.e., the conductive plate
102
), and hence the occupied volume of the antenna, is reduced in order to downsize the overall antenna.
Accordingly, there has been proposed an antenna structure for mobile wireless communications as shown in
FIG. 19
, which can provide a lower resonance frequency for the same occupied volume of the antenna.
As shown in
FIG. 19
, the conventional antenna for mobile wireless communications includes a conductive base plate
111
, a conductive plate
112
of a planar configuration, a conductive wall
116
, and two metal leads
113
and
114
. A voltage is applied to the conductive plate
112
from a supply point
115
, via the metal lead
113
. The conductive plate
112
is coupled to the conductive base plate
111
via the metal lead
114
. The conductive wall
116
is electrically coupled to the conductive plate
112
at one end thereof. Thus, the conductive plate
112
and the conductive wall
116
would together appear as if the conductive plate
102
in
FIG. 16
was bent downward near its open end. A predetermined interspace exists between the other end of the conductive wall
116
and the conductive base plate
111
. In this antenna structure, it is essential for the conductive wall
116
to be located at the farthest end of the conductive plate
112
from the metal lead
114
.
The use of the above-described conductive wall
116
makes it possible to obtain a downsized antenna for the following two reasons.
First, an increased current path length lowers the resonance frequency. Specifically, the resonance frequency is lowered by disposing the conductive wall
116
so as to increase the maximum value of the current path length in the opposite phase mode (FIG.
20
). Note that lowering the resonance frequency for the same occupied volume of the antenna is equivalent to downsizing an antenna while maintaining a constant resonance frequency. This is one reason why a downsized antenna can be realized by employing the structure shown in FIG.
19
.
Second, the resonance frequency can be lowered due to capacitive loading. The interspace between the conductive wall
116
and the conductive base plate
111
, which functions as shunt capacitance, is a factor in the lowering of the resonance frequency because the most intensive electric field resides at the open end of the conductive wall
116
.
FIG. 21
illustrates a specific implementation example of the conventional antenna for mobile wireless communications shown in FIG.
19
. Note that in the structure of
FIG. 21
, the dimensions of the conductive base plate
111
and the occupied volume of the antenna are the same as those of the structure of FIG.
18
. In other words, the conductive plate
112
has a rectangular shape with a width of 40 mm and a length of 30 mm. The conductive wall
116
has a rectangular shape with a width of 6 mm and a length of 30 mm. The metal leads
113
and
114
are 7 mm long each.
If the interval d is 4 mm, then the antenna shown in
FIG. 21
will have a central frequency of 1209 MHz in the case of a 50&OHg

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