Antenna apparatus

An antenna apparatus includes a rectangular conductor plate having a longitudinal length of one fourth the wavelength of a signal having an operating frequency and a line-shaped inverted F antenna mounted thereon. The line-shaped inverted F antenna is disposed at one end in the longitudinal direction of the conductor plate so as to be perpendicular to the longitudinal sides of the conductor plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to compact antenna apparatuses such as those 
used for portable radio terminals. 
2. Discussion of the Background 
In recent years, it has been important for portable radio terminals to be 
compact and thin for improved portability. As portable radio terminals 
have been made compact, however, the effectiveness of the antenna used 
therein may deteriorate. This tendency becomes especially prominent with a 
built-in antenna such as an inverted F antenna. 
FIG. 11 shows a conventional compact antenna apparatus in which a 
line-shaped inverted F antenna is disposed on a conductor plate. A 
line-shaped inverted F antenna 1101 is connected to a conductor plate 1102 
by a short-circuited line 1103 and a power-feed line 1104. A portable 
radio terminals having an operational frequency of 800 MHz is used as an 
example. The radio terminals, is shown simply by a conductor plate. The 
longitudinal length L of the conductor plate 1102 is set to one fourth the 
wavelength of a signal having the specified operational frequency, in the 
example, one fourth of the wavelength of an 800 MHz signal. 
FIG. 12 is a graph showing the radiation efficiency of the line-shaped 
inverted F antenna 1101 shown in FIG. 11. The horizontal axis indicates 
the longitudinal length L of the conductor plate 1102 shown in FIG. 11. It 
is understood from this graph that the efficiency becomes -3 dB or less 
when the longitudinal length of the conductor plate becomes one fourth the 
wavelength or less. This indicates that about half the power supplied to 
the antenna 1101 is lost in the antenna 1101. This deterioration may occur 
in a compact antenna such as a line-shaped inverted F antenna as a result 
of the compact conductor plate. 
As described above, as a portable radio terminals have been made compact, 
antenna performance may deteriorate. Especially with a compact antenna 
apparatus such as an inverted F antenna mounted on a conductor plate 
having a length of about one fourth the wavelength, the degree of 
deterioration is high and stable communication may be impeded. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a compact 
antenna apparatus having reduced deterioration and improved radiation 
efficiency. 
The foregoing object is achieved in one aspect of the present invention 
through the provision of an antenna apparatus comprising: a conductor 
plate formed of electrically conductive material and having at least of 
one side in the longitudinal direction with a length of one fourth the 
wavelength corresponding to a certain operating frequency, and having a 
short-circuited point disposed in the vicinity at least of one end to said 
side in the longitudinal direction with a length of one fourth the 
wavelength, and a power-feed point disposed in the vicinity at least of 
one end to said side in the longitudinal direction with a length of one 
fourth the wavelength; a first element connected at one end to said 
conductor plate at said short-circuited point; a second element connected 
at one end to said conductor plate at said power-feed point; and a third 
element connected at one end to said first element, connected to said 
second element such that the length from said power-feed point to another 
end of the third element is equal to one fourth the wavelength, and 
disposed substantially perpendicularly to said side in the longitudinal 
direction with a length of one fourth the wavelength of said conductor 
plate. 
In the antenna apparatus, a rectangular-shaped conductor plate formed of 
electrically conductive material and having two sides in a longitudinal 
direction with a length of one fourth the wavelength corresponding to a 
certain operating frequency; a short-circuited point disposed in the 
vicinity of at least one side of two sides substantially perpendicular to 
the two sides in the longitudinal direction of said rectangular-shaped 
conductor plate; a first line-shaped element connected at one end to said 
rectangular-shaped conductor plate at said short-circuited point; a 
power-feed point disposed in the vicinity of at least one side of the two 
sides substantially perpendicular to the two sides in the longitudinal 
direction of said rectangular-shaped conductor plate; a second line-shaped 
element connected at one end to said rectangular-shaped conductor plate at 
said power-feed point; and a third line-shaped element connected at one 
end to said first line-shaped element, connected to said second 
line-shaped element such that the length from said power-feed point to 
another end of the third line-shaped element is equal to one fourth the 
wavelength, and disposed substantially perpendicularly to the two sides in 
the longitudinal direction of said rectangular-shaped conductor plate. 
In the antenna apparatus, the power-feed point and said short-circuited 
point are disposed in the vicinity of a corner of said rectangular-shaped 
conductor plate. 
In the antenna apparatus, the third line-shaped element is disposed 
substantially at a right angle at a first disposed section near the other 
end and disposed substantially at a right angle at a second disposed 
section closer to the other end than the first disposed section. 
In the antenna apparatus, the other-end portion of said third line-shaped 
element disposed at the second disposed section is substantially 
perpendicular to the two sides in the longitudinal direction of said 
rectangular-shaped conductor plate. 
In the antenna apparatus, the portion from the first disposed section to 
the second disposed section of said third line-shaped element is 
substantially parallel to the two sides in the longitudinal direction of 
said rectangular-shaped conductor plate. 
In the antenna apparatus, the portion from the first disposed section to 
the second disposed section of said third line-shaped element is disposed 
toward said rectangular-shaped conductor plate. 
In the antenna apparatus, a rectangular-shaped conductor plate formed of 
electrically conductive material and having two sides in a longitudinal 
direction have a length of one fourth the wavelength corresponding to a 
certain operating frequency; a power-feed point disposed in the vicinity 
of at least one side of two sides of the rectangular-shaped conductor 
plate substantially perpendicular to the two sides in the longitudinal 
direction of said rectangular-shaped conductor plate; a first line-shaped 
element connected at one end to said rectangular-shaped conductor plate at 
said power-feed point; and a second line-shaped element connected at one 
end to said first line-shaped element, having the length from said 
power-feed point to the other end equal to one fourth the wavelength, and 
disposed substantially perpendicularly to the two sides in the 
longitudinal direction of said rectangular-shaped conductor plate. 
In the antenna apparatus, the power-feed point is disposed in the vicinity 
of a corner of said rectangular-shaped conductor plate. 
In the antenna apparatus, the second line-shaped element is disposed 
substantially at a right angle at a first disposed section near the other 
end and disposed substantially at a right angle at a second disposed 
section closer to the other end than the first disposed section. 
In the antenna apparatus, the other-end portion of said second line-shaped 
element disposed at the second disposed section is substantially 
perpendicular to the two sides in the longitudinal direction of said 
rectangular-shaped conductor plate. 
In the antenna apparatus, the portion from the first disposed section to 
the second disposed section of said second line-shaped element is 
substantially parallel to the two sides in the longitudinal direction of 
said rectangular-shaped conductor plate. 
In the antenna apparatus, the portion from the first disposed section to 
the second disposed section of said second line-shaped element is disposed 
toward said rectangular-shaped conductor plate. 
In the antenna apparatus, the third line-shaped element is disposed 
substantially at a right angle at a first disposed section near the other 
end, disposed substantially at a right angle at a second disposed section 
closer to the other end than the first disposed section, and disposed 
substantially at a right angle at a third disposed section closer to the 
other end than the second disposed section. 
In the antenna apparatus, the second line-shaped element is disposed 
substantially at a right angle at a first disposed section near the other 
end, disposed substantially at a right angle at a second disposed section 
closer to the other end than the first disposed section, and disposed 
substantially at a right angle at a third disposed section closer to the 
other end than the second disposed section. 
In the antenna apparatus, the third line-shaped element has a U shape or a 
J shape. 
In the antenna apparatus, the second line-shaped element has a U shape or a 
J shape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention are described below by referring to 
the drawings. 
FIG. 1 is a perspective view of an antenna apparatus according to an 
embodiment of the present invention. In FIG. 1, an antenna 101 and a 
conductor plate 102 are shown. The antenna 101 is mounted at an end of the 
conductor plate 102 through a short-circuited line 101a and a power-feed 
line 101b so as to be perpendicular to the two sides of the conductor 
plate 102 in the longitudinal direction. The antenna 101 is disposed so 
that it fits over the conductor plate 102. 
FIG. 2 is a graph showing results of measurement performed by the use of a 
radio antenna model formed based on the antenna apparatus shown in FIG. 1. 
A line 202 shows the radiation efficiency of the antenna 101. In the 
graph, the vertical axis indicates the radiation efficiency of the antenna 
101 and the horizontal axis indicates the length of the conductor plate 
102. 
For comparison, the radiation efficiency of a conventional antenna is also 
shown in the graph as a line 201. It is understood from the graph that a 
constant antenna gain is obtained irrespective of the length of the 
conductor plate in an antenna arrangement according to the present 
invention. Especially at a plate length of one fourth the wavelength, the 
efficiency is improved by about 2 dB as compared with the conventional 
method. 
The reason why efficiency is improved by changing the antenna arrangement 
is described below. When a line-shaped inverted F antenna is disposed on a 
board, since the antenna has a low profile, the amount of 
electromagnetic-wave radiation from the board becomes larger than that 
from the antenna itself. 
FIG. 3 roughly shows current flows on a board and an antenna in a 
conventional antenna apparatus to explain the foregoing event. Radiation 
from the line-shaped inverted F antenna itself is generated only from a 
current F, which flows perpendicularly to the board. Although a current G 
also flows in parallel to the board on the line-shaped inverted F antenna, 
since radiation from the current G is canceled by a current A, which leaks 
from the antenna to the board, it does not contribute to radiation. Since 
the current G flows in the opposite direction to that of the current A, 
the radiation field caused by the current G and that caused by the current 
A cancel each other. 
An electromagnetic wave radiated from the board is caused by a 
high-frequency current flowing from the antenna to the board. From the 
board, most radiation is produced at a current B, a current C, a current 
D, and a current E, which flows at the ends (surroundings) of the board. A 
high-frequency current is generally distributed more at an end of a board. 
The currents B and D contribute much to radiation and the currents C and E 
contribute, because the lengths of the portions where the currents C and E 
flow are short. 
As described above, radiation from the antenna itself is generated by the 
current F. The radiation resistance of the portion where the current F 
flows is about 600 milliohms with an assumption that the height of the 
antenna is one fiftieth the wavelength (the wavelength corresponds to the 
frequency of the signal used in the antenna). On the other hand, radiation 
is produced from the currents B, C, D, and F on the board. The 
corresponding radiation resistance is about 3000 milliohms when the board 
length is half the wavelength. Therefore, in such a case, it is obvious 
that the amount of radiation is larger from the board than from the 
antenna. 
Assume here that the board becomes short. For example, assume that the 
board length is one fourth the wavelength in the above model. The 
radiation resistance of the antenna is about 600 milliohms, which is the 
same as in the above case. On the other hand, the radiation resistance of 
the board is 1000 milliohms, which is one third that obtained when the 
board length is one half the wavelength. 
FIG. 4 roughly shows current distribution in a case when the board length 
is one fourth the wavelength. Compared to the cases in which the board 
length is long, it is understood that the current B and half the current D 
disappear when the board length is short, which means that a little 
current distribution contributes to radiation. As described above, when 
the board becomes short, the amount of electromagnetic radiation is 
reduced and the antenna performance deteriorates. 
An antenna arrangement according to the present invention is described 
below. FIG. 5 roughly shows the current distribution of an antenna 
according to the present invention. In this case, relatively high current 
distributions A and D remain, and greatly contribute to radiation. 
Therefore, as compared with the conventional antenna arrangement shown in 
FIG. 4, the amount of radiation from the board increases and as a result, 
the antenna radiation efficiency is improved. The radiation resistance of 
the board in this embodiment is 3000 milliohms when the board length is 
one fourth the wavelength, which is three times larger than that in a case 
in which the elements are arranged in the longitudinal direction of the 
board. 
In the inverted F antenna apparatus shown in FIG. 1, a line-shaped element 
of the antenna 101 is disposed perpendicularly at disposed sections near 
an end. The way the element is disposed is not limited to that in this 
case. The element is disposed toward the other end of the conductor plate. 
The direction in which the element is disposed is not limited to that 
direction. The element may be disposed over the conductor plate. 
FIGS. 6 to 10 illustrate modifications of the antenna apparatus shown in 
FIG. 1. These modifications achieve the same advantages as those of the 
antenna apparatus shown in FIG. 1. 
An antenna apparatus shown in FIG. 6 includes a power feed line 601 b. This 
antenna is called an inverted L antenna. The short-circuited line 601 a 
serves as a matching circuit in the inverted F antenna shown in FIG. 1. 
Since this matching circuit is not provided for the inverted L antenna 
shown in FIG. 6, matching between the antenna and the power-feed line 
deteriorates. Matching can be achieved in the inverted L antenna shown in 
FIG. 6 by providing a matching circuit (not shown) between the power-feed 
point of the antenna and the power-feed line. 
FIGS. 7 and 8 show antenna apparatuses bent in different directions from 
that in the antenna 101 shown in FIG. 1. In the antenna apparatus 701 
shown in FIG. 7, an antenna element is disposed in the direction away from 
a board 702. The antenna 701 includes a short-circuited line 701a and a 
power feed line 701b. In the antenna apparatus 801 shown in FIG. 8, an 
antenna element is disposed in a direction such that its tip is close to a 
board 802. The antenna 801 includes a short-circuited line 801a and a 
power feed line 801b. The elements are bent in a U shape with two right 
angles in these antennas. An operation of disposed sections is described 
below by referring to FIG. 7. Two sides 701c and 701e which are parallel 
in the U-shaped portion with two right angles are disposed such that they 
are always perpendicular to the longitudinal direction of the board 702. 
When this condition is satisfied, an antenna according to the present 
invention avoids current flow loss even if the antenna is disposed in any 
directions. In other words, under this condition, the currents flowing 
through antenna elements 701c and 701e do not cancel a current distributed 
on the board flowing in the longitudinal direction of the board. 
Therefore, the gain does not deteriorate due to the elements 701c and 
701e, because a current flowing in the longitudinal direction of the 
board, which is the main radiation source, is not reduced. In the 
embodiment shown in FIG. 1, although the portion corresponding to an 
antenna element 701d slightly cancel a current on the board 102, its 
amount is much less than that in the conventional antenna apparatus. In 
the embodiments shown in FIGS. 7 and 8, the amount of canceled current on 
the board becomes smaller than in the embodiment shown in FIG. 1. When the 
portion 701d becomes long, however, a current on the board 702 is canceled 
thereby. Therefore, when the portion 701d is sufficiently short, for 
example, when it is equal to or shorter than one twentieth the wavelength, 
the effect caused by the portion 701d can be ignored. 
FIG. 9 shows an inverted F antenna 901 in which an antenna element is made 
to have a coil shape so as to shorten its length. As compared with a 
folding method, this method reduces the operating frequency band of an 
antenna. However, radiation efficiency deteriorates little, since the 
amount of canceled current flowing in the longitudinal direction of the 
board decreases as compared with a folding method. 
FIG. 10 shows an antenna apparatus made by folding further the antenna 
element 101 shown in FIG. 1 to dispose the antenna near a corner of the 
board 102. With this structure, since a current G in FIG. 5, which cancels 
a current E, flows through a different position, radiation is generated 
from the current E. As a result, increased radiation is produced from the 
board 1002 and radiation efficiency increases. 
As described above, in a conventional antenna apparatus employing a compact 
antenna such as a line-shaped inverted F antenna, its performance 
deteriorates as the conductor plate becomes small in size. The radiation 
efficiency of the antenna deteriorates especially when the board length in 
the longitudinal direction is one fourth the wavelength. As proposed by 
the present invention, however, when an inverted F antenna or an inverted 
L antenna is disposed perpendicularly to the longitudinal direction of the 
conductor plate, the above deterioration of the antenna efficiency is 
reduced.