Patent Application
20150070224
1. Field of the Invention
The present invention relates to an antenna apparatus preferably for use in short-range communication and a communication terminal including the antenna apparatus.
2. Description of the Related Art
Radio frequency identification (RFID) systems are increasingly becoming popular as product management systems and billing and toll collection management systems. In such an RFID system, a reader/writer and an RFID tag wirelessly communicate with each other to exchange information. Each of the reader/writer and the RFID tag includes an RFID IC chip for processing a signal and an antenna for transmitting and receiving a radio signal. Predetermined information is transmitted between the antennas of the reader/writer and the RFID tag via a magnetic field or an electromagnetic field.
For example, FeliCa (registered trademark) that applies an RFID system to information communication terminals such as mobile telephones has been recently used. In Felica, a terminal itself is sometimes used as a reader/writer or an RFID tag. On the other hand, since communication terminals decrease in size and increase in functionality, there is not sufficient space for an antenna in the casings of the communication terminals. In order to solve this problem, for example, a configuration disclosed in WO2010/122685/A1 is sometimes used. In this configuration, a small coil conductor is connected to an RFID IC chip and a radio signal is transmitted from a conductive layer that is adjacent to the coil conductor and has a large area. The conductive layer functions as a radiation element (booster antenna) and is magnetically coupled to the coil conductor via an opening of the conductive layer. With this configuration, since a thin metal film can be used as the conductive layer, the conductive layer can be formed in narrow space between a printed circuit board and a terminal casing.
As the conductive layer (booster antenna), a metal film may be prepared as described above. Alternatively, in a case where the terminal casing is a metal casing, the metal casing itself may be used as the booster antenna. In this case, it is desired that the metal casing be connected to the ground of a circuit in the terminal casing. More specifically, it is desired that the metal casing be connected to the ground of a printed circuit board in the terminal casing. In the terminal casing, for example, a power supply circuit and a high-frequency signal processing circuit are formed. By using the metal casing as the ground, a ground potential in the terminal casing can become more stable. As a result, the operations of various circuits can become more stable.
However, in a case where the ground of the printed circuit board and the metal casing are connected, an antenna characteristic may be deteriorated in accordance with a connection method.
Preferred embodiments of the present invention provide an antenna apparatus capable of maintaining a radiation characteristic of a booster antenna connected to a ground conductor and a communication terminal including the antenna apparatus.
An antenna apparatus according to a preferred embodiment of the present invention includes a feeding coil connected to a feeding circuit, a booster antenna that includes a conductor at which a conductor aperture and a slit to connect the conductor aperture and an outer edge are provided and includes an area larger than a footprint of the feeding coil, a ground conductor facing the booster antenna, and a ground connection conductor that connects the booster antenna to the ground conductor. The conductor aperture is located at an offset position near the outer edge of the conductor. The ground connection conductor is disposed at a position on either side of the slit outside an area in which a current density of an induced current flowing through the booster antenna is in a range from a maximum value to about 80% of the maximum value or a position on one side of the slit in the area.
With this configuration, since a circuitous path for a current is not provided in an area (high current density area) in which the current density is in the range from a maximum value to about 80% of the maximum value, a loss becomes small and the deterioration of an antenna characteristic due to the connection of a booster antenna to the ground rarely occurs.
In order to further reduce a loss, the ground connection conductor is preferably disposed at a position on either side of the slit outside an area in which a current density of an induced current flowing through the booster antenna is in a range from a maximum value to about 50% of the maximum value or a position on one side of the slit in the area.
With this configuration, since a circuitous path for a current is not provided in an area (relatively high current density area) in which the current density is in the range from a maximum value to about 50% of the maximum value, a loss becomes smaller and the deterioration of an antenna characteristic due to the connection of a booster antenna to the ground rarely occurs.
The ground conductor is preferably a ground conductor pattern provided at a printed circuit board in a casing of an apparatus in which the antenna apparatus is embedded. The booster antenna is preferably a metal layer provided at the casing or a metal plate that is a portion of the casing.
With this configuration, the booster antenna can be electrically connected to the ground conductor and the need to newly dispose a booster antenna is eliminated.
The ground conductor is preferably a ground conductor pattern provided at a printed circuit board in a casing of an apparatus in which the antenna apparatus is embedded. The booster antenna is preferably a metal plate or a metal case that is disposed in the casing and shields a circuit located on the printed circuit board.
With this configuration, the booster antenna can be electrically connected to the ground conductor and the need to newly dispose a booster antenna is eliminated.
The slit preferably connects the conductor aperture and the outer edge of the conductor at a position at which the conductor aperture and the outer edge of the conductor are in closest proximity to each other.
With this configuration, the length of a path for a current that does not contribute radiation, that is, a current passing through the periphery of the slit and the booster antenna, is significantly reduced. This leads to the reduction in a loss.
A communication terminal according to a preferred embodiment of the present invention includes a feeding circuit, a feeding coil connected to the feeding circuit, a booster antenna that includes a conductor at which a conductor aperture and a slit to connect the conductor aperture and an outer edge are provided and includes an area larger than a footprint of the feeding coil, a ground conductor facing the booster antenna, and a ground connection conductor that connects the booster antenna to the ground conductor. The conductor aperture is located at an offset position near the outer edge of the conductor. The ground connection conductor is disposed at a position on either side of the slit outside an area in which a current density of an induced current flowing through the booster antenna is in a range from a maximum value to about 80% of the maximum value or a position on one side of the slit in the area.
According to a preferred embodiment of the present invention, since a circuitous path for a current is not provided in an area in which the density of a current flowing through a booster antenna is high, a loss becomes small and the deterioration of an antenna characteristic due to the connection of the booster antenna to the ground rarely occurs. As a result, an antenna apparatus with a long communication distance can be obtained. Furthermore, a directivity toward a high current density area can be achieved.
The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.
An antenna apparatus according to the first preferred embodiment of the present invention and a communication terminal according to the first preferred embodiment will be described with reference to the accompanying drawings.
Inside the metal cover 2 of the communication terminal 201, a feeding coil module is disposed so that a feeding coil 31 is arranged along the conductor aperture CA. The area of the metal cover 2 is larger than the footprint of the feeding coil 31, and functions as a booster antenna as will be described later. A surface on which the metal cover 2 is disposed (the back surface of the communication terminal) is directed toward an antenna of a reader/writer that is a communication partner.
Inside the casing 1, a feeding coil module is disposed so that it partly overlaps the conductor aperture CA. That is, a lens of a camera module and the conductor aperture CA are brought into alignment with each other so that the lens is externally exposed at the opening of the casing. Referring to
A capacitor to be connected in parallel to the connection portion 32 is provided at a circuit board. A resonant frequency is determined in accordance with an inductance determined by the feeding coil 31 and the magnetic sheet 39 in the feeding coil module 3 and the capacitance of the capacitor. For example, in a case where the feeding coil module 3 is used in NFC (Near Field Communication: short-range communication) such as Felica (registered trademark) and the HF band having a center frequency of approximately 13.56 MHz is used, the resonant frequency is set to approximately 13.56 MHz.
The number of windings (turns) of the feeding coil 31 is determined in accordance with a required inductance. In a case where the number of windings of the feeding coil 31 is one, the feeding coil 31 is a loop feeding coil.
As illustrated in
Since the coil window CW and the conductor aperture CA at least partly overlap in plan view of the feeding coil 31, a magnetic flux to be linked to the feeding coil 31 and an antenna in a communication partner can circulate through the coil window CW and the conductor aperture CA. In particular, when the circumferences of the coil window CW and the conductor aperture CA almost overlap in plan view of the feeding coil 31, a magnetic field generated by the feeding coil 31 can be effectively emitted from the metal cover 2.
One of the unique features of the present preferred embodiment of the present invention is that a ground connection conductor is disposed on either side of the slit SL outside a high current density area where the current density of an induced current flowing through the metal cover 2 (booster antenna) is in the range from its maximum value to about 80% (or about 50%) of the maximum value or on one side of the slit SL in the high current density area. First, the high current density area will be simply specified on the basis of a structure.
The ground connection conductor 6 to connect the metal cover 2 to the ground conductor 81 is disposed on one side of the slit SL in the first area.
An antenna characteristic that varies in accordance with a point of connection between the metal cover 2 and a ground conductor, that is, a position at which a ground connection conductor is located, and the number of the ground connection conductors will be described.
When the ground connection conductors were disposed in only the second area, the coupling coefficient was approximately 0.044, for example, as illustrated in
In the examples [1], [2], and [3], substantially the same distribution of density of a current flowing through the metal cover 2 was obtained. In the example [4], a current flowing through a ground conductor was generated as illustrated in circles in
Next, the change in antenna characteristic will be described focusing not on the number of the ground connection conductors but on the positions of the ground connection conductors.
Table 1 indicates the relationship between each of examples [5] to [10] and the presence of the ground connection conductor at positions (1) to (4) illustrated in
As is apparent from the comparison between the
On an inner surface of the metal case 9, the feeding coil module 3 is disposed so that the feeding coil 31 is arranged along the conductor aperture CA. Like in the first preferred embodiment, in the second preferred embodiment, the feeding coil module 3 includes a flexible substrate on which the feeding coil 31 is formed and a magnetic sheet (ferrite sheet). The area of the metal case 9 is larger than the footprint of the feeding coil 31 in the feeding coil module, and functions as a booster antenna. A surface on which the metal case 9 is disposed (the back surface of the communication terminal) is directed toward an antenna of a reader/writer that is a communication partner.
Thus, the metal case 9 on the printed circuit board 8 in a casing can be used as a booster antenna. When the same number of the ground connection conductors 6 are disposed at the same positions as the first preferred embodiment, an effect similar to that obtained in the first preferred embodiment can be obtained.
In the above-described preferred embodiments, the high current density area preferably is simply specified on the basis of a structure. That is, the first area, which includes the conductor aperture, the slit, and the feeding coil in plan view and is specified by a substantially straight line parallel to a portion of the outer edge of the metal cover connected to the slit, is defined as the high current density area. However, in this case, the constraint may be avoided. For example, the first area illustrated in
In the third preferred embodiment, an example in which the high current density area is determined on the basis of the range of the current density of an induced current flowing through a booster antenna will be described. In order to show the reason why the high current density area is determined on the basis of the numerical range of a current density, the relationship between the numerical range of the high current density area and a communication range will be described.
Each of a conductor aperture, a slit, and a feeding coil preferably has the same structure as that described in the first preferred embodiment. Non-limiting examples of calculation conditions for simulation are as follows.
The outer dimensions of the booster antenna: approximately 50 mm×approximately 80 mm
The outer dimensions of the ground conductor: approximately 50 mm×approximately 80 mm
The distance between the booster antenna and the ground conductor: approximately 5 mm (the booster antenna and the ground conductor overlap in plan view)
The size of the feeding coil: approximately 15 mm×approximately 15 mm
The distance between the end of the feeding coil and the end of the booster antenna: approximately 5 mm
The width of the slit: approximately 1 mm
The size of an opening of the booster antenna: φ approximately 3 mm
As is apparent from
Accordingly, in a case where a plurality of ground connection conductors are disposed for a booster antenna, it is important to determine an area in which the ground connection conductors are disposed on the basis of the value of a current density.
The reasons why the above-described results are obtained are as follows. In a case where the ground connection conductor is disposed in the area in which the value of a current density is equal to or greater than about 80%, almost all of currents generated at the booster antenna by the feeding coil flow to the ground conductor via the ground connection conductors and the amount of current flowing through the booster antenna is markedly reduced. In a case where the ground connection conductor is disposed in the area in which the value of a current density is less than about 80%, a sufficient amount of current flows through the booster antenna. Accordingly, the radiation effect of the booster antenna is increased and a communication range is increased. In a case where the ground connection conductor is disposed in the area in which the value of a current density is less than about 50%, the flow of a current to the ground conductor rarely occurs. Accordingly, the radiation effect of the booster antenna is further increased and a communication range is further increased.
Thus, in order to obtain the maximum possible communication range of approximately 30 mm, for example, in a case where the ground connection conductors are disposed on either side of the slit, the ground connection conductors are disposed outside the area in which the current density of an induced current flowing through the booster antenna is in the range from its maximum value to about 80% of the maximum value, for example. In order to obtain the maximum possible communication range of approximately 40 mm, for example, the ground connection conductors are disposed outside the area in which the current density of an induced current flowing through the booster antenna is in the range from its maximum value to 50% of the maximum value, for example.
The maximum possible communication range of approximately 40 mm, for example, is preferred in RFID communication. The maximum possible communication range equal to or wider than at least approximately 30 mm, for example, can be considered to be a practical level.
Each of a conductor aperture, a slit, and a feeding coil preferably has the same structure as that described in the first preferred embodiment. Non-limiting examples of calculation conditions for simulation are as follows.
The outer dimensions of the booster antenna: approximately 50 mm×approximately 100 mm
The outer dimensions of the ground conductor: approximately 50 mm×approximately 100 mm
The distance between the booster antenna and the ground conductor: approximately 5 mm (the booster antenna and the ground conductor overlap in plan view)
The size of the feeding coil: approximately 15 mm×approximately 15 mm
The distance between the end of the feeding coil and the end of the booster antenna: approximately 1 mm
The width of the slit: approximately 1 mm
The size of an opening of the booster antenna: φ approximately 3 mm
Referring to
A current density at the positions (A) and (E) is approximately 86% of its maximum value, for example. In a case where the ground connection conductor is disposed at these positions at which the current density is high, the maximum possible communication range becomes approximately 27 mm, for example. A current density at the positions (B) and (F) is approximately 80% of its maximum value, for example. In a case where the ground connection conductor is disposed at these positions, the maximum possible communication range of approximately 30 mm, for example, can be achieved. A current density at the positions (C) and (G) is approximately 62% of its maximum value, for example. In a case where the ground connection conductor is disposed at these positions, the maximum possible communication range of approximately 36 mm can be achieved, for example. A current density at the positions (D) and (H) is approximately 50% of its maximum value, for example. In a case where the ground connection conductor is disposed at these positions at which the current density is low, the maximum possible communication range of approximately 40 mm, for example, which is a sufficient communication range, can be achieved.
In a case where the ground connection conductor is disposed at the positions (A) and (I), (B) and (J), (C) and (K), or (D) and (L) between which no slit is disposed, the ground connection conductors have little effect on the maximum possible communication range.
As is apparent from the comparison with the results illustrated in
In the above-described preferred embodiments, a metal cover or a metal case is preferably used as a booster antenna. However, a metal layer located on the outer surface or the inner surface of a casing or a metal layer located in the casing may be used as a booster antenna. Alternatively, a metal plate (metal casing) that is a part of the casing may be used as a booster antenna. A metal case that shields a circuit located on a printed circuit board may be a metal plate.
While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-174490 | Aug 2011 | JP | national |
| 2012-126395 | Jun 2012 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13570521 | Aug 2012 | US |
| Child | 14547381 | US |
