DESCRIPTION
READER/WRITER
TECHNICAL FIELD
The present invention relates to a reader/writer that performs radio
communication with a wireless IC tag or the like.
BACKGROUND ART
A reader/writer that reads the data by receiving a radio wave from a wireless
IC tag is proposed (for example, see Patent Literature 1). Patent Literature 1
discloses that interactive communication occurs when a wireless IC tag comes close
to a reader/writer.
When a distance between the reader/writer and the wireless IC tag is
measured by the reader/writer, deceptive communication may be caused by a
malicious third party. More specifically, a person different from a wireless IC tag
holder herself/himself may perform communication by emitting a strong radio wave
from a position far from the reader/writer.
In general, a reader/writer receives a radio wave transmitted from a wireless
IC tag to detect a received level. When the detected received level is a
predetermined level or higher, the reader/writer determines that a wireless IC tag
is in a predetermined region and performs communication. The communication
performed at this time is called normal communication.
In this case, when a communication apparatus the transmission power of
which is improved by modification or the like of a wireless IC tag is used, a received
level reaches a predetermined level or higher even though a distance from the
reader/writer is long. In this manner, the reader/writer erroneously determines
that the wireless IC tag is in a predetermined region and communicates with the
wireless IC tag. This state is called abnormal communication.

When abnormal communication is performed from a distant position, for
example, in an application that handles money data or the like, the money data is
intercepted by a malicious third party to damage a person who provides a service or
a person who enjoys the service. In this manner, in a conventional reader/writer, a
malicious third party can disadvantageously perform communication from a distant
position.
CITATION LIST
Patent Literature
Patent Literature 1: Japanese Patent Unexamined Publication No. 2006-
229561
SUMMARY OF THE INVENTION
The present invention provides a reader/writer that can prevent a long
distance from being erroneously determined as a short distance even though the
reader/writer receives a strong radio wave transmitted from a distant position being
a predetermined distance or more away from the reader/writer.
The present invention includes a first antenna that receives a radio wave
transmitted from a radio wave transmitter, a second antenna that receives a radio
wave transmitted from the radio wave transmitter, and a received level detecting
unit that detects a received level of the radio wave received by the first antenna or
the second antenna. Furthermore, the reader/writer includes a determining unit
that determines whether communication is normal communication based on a level
difference between a received level value of the radio wave detected by the received
level detecting unit and received by the first antenna and a received level value of
the radio wave detected by the received level detecting unit and received by the
second antenna. The reader/writer further includes a configuration in which the

first and second antennas are arranged such that a first distance between the radio
wave transmitter and the first antenna is shorter than a second distance between
the radio wave transmitter and the second antenna.
With the configuration, even though a strong radio wave transmitted from a
distant position being a predetermined distance or more away from the
reader/writer is received, a long distance can be prevented from being erroneously
determined as a short distance.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 shows a block diagram showing a configuration of a reader/writer
according to a first embodiment of the present invention;
Fig. 2 shows a flow chart showing an operation of the reader/writer according
to the first embodiment;
Fig. 3 shows a graph showing a relationship between a distance between the
reader/writer and a wireless IC tag in the first embodiment and a reception field
intensity;
Fig. 4 shows a schematic view for explaining a configuration of abnormal
communication;
Fig. 5 shows a block diagram showing a configuration of a reader/writer in a
second embodiment of the present invention;
Fig. 6 shows a block diagram showing a configuration of a reader/writer
according to a third embodiment of the present invention;
Fig. 7 shows a configuration view showing a loop antenna according to the
third embodiment;
Fig. 8 shows a graph showing a relationship between a distance between the
reader/writer and a wireless IC tag in the third embodiment and a reception field
intensity;
Fig. 9 shows a block diagram showing a configuration of a reader/writer
according to a fourth embodiment of the present invention;

Fig. 10 shows a block diagram showing a state of a reader/writer used in a
fifth embodiment of the present invention,'
Fig. 11 shows a block diagram showing a configuration of a reader/writer
according to the fifth embodiment;
Fig. 12 shows a flow chart showing an operation of the fifth embodiment; and
Fig. 13 shows a block diagram showing a configuration of a reader/writer
according to a sixth embodiment of the present invention.
REFERENCE MARKS IN THE DRAWINGS
1 first antenna
2 second antenna
3 receiving unit
4 received level detecting unit
5 determining unit
6 reader/writer housing
7, 7a, 7b, 7c read surface
10, 10a, 10b, 10c reader/writer
31 antenna
32 antenna variable unit

11, 81 first loop antenna
12, 82 second loop antenna
41 first diversity antenna
41a, 41b, 42a, 42b loop antenna
42 second diversity antenna
45 distant position
46 wireless IC tag device
47 directional antenna
48 strong radio wave
71 printed circuit board

72 balun
73 loop element
74 connector

83 third loop antenna
84 forth loop antenna
85 fifth loop antenna
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described below based on embodiments with
reference to the accompanying drawings. The present invention is not limited to
the embodiments.
(FIRST EMEMPLARY EMBODIMENT)
Fig. 1 is a block diagram showing a configuration of a reader/writer according
to a first embodiment of the present invention. In Fig. 1, reader/writer 10 includes
first and second antennas 1 and 2 and receiving unit 3 that are incorporated in
reader/writer housing 6 serving as main body. Although not shown in Fig. 1, the
reader/writer also includes a transmitting unit, a power supply unit, a cable
communication unit communicating with an outside, and the like. First antenna 1
is relatively closely attached to read surface 7 of reader/writer housing 6. Read
surface 7 is one of the surfaces of reader/writer housing 6 and a surface to which
wireless IC tag 8 serving as a radio wave transmitter is brought close. When a
distance between read surface 7 and wireless IC tag 8 is smaller than a
predetermined length (for example, 5 cm), reader/writer 10 determines that
wireless IC tag 8 is present to start communication with each other.
Wireless IC tag 8 has various figures. In the embodiment, a configuration in
which a wireless IC tag is built in a cellular phone is employed. However, the
wireless IC tag may use various figures such as a card type.

First antenna 1 is connected to received level detecting unit 4 constituting
receiving unit 3 to detect a level of a received radio wave. As received level
detecting unit 4, an RSSI (Received Signal Strength Indication) circuit is used. In
the embodiment, second antenna 2 is arranged independently of first antenna 1.
Second antenna 2 is attached to a position that is away from read surface 7 further
than first antenna 1. A distance between first antenna 1 and read surface 7 is given
by A1, and a distance between second antenna 2 and read surface 7 is given by A2.
In this case, A1 < A2 is satisfied. Second antenna 2 is also connected to received
level detecting unit 4, and received level detecting unit 4 detects a level of a
received radio wave.
Determining unit 5 is connected to received level detecting unit 4.
Determining unit 5 determines, based on a level (B1) of a radio wave received by
first antenna 1 and a level (B2) of a radio wave received by second antenna 2, the
levels being detected by received level detecting unit 4, whether communication is
abnormal communication. More specifically, determining unit 5 determines
whether a distance between wireless IC tag 8 and reader/writer 10 is smaller than 5
cm (that is, whether a radio wave is transmitted from a distant position).
Fig. 2 is a flow chart showing an operation of reader/writer 10 according to
the first embodiment. A determining operation of determining unit 5 will be
described below with reference to Fig. 2. From the start of operation (S201), a
received level of first antenna 1 is monitored , and received level Bl of first antenna
1 is measured by received level detecting unit 4 (S202). It is determined whether
received level Bl is higher than predetermined received level BO (in this case, set to
a received level of a signal received at a position 5 cm away from the reader/writer).
More specifically, determining unit 5 determines whether Bl > BO is satisfied
(S203). When it is determined that Bl > BO is satisfied in step S203 (Yes in S203),
received level B2 of second antenna 2 is measured by received level detecting unit 4
(S204). Thereafter, difference D = Bl - B2 between the received levels of first and
second antennas 1 and 2 is calculated by determining unit 5 (S205). Thereafter,
determining unit 5 determines whether difference D between the received levels is

larger than predetermined value DO (S206). When it is determined that D > DO is
satisfied in step S206 (Yes in S206), it is determined that the communication is
normal communication, interactive data communication is performed (S207), and
the determining operation is ended (S208).
When it is determined that B1 < BO is satisfied in S203, and when it is
determined that D < DO is satisfied in S206, the flow returns to S202 to cause
received level detecting unit 4 to measure received level Bl of first antenna 1 again.
The reason why a radio wave transmitted from a distant position at a high
level can be discriminated by the above operation from a correct wireless IC tag at a
close position will be described below. Fig. 3 shows a relationship between a
distance between reader/writer 10 and wireless IC tag 8 and a received level of a
radio wave received by the antenna of reader/writer 10. Line 301 indicates a
relationship between a distance between reader/writer 10 and wireless IC tag 8 in
normal communication and intensity of a received level received by an antenna, and
line 302 indicates a relationship between a distance between reader/writer 10 and
wireless IC tag 8 in abnormal communication and intensity of a received level
received by an antenna. In a free space, received level P (mW) has a relationship
given by P α 1/(L2) with respect to distance L (m). Received level P attenuates in
inverse proportion to the square of the distance. It will be considered that, when a
distance between first antenna 1 and wireless IC tag 8 is 1 cm, first antenna 1 and
second antenna 2 are arranged such that a distance between second antenna 2 and
wireless IC tag 8 is 5 cm (i.e., a distance between the first and second antennas is 4
cm). In this case, as indicated by line 301, difference D1 between received level Bl
of first antenna 1 and received level B2 of second antenna 2 is given by Dl = 14 dB.
On the other hand, a radio wave from wireless IC tag 8 transmitted from a
position a long distance away from reader/writer 10 at a high level will be
considered. In this case, as indicated by line 302, when a distance between first
antenna 1 and wireless IC tag 8 is 1.00 m, a distance between second antenna 2 and
wireless IC tag 8 is 1.04 m. At this time, D2 = Bl - B2 = 0.3 dB is satisfied, value D
becomes small.

Therefore, determination threshold value DO of value D in determining unit 5
is set to a value between 14 dB and 0.34 dB (for example, 8 dB). When measured
value D is larger than threshold value DO, it can be determined that communication
is normal communication (i.e., wireless IC tag 8 is present near reader/writer 10) to
start data communication. When value D is smaller than DO, it is determined that
communication is abnormal communication, and the communication is stopped.
Fig. 4 is a schematic view for explaining a configuration of abnormal
communication. In Fig. 4, a malicious third party holds wireless IC tag device 46
installed at distant position 45 far away from reader/writer 10 and outputs strong
radio wave 48 from directional antenna 47 of wireless IC tag device 46. As in a
conventional technique, when a distance of wireless ID tag 46 is determined based
on only received level Bl of first antenna 1, radio wave 48 emitted from a distant
position at a high level cannot be determined as a radio wave from a distant
position. However, as described above, when the determination is made by using
the difference between the received levels of first and second antennas 1 and 2 as in
the first embodiment, even though a radio wave at received level Bl is received by
first antenna 1, it can be determined whether the radio wave is transmitted from a
distant position.
In the first embodiment, although the radio wave transmitter is explained as
wireless IC tag 8, when a device other than wireless IC tag 8 such as a bar code or
an IC card having a radio wave transmitting function is used, the same advantage
as described above can be obtained.
(SECOND EMEMPLARY EMBODIMENT)
Fig. 5 is a block diagram showing a configuration of reader/writer 10 in a
second embodiment of the present invention. In Fig. 5, according to the second
embodiment, reader/writer 10 has only one antenna 31 and antenna variable unit
32 to move antenna 31 in a direction away from read surface 7 of reader/writer
housing 6 (direction close to read surface 7) unlike in the first embodiment. As

antenna variable unit 32, for example, a motor, a magnet coil, or the like. The
component is driven to make it possible to vertically move antenna 31. The same
reference numerals denote the same parts as those in the first embodiment, and a
description thereof will be omitted.
In the second embodiment, in a state in which antenna 31 is brought close to
read surface 7, received level B1 is measured first, and antenna 31 is moved by
antenna variable unit 32. In a state in which antenna 31 is away from read surface
7, received level B2 is measured next. By difference D = Bl - B2 between received
levels at the two positions, as in the first embodiment, determining unit 5
determines whether communication is normal communication or abnormal
communication.
In the second embodiment, the reader/writer can be advantageously
simplified because the reader/writer can be configured by one antenna.
Furthermore, since two received levels are measured by the same antenna,
measurement accuracy can be avoided from being deteriorated by a difference
between antenna gains.
(THIRD EMEMPLARY EMBODIMENT)
Fig. 6 is a block diagram showing a configuration of reader/writer 10
according to a third embodiment of the present invention. In Fig. 6, in
reader/writer 10 according to the third embodiment, in place of first and second
antennas 1 and 2 in the first embodiment, first and second loop antennas 11 and 12
are used unlike in the first embodiment. The same reference numerals denote the
same parts as those in the first embodiment, and a description thereof will be
omitted.
Fig. 7 is a configuration view showing a loop antenna. Balun 72 that
performs unbalance-balance conversion is formed on printed circuit board 71, and a
balance output terminal of balun 72 is connected to both ends of loop element 73.
Loop element 73 is formed by bending a copper wire rod and arranged in such a

direction that a loop axis is parallel to printed circuit board 71. Connection
connector 74 to connect balun 72 to receiving unit 3 is connected to an unbalance
output terminal of balun 72.
The loop antenna has a loop element length that is sufficiently smaller than a
wavelength of an applied radio wave (a fraction of the wavelength) and is generally
called a small loop antenna. The small loop antenna is a type of a magnetic current
antenna and is characterized in that a variation in gain when a metal body is close
to the antenna is smaller than that of an electric current antenna (monopole
antenna or the like).
Fig. 8 shows an example of a relationship between a distance between
reader/writer 10 and a wireless IC tag and a reception field level. As described
above, although received level P α 1/(L2) is given in a free space, a phenomenon in
which antenna gains decrease is observed when antennas are brought close to each
other to have a distance that is a fraction of a wavelength of a radio wave. Fig. 8
shows a case in which a wavelength of 70 cm is used. As indicated by line 801,
decreases in antenna gain in monopole antennas are observed in a region in which
the antennas are brought close to each other to have a distance smaller than 4 cm.
On the other hand, as indicated by line 802, decreases in gain are not observed in
loop antennas. Therefore, when a loop antenna is used as reader/writer 10, a
received level obtained when reader/writer 10 is brought close to the wireless IC tag
can be accurately measured. In this manner, a radio wave emitted from a distant
position, i.e. abnormal communication can be more reliably discriminated.
(FOURTH EMEMPLARY EMBODIMENT)
Fig. 9 is a block diagram showing a configuration of reader/writer 10
according to a fourth embodiment of the present invention. In Fig. 9, in the fourth
embodiment, in place of first and second antennas 1 and 2 in the first embodiment,
first and second diversity antennas 41 and 42 are used unlike in the first
embodiment. The same reference numerals denote the same parts as those in the

first embodiment, and a description thereof will be omitted.
First and second diversity antennas 41 and 42 shown in Fig. 9 are arranged
in such a direction that two loop antennas 41a and 41b and two loop antennas 42a
and 42b have loop axes perpendicular to each other. In this arrangement, a plane of
polarization of a radio wave to be received can be changed, and a polarization
diversity effect is obtained. Therefore, received levels of two loop antennas 41a and
41b and two loop antennas 42a and 42b are measured, respectively, and large
measured values are employed.
In this manner, measurement accuracy of a received level can be improved, a
distance to wireless IC tag 8 can be accurately measured. The reason why the large
measured value is employed is to avoid a point where multi-path radio waves are
canceled out by fading to decrease a received level. Large measured values are
obtained from first and second diversity antennas 41 and 42, and difference D
between received levels is calculated by using the measured values. Based on value
D, as in the first embodiment, determining unit 5 determines whether
communication is normal communication or abnormal communication.
As described above, according to the fourth embodiment, since detection
accuracy of a distance can be improved, abnormal communication can be more
reliably discriminated.
When a diversity antenna is used as the antenna of reader/writer 10
configured by antenna variable unit 32 described in the second embodiment,
discrimination accuracy of abnormal communication can be improved.
(FIFTH EMEMPLARY EMBODIMENT)
According to the first to fourth embodiments, even though a malicious third
party communicates with reader/writer 10 by a wireless device that emits a strong
radio wave, abnormal communication by the malicious third party can be
determined.
According to the first to fourth embodiments, as shown in Fig. 10, when a

plurality of reader/writers 10 are arranged, even though wireless IC tag 8 performs
radio communication with one reader/writer 10b of reader/writers 10, other
adjacent reader/writers 10a and 10c do not communicate with wireless IC tag 8.
More specifically, in Fig. 10, three reader/writers 10a, 10b, and 10c described
in the first to fourth embodiments are arranged adjacent to each other while
keeping predetermined distances L therebetween. In this case, even though
distance L is arbitrarily set, when predetermined value DO is set to an appropriate
value, other adjacent reader/writers 10a and 10b do not perform radio
communication with wireless IC tag 8 when wireless IC tag 8 performs radio
communication with one reader/writer 10b of reader/writers 10.
For example, when L1 (distance between wireless IC tag 8 and first antenna
1 held by reader/writer 10b) = 1 cm and L2 (distance between wireless IC tag 8 and
second antenna 2 held by reader/writer 10b) = 5 cm, D = 14 dB is obtained. On the
other hand, when it is assumed that the position of wireless IC tag 8 does not
change, and distances between next reader/writer 10a and wireless IC tag 8 have
the following relationship. When L = 3 cm is given, L3 (distance between wireless
IC tag 8 and first antenna 1 held by reader/writer 10a) = about 7 cm and L4
(distance between wireless IC tag 8 and second antenna 2 held by reader/writer
10a) = about 9 cm are satisfied. In this case, a small value, i.e., D = 2 dB is
obtained. Therefore, when DO is set to a value (for example 8 dB) between 14 dB
and 2 dB, the next reader/writer can be avoided from communicating with the
wireless IC tag.
The fifth embodiment describes a configuration that is different from those of
the first to fourth embodiments and in which, even though reader/writers 10 are
proximally arranged adjacent to each other, each of reader/writers 10 can determine
communication with wireless IC tag 8 that is not above read surface 7 of
corresponding reader/writer 10 as abnormal communication.
Fig. 11 is a block diagram showing a configuration of reader/writer 10 in the
fifth embodiment of the present invention. In Fig. 11, reader/writer 10 is configured
by arranging receiving unit 3 and first loop antenna 81, second loop antenna 82,

third loop antenna 83, fourth loop antenna 84, and fifth loop antenna 85 that are
electrically connected to receiving unit 3 in reader/writer housing 6. Receiving unit
3 includes, as in the first embodiment, received level detecting unit 4 and
determining unit 5.
First loop antenna 81 is arranged at an upper part in reader/writer housing 6
and near read surface 7 and arranged at almost the center of read surface 7.
Receiving unit 3, second loop antenna 82, third loop antenna 83, fourth loop
antenna 84, and fifth loop antenna 85 are arranged at a lower part in reader/writer
housing 6 to surround receiving unit 3 from four directions. More specifically,
second loop antenna 82, third loop antenna 83, fourth loop antenna 84, and fifth
loop antenna 85 are arranged under first loop antenna 81. Second loop antenna 82
and fourth loop antenna 84 are arranged to face each other. Third loop antenna 83
and fifth loop antenna 85 are arranged to face each other.
According to the configuration shown in Fig. 11, as described in the first to
fourth embodiments, based on a difference between a received level of first loop
antenna 81 and a received level of second loop antenna 82, determining unit 5 can
determine whether communication is normal communication or abnormal
communication performed by a malicious third party.
For example, when a radio wave from wireless IC tag 8 comes from a
direction of third loop antenna 83, a received level difference that is a
predetermined level or more occurs between a received level obtained by third loop
antenna 83 and a received level obtained by fourth loop antenna 84. For this
reason, determining unit 5 can understand that wireless IC tag 8 is not located
above read surface 7 to make it possible to determine abnormal communication.
When determining unit 5 compares a received level obtained by third loop antenna
83 with a received level obtained by fourth loop antenna 84 to make it possible to
determine a direction from which a radio wave from wireless IC tag 8 comes.
Fig. 12 is a flow chart showing an operation of the fifth embodiment. The
operation according to the fifth embodiment will be described below with reference
to Fig. 12. From the start of operation (S121), received level detecting unit 4

monitors a received level of first loop antenna 81. Received level Bl of first loop
antenna 81 is measured by received level detecting unit 4 (S122). Determining unit
5 determines whether received level Bl is larger than predetermined received level
BO (in this case, set to a received level received at a position 5 cm away from the
antenna), i.e., Bl > BO is satisfied (S123).
When it is determined in step S123 that Bl > BO is satisfied (Yes in S123),
received level detecting unit 4 measures received level B2 of second loop antenna 82
(or any one of third loop antenna 83, fourth loop antenna 84, and fifth loop antenna
85) (S124). Thereafter, determining unit 5 calculates difference D = Bl - B2
between the received levels of first and second loop antennas 81 and 82 (S125).
Thereafter, determining unit 5 determines whether difference D between the
received levels is larger than predetermined value DO (S126).
When it is determined in step S126 that D > DO is satisfied (Yes in S126),
received level detecting unit 4 measures received level X1 of second loop antenna
82, received level X2 of fourth loop antenna 84, received level X3 of third loop
antenna 83, and received level X4 of fifth loop antenna 85 (S127). Thereafter,
determining unit 5 calculates received level difference Y1 = X1 - X2 between
received level X1 of second loop antenna 82 and received level X2 of fourth loop
antenna 84 or received level difference Y2 = X3 - X4 between received level X3 of
third loop antenna 83 and received level X4 of fifth loop antenna 85 (S128).
Thereafter, determining unit 5 determines whether difference Yl between
the received levels or difference Y2 between the received levels is larger than
predetermined value YO (S129). When it is determined in step S129 that difference
Yl between the received levels or difference Y2 between the received levels is
smaller than predetermined value YO (No in S129), normal communication is
determined to perform interactive data communication (S130). On the other hand,
when it is determined in S129 whether difference Yl between the received levels or
difference Y2 between the received levels is equal to or larger than predetermined
value YO (Yes in S129), abnormal communication is determined to stop the

communication. Thereafter, the flow returns to step S122. When it is determined
in step 122 that B1 < BO, the flow returns to step S122. When it is determined in
step S126 that D < DO is satisfied, the flow returns to step S124.
As described above, in the fifth embodiment, as described in the first
embodiment or the like, by determination made by determining unit 5 in step S126,
a radio wave transmitted from a distant position at a high level can be
discriminated from a radio wave from a correct wireless IC tag at a close position.
Furthermore, based on the received levels of second loop antenna 82, third loop
antenna 83, fourth loop antenna 84, and fifth loop antenna 85, by determination
made by determining unit 5 in step S129, it can be determined whether a radio
wave transmitted from wireless IC tag 8 comes from a position immediately above
reader/writer 10 or a position on the front, rear, left, or right of reader/writer 10.
In this case, the reason why it can be determined whether a radio wave
transmitted from wireless IC tag 8 comes from a position immediately above
reader/writer 10 or a position on the front, rear, left, or right of reader/writer 10 will
be described below. A relationship between a distance between reader/writer 10
and wireless IC tag 8 and received levels of radio waves received by first to fourth
loop antennas 82 to 85 is as shown in, for example, Fig. 3 or the like. In a free
space, the received level attenuates in inverse proportion to the square of the
distance. Therefore, the more a radio wave transmitted from wireless IC tag 8 is
close to the position immediately above reader/writer 10, the smaller difference Y1
between the received levels or difference Y2 between the received levels becomes.
In contrast to this, the more the radio wave transmitted from wireless IC tag 8 is
far away from the position immediately above reader/writer 10, the larger difference
Yl between the received levels or difference Y2 between the received levels
becomes. For this reason, when difference Yl between the received levels or
difference Y2 between the received levels is a predetermined value or more, it can
be determined that a radio wave transmitted from wireless IC tag 8 comes from the
front, rear, left, or right of reader/writer 10.
More specifically, in the fifth embodiment, first loop antenna 81 is arranged

at an upper part in reader/writer housing 6. According to this, due to a difference
between received levels based on a difference between a distance to first loop
antenna 81 and a distance to second loop antenna 82, a radio wave transmitted
from a distant position can be prevented from being erroneously determined as a
radio wave from a close position. The determination can be made due to not only
the difference between the received levels based on the difference between the
distance to first loop antenna 81 and the distance to second loop antenna 82, but
also a difference between received levels based on at least one of a difference
between a distance to first loop antenna 81 and a distance to third loop antenna 83,
a difference between a distance to first loop antenna 81 and a distance to fourth
loop antenna 84, and a difference between a distance to first loop antenna 81 and a
distance to fifth loop antenna 85.
In the fifth embodiment, second loop antenna 82 and fourth loop antenna 84
are arranged to face each other. Furthermore, third loop antenna 83 and fifth loop
antenna 85 are arranged to face each other. According to this, it is determined that
a radio wave that comes from wireless IC tag 8 comes from a side of reader/writer
10, and abnormal communication from wireless IC tag 8 arranged above read
surface 7 of next reader/writer 10 can be determined.
In the arrangement of first loop antenna 81, second loop antenna 82, third
loop antenna 83, fourth loop antenna 84, and fifth loop antenna 85 shown in Fig. 11,
second loop antenna 82, third loop antenna 83, fourth loop antenna 84, and fifth
loop antenna 85 are not arranged immediately below first loop antenna 81.
According to this, when second loop antenna 82, third loop antenna 83, fourth loop
antenna 84, or fifth loop antenna 85 performs radio communication with wireless IC
tag 8 located immediately above reader/writer 10, an antenna gain is not decreased
due to first loop antenna 81 or a circuit board supporting first loop antenna 81 that
is an obstacle.

(SIXTH EMEMPLARY EMBODIMENT)
From the above standpoint, for example, first loop antenna 11 and second
loop antenna 12 shown in Fig. 6 may be arranged as shown in Fig. 13. Fig. 13 is a
block diagram showing a configuration of reader/writer 10 according to a sixth
embodiment of the present invention.
In Fig. 13, first loop antenna 11 is arranged at an upper part in reader/writer
housing 6 and near read surface 7 and arranged at almost the center of read surface
7. Second loop antenna 12 is arranged at a lower part in reader/writer housing 6
and arranged at a position except for almost the center of read surface 7. More
specifically, second loop antenna 12 is arranged in a direction different from a
direction in which wireless IC tag 8 and first loop antenna 11 are arranged.
Therefore, when second loop antenna 12 performs radio communication with
wireless IC tag 8, first loop antenna 11 and the circuit board that supports the first
loop antenna 11 is not an obstacle in communication, and second loop antenna 12
can receive a radio wave at an appropriate received level.
In the sixth embodiment, only the arrangement shown in Fig. 6 is described.
However, the same concept can also be applied to the arrangement of first antenna
1 and second antenna 2 and the arrangement of first diversity antenna 41 and
second diversity antenna 42 that are shown in Figs. 1 and 9, respectively.
An installation position of receiving unit 3 need not be considered to achieve
the advantages of the present invention. Therefore, the installation positions of
receiving unit 3 shown in Figs. 1, 6, and 9 are only an example, and should not be
specified.
INDUSTRIAL APPLICABILITY
Since the present invention can determine abnormal communication
performed by a malicious third party, the present invention can be applied to a
communication apparatus that communicates with a wireless IC tag or the like.

We claim :
1. A reader/writer comprising:
a first antenna receiving a radio wave transmitted from a radio wave
transmitter;
a second antenna receiving a radio wave transmitted from the radio wave
transmitter;
a received level detecting unit detecting a received level of the radio wave
received by the first antenna or the second antenna; and
a determining unit determining whether communication is normal
communication based on a level difference between a received level value of the
radio wave received by the first antenna detected by the received level detecting
unit and a received level value of the radio wave received by the second antenna
detected by the received level detecting unit, wherein
the first antenna and the second antenna are arranged such that a first
distance between the radio wave transmitter and the first antenna is shorter than a
second distance between the radio wave transmitter and the second antenna.
2. The reader/writer according to claim 1, wherein
the determining unit determines that the communication is the normal
communication when the level difference between the received level value of the
radio wave received by the first antenna detected by the received level detecting
unit and the received level value of the radio wave received by the second antenna
detected by the received level detecting unit is larger than a predetermined value,
and the determining unit determines that the communication is not the normal
communication when the level difference is smaller than the predetermined value.
3. The reader/writer according to claim 1, wherein
the first antenna and the second antenna are configured by one antenna, and
the reader/writer has an antenna variable unit that changes the position of the one
antenna such that the one antenna is arranged at a position having the first

distance or a position having the second distance.
4. The reader/writer according to claim 1, wherein
the first antenna is arranged above the second antenna.
5. The reader/writer according to claim 1, wherein
the first antenna and the second antenna are arranged to face each other.
6. The reader/writer according to claim 1, wherein
the first antenna is arranged at an upper part in a main body, four antennas
including the second antenna are arranged at a lower part in the main body, and
every two antennas of the four antennas are arranged to face each other.
7. The reader/writer according to claim 1, wherein
the first antenna and the second antenna are loop antennas.
8. The reader/writer according to claim 1, wherein
the first antenna and the second antenna are diversity antennas.

First and second antennas (1,2) are arranged such that a distance between a
wireless IC tag (8) and a first antenna (1) is shorter than a distance between the
wireless IC tag (8) and a second antenna (2), a determining unit (5) determines
whether communication is normal communication based on a level difference
between a received level value of a radio wave received by the first antenna (1) and
a received level value of a radio wave received by the second antenna (2) is arranged
to determine abnormal communication from a distant position by a malicious third
party.