30-06-2012, 03:34 PM
An Electrically Small Spherical UHF RFID Tag Antenna With Quasi-Isotropic Patterns for Wireless Sensor Networks
[attachment=26323]
Abstract
An electrically small spherical UHF (911.25 MHz)
radio frequency identification (RFID) tag antenna is proposed. A
dipole-typed RFID tag antenna is wound into a spherical shape to
achieve small size and quasi-isotropic pattern. The overall diameter
of the antenna is 25 mm .
INTRODUCTION
UHF band (860~960 MHz) passive radio frequency identification
(RFID) technology has been widely used for
asset identification, retail item management, and tracking applications
due to low production cost with reasonable readable
range [1]. Recently, UHF passive RFID systems have been also
applied to wireless sensor applications [2], [3], where straight or
meandered dipole-typed RFID tag antennas were typically used.
The dipole-typed tag antennas, however, require a half-wavelength
in length and have null points in radiation pattern at both
ends. Therefore, a small antenna size is desirable because of the
need to be incorporated into a limited volume. Also, an isotropic
pattern is preferred to increase detectability regardless of the
tag’s position, which results in improvement of the communication
reliability between the tag and the reader.
ANTENNA GEOMETRY
Fig. 1 shows the geometry of the proposed RFID tag antenna.
As shown in Fig. 1(a), the initial design is a half-wavelength
straight dipole antenna with a short stub. The total length of
the initial dipole is 165 mm (0.5 at 911.25 MHz). The RFID
chip is located in the middle of the dipole. Next, as portrayed in
Fig. 1(b), the dipole antenna is packed onto a sphere’s surface,
where the diameter of the sphere is 25mm(0.076 ) and the electrical
size is a of 0.24. The left arm [Fig. 1©] and the right
arm [Fig. 1(d)] of the antenna are bent differently at five junctions.
The angle at each junction can be either 30 ( ) or 100
( ). Next, conjugate impedance matching between the RFID
chip and the miniaturized tag antenna is achieved by tuning the
short stub length and the overall dipole length.
[attachment=26323]
Abstract
An electrically small spherical UHF (911.25 MHz)
radio frequency identification (RFID) tag antenna is proposed. A
dipole-typed RFID tag antenna is wound into a spherical shape to
achieve small size and quasi-isotropic pattern. The overall diameter
of the antenna is 25 mm .
INTRODUCTION
UHF band (860~960 MHz) passive radio frequency identification
(RFID) technology has been widely used for
asset identification, retail item management, and tracking applications
due to low production cost with reasonable readable
range [1]. Recently, UHF passive RFID systems have been also
applied to wireless sensor applications [2], [3], where straight or
meandered dipole-typed RFID tag antennas were typically used.
The dipole-typed tag antennas, however, require a half-wavelength
in length and have null points in radiation pattern at both
ends. Therefore, a small antenna size is desirable because of the
need to be incorporated into a limited volume. Also, an isotropic
pattern is preferred to increase detectability regardless of the
tag’s position, which results in improvement of the communication
reliability between the tag and the reader.
ANTENNA GEOMETRY
Fig. 1 shows the geometry of the proposed RFID tag antenna.
As shown in Fig. 1(a), the initial design is a half-wavelength
straight dipole antenna with a short stub. The total length of
the initial dipole is 165 mm (0.5 at 911.25 MHz). The RFID
chip is located in the middle of the dipole. Next, as portrayed in
Fig. 1(b), the dipole antenna is packed onto a sphere’s surface,
where the diameter of the sphere is 25mm(0.076 ) and the electrical
size is a of 0.24. The left arm [Fig. 1©] and the right
arm [Fig. 1(d)] of the antenna are bent differently at five junctions.
The angle at each junction can be either 30 ( ) or 100
( ). Next, conjugate impedance matching between the RFID
chip and the miniaturized tag antenna is achieved by tuning the
short stub length and the overall dipole length.