by: P R Foster and R A Burberry
INTRODUCTION
As the frequency of choice for RFID devices rises into the microwave region, the problem of designing antennas to match the devices on the protected object becomes more acute.The objective of any such antenna must be to maximise the transfer of power into and out of the device on the protected object. This requires careful design to match the antenna to free space and to the foIlowing ASIC. The frequency bands considered in this paper are 435 MH2,2.45 G& and 5.8 GHz and the usage is for retail goods only. The antenna must
.be small enough to be attached to the required object
.have omnidirectional or hemispherical coverage
.must provide maximum possible signal to the ASIC
.have a polarisation such as to match the enquiry signal regardless of the physical orientation of the protected object,
.be robust
.be very cheap
The major considerations in choosing an antenna are
.the type of antenna
.its impedance
.RF performance when applied to the object 9
.RF performance when the object has other structures around it
POSSIBLE CONFIGURATlONS
There are two system configurations which can be used. The first is where the tagged objects are stored in a warehouse and a portable device, possibly hand-held, interrogates all the objects and the required object repIies. The other consists of an interrogating portal through which all objects are passed on entry and exit. The other major choice is between a rag which has a battery or one which is battery-less.
POSSIBLE ANTENNAS
There are several passible antenna types in these frequency ranges which may be used for RFID. Afew of these antenna types are listed in Table 1 where the emphasis is on small dimensions. Gain for such small antennas wili be limited and is dependent on the type of radiation patterns which can be omnidirectional with a peak gain of 0 to 2 dBi or directional where the radiation pattern ha5 a definite lobe and the peak gain might be 6 dBi. This gain affects the detection range of the antenna. The first three entries in the table are linearly polarised while the printed patch can be circularly polarised and the lag-conical is only circularly polarised. Because the orientation of the RFID tag is Iikely to be uncontrollable, the interrogator must be circularly polarised. A CP antenna for the tag would therefore produce 3 dB more signal.
