Research On UHF Antenna For NF RFID Reader

Recently, UHF NF RFID has drawn a lot of attentions since several of itsexcellent characteristics are taken into applications. It is proposed to be a suitablesolution for the issue that traditional UHF RFID system could not operate well whenthe tags are placed in or close to metals and liquids. Traditional UHF RFID, basedon electromagnetic waves propagating between reader and tags, is sensitive to theoperating environment. For the UHF NF RFID, the interaction between the readerand the tags can be either magnetic (inductive) or electric (capacitive), however theenergy associated with the magnetic coupled system is mostly stored in magneticfield. It can be only affected by objects with high magnetic permeability, which arenot common in everyday life. This is the reason why magnetic coupling, also used inLF/HF RFID systems, is able to operate near metals and liquids. Therefore, UHF NFRFID puts forward promising opportunities to develop the RFID system from palletlevel to item level tagging (ILT) in medical products, pharmaceutical logistics,bio-sensing applications and so on.For a reasonable design of UHF NF RFID reader antenna, the key point is togenerate strong and uniform magnetic field in an adequate interrogation zone. InLF/HF RFID system, an electrically small loop is used as the reader antenna, withthe reason the loop has a nearly constant current phase along it to provide strongmagnetic field along the axis. However it is not always the case in UHF band. Inorder to ensure the reading area, the size of the loop cannot be reduced. For anelectrically large loop, its perimeter is comparable with the operating wavelength.The currents distribution along the antenna experience phase-inversion and currentnulls, resulting in the weakening effects of magnetic field in the center.First in this paper, a novel printed-folded dipole for UHF NF RFID readerusing magnetic coupling is proposed. The strong and uniform magnetic field isexcited by two opposite-directed currents (ODCs). The folded dipole is printed onan FR-4substrate with the scale32cm×20cm. The bandwidth of the antenna rangesfrom826to853MHz. When the transmit power of the proposed antenna is17dBm,the reading area can be managed to35.3×24cm2on the surface for Impinj F43tag,and the reading distance is around5mm. For the case of ceramics ISO1800-6-C tag,and at the same time, the reading area increases to58×42cm2, and the readingdistance is around77mm at the center of the antenna. Both simulations andmeasurements show that the proposed antenna can provide a broad reading area witha very low transmit power of17dBm.Then a novel multilayer printed dipoles for reader are proposed. The antenna can be fabricated on a multilayer PCB with low cost. The proposed antennaresonates at842MHz, and can cover the China standard (840-845MHz). Thesimulated result shows that the current along the loop can stay in the same direction.Compared with other designs, the near-field magnetic field HZis much stronger, andit makes our design suitable for the NF RFID reader. The proposed antenna is testedand the factors, which have an effect on the reading ability, are taken intoconsideration. After the structure of the antenna has been improved, the magneticfield is stronger and more uniform. The amplitude of the magnetic field can beimproved by11dB.