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Printed: 23, March 2015

Rf identification includes a lengthy history [1,2] and is a vital identification technology as wireless assortment of information with no type of site [3]. RFID systems includes a readers transmitting RF energy to switch on and interrogate a transponder or tag that’s connected to the item to become identified. Passive UHF RFID tags don’t have an electrical source that belongs to them, and receive power in the readers RF field via their antenna directly attached to the integrated circuit nick. The tag identifies itself towards the interrogating readers by modulating the backscattered signal [3]. Recent advances in semiconductor industry and interest from leading retailers led to rapid development of UHF RFID which supplies longer read range (.5 to in excess of 10m) [4-7], when compared with well toned and broadly used LF/HF RFID with short read range (lt1m) [3].

Conventional planar passive UHF RFID tags are affected degradation in performance when mounted on, or perhaps in close closeness to, reflecting, lossy and capacitive physiques. Furthermore, tags might be contained within metal cages for transit [8]. This poor performance might be because of detuning from the resonant frequency from the tag antenna [8, 9]. The most popular option would be to create antennas for particular use, however as numerous objects requiring identification by passive RFID technology contain metal or liquid although it is undesirable to want specific tag tuning for every mounting. Therefore, there’s great interest to build up a passive RFID tag that’s in addition to the mounting platform in addition to being uncomplicated in structure and occasional profile. Several antennas happen to be suggested, designed for metal mounting they include folded dipoles, dipoles having a meander line separated in the ground plane with a dielectric substrate, planar inverted-F antenna (PIFA), and microstrip patch antennas [11-16]. However, they either have much talked about, or are complicated or costly to fabricate. The tag designs presented within this paper have low profile and uncomplicated manufacturing process, and could be fabricated on cheap common materials. While using impedance matching techniques described within this paper, platform tolerant tags with thickness under .003λ could be made with ease.

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The RFID ASIC utilized in these experiments is really a passive UHF read only transponder from EM Microelectronics (EM4222) [17], Impinj (monza3) [18]) and NXP (UCODE G2XM) [19].

planar antenna design on 3.4mm FR-4 substrate

Platform insensitive RFID tags designed on FR-4 substrate are equipped for tagging large objects for example pallets and containers in which the visibility and surface part of the tag isn’t of first concern, but low profile and sturdiness to mechanical shock and chemical attack is preferred.

Antenna Design and RFID Link Budget

The tag antenna design presented in Fig.1, includes a bent dipole by having an RFID ASIC (application specific IC) connected to the slot in the dipole center. The bent dipole is separated by 3.4mm FR-4 (r = 4.6, tan ¤ = .025) from the rear ground plane and by itself don’t present a great match towards the nick impedance. The readers EIRP is fixed by local rules and decays quickly by path loss between your readers and also the tag antenna. The ability offered at tag antenna is offered by:

The road loss between your readers and tag is offered by:

Where d is the size of the direct ray path, ‡n may be the reflection coefficient from the n-th reflecting object, and dn is the size of the n-th reflected ray path. Pt may be the output power the readers and Gt may be the gain from the readers antenna. Path loss thus remains proportional to d-n, with respect to the atmosphere, where n can differ between 1 and 6, [20].

The tag sensitivity the minimum signal strength required to switch on the tag IC in the tag location is offered by:

Where p may be the polarization efficiency, may be the impedance matching between your tag and also the nick, and Pth may be the nick power threshold sensitivity. And so the antenna port impedance needs to be conjugately matched towards the nick complex impedance for optimum power transfer. The bent dipole is near to a ground plane which cuts down on the real area of the antenna port impedance and can’t match the IC impedance. By making use of a carefully coupled metallic patch on a single layer because the bent dipole, strong fields are generated within the gap together. This boost the tangential fields around the antenna by controlling the length of the dipole and it is closeness from the patch, a great conjugate match is possible towards the ASIC.

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Fig. 1.Durable platform independent UHF RFID tag schematic diagram, Dimensions in mm.

The curvature dipole length can bond with half wave length or significantly less, with respect to the application and substrate thickness and dielectric qualities. The dipole needs to be relatively thick (3% of lambda) to provide appropriate impedance to complement the IC impedance. After choosing the parameters for that bent dipole the metallic patch width and length is adjusted to tweak the antenna resonant frequency and match the antenna port impedance to nick impedance.

Simulation and browse Range Measurements

To judge the woking platform independency from the design, CST Microwave Studio was utilized to simulate the tag response in three situations: (i) on the large expanded polystyrene tile (λ-λ), (ii) on the large metallic sheet (λ-λ), and (iii) on the water filled container (λ-λ-λ/2, water: r=81, tan¤ =.01S/m encircled with a 1mm thick polyimide container: r=3.5, tan¤= .003S/m). The resonant frequency of tags detunes near to water and metal, which at the best cuts down on the read range. Fig.2 shows the simulated return loss for that parasitically loaded bend dipole antenna described above, in which the resonance frequency continued to be within 1% from the frequency for that tag in isolation. This signifies minimal loading brought on by the trunk mounting structure and for that reason there’s minimal decrease in read range.

The simulated far fields from the tag are proven in Fig.3, and indicate a primary lobe directivity of two.7dBi and therefore the tag has good Omni-directional performance. The far field simulation results also indicate a rise in directivity for that tag around the large metallic plate (6.7 dBi), which leads to bigger read range finally there’s home loan business directivity for that tag around the large water bucket which might negatively modify the tag read range.

The tag dimensions succumbed Fig.1 were utilised to produce a prototype resonating at 915 MHz. The prototype tag read rang was measured in lab atmosphere utilizing a Gen2 readers with 500mW ERP output power along with a circular antenna with 4dBil gain. The utmost read range measured within the lab atmosphere was 6.3m at 915 MHz, using the range reducing to around 2m at 867 MHz and 954 MHz. The tag put on the polystyrene tile reliably covered a bandwidth well over 40MHz with read range near to maximum as proven in Fig.4. This bandwidth will cover the 26MHz needed by US rules. The tag bandwidth for near to maximum read range reduces to twenty-eight MHz when put on the big metallic plate, also is sufficient for all of us use. The tag read range elevated when put on a smaller sized metallic plate because the gain could be elevated by resonant ground currents. Around the water bucket, the read range reduced by about 30% to 4m because impure water is extremely absorbent to electromagnetic waves.

Fig. 2. Simulated return loss for durable platform independent UHF RFID tag on foam tile (λ-λ), on the large metallic sheet (λ-λ), as well as on a water filled container (λ-λ-λ/2)

Solid line: tag on foam tile, Doted line: tag on large metallic plate, Damaged line: tag on large water bucket.

Fig. 3. Simulated polar plot for durable platform independent UHF RFID tag on foam tile (λ-λ), on the metallic sheet (λ-λ), as well as on a water filled container (λ-λ-λ/2).

Solid line: tag on foam tile, Dotted line: tag on large metallic plate, Damaged line: tag on water bucket

Fig. 4. Measured tag read range on foam tile (λ-λ), on the metallic sheet (λ-λ), as well as on a water filled container (λ-λ-λ/2) at various readers transmitted frequency (MHz)

Solid line: tag on foam tile, Small doted line: tag on large metallic plate, Large damaged line: tag on small large water bucket, Small damaged line: tag on small metallic plate

low profile platform tolerant tag design

It’s desirable to build up thinner platform tolerant tags for cartons and smaller sized objects and you’ll be able to design tags for FR-4 substrates with under 1mm or .003λ thickness, in line with the principle in section II. However, the fringing fields between your bent dipole and also the ground increases for thin substrates and results in decrease in radiation efficiency that will reduce read range. A parasitically loaded bend dipole antenna was created and fabricated of .8mm FR-4 (r = 4.6, tan ¤ = .025) substrate, the tag antennas port impedance was conjugately matched towards the nick impedance using CST microwave studio, and browse range was measured within the lab atmosphere as defined in section II. The utmost read range measured for that prototype tag when compared to maximum read range measured in section II for tag on 3.4mm FR-4, demonstrated a 50%, 57%, 67% reduction correspondingly for (i), (ii) and (iii) above, this decrease in read range is much more severe for tags with thinner substrate. To beat this decreasing read range problem a tag is presented within the next section utilizing a multi layered design.

Multi layered antenna design, simulation results and browse range measurement