RESULTS

The tag measurement setup comprised a Rohde & Schwarz R&S®ZVB-20 vector network analyzer (VNA), a pair of linearly polarized transmit and receive horn antennas and fabricated prototypes of the tag (see Figure 6).16 A resonant peak at a particular frequency because of the presence of a resonating element is interpreted as a 1, signifying complete absorption; in the case of complete reflection, i.e., no resonating element, a 0 is encoded. Adding and removing resonating elements leads to the formulation of distinct bit combinations (see Figure 7). A graphical comparison of the numerically computed and experimentally measured results shows good agreement. Repeating, alternating and random bit sequences from tags on a 0.508 mm thick Rogers RT/duroid 5880 laminate are shown in Figure 8. Table 2 lists the properties of the laminate. Copper with a thickness of 0.035 mm was used as the radiator. The encoding capacity is determined by the number of resonating slot elements to obtain an equal number of resonances in the resulting frequency band. Figure 5a shows 12 octagonal resonating elements in a cobweb shape, corresponding to the RCS response in Figure 8a for a sequence of all 1s. Each cobweb shaped resonator represents a bit, resulting in a total capacity of 12 bits. Within the operating band from 6.5 to 18 GHz, there are 12 resonances with a 1:1 resonance-to-slot correspondence. The examples shown in Figure 8 illustrate the capability of the design to encode any 12-bit sequence. The RFID tag has a small 5.29 cm2 footprint, achieving a bit density of 2.26 bits/cm2. It is insensitive to a variety of polarization angles and is viable for use on non-rigid substrates.

Figure 6

Figure 6 RFID tag measurement setup.

Figure 7

Figure 7 RCS responses for various bit sequences created by adding or removing resonating elements.

 

A comprehensive comparison of this work with other reported results is summarized in Table 3. The table includes encoding capacity, bit density, polarization insensitivity, the flexible nature of the laminate, spectral bit capacity and spatial bit density. Encoding capacity is the number of bits stored for encoding data, which is 12 in this design. Bit density is the number of bits per unit area, usually measured in bits/cm2. The readability of the tag at different orientations with reference to the XY plane is indicated by polarization insensitivity. Spectral bit density is the number of bits per GHz, and spatial bit density relates to the number of bits per λ2, where λ is the wavelength.

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f8.jpg

Figure 8 Computed vs. measured RCS responses for tags with repeating (a), alternating (b) and random (c) bit sequences.

t2.jpg
 

CONCLUSION

A passive, polarization independent, chipless RFID tag with a compact size of 23 × 23 mm and 12-bit encoding capacity provides a bit density of 2.26 bit/cm2. The design mitigates high order harmonic components and possesses 1:1 slot-to-bit correspondence, enabling a total of 4096 items to be uniquely tagged.

Table 3

Acknowledgments

This work was financially supported by Vinnova, the Swedish Governmental Agency for Innovation Systems, and the University of Engineering and Technology in Taxila, Pakistan, through the Vinn Excellence Centers program and ACTSENA research group funding, respectively.

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