Microwave Journal
www.microwavejournal.com/articles/39178-chipless-rf-identification-tags-with-microstrip-patch-resonators

Chipless RF Identification Tags with Microstrip Patch Resonators

November 14, 2022

An RF identification (RFID) coding approach is based on the radar cross section (RCS) of a multi-patch chipless RFID tag. Combined with classical frequency coding, this increases the coding capacity. This work describes a six-resonator RFID tag using microstrip patches that has a high data capacity and low-cost. It can be directly printed on products such as personal IDs, credit cards, paper and textiles because it needs only one conductive layer. It is designed to operate over the range of 4.5 to 6.2 GHz, at frequencies allocated for RFID systems.

For several years, RFID technology has brought many innovations in the field of automatic identification of people and goods; however, for consumer product identification the barcode is still dominant.1 An RFID system consists of RFID tags and an RFID reader.2,3 RFID tags are attached to objects to be identified. Each RFID tag contains a unique tag identification number (tag-ID). The RFID tag contains electronic circuitry that stores the tag-ID and communicates wirelessly with the RFID reader. The RFID reader transmits an interrogation signal, which communicates with the RFID tag to obtain its unique tag-ID. The tag either actively transmits an RF response signal or passively reflects (backscatters) the interrogation signal. The tag response is captured by the RFID reader antenna and processed by the reader to extract its tag-ID.4-7

Chipless technology is of interest since its cost is lower than technologies with active chips and it enables operation under adverse environmental conditions where electronic chips can be easily destroyed. There has been much research focused on high performance chipless RFID tags.8,9 Numerous resonant topologies have been proposed, such as U-shape,10 L-shape,11 octagonal,12 slot,13 triangular patch,14 microstrip line15 and shorted dipoles oriented at 45 degrees.16

According to the encoding method, chipless RFID tags can be grouped into two main categories: 1) frequency signature based17-20 and 2) time domain reflectometry based chipless RFID.7,21-23

Recent development in the era of low-cost and compact communication systems has largely been due to the advent of small weight and size antennas that can provide good performance over a broad frequency range. The rectangular microstrip patch is an attractive choice. Its theory of operation is computationally simple. It is low-cost, easy to fabricate and is conformable. It enables low-profile structures of compact size that assure reliability, mobility and good efficiency.

This work describes a chipless tag comprising six patch resonators with coding based on control of its RCS magnitude over a 1.6 GHz bandwidth from 4.5 to 6.2 GHz. The effects of mutual coupling are also explored. Phase, along with frequency coding, enables increased coding capacity. Measurements validate simulation.

MICROSTRIP PATCH RESONATOR DESIGN

Figure 1

Figure 1 Microstrip patch antenna geometry.

Figure 2

Figure 2 Geometry of the chipless RFID tag using slots to reduce size. Lp = Wp = 16.5, W = 1.2 and G = 6 mm for a patch resonant at 4.6 GHz.

The microstrip patch is designed to operate over a narrow band. In the design of RCS magnitude-based chipless tags, resonant behavior is desirable. The design procedure is described by Karmakar.24 It is fabricated on a 0.76 mm thick Rogers RO4350B substrate (see Figure 1).

The width Wp and length Lp are given by the following equations:25

where

And ΔL is determined by

Where c is the speed of light in a vacuum, h is the substrate thickness and εreff is the effective permittivity given by the equation:

PARAMETRIC STUDY

The microstrip antenna’s patch length Lp and width Wp are calculated theoretically to be 17.2 and 21.7 mm, respectively, for resonance at 4.6 GHz, the target frequency of the lowest frequency patch. A parametric study of the dimensions of the patch antenna with slots (see Figure 2) is done to optimize performance and reduce size (Lp = Wp = 16.5 mm).

Figure 3

Figure 3 RCS magnitude over frequency vs. Lp and Wp (a) and W vs. G (b).

Figure 4

Figure 4 RCS magnitude (a) and phase (b) of the optimized 4.6 GHz design.

The parameter sweep section of the CST Studio Suite is used to optimize the various patch parameters. Changes to Wp, Lp and slot dimensions W and G have a significant impact on RCS. Figure 3a shows the resonant frequency as a function of Lp and Wp. The initial result, using dimensions analytically derived show resonance at f = 3.6 GHz. After optimization, resonance is achieved at f = 4.6 GHz with Lp and Wp both equal to 16.5 mm. For best results, W = 0.9 to 1.5 mm and G = 4 to 8 mm, as shown in Figure 3b. Figure 4 shows the final magnitude and phase response of the 4.6 GHz patch.



A similar procedure is used to determine the dimensions of the other patches in the multi-bit tag.

CHIPLESS RFID DESIGN SIMULATION AND MEASUREMENT

Four-Bit Patch Tag

In the case of a multi-patch tag, an array of narrowband patches performs the functions of signal reception, tag-ID data encoding and transmission of backscattered signals. The individual patches each have a different resonant frequency and backscatter. A unique frequency signature is produced by the array in the total backscattered signal. The RCS of the tag presents the frequency signature encoding the tag-ID data.

Figure 5

Figure 5 Layout (a) and manufactured (b) patch tags returning the ID 1111.

Figure 6

Figure 6 Measurement setup using a VNA in an anechoic chamber.

Two multi-patch tag designs are considered. The first is composed of four rectangular microstrip patch resonators. The four patch resonators have lengths Lp of 12, 13.5, 15 and 16.5 mm and resonate at 6.1, 5.7, 5.1 and 4.6 GHz, respectively (see Figure 5). Measurements are made in an anechoic chamber using a Keysight PNA-N5221A vector network analyzer (VNA) as shown in Figure 6. The horn antennas are separated by e = 30 cm. All resonant peaks up to r = 30 cm can be extracted. Simulated and measured results are shown in Figure 7.

Figure 7

Figure 7 Simulated vs. measured RCS magnitude of the 4-bit chipless tag.

Figure 8

Figure 8 Simulated vs. measured RCS magnitudes (a) and phase (b) for two tags with different IDs.

 

One or more rectangular patches can be printed on a tag, and one or more of these patches can be shorted, nulling its corresponding frequency. A data bit 1 or 0 is encoded according to the presence or absence of a notch in the spectral response, respectively. Four notches appear in the magnitude response as four 1’s of the tag with ID “1111,” while three notches appear for three 1s, with no notch for the 0 in the magnitude response of the tag with ID “1101” (see Figure 8a). Four phase transitions are seen in the phase response for the tag with ID “1111,” but no phase transition appears in the 0 for the tag with ID “1011” (see Figure 8b).

Six Bit Patch Tag

Data capacity can be achieved by adding more resonators. Six resonators with Lp = 12, 13.5, 14.5, 15.5, 16.5 and 17.5 mm create six notches in the spectrum corresponding to six resonant frequencies at 6.1, 5.7, 5.4, 5.1, 4.7 and 4.4 GHz. The simulated and the manufactured structures are shown in Figure 9. Simulation and measurement results are shown in Figure 10. Six resonances are clearly detected and easily distinguishable and show good agreement between measurements and the simulation.

Figure 9

Figure 9 Layout (a) and manufactured (b) patch tags designed to return the ID 111111.

Figure 10

Figure 10 Simulated vs. measured RCS magnitude of the 6-bit chipless tag.

 

The theoretical RCS definition is independent of the distance (r) between the tag and the antenna.26 In practice, the measured quantity is the backscattered power received at the antenna, which is highly dependent on the measurement setup. The Agilent PNA-N5221A VNA in a bistatic configuration is used to measure vertically polarized radiation in the frequency domain. In the frequency range of 2 to 8 GHz, the VNA delivers 0 dBm of power. Over the frequency band of interest, the two horn antennas each have a gain of 12 dBi. In the setup shown in Figure 6, measurements are made with e = 30 cm and r = 20 and 30 cm. With r at 30 cm, the notch at the highest frequency is not well defined, which implies degraded performance beyond that range (see Figure 11).

Figure 11

Figure 11 RCS magnitude of the 6-bit chipless tag placed at 20 and 30 mm from the transmit and receive horn antennas.

Figure 12

Figure 12 RCS magnitude (a) and phase (b) responses for tags with IDs of 111111 and 111011.

 

The chipless tag yields 6-bit data with a unique frequency signature. Comparing the results of the blue curve with those of the red curve in Figure 12a, the fourth notch disappears, creating a tag response with ID “111011.” Six phase transitions are seen in the phase response for the tag with ID “111111” in the blue curve, while only five are visible in the red curve in Figure 12b.



A comparison of this with other reported work is shown in Table 1.

CONCLUSION

A chipless RFID tag operating from 4.5 to 6.2 GHz transmits and recovers multi-bit data. Rectangular shaped resonators increase the bit-encoding capacity. Multi-patch RFID chipless tags use narrowband resonant structures to create notch frequencies. Although 6-bit data is encoded in the proposed tag, a higher capacity data structure can be designed by adding more resonators while maintaining the same operating bandwidth. The low-cost single-sided compact chipless RFID tag can be printed directly on many items.

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