Electromagnetic Simulation Software

Simulating Performance of a Low Cost,
Chipless RFID System in XFdtd
Introduction
An RFID tag system is examined using XFdtd EM Simulation Software to validate its
performance for use in situations where high volume usage requires very low-cost
components. This example is derived from a system described in a published journal
paper [1]. The RFID tag is constructed as a microstrip device with spiral resonators
tuned to specific frequencies that represent separate bits of the tag code. The
resonators are connected to two cross-polarized ultrawide band (UWB) monopole
disk antennas which receive and transmit the signal from the scanning system. The
system is demonstrated with a design capable of encoding six bits that are formed byresonances 100 MHz apart between 2.0 and 2.5 GHz. The testing is done using twocross-polarized log period dipole array (LPDA) antennas which transmit and receive thesignal from the RFID tag.
Device Design and Simulation
The six-bit RFID tag design is shown in XFdtd in Figure 1, where the red material
represents a 0.787 mm thick Taconic TLX0 substrate (relative permittivity 2.45, loss
tangent 0.0019) and the green material is perfect conductor material. The two crosspolarized UWB monopole disk antennas are visible at the left (horizontal) and right(vertical) of the center section holding six spiral resonators that form the six bits ofthe tag. In Figure 2, a close-up view of one of the spiral resonators is shown. The
dimensions of each spiral are modified slightly to shift the resonance by 100 MHz foreach successive spiral. A ground plane covers part of the back side of the RFID tag
under the spiral resonators but not the disk antennas.

Figure 1: The RFID tag consisting of two UWBmonopole disk antennas and six spiral resonators
is shown as a CAD model in XFdtd.

Figure 2: A detailed view of one of the spiral resonators is shown. The dimensions of the spiral arechanged slightly to shift the resonance of the spiraland create a different bit for the RFID tag.

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Simulating Performance of a Low Cost, Chipless RFID System in XFdtd
The system receives a signal from an external source that is polarized with one of thetwo antennas. The resonances from the spirals are then transmitted via the second,cross-polarized, antenna and received by a second external antenna in the matchingpolarization. The antennas are cross-polarized to reduce the amount of cross-talkbetween them and isolate the signal from the RFID tag.
The UWB monopole disk antenna shown in Figure 3 is analyzed alone initially. It isfound to have a typical monopole pattern with even gain for the co-polarized case in theazimuthal direction (Figure 4) and low cross-polarized gain (Figure 5). The return loss
for the antenna is at acceptable levels for a very broad frequency range, as shown in
Figure 6.

Figure 3: The UWB monopole disk antenna is shown
as a CAD model in XFdtd.

Figure 4: The co-polarized gain pattern of the UWBmonopole disk is uniform around the antenna.

Figure 5: The cross-polarized gain pattern of theUWB monopole disk antenna has very low gain
which aids to reduce any cross-talk interference in
the RFID tag.

Figure 6: The return loss of the UWB monopole diskantenna shows good performance over a broadfrequency range.

The dimensions of the spiral resonators, shownin Figure 7, at the center of the RFID tag are
tuned so that each one represents a single
bit from 2.0 GHz to 2.5 GHz in even 100 MHz
intervals.
Figure 7: A six element RFID tag is shown
which will produce a six-bit code for the RFIDtag.

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Simulating Performance of a Low Cost, Chipless RFID System in XFdtd
When simulated alone, the resonators are shown to produce deep nulls at the desired
frequencies in the S21 magnitude plots as shown in Figure 8. The phase of theS21 signal may also be used to signify a bit by detecting the phase shifts at certain
frequencies. This is demonstrated in Figure 9 where a steep phase shift is located ateach of the desired locations between 2.0 and 2.5 GHz.

Figure 8: The amplitude response of the RFID tag
when analyzed by itself shows clear definition ofthe six bits from 2.0 to 2.5 GHz. Here the responseof 000000 is shown.

Figure 9: The phase response of the RFID tag
when analyzed by itself shows steep phase shifts
at each of the six bit locations for the 000000 tag.

To test the performance of a full system, an external source is needed to send a signal
to the RFID tag and receive the coded response from it. This is handled by two crosspolarized LPDA antennas. One of the LPDA antennas is shown in Figure 10. The
LPDA is designed to transmit a strong co-polarized signal in the forward direction withlow cross-polarized gain. The co-polarized gain of the LPDA at 2.0 GHz is shownin Figure 11. The gain in the forward direction (peak gain location) is plotted versus
frequency in Figure 12. The initial design of the LPDA has less than desirable returnloss performance, so a matching circuit is added to the input port. The matching circuit
consists of a series inductor and a parallel capacitor added to the voltage source feed.
The result is much improved return loss over the frequency range of interest, as shownin Figure 13.

Figure 10: One of the LPDA antenna models isshown in XFdtd.

Figure 11: The pattern of the LPDA antenna has
strong forward gain and low cross polarization.

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Simulating Performance of a Low Cost, Chipless RFID System in XFdtd

Figure 12: The forward gain of the LPDA antenna
is above 7dBi over a broad frequency range.

Figure 13: The return loss of the LPDA antenna isgreatly improved by the addition of a simple matching circuit and shows very good performance overthe entire range of interest.

To test the entire system, two LPDA antennas are placed 5 cm away from the RFID tag
with one of the LPDA antennas aligned with the vertical monopole and one aligned withthe horizontal. The RFID tag is tested for three different bit combinations of 000000(all spirals resonating), 010101, and 111111 (all spirals shorted). The shorting of asingle spiral and thus the setting of a bit to 1 is performed by adding a small square ofconductor onto the spiral as shown in Figure 15. The output of the combined system
is displayed using the results of the 111111 tag as a reference as was done in theoriginal paper. This improves the visibility of the tag nulls as it removes the responses
introduced by the testing system. The adjusted amplitude plot for the 000000 tag isshown in Figure 16 and the nulls representing the six bits can be clearly seen as having
several dB of difference. The adjusted amplitude plot for the 010101 tag is shown inFigure 17 and again the three bits at 2.0, 2.2, and 2.4 GHz are represented by nulls at
those locations. Similarly, the phase response is normalized with the 111111 tag signal,and the 000000 tag phase is shown in Figure 18 while that of the 010101 tag is in
Figure 19; in each case the phase shifts at the bit locations are visible.

Figure 14: The full RFID system is shown withcross-polarized LPDA antennas for transmit andreceive and the RFID tag with cross-polarized UWB
disk antennas and the six spiral resonators. The tag is
5 cm away from the transmit/receive antennas.

Figure 15: The bits are set from 0 to 1 by theaddition of a short across the spiral arms. This
removes the resonance of the spiral from the frequency range of interest.

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Simulating Performance of a Low Cost, Chipless RFID System in XFdtd

Figure 16: The adjusted amplitude response of the RFID
system is shown for the 000000 tag. Here the responseof the tag is shown as after normalizing with the responseof the 111111 tag for clarity.

Figure 17: The adjusted amplitude response of the RFID
system is shown for the 010101 tag where the three 0 bits arevisible at 2.0, 2.2, and 2.4 GHz.

Figure 18: The adjusted phase response of the RFID system
is shown for the 000000 tag where the 0 bits are visible assteep phase shifts 100 MHz apart from 2.0 to 2.5 GHz.

Figure 19: The adjusted phase response of the RFID system
is shown for the 010101 tag where the 0 bits are visible assteep phase shifts at 2.0, 2.2, and 2.4 GHz.

SummaryThis example has demonstrated the performance of a low-cost, chipless RFID
tag system that uses spiral resonators to denote the bits. The performance of theresonators is shown to be quite good when tested alone with very clear indications of
the bit signal present in the S-parameter data. When combined into a full system withtransmit and reader antennas separated by several centimeters, the system produces
recognizable bit patterns, proving the design is feasible.

315 S. Allen St., Suite 416 | State College, PA 16801 USA | +1.814.861.1299 phone |+1.814.861.1308 fax | sales@remcom.com | www.remcom.com

Simulating Performance of a Low Cost, Chipless RFID System in XFdtd
Reference:[1] S. Preradovic, I. Balbin, N. C. Karmakar, and G. F. Swiegers, “Multiresonator-Based Chipless
RFID System for Low-Cost Item Tracking,” IEEE Trans. Microwave Theory and Techniques, vol. 57,
no. 5, pp. 1411-1419, May 2009.

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