Optimum Integration of passive UHF RFID Tag-Rectenna in a Single Feed Dual Band Antenna

Gianfranco Andía Vera

1, Shankar D. Nawale*

1, Yvan Duroc

2, and Smail Tedjini

1

1 Université Grenoble Alpes, LCIS, 50, rue Barthélémy de Laffemas BP54, 26902 Valence France,

gianfranco.andia-vera@lcis.grenoble-inp.fr, shankar.nawale@lcis.grenoble-inp.fr, smail.tedjini@lcis.grenoble-inp.fr

2 Université Claude Bernard Lyon 1, 15 boulevard André Latarjet 69622 Villeurbanne France,

yvan.duroc@univ-lyon1.fr

Abstract

In this paper, a design approach for the efficient integration of passive RFID tag operating at UHF 868MHz for

RFID communication and a rectifier operating at 2.45GHz for energy harvesting application, both in a single feed dual

band antenna is presented. Initially, the RFID chip and the rectifier element were characterized. Therefore, the step by

step design process for the integration of each element is carried over, for a maximum power transfer to the passive

RFID chip and the rectifying circuitry. The non-linear effects of rectifier and RFID chip are considered in a electric-

electromagnetic co-simulation, ensuring the conditions at the threshold activation power at the RFID chip. 36.9 % RF-

to-DC conversion efficiency is ensured at 2.45 GHz for -10dBm input power. The sufficient isolation is provided at

both frequencies to support an RFID communication and at the same time, harvesting from the 2.45 GHz band.

1. Introduction

Wireless Power Transmission (WPT), Electromagnetic Energy Harvesting (EEH), and passive Radio Frequency

Identification (RFID) are the concurrent technologies in the Internet of things era [1]. Given that the scope of the

passive RFID technology, nowadays, is not limited to the identification and has vast applications in the sensing of

environmental and human body parameters, data loggers, etc. [2]. These new applications have generated the

requirement of extra energy source for improving either the read range or the autonomy of these smart sensors [3][4].

In the idea to preserve the totally passive devices, EEH is the best solution where the rectenna can convert this

electromagnetic energy for the battery less devices and/or its attached sensors. Designs of these rectifying antennas for

EEH and WPT are widely explained in the literature. Many of these articles have presented rectenna designs at a single

band frequency, dual band frequency or even for wide band frequency [5]. Most of the rectennas are designed at ISM,

GSM and UMTS band frequencies. But, so far none of these approaches considers the RFID communication while EEH

is performed. The work presented here explains the step by step approach for the integration of a passive UHF RFID

chip working at the EU RFID band 868MHz and a rectifier circuit working at ISM 2.45GHz frequency, all in a single

feed dual band antenna. Therefore, the main objective is to the design an RFID tag-rectenna (RFID-TR), to harvest the

electromagnetic energy at the same time that an RFID communication is established.

This paper is organized as follows; In Section 2, the characterization of the RFID chip and the rectifier element

are presented to establish the RFID-TR design guidelines. After that, Section 3 introduces lumped parameter theory and

presents the process for dual band impedance matching by performing an electrical and electromagnetic co-simulation.

The simulated results of the studied RFID-TR and its performance are presented in section 4. Finally, some conclusions

are drawn in Section 5.

2. Characterization of RFID chip and Rectifying Diode

The architecture of the proposed RFID-TR is shown in Figure 1. A single rectifier diode is used to avoid the

complexity of the circuit. For an efficient integration of all the blocks, and given the non-linear behavior of the

rectifying diode and RFID chip, a characterization of those was performed for low input power. Here, the minimal

directive to accomplish the threshold activation level of the RFID chip is considered. Using the combination of two

measurement techniques [6], [7] the EM4325 passive RFID chip (TSSOP-8 package) and the HSMS 2680 Schottky

diode (SOT23 package) are characterized at -10dBm input power, for the frequencies of interest.

978-1-4673-5225-3/14/$31.00 ©2014 IEEE

Matching Network for Rectifier

Circuit

Matching Network for RFID

Chip

DC Pass Filter Rectifier Diode

RFID Chip

Energy harvesting Section at 2.45 GHz

RFID Communication Section at 868 MHz

Load

Antenna

Figure 1. Block diagram of the proposed RFID tag-rectenna

At the activation threshold in 868MHz, the measured chip impedance is 6.5 - j119 Ω. Figure 2 shows the

measured impedances for the RFID chip and Schottky diode. In order to consider the non-linear behavior of these

components, an equivalent model with closer impedance values to the measured ones is used in the simulation. The

RFID chip model is composed by three voltage multiplier stages composed by the same kind of Schottky diodes and

ended in a DC load. Additionally, a shunt resistor and a shunt capacitance represent the internal resistance of the RFID

chip and the capacitance of packaging, respectively. For the diode model, its Spice parameters are used. From

experimental results, it is worth noting that the impedance modulation of the RFID chip at 868MHz produces small

variations in the impedance at 2.45GHz. This impedance modulation will not produce considerable mismatching with

the impedance seen by the rectifier part.

Figure 1. (a) Measured impedance of the RFID chip and of its equivalent non-linear model at the activation

threshold power. (b) Measured impedance of the Schottky diode at -10dBm.

3. Design Procedure

Once the behavior pattern of the RFID chip and the Schottky diode is known at low input power, then it is

essential to design the optimized impedance matching between all the blocks of Figure 1. The impedance matching

section for the RFID chip is intended to be part of the antenna. Initially only a non-linear electrical simulation is carried

out for the circuit by considering the equivalent non-linear models above described. The RFID chip impedance at 868

MHz is easily matched with the conjugate of its reactance. At this stage, the main performance criteria observed was a

higher output DC voltage at the DC load of the RFID chip when -10dBm power arrives to the chip. This represents the

RFID tag part having 4.9 m theoretical read range [8] under 2 watts Effective Radiated Power (ERP). Then, later the

rectifier part at 2.45GHz is integrated.

(a)

0.868 GHz

2.45 GHzRFID Chip model

Schottky diode

RFID chip model

RFID chip

(b)

The matching section for the rectifier part is a High Pass Filter (HPF) conformed by a series capacitor and a

shunt inductor with respect to the antenna and rectifier. Better RF-to-DC conversion efficiency can be observed by

properly tuning this section. The shunt DC capacitor is used to filter the RF signals (fundamental and harmonics

produced by the diode and RFID chip [9]) to provide a constant and smooth output DC voltage. An output load

resistance of 1 kΩ is used to get the maximum output voltage across it.

3.1 Antenna Integration

An electrical-electromagnetic co-simulation is carried using CST Studio Suite to integrate the electrical circuit

above described with a single feed dual band antenna. Regarding the antenna structure, a modified L-F inverted antenna

is designed [10] in Rogers RO4003 substrate with 3.55 permittivity and 0.8 mm thickness. This structure allows using a

ground plane and radiator, all in a common face. The ground plane is useful for the rectifier circuitry. Figure 3 presents

the block diagram of the co-simulation and the structure of the optimized antenna. Each part of the planar inverted-F

structure is designed and optimized to be able to resonate at UHF band centered in 868MHz. The inverted-L part of [10]

becomes here a complete loop going back to the ground, each part of which, along with the distance between the inverted-F

are optimized to have a resonant at 2.45GHz. The capacitive impedances seen by the antenna are reduced by creating

inductive loops in the ground [8]. The entire system is optimized for a low input power when -10dBm is ensured at the

input of the RFID chip at 868MHz. The presented structure of the antenna considers the pad effects for the rectifier

circuitry. For simulation purposes an ideal directional coupler is used to integrate the S-parameters of the antenna with the

RFID chip model and the AC source.

1

2

3

4

5[S]

60.2 mm

62 mm

AC source

DC output voltage

ground

F-inverted

Modified L-inverted

Inductive loop

RFID chip

RFID DC load

Voltage multiplier

HPF

SchottkyRectifier DC load

DC capacitor

Antenna

1

2

3

45

Ideal coupler

(a)

(b)

(c)

Figure 3. Electric-electromagnetic co-simulation of the proposed RFID-TR. (a) Non-linear model of the RFID

chip. (b) Co-simulation integration of the RFID-TR. (c) Structure of the dual band single feed antenna with the

pads for the rectifier branch.

4. Simulation Results

Figure 4 gives the results of output DC voltage and RF-to-DC conversion efficiency with respect to input RF

power. The efficiency of the rectifier is calculated as DC output power (Pdc) divided by RF input Power (Pin). Where,

Pdc is calculated by considering DC voltage at the chosen 1kΩ load.

Figure 4. (a) Rectifier output DC Voltage and (b) conversion efficiency with respect to RF input power

(a) (b)

The output power levels for different input power levels are presented in Table 1. It is seen that, at -10dBm RF

input power the DC output power is 36.9 µW, which gives 36.9 % conversion efficiency.

Table 1. Comparison of output power at different input power

2.45 GHz RF Input Power

(dBm)

Output DC Power

(dBm)

Output DC Power

(µW)

0 -1.76 665.40

-5 -7.88 162.79

-10 -14.33 36.90

-15 -21.00 7.07

5. Conclusion

An optimized approach to design an RFID-TR in a single feed dual band antenna for UHF RFID communication

at 868MHz and energy harvesting rectenna at 2.45GHz, was presented. Starting from the characterization stage, the

work is followed by an electrical-electromagnetic co-simulation that considers the non-linear nature of the RFID chip

and rectifier diode. The simulated output RF-to-DC conversion efficiency is 36.9 % at -10dBm input power at 2.45GHz.

An enhancement in the DC output power of the rectifier is achieved due to the harmonics reflected by the RFID chip.

Further experimental results shall be validated to ensure the studied approach.

6. References

1. Y. Duroc and G. Andia Vera, “Towards autonomous wireless sensors: RFID and energy harvesting solutions,”

Internet of Things, Springer, 2014, pp. 233-255.

2. S. D. Nawale, N. P. Sarawade, “RFID Vapor Sensor: Beyond Identification”, 2012 Sixth International Conference on

Sensing Technology (ICST), 2012, pp. 248-253.

3. Z. Popovic, E. Falkenstein, R. Zane, “Low-Power Density Wireless Powering for Battery-less Sensors”, IEEE RWS

2013, pp. 31-33.

4. A. Georgiadis and A. Collado, “Improving Range of Passive RFID Tags Utilizing Energy Harvesting and High

Efficiency Class-E Oscillators”, 6th European Conference on Antennas and Propagation, 2011, pp. 3455-3458.

5. A. Boaventura, A. Collado,N.B Carvalho, A. Georgiadis, "Optimum behavior: Wireless power transmission system

design through behavioral models and efficient synthesis techniques," Microwave Magazine, IEEE , vol.14, no.2,

pp.26,35, March-April 2013

6. P.V Nikitin, K. V S Rao,R. Martinez, S.F. Lam, "Sensitivity and Impedance Measurements of UHF RFID

Chips," Microwave Theory and Techniques, IEEE Transactions on , vol.57, no.5, pp.1297,1302, May 2009

7. L.W. Mayer, A. L. Scholtz. "Sensitivity and impedance measurements on UHF RFID transponder

chips." Proceedings of the second international EURASIP workshop on RFID technology, Budapest. 2008.

8. G. Marrocco, "The art of UHF RFID antenna design: impedance-matching and size-reduction techniques," Antennas

and Propagation Magazine, IEEE , vol.50, no.1, pp.66,79, Feb. 2008

9. G. Andia Vera, Y. Duroc, S. Tedjini, "Analysis of Harmonics in UHF RFID Signals," Microwave Theory and

Techniques, IEEE Transactions on , vol.61, no.6, pp.2481,2490, June 2013

10. J. Liu, "Dual-band RFID tag antenna using coplanar inverted-F/L structure," RFID-Technology and Applications

(RFID-TA), 2010 IEEE International Conference on , vol., no., pp.96,99, 17-19 June 2010