A 45 µw Bias Power, 34 db Gain Reflection Amplifier Exploiting the Tunneling Effect for RFID Applications

Transcription

1 A 45 µw Bias Power, 34 db Gain Reflection Amplifier Exploiting the Tunneling Effect for RFID Applications Francesco Amato, Christopher W. Peterson, Brian P. Degnan and Gregory D. Durgin School of Electrical and Computer Engineering Georgia Institute of Technology, Atlanta, Georgia Abstract RFID applications have power constraints that limit RF tags to short range communications. This article presents the design procedures, validated by experimental results, to make a low-powered reflection amplifier that exploits the quantum mechanical tunneling effect to dramatically enhance the range of passive or semi-passive tags. A return gain of 34.4 db with bias power of 45 µw at 5.45 GHz and a return gain of 22. db with a bias power of 47 µw at 5.55 GHz have been observed for impinging RF power levels as low as -70 dbm. These results allow, for certain devices, a factor of 7 range improvement to the RFID link while keeping the bias power times lower than any other available reflection amplifier. This prototype could play a key role in enhancing RFID communication ranges without significantly affecting the low power budget typical for RFID technology; the design can be implemented in currently available semi-passive tags and opens the door for a new generation of long-range, passive/semi-passive transponders. Keywords RFID; reflection amplifier; quantum tunneling; tunnel diode; low powered RFID; backscatter radio; radio propagation; modulation factor. I. INTRODUCTION Radio Frequency Identification (RFID) is the most prominent application of backscatter modulation: an RFID reader transmits a radio frequency (RF) signal to a tag consisting of an antenna and an integrated circuit (IC). The circuit receives power from the reader and responds by modulating its load impedance Z L ; by varying the load Z L, the reflection coefficient changes. Modulation is possible by changing the amplitude or phase of the reflection coefficient at the interface between the tag antenna and the input load. The IC draws power either from the RF energy of the reader signal (passive tag) or from a battery (semi-passive tag); both passive and semi-passive tags are not equipped with amplifiers. Backscatter radio links are challenging because the tags involved in the communication with a reader have little or no power storage available; therefore, their use is restricted to short-range applications that collect power from the transmitter. The forward link is limited by the minimum signal strength required for the tag to power up. The return link is limited by the reader sensitivity to the backscattered power. Semi-passive tags are already return link limited and current results have shown ranges increased up to 2.25 m at UHF []. Passive RFID systems are currently forward link limited: the maximum distance is determined by the tag power requirements. Nevertheless, with the improving sensitivity of tag ICs [2], Fig. : State-of-the-art reflection amplifiers compared with the results obtained in this work. Comparisons have been made in terms of return gains and required biasing DC power. [3], the return link might become the weakest link in passive RFID communications as well. To increase the link ranges, improvements are needed either on the reader sensitivity or on the tag power requirements or both. On the tag side, [4] and [5] have suggested the use of reflection amplifiers with active loads to improve the communication range of RFIDs. Reflection amplifiers are characterized by a negative load impedance Z L that, at the cost of a certain amount of biasing power, can amplify the backscattered RF signal. Several successful prototypes based on field-effect transistors (FET) are available in the literature [6] [3] and early results obtained through the tunneling effect were presented in [4]. In Tab. I, the performances of FET reflection amplifiers are summarized and a comparison with the proposed prototype in terms of bias requirements and gain is plotted in Fig.. We present the design and testing of a reflection amplifier that improves both the forward and the return link of an RFID tag by exploiting the tunneling effect of tunnel diodes. Experimental results have shown that an impinging RF signal of -70 dbm is reflected with a return gain as high

2 TABLE I: Overview of reflection amplifiers available in literature. Reference DC Bias Power [mw] Gain [db] Bias voltage [V] Bias current [ma] RF Input power [dbm] Technology Freq. [GHz] This Work Tunnel diode [4] 5.45 This Work Tunnel diode [4] 5.55 [6] BFT25A [7] BJT BPF [8] FET NE32584c 5.8 [9] Silicon Avalance 7.4 [] FET CFY [] phmet 2.2 [2] OMMIC D0PH 2.2 [3] FET NE32584c 6.26 RFin RFout DC block tunnel diode Reflection Amplifier RF choke Demodulator Charge Pump or Battery modulated DC bias Microcontroller Fig. 2: Semi-passive or passive RFID tag block diagram with reflection amplifier used as single modulating load. as 34 db at the cost of a total biasing power of 45 µw. To the best of our knowledge, these are the highest gain and the lowest bias power requirements reported in the research literature encouraging further investigation to achieve link improvements in both current semi-passive and future passive RFID applications. Measurements have shown that a backscatter signal can be modulated by biasing on and off the reflection amplifier and that both amplitude shift keying (ASK) and phase shift keying (PSK) communications are possible. The use of the reflection amplifier as modulating load Z L for passive or semi-passive RFIDs is depicted in Fig. 2: a micro-controller, powered-up by a local coin battery, biases the tunnel diode on and off in order to modulate and amplify the RF signal impinging on the tag antenna. In passive tags, a charge pump can substitute the coin battery as the power requirements to bias the tunnel diode are extremely low. In this work, general considerations on the RFID link budget are followed by a survey of the state-of-the-art in reflection amplifiers (Section II). After introducing the fundamentals of a tunnel diode, the procedures to design a tunnel diode-based reflection amplifier are outlined (Section III) and experimental results at microwave frequencies are reported (section IV). II. RATIONALE The communication between a reader and a tag can be modeled as a monostatic or bistatic radar and can therefore be characterized by the radar range equation [5]. A link budget analysis for RFID systems is detailed in [6]. Assuming perfect matching, no fade margin, and line of sight between the reader and the tag, the forward link is limited by the tag sensitivity ( ) 2 λ P t = P T G T R G t, () 4πr max where P t is the tag sensitivity, P T is the power transmitted by the reader; G T R is the load-matched, free-space gain of the transmitter/receiver antenna; G t is the load-matched, freespace gain of the tag antenna; λ is the carrier-frequency wavelength; and r max is the maximum achievable reader-totag distance. After powering-up, a sufficient amount of power must be scattered back from the tag to the reader to transfer information. In a monostatic backscatter link, where a single antenna is used at the reader for both transmission and reception, the return link is limited by the reader sensitivity P Rmin. P Rmin = P T G 2 T RG 2 t where M is the modulation factor. ( λ 4πr max ) 4 M, (2) For an RFID tag switching its load Z L among two states (A and B), the modulation factor M in (2) is a function of the correspondent reflection coefficients Γ A and Γ B [7] with reflection coefficients defined as M = 4 Γ A Γ B 2, (3) Γ A,B = Z L A,B Zant Z LA,B + Zant, (4) with Z LA,B being the load impedance presented at the RF port of the tag in state A or B and Z ant the complex antenna impedance. A passive RFID tag can switch its loads between a short impedance (Z LA = 0, Γ A = ) and an impedance matched to the antenna (Z LB = Zant, Γ B = 0); resulting in a modulation factor M = Semi-passive tags, as they have less power constraints to power-up the IC, can switch their load between an open (Z LA =, Γ A = ) and a short (Z LB = 0, Γ B = ); resulting in a maximum achievable modulation factor M =.

3 Fig. 3: Reader requirements to guarantee a backscatter communication among passive or semi-passive tags of different sensitivities P t for maximum transmitted power P T = W. The horizontal line represents the minimum reader sensitivity currently available [8]. By combining () and (2), it is possible to highlight what reader sensitivity is required to read a tag at the maximum possible distance P Rmin = MP 2 t P T. (5) By fixing the maximum transmitted power P T to W, the minimum reader sensitivity P Rmin versus modulation factor M can be plotted (Fig. 3) for different values of tag sensitivity P t. The increasing sensitivity of passive RFID tag ICs will eventually make the reader the weakest node of an RFID system for both passive and semi-passive tags: passive tags (M = 0.25) with sensitivity of -24 dbm would have a longer communication range, but will require improved reader sensitivities or modulation factors higher than Data from semi-passive tags (M = ) with sensitivity of -27 dbm will not be received by currently available readers. One way to improve the return link consists in modifying the tag circuitry to increase the modulation factor. Values of M grater than one are possible by replacing the passive load Z L with an active one and a reflection amplifier can be used for this purpose. A. Related works The benefits of a reflection amplifier in RFID communications have been presented in [3]. The authors crafted two RFID transponders: a two-element array connected to two microstrip reflection amplifiers mounting pseudomorphic heterojunction FET (NE32584c) and a passive four-element array. When biased with the appropriate amount of voltage, the former topology showed a backscattering field level 4.5 db higher than the latter at a center frequency of 6.04 GHz. Advantages and disadvantages of using reflection amplifiers with RFID transponders have been highlighted in [2]. At K band (8-27 GHz), tuning at the wanted frequency can be difficult as parasitic components can produce a 2-3 GHz shift from the center frequency and an additional frequency shift (750 MHz) is observed when varying the biasing voltage level. The authors measured a maximum gain of 4 db at 2.2 GHz using a total biasing power of mw. Moreover, the return gain is completely absent outside the main band resulting in no amplification of unwanted signals. Amplitude modulation as well as phase modulation can be performed with transponders equipped with a reflection amplifier load: the amplitude of the return link signal can be modulated by simply turning on and off the biasing voltage of the load and a change in phase can be triggered by varying the bias level. Several researchers have built upon these early results suggesting different types of FET-based reflection amplifiers [6] [3]. Tab. I and Fig. give an overview of the results reported in literature and focus the attention on both the achieved gains and the power requirements: gains as high as 3.5 db have been reported for biasing power requirements falling in the range between 0.33 mw and W. III. DESIGN OF REFLECTION AMPLIFIERS WITH TUNNEL DIODES Tunnel diodes have been chosen to investigate the benefits of quantum tunneling in RFID applications. After a short introduction about their behaviour, this section illustrates the design procedures used to manufacture a tunnel diode-based reflection amplifier. A. Tunnel Diodes A tunnel diode is the first manufactured semiconductor where quantum tunneling was observed [9], [20]. As with other semiconductors, a tunnel diode simply consists of a p-n junction. However, the junction voltage barrier is extremely thin and is formed between two very heavily-doped regions (high concentration of donors and acceptors). Heavy doping results in the tunnel diode characteristic curve shown in Fig. 4. Normally, the forbidden energy gap of a p-n junction isolates the electrons on the two sides of the junction because their energy is not sufficient to surmount the potential barrier. If the junction region is thin enough, there is a finite probability that electrons of the n-type region tunnel to the empty states of the valence band in the p-type region when an external biasing voltage is applied [2]. It is from the quantum mechanical tunneling effect that the device takes its name. By applying low forward voltage, the tunneling effect occurs (region I of Fig. 4); tunneling charges produce a sharp increase of current with a slight variation of voltage up to a maximum current i. By increasing the applied bias, fewer and fewer electrons are able to tunnel through the barrier (region II of Fig. 4) and the amount of current decreases to i 2. With larger values of forward bias, the current remains small until the minority carrier injection takes place, giving rise to the normal exponential forward characteristic of a p-n junction diode (region III of Fig. 4). In Region II, the decreasing current as effect of the increasing bias gives to the tunnel diode a natural negative resistance R that can be used to design a reflection amplifier. A transmission line of impedance Z 0 terminating on a reflection amplifier with negative resistance R displays a reflection coefficient Γ greater than unity:

4 Current5[mA] I II i 5R5=55 slope III Forward5bias5[V] Fig. 4: Ideal characteristic curve of a tunnel diode. i 2 (a) Current [ma] V bias [V] Fig. 5: a) Tunnel diode equivalent circuit; b) MBD5057-E28 measured characteristic curve (at 300 K) used during the design process. (b) Ɵ L rl TABLE II: Coefficients a i a a 2 a 3 a 4 a 5 a RFin/out l l2 l3 l4 w C C2 TD L Vbias Γ = ( R) Z 0 R + Z 0 = R + Z 0 R Z 0, (6) and a return power gain s, in db, of: s = log Γ 2 = 20 log R + Z 0 R Z 0 (7) Therefore, when connected to a matched antenna, the reflection amplifier gives a modulation factor M with magnitude greater than and an amplified reflected signal. B. Simulation To assist with the design of the reflection amplifier, a testing platform for extracting SPICE models of tunnel diodes has been developed [22] by using MATLAB, a precision current source, and a picoammeter. In this work, tunnel diode model MBD5057-E28 [4] has been used and its small signal equivalent circuit has been defined (Fig. 5a) with L p = 0.4 nh and C p = 0.08 pf being the package parasitics reported in [23], C = 0.3 pf being the internal junction capacitance, and G a voltage controlled source defined by 8: i(v) = a v + a 2 v 2 + a 3 v 3 + a 4 v 4 + a 6 v 5 + a 6 v 6 (8) with the coefficients a i extrapolated through [22] and listed in Tab. II. The MBD5057-E28 measured characteristic curve is plotted in Fig. 5b: the voltage range spanning from 0.07 V to 0.29 V has a non-uniform slope resulting in two separated intervals with negative resistances: from 0.07 V to 0.6 V (R 465Ω) and from 0.2 V and 0.29 (R 426Ω). In the region between 0.6 V and 0.2 V, no significant RF amplification of an impinging signal is expected. For this particular device model, the reduced span of uniform negative Fig. 6: Schematic of the microstrip reflection amplifier. The final geometrical and electrical parameter are listed in Tab. III resistance region limits amplification to only low power RF signals. Although low power levels are typically encountered in RFID communications, a more uniform characteristic curve can extend the results obtained in this work to higher input power levels. In order to use a tunnel diode as reflection amplifier, the following criteria need to be met [24]: bias the tunnel diode in the negative resistance region of its characteristic curve; present the transmission line with a suitable negative resistance at the center frequency f 0 ; and reduce Γ to acceptable limits at frequencies away from f 0 ; maintain stability. Agilent Advanced Design System (ADS) has been used to design the reflection amplifier. The microstrip structure of the fabricated reflection amplifier is shown in Fig. 6. An external power supply, V bias, biases the tunnel diode at the desired voltage to amplify and reflect back the RF signal input. The radial stub isolates the biasing network from the input RF signal and the capacitor C 2 is used as DC block. Transmission lines of different lengths l i with fixed width w and a capacitor C are used for network matching and for tuning the circuit at the central frequency f 0. Setting f 0 = 5.8 GHz as the desired central frequency of operation, the parameters in Fig. 6 have been determined through simulations to obtain an amplified reflected RF signal. The final circuit parameters are listed in Tab. III. C. Fabrication The reflection amplifier has been realized on a 4 layer board [25]. The first inner layer (0.7 mil of copper) has been used as the microstrip line ground plane. The top layer with.4 mil of copper is separated from the ground plane by a 6.7 mil

5 TABLE III: Board dimensions and component values Parameter w l l2 l3 l4 θl rl Size 3.5 mil 50 mil 300 mil 0 mil 30 mil mil Component C C2 Reflection Amplifier Vector Analyzer Value [pf] A A Signal Generator Vbias + A A2 Bias Tee C2 VBias C 3 A3 Reflection Amplifier a) 2 Vbias 2 Spectrum Analyzer b) Fig. 8: Setups used to collect experimental data. a) Measurements on VNA E507B with avg. 6 and b) measurements with circulator, signal generator E8247C, and signal analyzer CXA-N9000A. Signal analyzer setup: resolution BW 3 khz, video BW 0 khz, avg., span MHz. RFin Tunnel Diode TABLE IV: Electrical properties of cables, attenuators and circulator at 5.8 GHz used for data collection a) b) Fig. 7: Fabricated reflection amplifiers with a) tunnel diode # and b) tunnel diode #2. FR4 substrate. The substrate (FR408 [26]) has a permittivity r = 3.66 and loss tangent tan δ = at 5 GHz. Via holes of 3 mil diameter and square pads of 5 mil side have been used. Two tunnel diodes (# and #2) have been mounted on two identical boards (Fig. 7). The tunnel diodes (MBD5057E28 [4]) operate up to 8 GHz and are fabricated using rapid thermal diffusion on Germanium substrate; Johanson Technology S603DS 0603 capacitors have been used. IV. E XPERIMENTAL RESULTS The measurement setups shown in Fig. 8 have been used to collect experimental data. An ammeter measured the current drawn by the amplifiers while a Vector Network Analyzer (VNA) displayed the resonant frequencies; a signal analyzer measured the RF level of a signal generator output amplified by the reflection amplifier. Attenuators A, A2 and A3 have been used to attenuate the RF signal impinging on the reflection amplifier and prevent the creation of harmonics. The electrical parameters of each additional component (cables, circulator and attenuators) used are listed in Tab. IV. Cable Cable 2 Attenuator A Attenuator A2 Attenuator A3 Part RG42B/U RG42B/U VAT-+ VAT-30W2+ VAT-30W2+ Att. [db] Circulator Insertion Loss [db] Isolation [db] CS Experimental results clearly show that the designed reflection amplifiers can be used in backscatter communications for both phase and amplitude modulations by varying the biasing voltage over time. The one port S-parameter data s, obtained for different biasing voltages, are shown in Fig. 9a and 9b for tunnel diode # and #2, respectively. When the biasing voltage is off, no amplification, nor resonance of the reflection amplifiers occur in the frequency band spanning from 3 GHz to 7 GHz. When the biasing voltage forces the tunnel diode into the negative resistance region, gains are observed. The two boards amplify an RF input signal at two slightly different frequencies (5.45 GHz and 5.55 GHz) as consequence of the different length of the device leads; moreover, parasitic components, such as bonding wire inductance, contribute to the detuning from the expected central frequency of 5.8 GHz. In Fig. 9a, it is highlighted how a reflection gain exists only around the center frequency while it is absent outside the main band that could otherwise give unwanted signal amplification. As expected by the characteristic curve in Fig. 5b, both the reflection amplifiers show an high gain for two biasing voltages (around 0.09 V and 0.22 V) and the amplification is small or absent for biasing voltages around 0.6 V. The effect of biasing on the s phase is shown in Fig.. A drastic change is evident when the biasing voltage is switched from 0 to 0.08 V for tunnel diode # and from 0 to 0.09 V for tunnel diode #2. The reflection amplifier mounting tunnel diode #, at the peak gain frequency of 5.45 GHz, has a phase of 40 for Vbias = 0.08 V and a phase of 9 with no bias ( φ = 49 ). The reflection amplifier mounting tunnel diode #2 at the peak frequency of 5.55 GHz, has a phase of 33 for Vbias = 0.09 V and a phase of 80 with no bias ( φ2 = 53 ). A useful way to test the reflection amplifiers consists in connecting the devices to a circulator to effectively separate

6 (a) Fig. : S-parameter phase sweeps for the two reflection amplifiers when no bias or optimal bias are applied. Power input P in = 50dBm (b) Return Gain [db] Tunnel Diode # Tunnel Diode #2 Fig. 9: S-parameter sweeps for reflection amplifiers mounting a) tunnel diode # and b) tunnel diode #2 at different biasing levels. Power input P in = 50 dbm. the input power from the output power; the measurement setup shown in Fig. 8b has been used to record the data reported in Figg., 2 and 3. The sensitivity of the manufactured reflection amplifiers to different levels of input power is shown in Fig.. The highest return gains (34.4 db and 22. db) are obtained for input powers of dbm and dbm respectively. The tunnel diodes have been biased at 0.08 V and 0.09 V and currents of 566 µa and 525 µa have been measured for a total biasing power of 45 µw and 47 µw respectively. These results represent, at the best of our knowledge, the highest gains and the lowest power requirements for any reflection amplifier ever reported in literature (Fig. and Tab. I). In Fig. 2, the bandwidths of the reflection amplifiers are shown. At frequencies away from the center, no amplifications are observed, preventing the device from amplifying unwanted signals. Finally in Fig. 3, the effects of the biasing voltage on the P in [dbm] Fig. : Return gains as function of the RF power, P in, at the reflection amplifier input. Tunnel Diode #: input frequency 5.45 GHz, V bias = 0.08 V, I bias = 566 µa. Tunnel Diode #2: input frequency 5.55 GHz, V bias = 0.09 V, I bias = 525 µa. output gains have been highlighted for both the amplifiers at the respective central frequencies (5.45 GHz, 5.55 GHz) and optimum input powers (-70 dbm, -60 dbm). As expected by the tunnel diode measured characteristic curve (Fig. 5b), two peak gains are observed at two different voltage intervals. V. CONCLUSIONS The tunneling effect exploited in the presented prototype gave high return gains (34.4 db and 22. db) with low bias power requirements (45 µw and 47 µw) at 5.45 GHz and 5.55 GHz respectively for RF input power levels as low as dbm and up to dbm. Differences in the results are attributed to fabrication errors and to different lengths of the tunnel diode leades.the same results can be obtained with

7 Return Gain [db] Tunnel Diode # Tunnel Diode # Freq [GHz] Fig. 2: Reflection amplifier gains as function of the input frequency. Tunnel Diode #: input power -70 dbm, V bias = 0.08 V. Tunnel Diode #2: input power -60 dbm, V bias = 0.09 V. Return Gain [db] Tunnel Diode # Tunnel Diode # V bias [V] Fig. 3: Reflection amplifier gains as function of the V bias. Tunnel Diode #: input frequency: 5.45 GHz, input power: - 70 dbm. Tunnel Diode #2: input frequency: 5.55 GHz, input power: -60 dbm higher RF input powers when employing tunnel diodes with a more uniform characteristic curve and encourage further investigation to achieve link improvements in both current semi-passive and future passive RFID applications. The high gains and low bias powers achieved in this work are due to the heavy doping of the p-type and n-type semiconductors forming the tunnel diode. The thin junction resulting from it gives a finite probability for electrons to tunnel through the potential barrier when an extremely low bias voltage is applied. At slightly higher voltages, the tunneling effect fades generating a natural negative resistance exploited in this work. The prototype can play a key role in enhancing RFID communication ranges without significantly affecting the low power budget typical for this technology. It can be implemented not only in currently available semi-passive RFID tag models, but it opens the door for a new generation of long range passive tags; moreover, retrodirective designs will benefit from the integration of tunnelig effect-based technologies and a new class of less power-hungry amplifiers will compete with currently available CMOS devices. ACKNOWLEDGEMENT This work was funded in part by National Science Foundation Grant # REFERENCES [] S. Thomas and M. Reynolds, A 96 Mbit/sec, 5.5 pj/bit 6-QAM modulator for UHF backscatter communication, in RFID (RFID), 202 IEEE International Conference on, Apr. 202, pp [2] S. Hemour and K. Wu, Radio-frequency rectifier for electromagnetic energy harvesting: Development path and future outlook, Proceedings of the IEEE, vol. 2, no., pp , Nov 204. [3] Impinj. (204, Nov.). [Online]. Available: products/tag-chips/monza-r6/ [4] F. Amato and G. D. Durgin, A tunnel diode reflection amplifier for RFID antennas, Apr 203, poster presentation, Orlando FL. [5] B. Smida and S. Islam, Full-duplex wireless communication based on backscatter amplifier, in Communications Workshops (ICC), 204 IEEE International Conference on, June 204, pp [6] J. Kimionis, A. Georgiadis, A. Collado, and M. Tentzeris, Enhancement of rf tag backscatter efficiency with low-power reflection amplifiers, Microwave Theory and Techniques, IEEE Transactions on, vol. 62, no. 2, pp , Dec 204. [7] P. Chan and V. Fusco, Full duplex reflection amplifier tag, Microwaves, Antennas Propagation, IET, vol. 7, no. 6, pp , Apr [8], Bi-static 5.8 GHz RFID range enhancement using retrodirective techniques, in Microwave Conference (EuMC), 20, pp [9] G. Dalman, F. Zappert, and C. Lee, Relaxing-avalanche-mode reflection amplifier, Electronics Letters, vol. 8, no. 9, pp , May 972. [] A. Lazaro, A. Ramos, R. Villarino, and D. Girbau, Time-domain UWB RFID tag based on reflection amplifier, Antennas and Wireless Propagation Letters, IEEE, vol. 2, pp , 203. [] H. Cantu, V. Fusco, and S. Simms, Microwave reflection amplifier for detection and tagging applications, Microwaves, Antennas Propagation, IET, vol. 2, no. 2, pp. 5 9, Mar [2] H. Cantu and V. Fusco, A 2 GHz reflection amplifier MMIC for retro-directive antenna and RFID applications, in MM-Wave Products and Technologies, The Institution of Engineering and Technology Seminar on, Nov. 2006, pp [3] S. Chung, S. Chen, and Y. Lee, A novel bi-directional amplifier with applications in active van atta retrodirective arrays, IEEE Trans. Microwave Theory and Techniques, vol. 5, no. 2, Feb [4] Aeroflex Metelics. (2005, Dec.). [Online]. Available: Series Planar Back Tunnel Diodes.pdf [5] C. A. Balanis, Antenna Theory: Analysis and Design, 2nd ed. New York: Wiley, 997. [6] J. Griffin and G. Durgin, Complete link budgets for backscatterradio and RFID systems, Antennas and Propagation Magazine, IEEE, vol. 5, no. 2, pp. 25, Apr [7] J. T. Prothro and G. D. Durgin, Improved performance of a radio frequency identification tag antenna on a metal ground plane, Master s thesis, Georgia Institute of Technology, Atlanta, [8] Motorola. (202). [Online]. Available: com/web/business/products/rfid/rfid\%20readers/fx9500/ Documents/FX9500 Specifications.pdf

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