1. Ph.D. Candidate: Yunlei Li Advisor: Jin Liu 9/10/03
2.45 GHz Low Power Rectenna Design for Wireless Sensor & RFID Applications
Ph.D. Candidate: Yunlei Li
Advisor: Jin Liu
9/10/03

2. Outline
Introduction
Rectifier
Antenna
System
Conclusion

3. Radio Frequency Spectrum
Wavelength
Band designation
Wireless sensor & RFID applications
kHz
10-1km
LF (low frequency)
LF RFID: Passive IC tag
inductive coupling
kHz
MF (medium frequency)
MF RFID: Passive IC tag
3-30 MHz
100-10m
HF (high frequency)
HF RFI D: Passive IC tag
Inductive coupling
(6.78 MHz, MHz, MHz)
MHz
10-1m
VHF (very high frequency)
Wireless sensor & Active RFID
MHz
1m-10cm
UHF (ultra high frequency)
Wireless sensor & Active RFID: transceivers
(315, 433, 868, 915, 2450 MHz)
3-30 GHz
10cm-1cm
SHF (super high frequency)
5.8 GHz Active RFID
Beamed microwave power transmission

4. Electromagnetic Power Transmission
RF Power launched through electromagnetic waves by an antenna
 = c/f
Near field: the area from the antenna to the point where the electromagnetic field forms at a distance of D </2
Far field: The area after the point at which the electromagnetic wave has fully formed and separated from the antenna at a distance of D </2

5. RF Power Transmission:near field
Passive RFID tag
Inductive Coupling (transformer effect)
Energy in magnetic field strength
Coil antennae
Reader-> transponder Power & data
Transponder-> reader Data back by load modulation

6. RF Power Transmission:far field
Friis Transmission Equation
Pr=PtGtGr2/(4R)2
Pr: Receiveded power
Pt:Transimitted power Gt:Transimitter antenna gain
Gr: Receiver antenna gain
R: Transmission distance
Calculated received power
Assuming Gt=20dB, Gr=10dB

7. Rectenna: RF to DC Conversion
Rectenna element= Rectifier+Antenna
Frequency reflecting plane
Dipole or patch antenna
Microwave low pass filter
Schottky barrier diode
Low pass filter passing DC
Load resistor
Applications
Wireless power transmission between space and earth
high power Rectenna array
Wireless sensor (GAP4S) & long range RFID
Low power Rectenna used to
convert RF power to DC to
charge a battery or big Cap
Performance Goal-high efficiency
Overall efficiency o=DC output power/incident RF power
>85% (high power & optimized load)
Conversion efficiency c=DC output power/(incident RF power-reflected RF power)
>90% (high power & optimized load)

8. Rectifier/RF Detector
Single diode
Voltage doubler

9. Schottky Diode Equivalent circuit of a Schottky diode
Rj=0.026/IT, IT=Is+Ib
Is=diode saturation current, a
function of barrier height
Ib=external applied bias current
Cj=diode junction capacitance
Lp, Cp=Parasitic inductor & Cap
Rs=Parasitic resistance representing losses
Voltage sensitivity of a diode in mV/W
2=0.52/(IT(1+2Cj2RsRj)(1+Rj/RL))
N-type
Low Rs
External bias (High barrier, low Is)
High flicker noise
P-type
High Rs
Zero bias (low barrier, high Is
Low flicker noise

10. Low pass filter for better efficiency

11. Microstrip Patch Antennas

12. Radiation performance of single layer patch

13. Microstrip Patch Array

14. Hybrid-Ring Coupler
Hybrid ring coupler to split powers from the input to two outputs
Power split ratio
Note: there is an upper limit on line impedance of about 150  for many microstrip transmission lines

15. Gain of Microstrip Patch Array
The Maximum gain of a microstrip phase 2nx2m array
GdB=10log(4A/2)-(D1+D2)/2
A=D1*D2
D1=effective width of the uniformly spaced array
D2=effective height of the uniformly spaced array
=attenuation in dB per unit length of a 50 ohm transmission line being used in the monolithic feed [A typical value of  is 0.4dB/ft for a 50 ohm microstrip line on 1/32th-in (0.794mm) Teflon fiberglass at 2.2 GHz

16. System Design
Monolithic Integration of rectenna (antenna array with rectifier) with RF detector
Impedance matching of patch antenna (or antenna array) to the input of the rectifier using corporate feed network
Ring coupler to split power from antenna to rectenna and demodulator separately to maintain 8dB power split ratio
Use single diode rectifier to maximize efficiency of the Rectenna
Use voltage doubler detector to maximize its voltage for better demodulation
Ring coupler isolate the Rectenna & Detector and allow separate impedance matching network design

17. Conclusion
A 2.4GHz low power Rectenna & detector will be designed and simulated with ADS
The system will be monolithically integrated onto a single circuit board
A high gain patch antenna array boosts the power level at the input of the Rectenna for better power conversion efficiency
A hybrid ring coupler is used to divide the power between Rectenna and detector
The system implements a key RF front end for GAP4S wireless sensor system