5.

8GHz Circularly Polarized Rectennas Using Schottky Diode

and LTC5535 Rectifier for RF Energy Harvesting
T.M. Chiam
1
, L.C. Ong
1*
, M.F. Karim
1
, Y.X. Guo
2
1
RF & Optical Department, Institute for Infocomm Research, Agency for Science, Technology & Research
1 Fusionopolis Way, #21-01 Connexis (South Tower) Singapore 138632
*ongmichael@i2r.a-star.edu.sg
2
ECE Department, National University of Singapore

Abstract 5.8GHz circularly polarized single element patch
antenna and 2x2 antenna array for RF energy harvesting were
designed and fabricated. A comparison between Schottky diode
and LTC5535 for RF energy harvesting rectenna (rectifying
antenna) was analyzed at WLAN frequency of 5.8GHz. Both
rectennas are printed on Rogers 4003C substrate. Maximum
gain at boresight tip was measured to be 3.4dBi and 8.0dBi for
circular polarized single element patch antenna and array patch
antenna respectively. And the measured return loss of the two
types of antennas was measured to be -10.3dB and -20.3dB
respectively. The maximum output DC voltage reading is
72.5mV for Schottky diode and 428.3mV for the LTC5535
rectifier, at a distance of 15cm from the transmitting reference
antenna.

Index Terms Circular polarization, circularly polarized
antenna, microwave power transmission, rectenna, rectenna
array, rectifier, RF energy harvesting.
I. INTRODUCTION
The use of rechargeable batteries has significantly
increased as many applications impose mobility and
autonomous working. These wireless devices are growing in
many applications such as mp3 players, mobile phones,
sensor networks, etc. Stand-alone devices such as wireless
sensor nodes located in difficult access environments have to
work without the human intervention for many years. In these
situations, many troubles arise such as battery replacement or
recharging, in addition to the size and weight. The problem
increases tremendously when the number of devices is large
and distributed in a wide area or located in inaccessible
places. Therefore, a feasible method is needed to supply
energy to remote sensors.
There are digital systems and sensor devices with ultra low
power consumptions for energy harvesting purposes [1]. This
makes feasible the development of low power harvesting
systems. Research work have been conducted on
electromagnetic energy scavenging on high frequencies, from
2GHz to 18GHz, the unlicensed 2.4GHz and 5.8GHz ISM
band [2][3]. Also, technologies such as RFID use similar
technology to work at 13.56MHz or UFH-ISM frequencies
[4][5][6]. Most of them use their own RF transmitter to
supply power. In addition, the distance between the RF
transmitter and the harvesting unit is relatively short [7]. And
in the ISM band, the transmitted power allowed is low.
Generally, this type of devices harvest RF energy, as shown
in Figure 1. The system comprises of an antenna, impedance
matching network, rectifier circuit, low pass filter, storage
element and control unit [2][8]. Most of the rectennas and
their matching networks are designed to have a large
bandwidth [9][10]. Recently, circular polarized (CP) antennas
have become one of the important characteristics in designing
rectennas [11][12] as it avoids changes in the output voltage
due to the rotation of the transmitter or receiver.
In this paper, a new 5.8GHz circularly polarized single
element and 2x2 rectenna array have been designed and
fabricated. The Schottky diode and LTC5535 RF detector are
used for the rectification and their performances compared in
terms of the output DC voltage.

II. RECTENNA STRUCTURE
The block diagram for the rectenna is shown in Figure 1.
The antenna and rectifier circuits were first designed and
measured separately. Finally, they are combined to form the
complete rectenna.




Fig. 1. Block diagram of the rectenna circuit

A. Antenna Design
The circular polarized single patch antenna is designed on
RO4003C substrate with a dielectric constant of 3.38, loss
tangent 0.0027, and a thickness of 32mils as shown in Figure
2. The dimensions of the 50 feed line is 12.7mm by 0.2mm
and it is matched to a 50 line via a quarter wave
transformer.








Fig. 2. Schematic of the single element CP antenna
Antenna
Impedance
Matching & Filtering
Rectification Load
r
R
12.7mm
To SMA
/ 4
978-1-4244-2802-1/09/$25.00 2009 IEEE
32
The radius, r of the first patch is 4.2mm and the second
circular patch, R is 5.1mm, this is added to optimize the CP
performance and return loss of the antenna. The simulation
result of the antenna shows a return loss of -11dB in Figure 3
at 5.8GHz.












Fig. 3. Simulation result of the single element CP antenna

The antenna array configuration is designed to increase the
gain of the single element antenna. This increase in gain
corresponds to improved power efficiency. Antenna array of
2x2 configurations is designed as shown in Figure 4. The
characteristic impedances of each line are calculated to give
the widths for the transmission lines. T1 and T2 are
essentially the same power combiner. The 100 line bodies
of T1 and T2 form the arms of T3 resulting in the final
combination of all power from the antennas.












Fig. 4. Schematic of the 2x2 CP array antenna













Fig. 5. Simulation result of the 2x2 array CP antenna

The 100 line between the 50 and 200 strips is a
quarter wave transformation to match the lines so that no
power is reflected away from the output. Length of the
microstrip lines on the T-junction had no bearing on the
matching. The simulation result of 2x2 array shows return
loss of -12.2dB at 5.8GHz as shown in Figure 5.

B. Rectifier Circuit
The purpose of the rectifier is to directly convert
microwave RF energy into DC electrical energy. Schottky
diodes are preferred because of the low voltage drop and
higher speed. In addition, Schottky diodes consume the least
amount of power due to conduction and switching.
Agilents HSMS-286x family of DC biased detector diodes
have been designed and optimized for use from 915MHz to
5.8GHz. They are ideal for RFID and RF tag applications as
well as for large signal detection, modulation, RF to DC
conversion or voltage doubling. HSMS-2862 diode of the
series has been chosen for this rectifier circuit. This circuit
required the parasitics of up to about 6GHz. This was simply
carried out by modifying the parasitic values and comparing
against the return loss graph supplied in the datasheet. The
next step was to create a matching input circuit for the diode.
One diode is grounded while the other diode is directly
connected to the matching network. The matching is done for
a 5.8GHz input.
The second rectifier circuit as shown in Figure 6 makes use
of a Linear Technology IC chip, LTC5535. The LTC5535 is
an RF power detector with adjustable gain and 12MHz
baseband bandwidth for RF applications operating in the
600MHz to 7GHz range. A temperature compensated
Schottky diode peak detector and output amplifier are
combined in a small thin SOT package.













Fig. 6. Schematic of the rectifying circuit using LTC5535

The supply voltage range is optimized for operation from a
single cell lithium-ion or three cell NiMH battery. The RF
input voltage is peak detected using an on-chip Schottky
diode. The detected voltage is buffered and supplied to the
Vout pin. The LTC5535 output amplifier gain is set via
external resistors. The initial starting voltage of 200mV can
be precisely adjusted using Vos pin. The LTC5535 operates
with input power levels from -32dBm to 20dBm.
50
50 50
50
200
100
50
100
100
T1
T2
T3
1 2 3 4 5 6 7 8
-12
-10
-8
-6
-4
-2
0

R
e
t
u
r
n

l
o
s
s
,

S
1
1

(
d
B
)
Frequency (GHz)
1 2 3 4 5 6 7 8
-14
-12
-10
-8
-6
-4
-2
0


R
e
t
u
r
n

l
o
s
s
,

S
1
1

(
d
B
)
Frequency (GHz)
33
III. MEASUREMENTS RESULTS & DISCUSSIONS
The fabricated single element and 2x2 array are shown in
Figure 7, and Figures 8(a) and (b) respectively. The return
loss, S11 is measured with a vector network analyzer (VNA)
which is calibrated using the short-open-load-thru (SOLT)
calibration kit. At 5.8GHz, the measured return loss S11 for
the single element is -10.3dB and for the array is -20.3dB.
These are shown in Figures 9(a) and (b) respectively.










Fig. 7 Single element antenna











(a)











(b)
Fig. 8. Fabricated structures of the rectennas; (a) Schottky
diode, and (b) LTC5535 IC

The radiation pattern of the circular polarized patch
antenna was measured in an anechoic chamber as shown in
Figures 10(a) and (b). Since it is a circular polarized antenna,
the radiation patterns are nearly identical on both planes of
the antenna. A standard gain horn antenna was used as a
reference transmitter. The rectenna was rotated at the azimuth
plane from -180 to 180. The maximum gains at boresight
measured are 3.4dBi and 8.0dBi respectively for the single
element patch and circular polarized patch array antenna.













(a)













(b)
Fig. 9. Measured results of the antenna; (a) single element, and
(b) 2x2 array antenna

The setup for the DC voltage measurement from the
rectenna is shown in Figure 11. The input RF power is
generated by the Agilent signal generator E4438C which is
connected to the reference horn antenna. The rectenna is
connected to the voltmeter which converts the RF power to
DC output voltage. The input power levels are measured
from -10dBm to 16dBm at a distance of 15cm. The maximum
output from the 2x2 patch antenna array is 72.5mV and
428.6mV using the Schottky diode and LTC5535
respectively. Table I shows the RF input power and DC
output voltages for both the Schottky diode and LTC5535.













(a)
5.0 5.2 5.4 5.6 5.8 6.0
-20
-15
-10
-5
0
R
e
t
u
r
n

L
o
s
s
,

S
1
1

(
d
B
)
Frequency, (GHz)
0
Single Element Antenna
H-Plane, Co-pol
E-Plane, Co-pol
H-Plane, X-pol
E-Plane, X-pol
f = 5.8 GHz
10 dBi
-90
o
180
o

90
o
= 0
o
5.0 5.2 5.4 5.6 5.8 6.0
-20
-15
-10
-5
0

R
e
t
u
r
n

l
o
s
s
,

S
1
1

(
d
B
)
Frequency, (GHz)
34












(b)
Fig. 10. Measured radiation pattern of; (a) single element, and
(b) 2x2 array antenna.













Fig. 11. Measurement setup for the DC voltage output from the
rectenna in Anechoic Chamber















IV. CONCLUSIONS
A circularly polarized single element and 2x2 rectenna
arrays for RF energy harvesting at 5.8GHz have been
designed and fabricated on a thin low-loss microwave
laminate. The measured return loss of the single element is
-10.3dB and the 2x2 antenna arrays is -20.3dB. The
maximum gain at boresight measured is 3.3dBi and 8.0dBi
for the circular polarized single element patch antenna and
patch array antenna respectively. The 2x2 array rectenna
outputs a DC voltage of 72.5mV over a distance of 15cm
when 16dBm microwave power was transmitted at 5.8GHz.
Using the same method, LTC5535 array rectenna outputs a
DC voltage of 428.3mV. Therefore, its performance was
better in terms of higher DC output voltage.

REFERENCES
[1] L. Mateu and F. Moll, "Review of Energy Harvesting
Techniques and Applications for Microelectronics", Proc. SPIE,
Vol. 5837, pp. 359-373, 2005.
[2] Z.B. Popovic, "Wireless Powering for Low-Power Distributed
Sensors", Serbian Journal of Electrical Engineering, Vol. 3, No.
2, pp. 149-162, 2006.
[3] J.O. McSpadden and K. Chang, "A Dual Polarized Circular
Patch Rectifying Antenna at 2.45GHz for Microwave Power
Conversion and Detection", IEEE MTT-S Microwave
Symposium Digest, Vol. 3, No. 23-27, pp. 1749-1752, May
1994.
[4] B. Jamali, D.C. Ranasinghe and P.H. Cole, "Analysis of UHF
RFID CMOS Rectifier Structures and Input Impedance
Characteristics", Proc. SPIE, Vol. 6035, pp. 102-112, 2005.
[5] H. Yan, J.G.M. Montero, A. Akhnoukh and L.C.N. de Vreede,
"An Integration Scheme for RF Power Harvesting", Proc. SAFE
2005 Conf., 2005.
[6] Bing Jiang, Smith, J.R., Philipose, M., Roy S., Sundara-Rajan,
K., and Mamishev, A.V. "Energy Scavenging for Inductively
Coupled Passive RFID Systems", Proc. IEEE IMTC 2005, Vol.
2, No. 16-19, pp. 984- 989, 2005.
[7] Shinji Mikami, Tetsuro Matsuno, Masayuki Miyama, Hiroshi
Kawaguchi, Masahiko Yoshimoto and Hiroaki Ono, "An Energy
Harvesting Wireless-Interface SoC for Short-Range Data
Communication", IEEJ Trans. EIS, Vol. 126, No. 5, pp. 565-
570, 2006.
[8] M. Latrach, J. Zbitou, and S. Toutain, "Low Cost Microwave
Rectifier for Low and High Powers", Proc. URSI, 2002.
[9] J. Zbitou, M. Latrach and S. Toutain, "Hybrid Rectenna and
Monolithic Integrated Zero-bias Microwave Rectifier", IEEE
Trans. on MTT, Vol. 54, No. 1, pp. 147-152, 2006.
[10] Mi. Minhong, M.H. Mickle, C. Capelli and H. Swift, "RF
Energy Harvesting with Multiple Antennas in the Same Space",
IEEE Antennas and Propagation Magazine, Vol. 47, No. 5, pp.
100-106, 2005.
[11] J. Heikkinen and M. Kivikoski, Low-profile Circularly
Polarized Rectifying Antenna for Wireless Power Transmission
at 5.8GHz, IEEE Microw. Wireless Comp. Lett., Vol. 14, No. 4,
pp. 162-164, 2004.
[12] M. Ali, G. Yang, and R. Dougal, A New Circularly Polarized
Rectenna for Wireless Power Transmission and Data
Communication, IEEE Antennas Wireless Propag. Lett., Vol. 4,
pp. 205-208, 2005.
[13] B. Strassner and K. Chang, Highly Efficient C-band
Circularly Polarized Rectifying Antenna Array for Wireless
Microwave Power Transmission, IEEE Trans. Antennas
Propag., Vol. 51, No. 6, pp. 1347-1356, 2003.


0
Antenna Array
H-Plane, Co-pol
E-Plane, Co-pol
H-Plane, X-pol
E-Plane, X-pol
f = 5.8 GHz
10 dBi
-90
o
180
o

90
o
= 0
o
TABLE I
Power Input (dB m) Schottky Diode (mV) LTC5535 (mV)
16 72.5 428.3
14 60.2 362.5
12 48.8 305.2
8 26.5 227.6
4 13.7 188.7
0 3.8 167.9
-5 16.1 161.5
-10 1.1 158.1
35