INTERNATIONAL JOURNAL OF COMMUNICATIONS
Volume 8, 2014
An Enhan
E
ced RF
F Enerrgy Haarvestiing Syystem ffor
Low
w-Poweer Wirreless Sensor
S
rs in 900 MH
Hz Bannd
Moh
hammed M. Bait-Suwailam
B
m, Zia Nadir
A
Abstract—This paper describes a technique to enhance eneergy
connversion module for Radio Frrequency (RF) energy harvestting
sysstem at 900 MH
Hz band. The function
f
of the energy converssion
moodule is to conveert the RF signaals into direct-cu
urrent voltage at the
givven frequency band.
b
The propo
osed design is capable of charg
ging
sm
mall power consu
umption sensorss by capturing ambient
a
RF wav
ves.
The technique com
mprises the use of high-profile monopole anten
nna
array with a reflecctor at the frontt-end of the harv
vesting circuitry
y in
ordder to maximizee the RF energy
y received by th
he antenna systeem.
Furrthermore, a prrototype of thee proposed RF energy harvestting
sysstem was simulaated, built and teested in laborato
ory. The propo
osed
dessign is compareed with a refereence case that uses
u
only a sin
ngle
moonopole antenna element at both transmitter and receiver sides. The
T
achhieved results sh
how better perfformance. The designed
d
system
m is
useeful in many wirreless application
ns and can easily
y be integrated with
w
othher high-gain an
ntenna systems to
t further maxim
mize the harvessted
eneergy.
K
Keywords— Electromagnetiics, energy harvesting,
h
eneergy
scaavenging, radio-ffrequency harvesting, and recten
nna.
Fig. 1: Poteential radio-frequeency (RF) sources.
Although the
electronnic devices andd wireless senssor networks. A
amountt of energy hharvested from
m Radio Freqquency (RF)
w, neverthelesss, in many insttances only a
waves iis relatively low
few millliwatts of pow
wer is needed too power wireless sensors.
The cooncept of RF energy harvessting system is shown in
Fig.2, w
which consistts of a radiaating system, a matching
networkk, RF-DC convversion and loaad circuits.
I. INTRO
ODUCTION
N
owadays, wireless
w
deviices are gro
owing in maany
applicationss like mobile ph
hones or senso
or networks. Su
uch
wirreless devices provide efficcient and practtical solutions to
connsumer, industtrial, and militaary needs. Thiss dramatic grow
wth
in wireless appliccations producees a large usag
ge of batteries that
t
or empowering most wirelless
givves alternativee solutions fo
devvices. Howev
ver, such batteeries pose co
onstraints due to
pacckaging size, operational tim
me, and their deposition cau
use
envvironmental isssues. Furtherm
more, many applications,
a
like
l
wirreless sensor nodes
n
are locaated in difficullt or inaccessiible
plaaces so their battery
b
mainten
nance becomes a crucial isssue.
Thhis problem mo
otivated lots off researchers to
o think of infin
nite
pow
wer supply resources or enh
hanced power over a period
d of
tim
me. Fig.1 dep
picts several potential sou
urces of pow
wer
harrvesting aroun
nd us, includ
ding ambient electromagneetic
waaves. It is indeeed possible to capture suffficient energy by
harrvesting the energy
e
from, for example,, vibration frrom
peoople walking or
o even automo
obile heat, wind or broadcasting
waaves, etc..., to power
p
very smaall devices such as wearable
Fig. 2: RF
F harvesting system
m block diagram.
Curreently, many ttechnologies hhave been devveloped that
attemptt to overcomee the limitatioons imposed on wireless
devices . For instancee, recent advaancements in rechargeable
batterie s and use oof double-layeer capacitors, as well as
technoloogies that harvvest solar, windd, or even kineetic energy to
name a few have beenn effective at ssatisfying a largge portion of
the estaablished markeet need. Moreoover, in some ddeployments,
owing tto the sensor loocation, batteryy replacement may be both
practicaally and econnomically inffeasible, or m
may involve
signific ant risks to huuman life [1]. A
Another alternaative is to get
an advaantage of thee electromagneetic RF wavees that exist
around us. Such wavees can propagaate through the surrounding
M
Mohammed M. Bait-Suwailam iss with ECE dep
partment, Collegee of
Enggineering, Sultan Qaboos universiity, Muscat 123, Sultanate of Om
man
(corrresponding Autho
or: +968-24142571; email: msuwailem@squ.edu.om)
Z
Zia Nadir is with ECE department, College of Engineeering, Sultan Qab
boos
university, Muscat 12
23, Sultanate of Om
man (email: nadir@
@squ.edu.om)
ISSN: 1998-4480
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INTERNATIONAL JOURNAL OF COMMUNICATIONS
Volume 8, 2014
space and hence can be captured by an antenna. This kind of
energy harvesting is still a new research direction and is of
great interest to engineers and researchers worldwide. Several
sources for RF energy harvesting exist, which can be
classified into three categories: intentional sources, anticipated
ambient sources, and unknown ambient sources. The
intentional sources are in fact dedicated power transmitters
that provide controlled amount of power. Unknown ambient
RF sources can still provide minimal amount of power,
however, with no control and no knowledge of the sources of
such transmitted energy. The levels of power RF available in
the ambient environment are low, which requires a matching
circuit to minimize the losses by reflection and to maximize
the transmission of power towards the electronic device [2].
The main objective of an RF energy harvesting system is to
convert the RF power from the space into usable electrical
energy source. When harvesting energy in the GSM or WLAN
band, one has to deal with very low power density levels.
Therefore, neither GSM nor WLAN are likely to produce
enough ambient RF energy for wirelessly powering miniature
sensors, unless a large area is used for harvesting.
Alternatively, the total antenna surface can be minimized if
one uses a dedicated RF source, which can be positioned close
(a few meters) to the sensor node, thereby limiting the
transmission power to levels accepted by international
regulations [3].
In order to achieve that, a system with several stages is
required. The RF power is generated from a transmitting
antenna (i.e., the ambient RF waves), which are then captured
by a receiving antenna as an input feeder to the harvesting
circuitry. In this work, we propose the use of linearly
polarized antenna system for ease of impedance matching.
Second step is to transfer the power received by the antenna to
the circuit with minimal losses. Therefore a matching circuit,
if needed, is placed at the input stage of the harvesting circuit
to minimize the mismatch between the antenna and the next
component of the circuit. Furthermore, a voltage booster
circuit is added to the device as an amplifying unit for the
voltage input in order to increase the output voltage of the
system. Since most of the consumer devices require a DC
supply unit to charge, a rectifier circuit takes its function to
convert the AC voltage into DC and stores it in a storage unit.
Significant amount of research work have been conducted in
this exciting field ranging from antenna design to highefficiency rectifying devices. The RF-DC conversion
efficiency of the rectenna with a diode depends on the
microwave power input intensity and the connected load. We
need to get the optimum microwave power input intensity and
the optimum load to maximize the efficiency. When the power
or load is not well matched, the transferred power is not the
optimal power and the efficiency becomes quite low. The
efficiency is also determined by the characteristic of the diode;
the diode has its own junction voltage and breakdown voltage.
If the input voltage to the diode is lower than the junction
voltage or is higher than the breakdown voltage, the diode
does not show a rectifying characteristic. As a result, the RFDC conversion efficiency drops with a lower or higher input
ISSN: 1998-4480
than the optimum [4].
It is instructive to mention here that tremendous amount of
work have been allocated to antenna design of such harvesting
systems [5]-[9]. Many of such research attempts have adopted
commercially available antennas or even designed ones that
are electrically-large. As such, this imposes constraints on the
usability of such systems. In [10], a conceptual view of
enhancement mechanism of RF energy harvesting was briefly
introduced. In this paper, we provide a thorough analysis of a
cost effective and efficient antenna design to maximize the
amount of power that is captured by the antenna system of the
harvesting circuitry. An appropriate receiving antenna design
is imperative since the antenna characteristics, such as gain,
radiation pattern, and impedance bandwidth, can affect the
amount of harvestable energy [11]. The enhancement is
achieved here through incorporating a monopole antenna array
designed at 900 MHz band with a reflector to maximize the
amount of captured energy by the antenna system. Several
sensitivity analysis and experiments were carried out in
laboratory and results are comprehensively presented in this
paper. For comparison purposes, single antenna elements at
both transmitter side and receiver end of the harvesting circuit
were adopted. In this work, an enhanced RF energy harvesting
circuitry is designed and tuned to work at the 900 MHz
frequency band. However, for broadband applications it is
necessary to have a broadband antenna and a broadband
matching unit for the diode [12], which is currently under
investigation.
This paper is organized as follows. Section two explores the
proposed energy harvesting system. Both the circuit layout
and fabricated prototypes are further discussed. Section three
describes procedure of the experimental work that was
conducted in controlled laboratory environment. The results
are also discussed and several case studies quantifying the
performance of the energy harvesting circuitry are explored.
Finally, section four concludes this research work with a brief
summary of the findings.
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INTERNATIONAL JOURNAL OF COMMUNICATIONS
Volume 8, 2014
II. RF
R ENERGY HARVESTING
A
SYSSTEM
use of ddiodes, it is insstructive to stuudy the effect oof the choice
of diodee on the amouunt of harvested energy. For this study, a
7-stage voltage boosster circuit waas used. An eexperimental
study b ased on the usse of the 7-stagge booster circcuit had been
conductted in laboratoory in order too investigate tthe effect of
diodes oon performancce.
Severral observatioons were notted. First of all, a big
improveement on the aamount of harrvested output voltage was
achieveed when using silicon carbidde (SiC) schottkky diodes as
comparred to the norm
mal schottky ddiodes. In factt, the output
from an input oof just 32 mV
voltage can reach a vaalue of 1.6 V fr
within ccouple of minuutes. The empployed SiC schhottky diodes
were trremendously aable to resolvve the problem
m of having
small am
mount of AC vvoltage at the front-end of thhe harvesting
circuitryy. The powerr lost within the circuit is due to the
dissipattion of power iin the internal resistance of tthe diodes of
the circcuits. The finddings of achieeved gain from
m the circuit
based oon choice of diode are disccussed next. F
For an input
voltage of 32 mV, thhe achieved gaain from the ccircuit that is
based oon normal schhottky diode w
was nearly 14 dB and the
power llost was 8 μW
W. However, this result waas more than
double when SiC schottky diodes are used andd the loss-in
power ddecreased to 0.08 μW.
In suummary, the abbove discussioon shows that S
SiC schottky
diodes are most suitaable for very high frequenccy switching
applicattions. This is because of thhe fact that thhe switching
losses iin SiC schottkky diodes are nnegligible and such diodes
have low
w resistivity. T
The power loss in boosting ddiodes is due
to both conduction annd switching loosses. The connduction loss
ward voltage drrop, whereas
is mainnly caused by tthe diode forw
the swittching power lloss is due to thhe reverse recoovery energy
loss. O
Overall in SiC schottky dioddes, the switcching loss is
negligibble, because thhe reverse recovvery current is nearly zero.
B
Based on thee block diagram of the haarvesting systtem
desscribed in secttion I (see Fig
g.2), Fig.3 show
ws the schemaatic
layyout of an enerrgy harvesting
g system. It is worth
w
noting that
t
by increasing number of voltage multiplier stages, a
utput voltage of
o the harvesting
subbstantial increase of the ou
cirrcuit can be deeveloped depen
nding on the type
t
of choicee of
dioodes and capaacitors. In this work, 3- and
d 7-stage voltaage
booster circuits are
a considered. Prototypes of such harvesting
uits were modeeled
devvices with the 3- and 7-stagee booster circu
andd manufactureed in-house. The harvestin
ng circuits were
w
tunned to operate at a frequenccy of 868 MHzz. An anticipaated
RF
F source was used
u
at transmiitter side to pro
ovide a sufficiient
RF
F energy in ord
der to quantify the performan
nce and efficien
ncy
of the proposed harvesting
h
circcuit. To increaase the DC pow
wer
he efficiency of
o conversion RF/DC,
R
a speccial
harrvested and th
RF
F source can be used to feed
d certain devicees [2]. Below are
thee details of blo
ocks of the RF harvesting cirrcuitry adopted
d in
thiis work.
Fiig. 3: System diagrram of RF harvester.
A
A. Voltage bo
ooster circuit
The multiplierr acts as a charrge pump to eff
ffectively boostt as
weell as rectify th
he input voltag
ge. The adopted
d circuit is a half
h
waave version of
o the circuitt and depend
ds heavily up
pon
im
mpedance matching for maxiimum wirelesss power transffer.
Thhe design hence is a non-linear system where the inp
put
voltage at each stage
s
of the booster circuit haas an exponential
wer [13]. Fiig.4 depicts the
rellationship witth output pow
schhematic layout of the 7-stage voltage boosteer circuit.
C. P
Proposed antennna design
In orrder to enhancee the captured energy, we prropose to use
a conduucting reflectoor along with a monopole anntenna array.
Fig.5 sshows the cirrcuit prototype after incorpporating the
proposeed enhancemennt technique. A 3-stage volltage booster
circuit w
was used for proof of conccept, although higher-order
booster circuits wouuld result in dramatic increease on the
harvesteed energy. In this work, onnly two monoppole antenna
elementts are adopted. In addition too that, we propose the use
of a reeflector alongg with the anntenna array. This should
substanntially focus m
more the ambbient RF enerrgy into the
The reflector
antennaa system, and hence fed intoo the circuit. T
used heere is an aluminnum foil sheett placed at the back side of
a paraabolic-shaped thick paper.. In brief, this should
collectivvely increase the directivityy of the monoppole antenna
system in one direcction instead oof being omnni-directional
along thhe azimuthal pllane.
Fig. 4: Schematic
S
of the 7-stage voltage boo
oster circuit.
B
B. Effect of ch
hoice of diode on harvested energy
e
A
As the voltagee booster circuitry in this work constitutes the
ISSN: 1998-4480
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INTERNATIONAL JOURNAL OF COMMUNICATIONS
Volume 8, 2014
schottkyy BAT41 diodde and 560 pF
F ceramic cappacitors were
used. A
As discussed beefore, schottkyy diode offers low forward
voltage and high switcching speed annd is considereed as an ideal
componnent for RF eneergy harvestingg [14].
Fig. 5: A snapshot of the designed antenna
a
array along
g with the conductting
refleector.
Fiig. 8: A built-in prrototype of an RF eenergy harvester. N
Note that the
anttenna system is noot shown.
F
Fig.6 shows th
he simulated radiation
r
patteern (both E-plaane
(Fiig.6) and H-p
plane (Fig.7)) of a single antenna
a
with and
a
witthout the refleector. For this purpose,
p
open source simulaator
waas used to geenerate the rad
diation pattern
n mimicking the
depployed antennaa system in one of the scenarrios considered
d in
thiis study. As caan be seen from
m the two figu
ures, the reflecctor
willl guide the pattern
p
to be more
m
directive and collectiv
vely
foccus received po
ower towards the
t harvesting circuitry.
c
Upoon boosting thee output voltagge, a load capaacitor with a
value eqqual to 2.2 mF
F is used to stoore the output power. That
capacitaance has been cchosen after seeveral trial-andd-error stages
perform
med to achievee an optimizeed capacitancee value. The
ground plate is imporrtant here to sshield the circuuit from any
externall electromagnnetic interferennce and to prrovide better
perform
mance for the monopole anntenna array. In practical
cases, tthe amplitude of the AC ssignal will bee divided by
couplinng capacitors annd the junctionn capacitance oof the diode.
Also thhe reverse leeakage currennt of the dioode and the
resistannce of the dioode will limitt the feasible DC output
voltage . Therefore, too obtain an opttimal output vvoltage, large
couplinng and charge-sstorage capacittance is preferrred [15].
III. EXPER
RIMENTAL SETU
UP AND RESULT
TS
F source Agileent, N9310A
In thhe experimentaal setup, an RF
RF signal from a monopole anntenna at the
was useed to supply R
transmittting side. A ffrequency of 8868 MHz was used so that
no interrference can reesult from licennsed frequencyy bands used
by otheer agents. Att the receiverr front-end siide, another
monopoole antenna hhad been usedd in order to capture the
radiatedd emissions. F
For optimal siggnal strength, the sending
and recceiving antennaas have been chosen identiccal in shape,
size andd with same ppolarization. Fuurthermore, it is important
to placee the receivingg antenna at thhe far field reggion to make
sure thhat the captureed energy is only due to the radiated
magnetic wavees (i.e., no reaactive field). T
The receiving
electrom
antennaa is connected to the harvestting circuitry. Fig.9 shows
the scheematic of the measurement setup as condducted in the
laboratoory. A Spectruum Analyzer, A
Agilent N93200B, was used
at the reeceiver side inn order to dispplay the captureed energy or
power.
Fig. 6: Simulated
d E-plane radiation
n pattern of a singlle monopole anten
nna
elem
ment (a) with and (b)
( without a reflecctor.
Fig. 7: Simulated
d H-plane radiation
n pattern of a singlle monopole anten
nna
elem
ment (a) with and (b)
( without a reflecctor.
A
An image of the
t built-in prrototype with a 3-stage voltaage
booster circuit iss shown in Fig.8. Notice thatt each two diod
des
ors form a sin
ngle boosting stage. Along the
andd two capacito
edgge of the prin
nted circuit board, an SMA
A connector was
w
solldered to conn
nect the anten
nna system with the remaining
parrts of the cirrcuit. In the three
t
stage vo
oltage multipliier,
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INTERNATIONAL JOURNAL OF COMMUNICATIONS
Volume 8, 2014
harvestiing circuit. Thhe input RMS vvoltage and thhe output DC
voltage were recordedd and the resultts are as shownn in Fig.11.
Fig. 9: Measurem
ment setup with distance controlled using
u
measuring ru
uler.
In this experiiment, the ou
utput power of
o the RF sig
gnal
gennerator was co
onsidered at a frequency off 868 MHz with
w
pow
wer amplitudee strength equal to 20 dBm. One
O of the cruccial
reqquirements for the energy haarvesting circuiit is to be ablee to
operate with weeak input RF power. For a typical 50 Ω
0 dBm receiv
ved RF signaal power meaans
anttenna, the 20
am
mplitude of 0.1
1 W. As the peak
p
voltage of
o the AC sig
gnal
obttained at the antenna is gen
nerally much smaller than the
dioode threshold, diodes with lowest possiblee turn on voltaage
aree preferable [1
1]. The expectted receiving signals
s
should be
lesss than the traansmitted sign
nals due to pro
opagation lossses.
Onne of these lossses is the path loss. In this stu
udy, this loss was
w
callculated using Friis equation
n, which is soleely used to stu
udy
RF
F communicatiion links [16]. This transmiission formulaa is
exppressed as:
,
Fig. 10: Spectrum view
w of the captured aantenna resonance frequency.
ments were taaken in order to study the
Diffeerent measurem
effectivveness of the pproposed enhanncement methood. This can
be donee by measurinng the output DC voltage for different
cases. T
The results w
were taken for one antenna without any
enhanceement, one anntenna with thhe reflectors, tw
wo antennas
only annd two antennaas with the refllectors. Fig.12 summarizes
and com
mpares the resuults between thhe four cases.
(1
1)
r
powerr at the receiveer side (in wattts),
whhere Pr is the received
Pt is the transmittted power (in watts), λ is th
he wavelength
h of
d signal (in meter), Gt and Gr are
thee transmitted
traansmitting/receiving antenna gains, and d is the separation
disstance betweeen the transm
mitting/receivin
ng antennas (in
meeter). It is clearr that the receiived signal streength is inverssely
prooportional to the square of
o the distan
nce between the
traansmitting and
d the receivin
ng antennas. Furthermore,
F
the
Federal Commun
nications Comm
mission (FCC)) regulations haave
he allowable transmitted signal
s
power on
sett limits on th
speecific frequen
ncies. For calibration
c
pu
urposes, seveeral
meeasurements were
w
conducted
d in a non-aneechoic laborato
ory
envvironment to validate this relationship for
f the propossed
harrvesting circuitt.
In the first meeasurement settup, one antenn
na was used as
a a
traansmitter and another
a
was ussed as a receiv
ver. The captured
eneergy was meaasured directly
y after the reeceiving anten
nna
usiing the spectrrum analyzer (see Fig.10). Then, measured
recceived power data were con
nverted to RM
MS voltages using
Ohhm's Law Poweer equation:
,
Fiig. 11: Input AC vvoltage Vin and outpput DC voltage Voout of the RF
harvesting circcuit as a function oof separation distannce.
(2)
whhere Pr is the received powerr, Z0 is equal to
o 50 Ω, and Vinn is
thee RMS input vo
oltage at the front end of harv
vesting circuit.
At the end, the output
o
voltage was measured at the end of the
ISSN: 1998-4480
Figg. 12: Measured ouutput voltage as a ffunction of separattion distance
betweenn transmitting and rreceiving antenna system without a load capacitor.
5
INTERNATIONAL JOURNAL OF COMMUNICATIONS
Volume 8, 2014
A
As can be seen
n from Fig.12, the recorded DC
D output voltaage
at a separation distance of 25
2 cm between
n single anten
nna
d at both transm
mitter/receiverr (Tx/Rx) sidess is
eleements located
appproximately eq
qual to 570 mV.
m This resullt is without any
a
enhhancement but when reflecttors were inco
orporated at both
Txx/Rx sides, the output voltagee was increased
d to reach 1.06
6V
in couple of seco
onds. Moreoveer, adding ano
other antenna will
w
put voltage mo
ore. The use of
o two monop
pole
inccrease the outp
anttennas has incrreased the outp
put voltage to 630 mV (without
refflectors) and to
o reach 1.27 V with the use of two reflecttors
witth both transm
mitting/receiving antennas. It is clear from
ressults above th
hat enhancem
ment of energ
gy harvesting is
attaainable from th
he proposed tecchnique.
Other measurrements had also
a
been carrried out with
h a
cappacitive load at
a output of thee harvesting ciircuit. The outp
put
voltage withoutt the loading
g capacitor is
i instantaneo
ous.
ng a capacitorr at the outp
put stage of the
Hoowever, addin
harrvesting circuiit makes the reading
r
more stable. Hence, it
proovides more reeliable reading
gs, however, att the cost of tiime
as it does take some time to stabilize the measured outp
put
g delay at eaach
voltage due to the uncontrolllable charging
nce. Nevertheless, capacitorrs with minim
mal
inddividual distan
chaarging delay will
w overcome this issue. In fact, the issuee of
conntrolling the ch
harging delay of
o load capacittors in the conttext
of this study is an
a important parameter,
p
and is the subjectt of
futture work.
F
Fig.13 depicts the measured
d results obtain
ned when using
ga
loaad capacitor. Data were taken for seeveral separation
disstances betweeen transmitting antenna and receiving
r
anten
nna
sysstem. Comparison is made with a referen
nce circuit wh
hen
adoopting single antenna
a
elemen
nts at both Tx//Rx sides. As can
c
be seen, capacittors require laarge amount of
o time to reaach
ults
staabilized outputt DC voltage when comparring these resu
witth those obtain
ned in Fig.12 without
w
using th
he load capacittor.
Hoowever, results obtained when
w
using thee load capaciitor
connfirm the prop
posed techniqu
ue that is addiing the reflecttors
andd adopting a high-gain
h
anten
nna array increease the receiv
ved
inpput (AC) and output
o
(DC) volltage of the harrvesting circuitt.
The eelectrical energgy stored in thhe load capacittor is studied
next. T
The stored eenergy is givven by the well-known
express ion,
,
(3)
where WE, C and V are the storeed energy by the load (in
Joule), tthe capacitance of the load ccapacitor (in Faarad) and the
DC outtput voltage (iin Volt), respeectively. As eexpected, the
strengthh of stored enerrgy decreases as far as the ouutput voltage
decreas es (see Fig.14)). For instance,, at a separationn distance of
25 cm bbetween the traansmitter and thhe RF harvesteer circuit, the
energy stored from one antenna w
without any eenhancement
reachedd to about 213 μJ. As the recceived power inncreases, the
output voltage startss to graduallyy increase, whhich in turn
increasees the stored ennergy in the caapacitor. On thhe other side,
the enerrgy stored, wheen using two m
monopole antennna elements
with thee reflectors reaached to 850 μJJ, which is aboout four folds
higher tthan the previious case. At a separation ddistance of 2
meters, the energy is ddropped to 1.85 μJ, due to thhe drop in the
receivedd power densiity. Fig.14 alsoo depicts the eenergy stored
as a fun
unction of sepaaration distancce between thee transmitter
and recceiver for bothh cases, namelyy: proposed prrototype and
referencce case withouut any reflectoors. The normaalized stored
energy as a ratio betw
ween the two ccases is in the rrange of 1:4.
That raatio starts to ddecrease as farr as separationn distance is
increaseed. This is attrributed to effeectiveness of tthe reflectors
as well as the decay of the received ppower strengthh.
It is worth highligghting here thhat more enhaancements in
terms oof captured poower density aat the front-ennd of the RF
harvestiing system annd captured ennergy is conceeivable when
using a high-gain anteenna array systtem.
Fig. 144: Measured storedd energy from a tw
wo element antennaa system with
reflector compaared with an antennna without a reflecctor
IV. CONCLUSION
In thhis paper, a technique foor enhancing RF energy
harvestiing systems was proposedd. The propoosed system
consistss of monopole antenna array,, a 3-stage volttage booster,
Fig. 13: Measu
ured output DC volltage for the antenn
na array and singlee
antennaa at both Tx/Rx sid
des using a load caapacitor.
ISSN: 1998-4480
6
INTERNATIONAL JOURNAL OF COMMUNICATIONS
Volume 8, 2014
[8]
rectifying circuit and a unit storage device. To further enhance
the captured energy, an in-house made conducting reflector
was used. Moreover, the performance of 7-stage voltage
multiplier circuit on the overall captured output voltage was
emphasized. Several design constraints related to
manufacturability of such systems were also discussed with
more emphasis given to the choice of diodes.
The experimental results, based on this study, show that the
power harvested by one antenna decreases when separation
distance between transmitting antenna and receiving circuitry
increases. Furthermore, the use of high-gain antenna array as
well as RF reflector at the input of the harvesting system has
resulted in a substantial increase in the harvested power.
Currently, more work is being carried out to investigate the
effect of load capacitors delay on performance as well as the
use of high-gain planar antennas in order to maximize the
captured energy by the harvesting circuitry.
[14]
ACKNOWLEDGMENT
[15]
[9]
[10]
[11]
[12]
[13]
The authors would like to thank SQU for providing the
financial resources to accomplish this research work. The
authors would also like to thank Ahmed Al-Khayari, Hamad
Al-Khayari, Sulaiman Al-Nabhani, Maryam Al-Lawati and
Manal Al-Busaidi for their valuable support in performing
several experimental setups.
[16]
Mohammed M. Bait-Suwailam (MIEEE) received the B.E. degree in
electrical and electronics engineering from Sultan Qaboos University,
Muscat, Oman, in 2001, the M.A.Sc. degree from Dalhousie University,
Halifax, NS, Canada, in 2004, and the Ph.D. from the University of
Waterloo, Waterloo, ON, Canada, in 2011, both in electrical and
computer engineering.
He was a Lecturer with Sultan Qaboos University, in 2001, where he
was involved in teaching, academic advising and research. While
pursuing his masters and doctorate, he was a Teaching/Research
Assistant. Presently, he is an Assistant Professor with the Electrical and
Computer Engineering Department, Sultan Qaboos University. He has
authored/co-authored more than 25 papers in refereed journals and
conference proceedings. His current research interests include
computational electromagnetics, energy harvesting, and applications of
electromagnetic bandgap structures and metamaterials in radiating
systems, guiding structures, and EMI/EMC related problems.
Dr. Bait-Suwailam is a member of IEEE and serves in several
journals/conferences committees. He is a recipient of two scholarships
from Sultan Qaboos University and the International Student Doctoral
Completion Award from the University of Waterloo in 2011.
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Z. Nadir (MIEEE, MIEAust, MECTI), received the B.Sc.(Honors)
degree in Electrical Engineering with specialization in communication
Systems from UET Pakistan in 1989, the D.E.S.S. in microwaves and
micro-electronics, D.E.A. in Electronics and the Ph.D. in ElectronicsEMC from Laboratory of Radio Propagation and ElectronicsLRPE(renamed as TELICE), University of Sciences and Technologies,
Lille-1 France in 1996,1997 & 1999 respectively. From 1990 to 1994 he
worked in WAPDA, the electricity department of Pakistan. In 1999, he
joined the High Voltage and Short Circuit, Laboratory, IslamabadPakistan as Assistant Director (Technical).
He was involved mainly in testing procedures, standards and
performing tests on Transformers, switchgears, Insulators, Power
Transformers etc. He was a visiting researcher in the area of RF
electronics at iradio laboratory, University of Calgary, Canada. His
current research is in the areas of RF electronics, Microwave-RF
propagation studies, and computational electromagnetics.
Dr. Nadir is member of Pakistan Engineering Council, IEEE USA,
Engineers Australia and ECTI-Thailand. Since 2001, he is attached as
faculty member with the department of Electrical and Computer
Engineering, Sultan Qaboos University Muscat, Oman. He has achieved
several awards and certificates while serving the academia and industry.
7