International Journal of Electric & Computer Sciences IJECS-IJENS Vol: 11 No: 01
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The Behavior of the Dielectric Properties of
Paddy Seeds with Resonance Frequencies
Manjur Ahmed, Fareq Malek, Ee Meng Cheng# , Ahmad Nasir Che Rosli, Mohammad Shahrazel
Razalli, Hasliza A. Rahim, R. Badlishah Ahmad, Rusnida Romli, Mohd Zaizu Ilyas, Muhamad Asmi
Romli
School of Computer and Commun icat ion Engineering
#
School of Mechatronic Engineering
Universiti Malaysia Perlis (UniMAP)
No 12 & 14, Jalan Satu, Taman Seberang Jaya, Fasa 3
02000 Kuala Perlis, Perlis, Malaysia
Manjur Ahmed ( manjur_39@yahoo.com ), Fareq Malek ( mfareq@unimap.edu.my )
Ee Meng Cheng (emcheng@unimap.edu.my ), Ahmad Nasir Che Rosli ( ahmadnasir@unimap.edu.my )
Mohammad Shahrazel Razalli ( shahrazel@unimap.edu.my )
Hasliza A. Rahim ( haslizarahim@unimap.edu.my )
R. Badlishah Ahmad ( badli@unimap.edu.my )
Abstract--
the market price of agricultural commodities. In the
processing of agricultural products, dielectric
properties of the materials is important information
for efficient processing, achieving desired behavior of
the materials and in obtaining desired high-quality.
The dielectric properties of agricultural or biological
materials have much significance in processing of
agricultural commodities and food. Concept and idea
regarding the dielectric properties of agricultural
commodities and insects enables us to understand the
interactions between electromagnetic wave and
vegetation, in order to inspire the development of RF
treatments for agricultural products [1]. The dielectric
property of agricultural products is a function of
moisture content, frequency of the applied
electromagnetic field, the temperature of the
materials, and density and structure of the materials
[2-9]. In granular and particulate materials, the bulk
density of the air-particle mixture is another factor
The dielectric properties of two sample
paddy seeds D219 and D222 were measured using
microwave perturbation technique in a frequency range
9-12.2GHz of 33 frequency at interval of 100KHz, at a
constant temperature (20oC), same bulk density and
moisture content (nearly same, 10.8% and 11% dry
basis). The dielectric constant of D219 sample is higher
than D222 before 9.8 GHz then decreases progressively
when frequency increases. The loss factor of D219 also
decreases sharply if compared with D222. The deviated
values have been ignored as noise. The deviated values
were presented due to the shape of the samples, unequal
perturbation which attribute to full height insertion in
cavity and shape perturbation of the cavity. This paper
relates to a characterization system for various
categories of paddy seeds to store for post harvesting.
Index Term-- Complex permittivity, Cavity resonant
technique, Q-factor, Resonance frequency, unequal
perturbation, microwave treatment protocols.
Nomenclature
Q
quality factor
index c for empty cavity and
index s for sample inserted into cavity
ε
complex permittivity
ε′
dielectric constant
ε″
dielectric loss factor
resonance peak frequency
1.
INT RODUCT ION
Complex permittivity of agricultural or biological
materials is one of the most important characteristics
for determining quality. It is important in determining
the proper time for harvest and the potential for safe
storage. It is also an important factor in determining
f
V
r2
frequency bandwidth difference
resonance frequency,
index c for empty cavity and
index s for sample inserted into cavity
volume
index c for empty cavity and
index s for sample inserted into cavity
regression coefficient
that influences the permittivity [4][10-14] .
Rice (Oryza sativa L.) is the most important staple
food in Asia and a major property for trading
international and domestic market. Many studies have
been conducted to explore the possibility of using
electromagnetic energies to different agricultural
commodities. However, most of the studies did not
focus primarily to the post harvest rice seeds.
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Cavity resonant technique is the most accurate
method in measuring permittivity and permeability of
low dielectric factor materials [5]. Resonant cavities
are high Q structures that resonate at certain
frequencies. A sample material affects the center
frequency (f) and quality factor (Q) of the cavity.
From these parameters, the complex permittivity (ε)
or permeability (μ) of the material can be calculated at
a particular frequency [6-8]. Measurement of
dielectric properties of paddy as a function of
frequency, moisture content and bulk density presents
a great importance in leading further development and
improvement in RF and microwave treatment
protocols. These protocols will help for controlling
seed born fungi and insects, both on the surface and
inside the post harvest rice seeds, with minimal losses
of seed qualities. The present study therefore, was
undertaken to measure the dielectric properties of
paddy samples using cavity resonance method having
same density (0.6634 gm/cm3 ) and temperature
(20o C); 33 frequencies between 9 to 12.2GHz with the
step frequency of 100kHz. The frequency range was
chosen due to the frequency range of the cavity (8.212.2 GHz). The small step frequencies are helpful for
analyzing the final results with others. The data
accusation in this cavity method was different from
others. Good agreement with the known data has been
found with other researchers [14]. Then experimental
data has been analyzed and discussed.
2. M ICROWAVE CAVIT Y PERT URBAT ION THEORY
The complex relative permittivity,
of a material
can be expressed in the following complex form [2]:
(1)
The real part ε′ is the dielectric constant, represents
stored energy when the material is exposed to an
electric field, while the dielectric loss factor ε″, which
is the imaginary part, mainly influences energy
absorption and attenuation [2-3]. The quality factor
(Q) can be expressed as [12],
(2)
Where
be the resonance peak frequency and
is the 3dB bandwidth that is found by difference
of higher and lower frequency in bandwidth. The
dielectric constant (ε′) and dielectric loss factor (ε″)
are then calculated for material perturbation in cavity
as [12]:
(3)
(4)
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where V, Q and f be the volume, Q-factor and
resonance frequency; the index C and S denote the
empty cavity and sample inserted in cavity,
respectively. For measurements of lossy materials, the
filling factor (the ratio of stored electromagnetic
energy in the sample on resonance to the energy in the
entire volume of the resonator) must be small to
enable a reasonable Q-factor (e.g. 100 or more) to be
obtained. Otherwise the resonance may be too small
to measure, as it will be severely damped [13]. For
getting the small filling factor, sample should be small
so that it occupies only a small fraction of the cavity
volume.
It has been shown by mathematically that, if any
sample is inserted into the cavity (increase in
permittivity, or permeability, ), will decrease the
resonance frequency and increase in s tored energy of
the perturbed rectangular cavity [2]. That means,
when a sample is inserted into the cavity, the resonant
frequency will decrease due to the attenuation of the
transmitted wave and hence decreases the
transmission coefficient S12 .
3.
M ET HODOLOGY
3.1. Experimental Setup
The rectangular cavity which was used in
experiment is illustrated in fig. 1. w, h, l are the
width, height and length of the cavity, respectively.
Meanwhile a, b, c are the width, height and length of
the sample under test holder, respectively. The
operating frequency of the cavity ranges from 8.2 to
12.4 GHz and resonance microwave frequency is
located at 9.8 GHz (in air). The dimensions of the
cavity are 23mm10mm101mm. One end of the
cavity (port 1) was connected to the Vector Signal
Generator while the other to the Spectrum Analyzer
(port 2) with the coaxial SMA‟s connectors [3] as
shown in fig. 1. The Vector Signal Generator (Agilent
E8267D, 250kHz-20GHz) give off the required
frequency at a defined power level and Spectrum
Analyzer (Agilent E4405B, 9kHz-13.2GHz) display
the response of that frequency via materials. The
paddy was measured with 0.1mw power and 9 to 12.2
GHz frequency ranges.
Previously, cavity perturbation was used to measure
the complex permittivity of many solid dielectric
materials; it is quite easy to prepare the sample of
these materials. In this work, the agricultural material
under test is paddy and it is hard to measure the
dielectric property due to its non-homogeneous shape,
bulk density variation, moisture content and existent
of the air gap those influences to the dielectric
properties [4]. To resolve these problems, one sample
holder is being used for keeping constant of the bulk
density. The sample holder is made by eccostack
which has dielectric constant of 1.01 (approximate to
dielectric constant of air). The dimensions of the
rectangular sample holder are 22mm×10mm×38mm.
Again, a tuner was used to tune the input frequency of
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the cavity that‟s comes from vector signal generator.
This tuner is helpful for getting the optimum
resonance of frequency.
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Vector Signal Generator and Spectrum Analyzer
respectively. Samples were measured in the range of
9GHz to 12.2GHz with step frequency of 100 KHz to
get the more relationships between frequencies and
dielectric properties.
Fig. 1. Schematic geometry of the rectangular waveguide resonance
cavity with MUT .
3.2. Sample Preparation
Paddy seed (Oryza sativa L.) used in this study was
obtained from Perlis, Malaysia. Two types of paddy
seed sample D219 and D222 was used. The seeds
were cleaned manually to remove all foreign matter
such as dust, dirt, stone, and chaff. The moisture
contents of the two sample seeds are 10.8% and 11%
that are nearly same. Moisture contents of the
samples were determined by standard oven method
(dry basis). As described previously, the dielectric
properties of agricultural commodities changes with
variation of moisture content and bulk density [6]. In
order to maintain the bulk density as a constant, the
weight of the empty sample holder and sample holder
with sample were measured. First of all, both samples
were ground by an electric blender to shatter the
paddy seeds in order to form flour. The samples were
ground on purpose to minimize the air gap among the
paddy seed.
3.3. Experimental Procedure
The dielectric properties are changed with the
variation of bulk density and moisture content [5]. To
maintain the bulk density as a constant, the empty
sample holder was measured with a digital weight
meter (2.71gm) and then measured the sample holder
with samples so that all samples become same weight
(3.71gm). The small sample will give the lower filling
factor and hence higher Q-value. After that, the
volume of samples were acquired by calculating the
volume difference of sample holder and sample
occupied respectively Therefore, the density of the
samples can be maintained as a constant (ρ = 0.6634
gm/cm3 ).
The sample holder with sample was inserted in the
middle of the rectangular resonance cavity to gain the
best electric field. A tuner has been used to get the
optimize resonance frequency. Two adapters were
connected to the tuner and other end of waveguide
and these adapters were therefore, connected to the
Fig. 2. Experimental data accusation system in the cavity resonance
method.
The transmission coefficient S12 versus frequency
spectrum has been monitored in the spectrum
analyzer. The tuner has been set by sliding to get the
optimal value of the peak frequency. The bandwidth
can be determined by simply locate the frequency
corresponding to the value S12 which is 3 dB less
than peak value as seen in Fig. 2 in which resonance
frequency and bandwidth frequencies are represented
by
,
and
respectively. The quality factor,
Q is calculated using equation (2). Evidently, it is
easy to monitor the resonance frequency shift and the
3dB bandwidth from the visual S12 parameter and thus
to find the change of Q factor. From Fig. 3, it can be
seen that the transmission coefficients (S12 ) has been
attenuated when sample is inserted and every
resonance frequency have a different peak S 12 value ,
either in empty or sample occupied holder .
4. RESULT S AND DISCUSSION
4.1 Variations in attenuation of the resonance
frequencies
Previously described that, if any sample is inserted
into the cavity, it will decrease the resonance
frequency and increase the stored of energy using
perturbed rectangular cavity [2]. It can be seen from
Fig. 3 that the S12 values of the cavity, when paddy
seed grind is inserted with a sample holder, then
decreased with respect to the empty cavity.
Consequently, the resonance frequencies are also
attenuated due to the changes in S12 parameters. The
dielectric constant (energy stored) determines the
electric field distribution and the phase of waves
traveling through the material. Dielectric loss factor,
which is the imaginary part of complex permittivity,
mainly influences energy absorption and attenuation
[8-9]. However, the quality factor (Q) also decreases
with resonance frequency if compared the case of
sample inserted in cavity with the case of empty
cavity (later will be discussed with Fig. 6).
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decreases with increase in frequency. However it does
not decrease as sharp as D219.
Dielectric loss factor
of paddy samples has
been shown in Fig. 5. The loss factor of the paddy
D219 decreases sharply than D222 when frequency
increases.
Fig. 3. Variations in the attenuation of the resonance frequencies
with empty cavity when sample of paddy seed is inserted.
Fig. 5. Dielectric loss factors of the whole paddy flour having same
density (0.6634 gm/cm 3) and temperature (20 o C); 33 frequencies
between 9 to 12.2GHz at interval 100KHz.
It can be observed that the values of dielectric
constant and loss factor are low; the highest value of
and
for D219 and D222 has been found as:
Fig. 4. Dielectric constants of whole paddy flour. Same density
(0.6634 gm/cm 3 ), moisture (10.8 and 11 %db) and temperature
(20 o C) at various frequencies; 33 frequencies between 9 to 12.2GHz
at interval 100KHz.
T able I
T he highest value of
The
highest
value between
4.2 Dielectric properties of paddy samples
Variation of typical dielectric constants
of
paddy D219 and D222 with frequency are presented
in Fig. 4.
At 9-9.7GHz, the D219 paddy exhibited
significantly higher values of dielectric constants ( )
as compared to the D222 paddy; while from 9.812.2GHz, the dielectric constants of D222 paddy
exhibits higher values than D219 paddy. Although the
bulk density (0.6634 gm/cm3 ) and temperature (20o C)
remains constant; moisture may play a major
contributor to the dielectric properties. Nevertheless,
the moisture contents of the samples are 10.8% db and
11% (dry basis) respectively. There are slight
differences of moisture contents in the samples. Even
though at frequency 9-9.7GHz, the dielectric
constants of D219 sample is higher than D222 and
vice-versa for frequency range 9.8-12.2GHz; but
of
D219 decreases sharply when frequency increases.
Conversely, the dielectric constant of D222 sample
and
Sample
At
P addy D219
2.6
9.6E-7
P addy D222
2.44
6.7E-5
9 to 12.2 GHz
9 GHz
Lower values of
and
indicate that there is
only weak interaction appear between the paddy
samples and electromagnetic energy. Therefore stored
energy and influence of the electric field in the
samples is insignificant. The energy absorption and
attenuation exhibit insignificant variation under the
influence of the alternating electric field. The
measured values are slightly higher than the known
value in literature [10-11]. Meanwhile, the
values
are lower than the value from literature [10-11] where
the literature states that rice has
and
at 2.45GHz [10][11]. Again it was found
at 500-2500MHz that the dielectric constant of Indian
Basmoti rice flour (totally pure and dry) are
(mean value) [14]. Nelson [15] found that
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dielectric constant of whole wheat flour which has
12% of moisture and 0.78 g/cm3 of density is
approximately 2 at 27, 40, 915, 1800MHz
frequencies.
31
The second degree of polynomial relationships are the
best fit for sample D222 to relate the Q-factors with
resonance frequencies (f). The equations are,
(7)
4.3 Deviation of Data
The data (frequency, S-parameter) accusation from
spectrum analyzer is quite difficult to mark the
accurate 3dB bandwidth. Despite of the difficulties,
the resonance frequency, frequency bandwidth or Qfactor can still be calculated from this method by
using equation 2. However, some of the calculated
data have deviated from its probable values of
literature [10-11][15]. For example, the loaded Qresonance at 9.2GHz is greater than empty Qresonance; which is undesirable. Deviation of data can
be seen in Fig. 5. It is being seen from Fig. 5 that
some data on behalf of each frequency has been
eliminated as undesired value.
In rectangular resonance cavity, the measured
parameters are dependent on the volume, geometry,
mode of operation of the cavity, permittivity, shape,
dimensions, and location of the object in the cavity
[16-17]. It is apparently supposed that the lower value
and some deviated value of dielectric properties
especially on loss factor is for variation of particle
shape and it should be found an optimal place in
cavity to get the optimal resonance. The loss factor of
paddy D219 has more degraded with respect to paddy
D222. The „logarithmic‟ and „moving average per
two‟ regressions are best fit for paddy D219 and
D222 respectively to correlate the dielectric loss
factors with frequencies (f). The equations are,
(5)
Where r2 =0.977.
(6)
Where p = period of 2 (p>1 is an arbitrary choice, t is
the timeline and
.
The uncertainties of the quality factor (Q) for both
samples of paddy seed can be observed in Fig. 6.
According to the observation of Nelson [4], the two
measured cavity parameters, shift of resonant
frequency and change in Q factor, dependent on
permittivity, shape, and volume of an object placed in
the cavity, as well as the orientation and location in
the cavity. Although a tuner was used to get the
resonance frequency which corresponding to
significant peak of S12, the shape and material
perturbation in cavity must be taken into account for
measuring the dielectric properties. In addition, the
resonator cavity has experienced an unequal
perturbation due to full height insertion of the samples
in the cavity. Full height insertion of samples causes
the interruption of electromagnetic field distribution.
Where r2 = 0.848.
(8)
Where r2 = 0.794.
Fig. 6. Q-factor of empty cavity and sample (D222) inserted in
cavity that is showing uncertainties.
5.
CONCLUSIONS
The perturbation technique of cavity was used by
which the dielectric properties of two sample paddy
seeds D219 and D222 were measured at a constant
temperature (20o C), bulk density and moisture content
in a frequency range 9-12.2GHz and 33 frequencies
were measured with step frequency of 100kHz. The
dielectric constant of D219 sample is higher than
D222 sample within frequency range 9-9.7GHz.
Meanwhile, the dielectric constant of D222 sample is
higher than D219 within frequency range 9.812.2GHz. But it decreases gradually when frequency
increases. The loss factor of D219 and D222 samples
also decreases when frequency increases. However
loss factor of D219 decreases with greater slope than
D222. Also, the difficulties of this method have been
discussed. There have some uncertainties of Q-value
due to unequal perturbation for full height insertion of
samples, shape of samples and orientation of the
samples. This study can be extended by characterizing
dielectric properties of paddy which attribute to other
parameters such as temperature, bulk density, and
moisture content and so on. This characterization can
provide the important information for further
development, improvement and scaling-up of RF and
microwave treatment protocols .
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