Indi an Journ al of Radio & Space Physics
Vol. 28 , October 1999, pp. 244-246
Dielectric properties of Bamboo and Canna plants leaves at 9.8 GHz
T J Bhoopathy
I ,
A Anandavadivel 2 & P V Mohanamani I
'Postgraduate Research Department of Physics, Pachaiyappa's College, Chennai 600030
2
Department of Applied Physics, Sri Venkateswara College of Engineering, Sriperumbudur 602 105
Received II Jalluary 1999; revised received 26 May 1999
The complex permittivity of the tropical plants leaves, viz., Bamboo and Canna,has been measured at 9.8 GHz using
thin sheet waveguide method . The experimental measurements are compared with the results of predi cted dielect ric model s.
The experimental values are found to agree well with one of these models.
1 Introduction
Monitoring of vegetative canopy using microwave
remote sensing method has received reasonable
attention for the past two decades . The success of thi s
method depends on the ability to correlate the key
vegetative properties, dielectric properties and
orientations to the observed measurements . Extensive
work has been done both on experimental and
theoretical studies of diel ectric properties of plant
samples l. In most of the theoretical methods available
in the literature 2-S, the plant, as a whole, is considered
to be the dielectric mixture having components such
as trunk, stem, leaves, branches, etc. that acts as
scatterer of EM wa ves. Hence, to predict the dielectric
properties of the plant as scatterer, there is a need to
study the dielectric properties of leaves and similar
vegetative components. This paper reports the
measurements of dielectric constant of Bamboo
(Bamboosa valgorisa) and Canna(Canna Indica
orientalis) plants leaves. Number of models for the
prediction of dielectric properties of vegetative
samples are availau ic in the literature, vi z. (i) twophase ellipsoidal mixture model by r\',;,:!;,::- :' Tld Van
Santen J and deLoor4, (ii) two-phase confocal
ellipsoidal model by Tinga et al s., (iii) two-dielectTic
6
mixing model by Fung and Fung and (iv) dual
7
dispersion model by Ulaby and EI -Rayes The Fung
and Fung and Ulaby- EI-Rayes model s are employed
in the present study. Carl son 8 used the waveguide
resonator technique to measure the dielectric constant
at 8.5 GHz as a function of moisture content. Similar
9
measurements at 9.5 GHz were reported by Tan . The
thin sheet waveguide method suggested by Sarabandi
and UlabylO has been used to obtain the complex
dielectric constant in the present work, because this
method is suitable for the high loss materials like
moist vegetation. The experimental values are
compared with the Fung and Fung6 and Ulaby-El7
Rayes mode ls to predict the suitability of the present
method of measurement.
2 Method and materials
The X-band Gunn oscillator (Mode l XM251 of
Scientific Instrument Company, Allah abad) has been
used with isolator, frequency meter, bi-directional
coupler and slotted waveguide section as shown in
Fig.l . A small piece of leaf sample is inserted
between the flanges of the wavegui de and slotted
) .
,
~
I SLOTJU>WAVEGUTOE
2 SAMPLE
J DlR£cnONAL C"OliI'UR
" MATCHEDTER.-..uNATOR
Fi g. I- Micro wav e ci rcuit loyo ut for measurement of compl ex
dielectric con stant
BHOOPATHY el . at. : DIELECTR IC PROPERTIES OF BAMBOO &CA
•
section without making much damage to the sample.
The modulated and amplified microwaves are fed into
the directional coupler and then on sample at slotted
waveguide. The voltage standing wave .ratio(YSWR)
and distances of the minimum field with (Zlllin) and
without (zo) sample, are measured 11. Using these
values, the complex reflection coefficient (r-) and
complex dielectric constant(Er) were calculated as
suggested by Sarabandi and Ulabi o. This experiment
was carried out at the room temperature of 25.5°C
with a frequency of 9.8 GHz
About 150
measurements were taken with each type of leaf
samples. The initial weight of the samples were taken
and then were used at the slotted section of the
waveguide , and the YSWR values were measured
for the calculation of complex di electric constant.
Finally, the samples were slowly dried and weighed to
245
Er = Non-disperse dielectric constant
= 1.7 - O.74m+6.16m 2
VI = Yol .fraction of free water
= m (O .55m -0 .076 )
Vb = Yol. fraction of bulk vegetation-bound water
mixture
m
Gravimetric moisture content on wet weight
basis
Here,
(J
2
is taken as 1.27 S/m and Vb as 4.64
m /(l +7.36 m
2
)
T he 'aI Ul~ ~ of complex dielectric constant fro m
Ulaby-EI-Rayes model fit well with the experimental
20.00 ~------------------------------,
calculate the gravimetric moisture content on wet
weight basis .
3 Comparison of measured values with the
dielectric models
A LEAVES
FUNG AND FUNG MODEL
ULABY-ELRAYES MODEL
PRESENT VALUES
1
2
~'~.oo
+
I-<
CI)
Z
0
3.1 Fung and Fu ng model
Fro m Fung and Fung model 6 the compl ex dielectric
constant is represented as
+
+
+
;2
G
+
p.J
....l
p.J
is
[; r=
2
u
u Jo.oo
~.OO
+
[em I 2) rea l (Ew) + 1.5] -i [ (m I 3) imago Ew]
where,
0.000 .00
m =·Gravimetric moi sture content with respect to wet
weight
0.20
0 .30
o. ~o
0 .50
0 .60
Fig. 2- Dielectric constant vs gravimetric moisture content of
Bamboo leaves (thickness 0.015 cm)
Ew= Complex dielectric constant of water at gIVen
requency
= 4.9 + { 75/[1 + i ifl 18)]}
f =Frequency in GHz
0 . 10
GRA VIMETRIC MOISTIlRE CONTENT
14.00
~
12.00
~------------------------------,
2
+
FUNG AND FUNG MODEL
ULABY -ELRA YES MODEL
PRESENT VALUES
10.00
CI)
3.2 Ula by - EI-Rayes model
The complex dielectric constant according to
7
Ubby-EI-Rayes model is given by
[; r = E r+ VI [4 .9+{75/[l-i-iif 118)]} ]-i (18
x 2.9+55/[ 1+ifIO.18)]o5
(J
Ij)+Vb
~
U
~
....l
p.J
o
8 .00
2
6.00
• . 00
+
2 .0 0
where,
f
(J
= Frequency in GHz
= Ionic conductivity of free water in Sim
0.00 +.-......,.-r.-,,.,..,...-rn"TTT....-rrTTT,.,..,-TTT,,.....~.-r-c~n-n~0:--:l50
O.
0 .10
o. 0
0 ..30
0.-«>
.
GRA VIMETRlC MOISTURE CONTENT
Fig. 3-Dielectric constant vs moisture content of Canna leaves
(thickness 0.035 cm)
246
INDIAN J RADIO & SPACE PI-IYS , OCTO BER 1999
values as shown in Figs 2 and 3. The soli d hnes
correspond to the calculated values of di electric
constant using tl:.e fung and Fung 6 and Ulaby-El Rayes 7 models.
4 Result and discussion
The preser. t study shows that no significant
difference is observed in the dielectric properties of
leaf samples with different sides fac ing the source
wave. Figures 2 and 3 show that the observed
complex diel ectric constant increases with the
increase of water content. This confim1s that water
content (bound and free states) plays an important
role. The dielectric properties of leaves are not the
linear function of moisture content. However, our
results show ~he applicability of the dual dispersion
model of Ulaby-EI-Rayes to these particular plants
leaves . T he main difficulties encountered with in this
experiment were the nature and size of the samples
and also the water content. The water content 'of
newly plucked Canna leaves drops quickly; hence, the
moisture content above 0.4 is difficult to obtain. The
sensitivity of the electronic balance limits the
precision of the observation with moisture content
less than 0.1. Hence, the overall accuracy of
measurements is estimated to be within 8 to 9%. Test
measurement with the thi n specimen of Te fl on agrees
with thi aCGuracy . From the models th us selected, the
Ulaby-EI-Rayes mod el fits well with the ex perimental
values as compared to the Fung and Fun g model.
Therefore, to estimate the dielectric consta nt and loss
factor of Bamboo and Canna plants leaves the UlabyEl -Rayes dual di spersion model can be used .
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