Metamaterial-based Electrically Small Antenna
Designed for GSM and ISM Applications
Latheef Ahmed Shaik(1), Chinmoy Saha (1), Jawad Yaseen Siddiqui (2)
(1)
Department of Avionics, Indian Institute of Space Science & Technology,
Thiruvananthapuram - 695547, Kerala, India
yaadein.latheef@gmail.com,csaha@ieee.org
(2)
Institute of Radio Physics and Electronics, Calcutta, India
jysiddiqui@ieee.org
Abstract: The demand for small, compact, low cost antennas
operating at multiple frequencies has increased tremendously
over the past years due to the need for reduced antennas in the
modern wireless communications and military applications. Here
we present an entirely planar metamaterial based electrically
small antenna by combining small planar CPW fed monopole
antenna with a SRR unit cell positioned in the close proximity to
the monopole structure, thus creating a composite compact
planar structure for multiband applications. The SRR unit cell
exhibits the negative permeability over a frequency band, and
thereby producing magnetic coupling to the monopole structure
that starts to radiate new bands.
Keywords: Metamaterial, electrically small antennas, multiband
antennas, printed antennas
I.
INTRODUCTION
The market for the small, compact, low cost antennas
operating at different frequencies has increased tremendously
due to the increased sophistication of the military and modern
wireless communication domains [1]. Microstrip antennas
provide the compactness has the difficulty of miniaturization
as the resonant frequency is determined by the dominant mode
of the cavity. In order to miniaturise the patch antenna several
methods employed are placing shorting pins or plates [2],
placing slots on the radiating patch [3], or fractalizing the
radiating edges [3], folding the patch into the multilayered
structures [4], using high dielectric constant substrates [2] and
so forth. The above methods have their own shortcomings.
The emergence of the artificial materials provides the new
ways of achieving the miniaturization. The left-handed
materials are the subclass of the artificial materials which
exhibit simultaneous negative permittivity and permeability
over certain band of frequencies [5-6]. These exotic properties
of the metamaterials can be used to alter or enhance the
radiation characteristics of the antennas and as well as the
microwave circuits. Several miniaturized or electrically small
antennas have been proposed by placing a metamaterial
resonant structure near to the radiating element [7-9]. The idea
behind this approach is that the radiator is sensitive due to the
presence of the resonator like structure due to the coupling,
and the resonator structure affects the radiation characteristics
of the radiator by acting like an effective shell enclosing the
radiator and hence the term metamaterial-based antenna [7].
The metamaterial resonator utilized here is the split ring
resonator (SRR) which consists of two coaxial rings with
splits in the rings located at the diametrically opposite sides.
By exciting the structures with the axial magnetic field, it acts
as resonant LC circuit and exhibits the negative permeability
over certain band of frequencies above the resonance
frequency of the structure [5].
In this paper we utilize the electrically small planar
monopole antenna loaded with the SRR unit cell to provide
the planar metamaterial-based electrically small antennas. Due
to the SRR unit cell exhibiting the negative permeability over
a frequency band, which producing magnetic coupling to the
monopole structure thereby the monopole starts to radiate at
new bands. The combination of the monopole and the SRR
results in the multiband operation, the resonances either of the
monopole or the SRR can be tuned independently as the
elements are linked due to the coupling. Hence the design of
the overall antenna is the superposition of the monopole
antenna with the SRR unit cell with minimal modifications.
The multiband operational frequencies of the antenna are
selected as the ISM (2.4 GHz) and the GSM (1.6 GHz) band
due to their vast spread of application domains. The monopole
antenna designed here is fed using the coplanar waveguide
feeding technique, and here we compare the two antennas in
which the resonant frequencies of the monopole antenna and
the SRR element are interchanged. The proposed structures
are modelled on a RT duriod laminate of dielectric constant
εr=2.33 with loss tangent tanδ =0.0012 using a commercial
available electromagnetic simulator HFSS [10]. The paper is
organised as follows: section II describes the design
considerations of the two antennas where the antennas are
modelled in the simulator and the section III presents the
results and the discussion on the electrically small criteria of
the antennas.
II.
PROPOSED ANTENNA DESIGN
The proposed antennas are presented in Fig. 1. The antennas
are names as J-antenna and L-antenna because of the shape of
the feed used in the design. Here the monopole antenna is fed
using coplanar feeding technique and the centre strip is
extended to form the feed. The resonance frequency of the
monopole is determined by the length of the feed above the
ground plane i.e., it resonates when its length is closer to λ/4,
the bending of the feed may slightly shift the resonance
frequency and coming to the metamaterial resonator circuit,
the resonance frequency of the SRR is computed by the
rigorous calculations presented in [11].
(a) J-Antenna
Figure 2: represents the square split ring resonator. The grey shade
indicates substrate and the copper shade represents the metallization. For
the J-Antenna the SRR dimensions are: rext = 5mm, c = 0.6mm, d = 0.4
mm, g = 0.4 mm. For the L-Antenna the SRR dimensions are given as:
rext = 6.5 mm, c = 0.8mm, d = 0.8mm, g = 0.5mm.
The negative permeability of the SRR element can be shown
by the procedures described in [12-14], which are not shown
here. The SRR exhibits the magnetic resonance, and hence a
negative permeability region, acting as a metamaterial sample.
We exploit this property to enhance the radiation
characteristics of the antenna by disposing this resonator
element in the vicinity of the monopole so that it gets
magnetically coupled to it and thereby the composite structure
radiate multiband. The inclusion of the SRR into he antenna
slightly affects the resonance frequency of it, which give the
advantage of treating the antenna and the SRR as the
individual elements. Some fine tuning is done to bring back
the shift in the resonance frequency.
The antenna are simulated on RT duriod laminate of
dielectric constant εr = 2.33 with loss tangent tan δ = 0.0012
with height of the substrate as h = 1.575mm and the
metallization thickness as 35micron. In the J-antenna case the
monopole is designed at the 1.6 GHz and the corresponding
SRR at 2.4 GHz, whereas in the case of the L-antenna the
monopole is designed at the 2.4 GHz and the SRR at 1.6 GHz.
The outer dimensions of the antenna are kept the same to
compare the compactness of the antenna.
III.
(b)
L-Antenna
Figure 1: CPW fed strip monopole antenna loaded with SRR in proximity
of the feed (a) J-Antenna, (b) L-Antenna. The dimensions are: L = 20mm,
a = 50mm, b = 50mm, W = 0.3mm, S = 5mm, L1 = 18.4mm, L2 = 13mm,
L3 = 13.4mm, x = 1mm, y = 1.3mm, L4 = 21mm, L5 = 15mm, x1 = 1mm,
y1 = 1.2mm, t = 0.035mm and h= 1.575mm. The dimensions of the
respective SRRs are presented in the Fig. 2 caption.
SIMULATED RESULTS
We have plotted the reflection coefficient of the monopole
antennas with and without the SRR in Fig 3, which illustrates
that the antennas resonance is slightly altered by the inclusion
of the SRR. In the Fig. 3(a), we can observe the reflection
coefficient of the J-antenna where the structure radiates at two
different frequencies and the lower resonance at 1.6GHz
corresponds to the monopole and the upper resonance at
2.4GHz to the SRR. In the absence of the SRR the antenna
resonates at single frequency. The higher order mode at
4.8GHz is created in the composite antenna case. In the
monopole alone case higher order mode is present but the
strength of it is poor, coming to the composite case the
interaction between the SRR and the monopole has
strengthened the higher order mode. Similarly the Fig. 3(b),
represents the reflection coefficient of the L-antenna where
the lower frequency at 1.6GHz corresponds to the monopole
antenna and the upper frequency to the SRR at 2.4 GHz. One
another point to be noted is that the matching bandwidth (-10
dB) of the SRR is greater in the case of the J-antenna as the
SRR interacts with more length of the monopole when
compared to the L-antenna.
With SRR
Without SRR
0
0
330
30
-2
-4
300
60
-6
-8
-10
270
90
-8
-6
-4
240
120
-2
210
0
150
180
The radiation patterns of the composite monopole and the
monopole alone in both the planes are presented in the Fig. 4.
(a)
With SRR
Without SRR
0
S11 , dB
0
0
-5
-5
-10
-10
-15
330
30
300
60
-20
-15
270
90
-20
-20
-15
Monopole with SRR
Monopole alone
-25
-10
-30
240
120
-5
0
1
2
3
4
5
6
0
210
150
180
Frequency, GHz
(a)
(b)
0
Figure 4: Radiation pattern of the J-antenna at the lower resonance frequency
of the composite antenna i.e., 1.6 GHz (a) indicates the field pattern in the XZ
plane (b) indicates the field pattern in the XY plane
-5
S11, dB
-10
-15
-20
Monopole with SRR
Monopole alone
-25
-30
-35
0
1
2
3
Frequency, GHz
(b)
4
5
6
The radiation pattern of the monopole is not affected by
the inclusion of the SRR, as observed in the Fig. 4. Similarly
the radiation pattern of the L-antenna can be plotted and
verified that the radiation patterns is slightly altered with the
inclusion of the SRR and hence the radiation pattern of the Lantenna is not included here. The radiation patterns are as
expected exhibiting omni-directional in one plane and
directional radiation pattern in the other.
As a thumb rule the antenna is said to be electrically
small
when the size of it is less than λ/10, this measure of the
Figure 3: Reflection coefficient of (a) J-antenna (b) L-antenna. In both the
figures the reflection coefficient is plotted for both the composite case as well specifying is not stringent, and hence the theoretical Chu limit
as the monopole alone.
is used to specify the compactness of the antenna ([7], [1517]). To specify the antenna as electrically small antenna, we
have to calculate the quality factor Q of the antenna and
compare how close to the Chu limit. To compute the Q value
of the system at the resonance frequency f0, the following
expressions are used:
2
𝑄𝑉𝑆𝑊𝑅 (𝜔0 ) =
𝑄𝑐ℎ𝑢 (𝜔0 ) =
𝐹𝐵𝑊𝑉𝑆𝑊𝑅 (𝜔0 )
1
𝑘𝑎
𝑄𝑟𝑎𝑡𝑖𝑜 (𝜔0 ) =
1
+ (𝑘𝑎)3
(1)
(2)
2
𝐹𝐵𝑊𝑉𝑆𝑊𝑅 (𝜔0 )×𝑄𝑐ℎ𝑢 (𝜔0 )×𝑅𝐸
(3)
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IV.
CONCLUSION
We have presented the planar electrically small antennas
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