Compact triband circularly polarized planar slot antenna loaded with split ring resonators

Abstract

In this article, a coplanar waveguide (CPW) fed triband circularly polarized (CP) planar slot antenna loaded with split ring resonators (SRRs) is presented. The truncated slot antenna resonates at 4.15 GHz, which gives two orthogonal degenerate modes to produce circular polarization at the first band. The second and third band resonances are achieved at 4.77 GHz and 5.1 GHz respectively due to the loading of SRRs on the slot antenna. The electric fields produced by the single and multiple split gaps in each ring of SRR1 and SRR2 produce CP at the second and third band. All three bands are tuned independently to achieve optimized axial ratio bandwidth. The antenna is fabricated and verified experimentally. The measured results give impedance bandwidth of 64.54% and axial ratio bandwidths of 11.76%, 1.9%, and 3.87% at first, second, and third band, respectively.

Full text

RESEARCH ARTICLE
Compact triband circularly polarized planar slot antenna
loaded with split ring resonators
Puneeth Kumar Tharehalli Rajanna | Karthik Rudramuni |
Krishnamoorthy Kandasamy
Department of Electronics and
Communication Engineering, National
Institute of Technology Karnataka,
Mangalore, India
Correspondence
Puneeth Kumar Tharehalli Rajanna,
Electronics and Communication
Engineeering, National Institute of
Technology Karnataka, Surathkal,
Mangalore, Karnataka 575025, India.
Email: puneeth.tc@gmail.com
Abstract
In this article, a coplanar waveguide (CPW) fed triband circularly polarized
(CP) planar slot antenna loaded with split ring resonators (SRRs) is presented. The
truncated slot antenna resonates at 4.15 GHz, which gives two orthogonal degener-
ate modes to produce circular polarization at the first band. The second and third
band resonances are achieved at 4.77 GHz and 5.1 GHz respectively due to the
loading of SRRs on the slot antenna. The electric fields produced by the single and
multiple split gaps in each ring of SRR1 and SRR2 produce CP at the second and
third band. All three bands are tuned independently to achieve optimized axial ratio
bandwidth. The antenna is fabricated and verified experimentally. The measured
results give impedance bandwidth of 64.54% and axial ratio bandwidths of
11.76%, 1.9%, and 3.87% at first, second, and third band, respectively.
KEYWORDS
axial ratio, circular polarization, slot antenna, split ring resonator
1|INTRODUCTION
Circularly polarized (CP) antennas are preferred over
linearly polarized antennas in mobile wireless communi-
cation systems because of their supremacy in the modern
era.
1
Mobile applications require compact circularly
polarized antennas which operates multiple bands simul-
taneously. Many dual-band CP antennas are designed
using patches and slots.
2,3
Planar slot antennas with the
loading of parasitic elements like patches, stubs and slots
are designed to achieve dual band CP.
4-7
The stubs and
slot loaded designs have complex feeding mechanisms to
achieve circular polarization. To effectively utilize antenna
designs in wireless applications, it is required to design a
simple feeding structure. The compact antennas with sim-
ple feeding structures are required to fit the antenna into
mobile devices. Dual band antennas are designed with
compactness by loading of strips with vias,
8,9
but the fabri-
cation process becomes complicated. Another solution to
achieve dual band CP is using metamaterials. Metasurfaces
(MTSs) are the special class of metamaterials with sub-
wavelength metal or dielectric patches. Some dual-band
CP antennas are designed using electromagnetic band gap
(EBG) structure,
10
high impedance surface (HIS),
11
slot
based metasurface,
12
and stacked metasurfaces.
13
In MTS
based designs, the antenna structure is large and losses
will be more due to metasurface patches. The multiband
antenna with a single CP bandisproposedinReference
14 and triband CP antenna is proposed in Reference 15.
In Reference 16, a hexagonal slot with L shaped loaded
strips is designed to achieve triple band CP. A triple-
wideband triple-sense CP antenna is designed in Reference 17
based on square slot antenna with L shaped radiator.
In Reference 18, a square slot antenna with unequal u
shaped radiator is proposed to achieve dual band dual
sense CP application. In the above designs, it is difficult
to tune the operating bands independently. Split ring reso-
nator (SRR) and complementary split ring resonator
Received: 27 March 2019 Revised: 7 August 2019 Accepted: 7 August 2019
DOI: 10.1002/mmce.21953
Int J RF Microw Comput Aided Eng. 2019;e21953. wileyonlinelibrary.com/journal/mmce © 2019 Wiley Periodicals, Inc. 1of9
https://doi.org/10.1002/mmce.21953

(CSRR) are the good candidates for the design of multiband
antennas for independent tuning.
19,20
The cavity-backed SRR
loaded crossed dipole antenna is designed in Reference 21 to
achieve dual band CP. CSRR is loaded as superstrate at
open-ended waveguide to achieve dual band CP.
22
Apair
of SRR is loaded at the backside of the slot antenna to
achieve dual band CP.
23
Recently a triband circularly
polarized antennas are presented in References 24 and 25.
Here, the focus of the proposed design is to achieve
triband CP with the independent tuning of each band.
In this paper, A novel single layer CPW fed corner trun-
cated planar slot antenna loaded with a pair of SRR for
triband CP is presented. The perturbed slot generates CP in
the first band and two SRRs produce CP in second and third
band respectively. The bands are tuned independently to
each other. The perturbed slot generates two orthogonal
degenerate modes that produce circular polarization in the
first band. The split gaps in the SRR1 and micro-splits in
SRR2 generate orthogonal electric fields, which are required
to produce circular polarization in the second band and third
band. The designed antenna gives good impedance matching
and axial ratio in all three bands.
2|ANTENNA GEOMETRY AND
DESIGN
The proposed antenna is designed using Rogers 4003C sub-
strate (ε
r
= 3.38 and tanδ= 0.0027) with a thickness of
1.52 mm and it is shown in Figure 1. It consists of a single
layer CPW fed corner truncated slot antenna and a pair of
SRRs. The slot and SRRs are etched on top of the substrate.
The resonance frequency of the slot antenna is calculated by
using Equation (1).
23
fr=c
2Slffiffiffiffiffiffiffiffiffiffiffi
2
εr+1
rð1Þ
where c is the speed of light in air and S
l
=(a+s) is the
length of the square slot.
The SRR has radii of R
1
and R
2
(R
2
=R
1
-c-d) forming
the outer and inner rings with a strip width of c. The spacing
between rings is d and the similar gaps in both rings are
g
1
=g
2
. The SRR
1
resonates at 4.77 GHz with parameters
given by R
1
= 3 mm, R
2
= 2.4 mm, c= 0.5 mm,
d= 0.2 mm, g
1
=g
2
= 0.7 mm. The resonance frequency
26
of the rotated SRR is given by
f0=1
2πffiffiffiffiffiffiffiffiffiffiffiffi
LtCeq
pð2Þ
where, C
eq
=((π+q)
2
-θ
2
/2(π+ q)) r
avg
C
pul
,q=C
g
/(r
avg
C
pul
), C
g1
=C
g2
=C
g
is the split gap capacitance of the
rings, r
avg
is the uniform average dimension of the inner and
outer ring of SRR, C
pul
is capacitance per unit length
between inner and outer rings of SRR, θis the angle of
FIGURE 1 Geometry of the proposed antenna W
g
= 70 mm,
S= 28 mm, a= 10 mm, W
f
= 3 mm, g= 0.7 mm, L
f
=33mm
FIGURE 2 Simulated Reflection
coefficient and axial ratio of truncated slot
antenna for different values of a. A,
S11. B, Axial ratio
2of9 THAREHALLI RAJANNA ET AL.

rotation of the inner ring with respect to the outer ring and L
t
is
the total equivalent inductance of circular rings with circumfer-
ence l = 2πr
0
-g having thickness c
1
=c
2
=cis given by,
Lt=0:0002l2:303log10
4l
c
−2:451

ð3Þ
The capacitances C
1
and C
2
are equal when two split
gaps of SRR are aligned in the same axis. When the inner
ring is rotated 90
0
with respect to the outer ring, the values
of C
1
and C
2
become unequal. C
1
and C
2
are calculated
as, C
1
=(π-θ)r
avg
C
pul
and C
2
=(π+θ)r
avg
C
pul
.The
micro-splits
27
are etched on SRR2 to achieve the third band
with CP. The micro-split gap dimensions are g
3
=g
4
=
g
5
=g
6
=g
c
= 0.3 mm. The split gaps, which are orthogo-
nal to each other with respect to the conventional axis of
SRR generate orthogonal electric fields that are required
for circular polarization. The gaps are optimized to achieve
good axial ratio at the second and third band.
FIGURE 3 Simulated reflection
coefficient and axial ratio of antenna
design process. A, Reflection
coefficient. B, Axial ratio
FIGURE 4 Simulated Electric field
distribution at 4.4 GHz. A, ωt=0
0
.B,
ωt=90
0
. C, Surface current distribution of
SRRs at 4.77 GHz and D, 5.1 GHz
THAREHALLI RAJANNA ET AL.3of9

3|OPERATING PRINCIPLE
The corner truncation for CP wave in microstrip patch anten-
nas is common in the literature.
14
The proposed work pre-
sents a truncated slot antenna with a pair of SRRs for triband
CP. The truncated slot antenna generates two orthogonal
degenerate modes to produce CP in the first band by opti-
mizing the truncation. The simulated reflection coefficient
and the axial ratio of the slot antenna for different trun-
cated values are shown in Figure 2. The axial ratio of the
slot antenna increases by increasing the truncation of the
slot. The value of truncation is optimized to a= 10 mm,
further increase of truncation deteriorates the axial ratio
value. The SRRs are placed at the two diagonal corners of
the slot antenna and it is excited by the axial magnetic
field of the slot gives second and third band respectively.
The design process of the proposed antenna is shown in
Figure 3.
The CPW fed perturbed slot antenna (case a) resonates
at 4.15 GHz with circularly polarized waves. The axial
ratio plot in Figure 3B shows that the perturbed slot
generates CP at its resonance frequency. The corner-
perturbed slot with a rotated SRR (case b) resonates at
4.77 GHz produces circular polarization at the second
band. To achieve triband operation, a micro-split SRR is
placed at another corner of slot antenna (case c), which
resonates at 5.1 GHz and achieves CP in the third band.
The electric field distribution of the truncated slot antenna
at different instances of time is shown in Figure 4. From
the figure, it is observed that the truncated slot antenna
radiates LHCP wave in the +z direction. To get RHCP
wave, the corners should be truncated at the opposite side
of the slot antenna. The surface current distribution of
SRRs at 4.77 GHz and 5.1 GHz are shown in Figure 4C,D.
The two SRRs are also radiating LHCP wave at their
corresponding resonance frequency. The optimized perfor-
mance of axial ratio in all the bands is achieved by inde-
pendently tuning the slot and SRRs modes.
The equivalent circuit of a pair of SRR loaded slot
antenna
26
is shown in Figure 5A. Where R
s
and G
s
are the
losses in the feed and substrate. L
s
is feed inductance, C
s
is
feed gap capacitance, L
sh
and C
sh
represent the slot. The
simplified equivalent circuit of the slot antenna loaded with
SRRs is shown in Figure 5B. L
e
’and C
e
’are the effective
equivalent inductance and capacitance of the SRR
1
.L
n
’and
C
n
’are the effective equivalent inductance and capacitance
of the SRR
2
.M
1
and M
2
are the mutual coupling coefficients
between the slot and SRRs. The calculated resonance fre-
quency of SRR using Equation (2) is f
0
=4.65GHz. The
corresponding values of L
e
’,C
e
’,L
n
’,andC
n
’are 9.14 nH,
0.128 pF, 9 nH, and 0.103 pF, respectively. The reflection
coefficient comparison between EM simulation and circuit
FIGURE 5 Equivalent circuit
modeling of proposed antenna. A,
SRR loaded CPW fed slot antenna. B,
Simplified equivalent circuit of
proposed antenna. C, The simulated
reflection coefficient comparison
between circuit simulation and EM
simulation of the proposed antenna
4of9 THAREHALLI RAJANNA ET AL.

simulation is shown in Figure 5C. It is observed from the
figure that the simulations are in good agreement in each
frequency band. The values of L
sh
and C
sh
are 0.13 nH and
1.37 pF.
The parametric study of the proposed antenna to analyze
the sensitivity of parameters is shown in Figure 6. The
results show that the value of truncation plays a vital role in
achieving optimized axial ratio in the first band. The ring
radius and micro-split gaps in the ring are responsible for the
tuning of a second and third band. The variation in reflection
coefficient and axial ratio for S
l
is shown in Figure 6A,B.
The resonance shifts due to the variation in the electrical
dimension of the slot antenna. There is a substantial varia-
tion in the axial ratio of the first band due to the perturbation
effect, but the second and third bands are not affected by this
perturbation. The reflection coefficient and axial ratio due to
the variation of the ring radius are shown in Figure 6C,D. It
is noticed from the figure that, there is a significant variation
in both impedance and the axial ratio of the second band due
to the variation of electrical dimensions of the ring. The first
and third bands are not affected by SRR
1
. The gap variation
of micro-split SRR changes the resonance and axial ratio of
the third band and it is shown in Figure 6E,F. This variation
is due to the change in the capacitance value of the micro-
splits. There is no significant effect on the slot and SRR1
resonance due to these micro-splits. This shows that the
bands can be tuned independently to desired resonance with
SRR dimension and micro-splits.
FIGURE 6 Simulated reflection
coefficient and axial ratio due to variation
of S
l
,R
1
and split gap g
c
.A,S
11
due to
variation of S
l
. B, Axial ratio due to
variation of S
l
.C,S
11
due to variation of
R
1
. D, Axial ratio due to variation of
R
1
.E,S
11
due to variation of gc
.
F, Axial
ratio due to variation of g
c
THAREHALLI RAJANNA ET AL.5of9

4|EXPERIMENTAL RESULTS AND
DISCUSSION
The proposed triband CP antenna is fabricated to verify the
simulated design as shown in Figure 7A and its measure-
ment set-up is shown in Figure 7B. The comparison is made
between simulated and measured impedance bandwidth
(S
11
<−10 dB), axial ratio bandwidth (AR < 3 dB) and
broadside gain. The simulated and measured reflection
coefficients are shown in Figure 8A and the simulated and
measured axial ratios are shown in Figure 8B. The simulated
impedance bandwidth of 60.44% is achieved from 2.84 GHz
to 5.30 GHz and the measured impedance bandwidth of
64.54% is achieved from 2.77 GHz to 5.41 GHz. The simu-
lated and measured axial ratio bandwidth of 13.15%
(3.55 GHz-4.05 GHz) and 11.76% (3.6 GHz-4.05 GHz) is
achieved at the first band, 2.33% (4.66 GHz-4.77 GHz) and
1.9% (4.67 GHz-4.76 GHz) is achieved at the second band,
FIGURE 8 A, Simulated and
measured reflection coefficient. B,
Simulated and measured axial ratio
FIGURE 9 Comparison between
simulated and measured gain and
simulated radiation efficiency A, gain
and B, radiation efficiency
FIGURE 7 A, Prototype of the
fabricated antenna. B, Radiation pattern
measurement set-up for the proposed tri-
band CP antenna
6of9 THAREHALLI RAJANNA ET AL.

4.47% (5.03 GHz-5.26 GHz) and 3.87%(5.06 GHz-5.26 GHz)
is obtained at the third band. The simulated and measured
broadside gain is shown in Figure 9a. The broadside peak
gain of 2.88 dBic at the first band, 1.96 dBic at the second
band and 2.96 dBic at the third band is achieved at
3.9 GHz, 4.7 GHz, and 5.2 GHz, respectively, with 1 dB
variation over axial ratio bandwidth. The simulated radia-
tion efficiency of around 58% at the first band, 52% at the
second band and 51% at the third band are obtained and
the graph is shown in Figure 9b. The comparison is made
between simulated and measured radiation patterns of the
proposed triband CP antenna in both xz and yz planes are
shown in Figure 10. The figure shows that the simulated
and measured radiation patterns are bidirectional and well
in agreement with co-pol and x-pol levels in all the bands.
The RHCP level is −12 dB below LHCP level in the first
band, −20 dB at the second and third band respectively.
The LHCP is obtained at an inclined angle of 20
0
,20
0
and 32
0
with respect to broadside direction at the first,
second and third band respectively. This is due to the
truncation effect of the slot antenna and gap capacitance
of the micro-splits. The comparison table of the proposed
antenna with previous literature is shown in Table 1.
5|CONCLUSIONS
A triband circularly polarized planar slot antenna loaded
with a pair of SRRs is proposed. The truncated slot achieves
CP in the first band and the CP is obtained at the second
band due to the presence of SRR
1
. The micro-splits etched
FIGURE 10 Simulated and
measured radiation pattern of
proposed antenna at 4.15 GHz,
4.77 GHz, 5.1 GHz. A, xz plane. B,
yz plane. C, xz plane. D, yz plane. E,
xz plane. F, yz plane
TABLE 1 The performance comparison of the proposed antenna
Reference Size Impedance bandwidth (%) Axial ratio bandwidth (%) Gain(dBic)
Independent band and
polarization sense tunability
5 0.201λg× 0.201λg2.3, 7.3 0.62, 1.41 6.4, 7.9 No
7 0.42λg× 0.42λg0.69, 5.53 3.79, 1.43 1.43, 5 No
17 1.25λg× 1.25λg44, 70.9 35.9, 44, 6.3 6, <4.2 No
23 1.48λg× 1.48λg13.15, 14.88 3.1, 4.2 5.9, 6.2 Yes
28 0.68λg × 0.65λg 70.6 12, 10, 4.4 <3.13 No
29 0.48λg× 0.48λg3.37, 4.66, 7.49 0.625, 1.25, 0.63 4.2, 5.5,4 No
30 0.41λg× 0.49λg15.5, 22.5 7.34, 15.81 2.48, 3.09 No
proposed 1.78λg× 1.78λg64.54 11.76, 1.9, 3.87 2.88, 1.96, 2.96 Yes
THAREHALLI RAJANNA ET AL.7of9

on SRR
2
produce CP in the third band. The proposed
antenna has a single layer with simple CPW feeding mecha-
nism so that it is easy to fabricate and integrate with micro-
wave circuit. The resonance frequency and polarization
sense of each band can be tuned independently. This tri-band
CP antenna has good impedance bandwidth with 3 dB axial
ratio bandwidth of 450 MHz, 90 MHz, and 300 MHz in first,
second, and third band, respectively. The proposed antenna
is useful for IEEE 802.11 and Wi-Fi application.
ORCID
Puneeth Kumar Tharehalli Rajanna https://orcid.org/
0000-0002-1633-019X
Karthik Rudramuni https://orcid.org/0000-0002-7109-
5989
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AUTHOR BIOGRAPHIES
Puneeth Kumar Tharehalli Rajanna
received his B.E. degree in telecommuni-
cation engineering and M. Tech degree
in digital communication engineering
from Visveswaraya Technological Uni-
versity, Belgaum, Karnataka, India, in
2009 and 2013, respectively. He is
currently working toward his Ph.D. in electronics and
communication at the National Institute of Technology
Karnataka, Mangalore, India. His field of research
includes metamaterial-based antennas, microwave
antennas, and microstrip antennas
Karthik Rudramuni received his
B.E. degree in electronics and communi-
cation engineering and M. Tech degree
in RF and microwave engineering from
Visveswaraya Technological University,
Belgaum, Karnataka, India, in 2012 and
2015, respectively. He is currently work-
ing toward his Ph.D. in electronics and communication at
the National Institute of Technology Karnataka, Mangalore,
India. His field of research includes metamaterial-based
antennas, Goubau line-based leaky wave antennas, and
microstrip antennas
Krishnamoorthy Kandasamy received
his B.E. degree in electronics and com-
munication engineering from Bharathiar
University, Coimbatore, India, in 2003,
his M.E. degree in communication sys-
tems from the College of Engineering,
Guindy, Anna University, Chennai,
India, in 2007, and his Ph.D. in electrical engineering from
IIT Bombay, Mumbai, India, in 2016. He is currently an
assistant professor at the Department of Electronics and
Communication Engineering, National Institute of Technol-
ogy Karnataka, Surathkal, India. His current research inter-
ests include metamaterials, antenna engineering, microwave
integrated circuits (MICs), and monolithic MICs
How to cite this article: Tharehalli Rajanna PK,
Rudramuni K, Kandasamy K. Compact triband
circularly polarized planar slot antenna loaded with
split ring resonators. Int J RF Microw Comput Aided
Eng. 2019;e21953. https://doi.org/10.1002/mmce.
21953
THAREHALLI RAJANNA ET AL.9of9