SRR-based Substrates for Microstrip Antennas
Adnan Sondas, Mustafa H. B. Ucar, Yunus E. Erdemli
Electronics & Computer Education Dept.
Kocaeli University
Kocaeli, Turkey
{asondas, mhbucar, yunusee}@kocaeli.edu.tr
Abstract—Microstrip antennas over artificial substrates
composed of split-ring resonator (SRR) elements are introduced
in the paper. It is numerically demonstrated that the proposed
SRR-based substrates provide miniaturization, bandwidth
enhancement, and multi-frequency operation for patch, dipole
and slot arrays.
well-known EM simulator based on the finite-difference
time-domain (FDTD) method. In the paper, optimum
simulation results based on numerous parametric studies are
presented.
Keywords− split-ring resonator elements; artificial substrates;
microstrip antennas; miniaturization; bandwidth enhancement;
multi-frequency operation
In the paper, we first consider miniaturized microstrip patch
antenna (MPA) designs over an SRR-based substrate, offering
multi-frequency improved-bandwidth operation. A dual-layer
SRR structure is then demonstrated to provide bandwidth
enhancement for a printed dipole array. Finally, a dualfrequency printed slot array design over a single-layer SRR
structure is presented.
I.
INTRODUCTION
Multi-functionality plays a major role in today’s communication systems where size is the main limiting factor in
designing integrated transmitter/receiver circuitry. In this
context, electronically reconfigurable printed antennas were
previously considered in polarization-diverse or multi-band
applications [1, 2]. In those studies, antenna element itself is
reconfigured via appropriate switching. Alternatively, one can
reconfigure the substrate or tailor the frequency response of
the substrate to achieve enhanced antenna operation [3]. In
this study, we propose artificial substrates composed of
split-ring resonator (SRR) elements providing miniaturization,
bandwidth enhancement and multi-frequency operation for
microstrip antennas.
SRR elements with inherent μ−negative behavior [4] were
used as building blocks of metamaterial structures in various
filter applications [5, 6]. These elements were also been
utilized as composite substrates providing miniaturization for
patch antennas [7−9]. Recently, a split-ring microstrip antenna
element has been proposed for dual-band WLAN applications
[10]. In this paper, we introduce an SRR-based substrate
offering miniaturization, bandwidth enhancement and multifrequency operation for a microstrip patch antenna. The
frequency-tuning is achieved by means of several metallic
loadings placed appropriately between the rings of each SRR
element. Similar loading configurations were previously
employed in a variety of SRR filter structures [11−13]. In
practice, those loadings can be replaced by on/off switches for
active frequency-tuning. In the paper, we also present a dualfrequency, dual-polarized 2×2 patch array design over the
SRR substrate. Besides the patch, we also consider SRR-based
substrates for printed dipole and slot arrays. The preliminary
results demonstrate bandwidth enhancement for dipoles and
dual-band operation for slots due to SRR elements.
We note that the full-wave analysis of the proposed designs
were carried out using CST Microwave Studio®, which is a
This work has been sponsored by the Scientific and Technological
Research Council of Turkey (TUBITAK) under Project No. 107E198.
II.
MICROSTRIP ANTENNAS OVER SRR SUBSTRATES
A. Microstrip Patch Antenna Design
The proposed MPA/SRR configuration is depicted in
Fig. 1. As shown, the MPA excited by a coaxial feed is placed
over a composite substrate composed of eight SRR slabs
backed by the ground plane (GP). Each SRR slab contains five
SRR elements, and each SRR element includes three pairs of
metallic loadings (s1, s2, s3) appropriately inserted between the
rings (Fig. 2) for frequency-tuning purposes. To achieve
desired antenna performance, the SRR splits as well as the
SRR slabs are placed along the same direction as the E-field
excited on the antenna element.
The return loss characteristics of the MPA/SRR designs are
displayed in Fig. 3. As seen, while the stand-alone MPA has a
resonance around 5.95 GHz, the inclusion of the SRR
substrate (w/o loadings) results in reduced resonance
frequency of 4.4 GHz, providing miniaturization [8] with a
ratio of 1.35. More importantly, the metallic loadings allow
for frequency tuning. As seen in Fig. 3, the patch operation
can be adjusted by an appropriate loading. The loadings also
provide bandwidth enhancement; in particular, the MPA/SRR
with s3 loadings results in 13% S11 bandwidth as compared to
the bandwidth of 5% for the case without loadings (a 50Ω
system with |S11|< −10 dB criterion is considered).
We remark that the pattern/gain characteristics are almost
not altered due to the loaded SRR substrate. In Fig. 4, the
broadside radiation pattern at 5 GHz is displayed for the
MPA/SRR configuration with s3 loadings. Also, as seen in
Table I, the MPA/SRR designs offer a directive gain of 8 dBi
on the average, which is only 1 dB less than that of the
stand-alone MPA. We also note that the computed radiation
efficiencies for all of the MPA designs presented here are
better than 98%.
εr
TABLE I.
r
DIRECTIVE GAIN PERFORMANCE OF THE MPA/SRR DESIGNS
Patch
h
MPA/SRR Designs
W
GP
Feed
L
t
t
s
SRR slabs
d
d
εr
x
y
Figure 1. Proposed MPA/SRR configuration: L=W=21, r=5, h=4.5, t=0.5,
s=3, d=1, (all in mm), εr=2.2.
s1
w
s1
g
s2
s2
L2
L1
g
s3
s3
Figure 2. Proposed tunable SRR configuration: L1=3.5, L2=2.5, w=g=0.25
(all in mm).
0
-5
S11 (dB)
-10
MPA/SRR
w/o loading
-15
MPA
w/o SRR
MPA/SRR
w/ s1
-20
MPA/SRR
w/ s2
-25
MPA/SRR
w/ s3
-30
3
3.5
4
4.5
5
5.5
6
6.5
7
Frequency (GHz)
z
Gain
w/o SRR
5.95 GHz
w/ SRR
4.4 GHz
SRR + s1
4.65 GHz
SRR + s2
4.85 GHz
SRR + s3
5.0 GHz
D0 (dBi)
8.99
7.88
8.03
8.09
8.14
In practice, low-loss and high-isolation surface-mounted
on/off switches can be employed in lieu of those metallic
loadings for dynamic control of the operational frequency. We
also note that besides the vertical coaxial feeding, microstripline excitation was considered in the MPA/SRR designs.
Similar miniaturization as well as frequency-tuning performance yet less bandwidth is observed in the latter case as
compared to the former case.
B. Microstrip Patch Array Design
In the light of the MPA/SRR design presented above, a
dual-frequency, dual-polarized 2×2 patch array was designed.
The array configuration is displayed in Fig. 5. As shown, the
array consists of two subarrays, each consisting of two patches.
The subarray #1 (SA−1) includes larger patches (outer ones
labeled as 1 and 2 in Fig. 5), each backed by four unloaded
SRR slabs for miniaturization purposes. The subarray #2
(SA−2) is composed of smaller patches (inner ones labeled as 3
and 4 in Fig. 5). In order to obtain dual-polarization, the SA−1
and SA−2 are fed accordingly so as to excite polarization along
x−direction and y−direction, respectively. In addition, the
SA−1 and SA−2 are placed appropriately so that potential
parasitic interaction between subarrays would be minimized.
The return loss characteristics of the patch array are
displayed in Fig. 6 along with the corresponding radiation
patterns in Fig. 7. As seen, the SA−1 operates around 3.3 GHz
(S11 & S22), and the SA−2 at 5.5 GHz (S33 & S44), resulting in
dual-frequency operation. As the array offers a directive gain
of 11 dBi on the average, the polarization profile of the array
is clearly observed from the patterns where the SA−1 provides
x−directed and the SA−2 y−directed polarization. We also note
that the array performance presented here is obtained when the
subarrays are excited one at a time. However, similar array
performance is observed in the case of synchronous excitation
of the subarrays.
Figure 3. Return loss charactersitics of the MPA/SRR designs.
f =5 GHz
D0=8.14 dBi
Figure 4. Radiation pattern of the MPA/SRR design with s3 loadings.
C. Printed Dipole Array Design
We now consider printed dipole array (PDA) performance
in presence of a dual-layer SRR substrate. The unit cell
configuration for the infinite-periodic PDA over the SRR
structure is displayed in Fig. 8 along with the parameters of the
SRR layers (namely, SRR−1 and SRR−2) defined in Fig. 9.
The physical parameters of the SRR−2 are chosen to be 1.5
times larger than those of the SRR−1 with noting that the inner
split of the SRR−2 is closed. As shown in Fig. 8, the
concentrically placed SRR elements are located underneath the
probe-feed of the dipole. Unlike the patch configuration
(Fig. 1), the SRR planes now are in parallel to the plane of the
dipole. Similar to the patch excitation, the probe-feed is located
along the same direction as the SRR splits to achieve optimum
array performance.
d
Feed
l
z
Dipole
SRR−1
SRR−2
t
h2
t
h1
y
x
εr
εr
Dx
r
GP
1
L2
Dy
W
L1
d
3
Figure 8. Unit cell configuration of the PDA over dual-layer SRR substrate:
Dx=66.24, Dy=20.7, h1=8.99, h2=2.99, t=0.5, l=59.52, d=1
(all in mm), εr=3.
L1
L
4
d
2
w
W2
g
L1
Figure 5. 2×2 patch array configuration; perspective view (top), top view
(bottom). L=111, W=62, L1=21, L2=32, W2=30, d=10, r=15 (all in mm). All
other physical parameters are same as those in Fig. 1 and Fig. 2.
L2
g
SRR−1
SRR−2
0
Figure 9. Dual-layer SRR parameters: L1=11.4, L2=6.6, w=1.8, g=0.6
(all in mm). SRR−2 parameters are 1.5 × those of SRR−1.
S-parameters (dB)
-5
-10
S33
S44
S11
S22
-15
-20
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
Frequency (GHz)
Figure 6. Return loss characteristics of the 2×2 patch array design.
Fig. 10 displays the real part of the input impedance for the
PDA with and without the dual-layer SRR substrate. As shown,
the impedance peak of the stand-alone PDA at 3 GHz is split
into two peaks around 2.2 GHz and 3.1 GHz due to inclusion
of the SRR substrate. More importantly, after the second peak,
the impedance levels maintain almost constant around 40 Ω
over a 1 GHz band. This almost matched impedance profile
reflects to 23% S11 bandwidth (4.6−5.8 GHz) as can be seen in
Fig. 11 where the return loss characteristics are displayed. In
addition, Fig. 12 shows the radiation pattern (due to only one
unit cell) of the PDA/SRR design at 5 GHz, demonstrating an
almost broadside characteristic with the directive gain of 6.7
dBi per unit cell. Also, the PDA/SRR design provides 1 dB
more gain (on the average) than the stand-alone PDA does over
the band of interest.
We note that many parametric studies were employed to
achieve the optimum PDA design presented here. In those
studies, the critical parameters affecting the array performance
were SRR dimensions, number of SRR layers, distance
between the dipole and the SRR−1, distance between the SRR
layers, placement of SRR elements with respect to the feed,
presence/absence of rings splits, and the cavity height.
f=3.3 GHz
D0=11.5 dBi
f=5.5 GHz
D0=10.9 dBi
Figure 7. Radiation patterns of the 2×2 patch array design.
To understand the role of the SRR substrate in achieving
the improved PDA performance, we carried out additional
analyses based on the study reported in [3]. In [3], it is
demonstrated that bandwidth as well as radiation performance
of a PDA is improved due to an artificial substrate composed
of fan-like frequency-selective surface (FSS) elements. The
improvement is achieved due to relatively-flat (around 0o)
reflection coefficient phase response of the FSS substrate at the
expense of the corresponding amplitude response. When a
printed antenna is placed over such a substrate, the antenna
performance can be enhanced since the reflected field becomes
in phase with that directly radiated by the antenna itself [3].
Here we have considered the SRR substrate providing similar
bandwidth as well gain enhancement as reported in [3]. To
evaluate the reflection performance of the SRR substrate, we
consider two configurations (one without the SRR substrate,
config. #1, and the other with the SRR substrate, config. #2) as
depicted in Fig. 13. Fig. 14 shows the corresponding reflection
coefficient (Γ) characteristics over the frequency band of 3.5−6
GHz. As seen, similar relatively-flat phase (−90o< ∠Γ< 90o)
responses are obtained for both of the structures. However, the
significant difference is observed in the amplitude responses.
As expected, a full-reflection (i.e., |Γ|=1) from the GP is
obtained over the whole band in absence of the SRR substrate.
On the other hand, the SRR substrate provides reflections with
decreasing amplitude response (0.2< |Γ|< 1) over 4.5−6 GHz
band. In fact, this amplitude variation along with the smooth
phase response of the SRR-substrate’s reflection coefficient is
the main reason behind the improved bandwidth as well as
radiation performance of the dipole array.
f =5 GHz
D0=6.7 dBi
Figure 12. Radiation pattern for the PDA/SRR design.
Γ
Γ
Config. #1
Config. #2
εr
εr
t
t
SRR−1
h
h1
z
GP
GP
600
x (y)
Figure 13. Unit cell geometries for the configurations without (left) and with
(right) the SRR substrate; side views are shown. εr=3, h1=8.99, h2=2.99, t=0.5,
h=12.48 (all in mm). Γ represents the total reflection computed at the aperture
level when each structure is excited with an x-polarized incident plane wave.
500
400
Rin (Ohm)
SRR−2 h2
t
εr
PDA
300
1
200
PDA/SRR
0.8
100
Γ
50
0
0.6
0.4
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
Frequency (GHz)
Figure 10. Real part of the input impedance (Rin) for the PDA with and
without the dual-layer SRR substrate.
Config.#1
Config.#2
0.2
0
3.5
4
4.5
5
5.5
6
5.5
6
Frequency (GHz)
180o
90o
0
PDA
S11 (dB)
-5
∠Γ 0o
-90o
PDA/SRR
23%
Bandwidth
180o
3.5
-10
4.5
5
Frequency (GHz)
Figure 14. Reflection coefficient (Γ) characteristics of the configurations in
Fig. 13; magnitude response (top) and phase response (bottom).
-15
-20
1
4
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
Frequency (GHz)
Figure 11. Return loss characteristics of the PDA designs with and without
the SRR substrate.
We note that the reflection analysis presented above was
carried out using a well-validated periodic array simulator
[2, 3] based on the finite-element/boundary-integral (FE/BI)
method in conjunction with the fast spectral-domain algorithm
(FSDA) [14].
D. Printed Slot Array Design
Finally, we consider infinite-periodic printed slot array
(PSA) performance in presence of a similar SRR substrate. In
this case, we propose two PSA designs with single-layer SRR
structures, namely PSA/SRR−1 and PSA/SRR−2, of which the
unit cell configurations are depicted in Fig. 15 and Fig. 16,
respectively. Similar to the dipole configuration, the SRR layer
is in parallel to the plane of the slot, and the probe-feed is
positioned along the same direction as the SRR splits to
achieve desired array performance.
Fig. 17 displays the real part of the input impedance for the
PSA with and without the SRR substrates. As seen, the
impedance peak of the stand-alone PSA around 5.5 GHz is
split into two peaks due to inclusion of the SRR substrate. The
PSA/SRR−1 design has resonance peaks around 3.1 GHz and
5 GHz, while the PSA/SRR−2 design provides resonances
around 2.9 GHz and 5.5 GHz as shown in Fig. 17. Hence, the
proposed PSA/SRR designs offer dual-frequency array
operation. Unlike the dipole design, the slot designs have
impedance levels in the range of 100−300 Ohms, which would
require an additional balun/feed structure [2] to be deployed
within the cavity in order to match with 50Ω system.
h
Slot
εr
GP
Dx
Dy
l
w
L3
g
s
L1
g
L2
Figure 15. Unit cell geometry of the PSA/SRR−1 configuration; perspective
view (top), bottom view (bottom). Dx=31.2, Dy=8.4, h=7.1, t=0.6, L1=6.84,
L2=7.2, L3=4.32, g=0.36, w=1.08, l=29.4, s=0.6 (all in mm), εr=2.2.
Note that the optimum PSA designs presented here are the
products of several parametric studies. In those studies, the
parameters of interest were SRR dimensions, number of SRR
elements, distance between the slot aperture and SRR elements,
placement of SRR elements with respect to the feed, placement
of the splits with respect to the slot as well as to the feed,
presence/absence of rings splits as well as inner/outer ring
elements, and the cavity height. Those parameters primarily
determine impedance levels and frequency separation between
the resonance peaks.
Slot
Feed
SRR
v
CONCLUSION
In the paper, we have introduced artificial substrates
composed of SRR elements that enhance microstrip antenna
performance. In particular, it has numerically been
demonstrated that the composite SRR substrate allows for
miniaturization, multi-frequency operation and bandwidthimprovement for the microstrip patch antenna. Also, a similar
substrate has been employed under the 2×2 patch array,
providing a dual-polarized, dual-frequency operation. In
addition, the dual-layer SRR substrate has been shown to
provide bandwidth-enhancement for the printed dipole array,
while the single-layer SRR structure has offered dualfrequency operation for the slot array. We note that the FDTDbased CST Microwave Studio was utilized in full-wave
analysis of the proposed designs.
The work presented here is a preliminary phase towards the
development of an electronically switchable multi-functional
antenna panel (ESMAP). In the practical implementation,
low-loss and high-isolation surface-mounted switches are
projected to employ in antenna elements and/or in the
SRR-based substrate. We remark that difficulties such as
mounting as well as biasing of the switches would require
careful assessment during implementation phase.
GP
Figure 16. Unit cell geometry of the PSA/SRR−2 configuration: v=7 mm,
all other parameters are same as those in Fig. 15.
350
300
PSA/SRR−1
250
Rin (Ohm)
III.
t
Feed
SRR
200
PSA
150
100
PSA/SRR−2
50
0
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
Frequency (GHz)
Figure 17. Real part of the input impedance (Rin) for the PSA with and
without SRR substrates.
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