Surface waves minimisation in Microstrip Patch
Antenna using EBG substrate
Naveen Jaglan
Samir Dev Gupta
Dept. of Electronics & Communication Engineering
Jaypee Institute of Information Technology
A-10, Sector-62
Noida, India
naveenjaglan1@gmail.com
Dept. of Electronics & Communication Engineering
Jaypee Institute of Information Technology
A-10, Sector-62
Noida, India
samirdevgupta@yahoo.co.in
power to the space waves is enhanced. In this paper properties
of EBG are utilized in substrate design and its band-gap is
obtained with the aid of dispersion diagram [3]. Further the
effect of EBG substrate on mutual coupling is analyzed and
significant improvement in return loss and bandwidth is
achieved.
Abstract— Bandwidth enhancement is a key research area in
design of microstrip patch antennas. Electromagnetic Band Gap
(EBG) structures can assist in bandwidth enhancement by
effectively reducing surface waves excitation in patch antennas.
Unlike normal surface, EBG surface are selective in supporting
surface waves. The same is affected in this paper to design patch
antenna with its resonant frequency lying in the band gap of
EBG substrate. The paper analyses the performance of single
patch antenna and patch antenna array embedded with EBG
substrate. It is observed that the performance of the realized
antenna is found to be better as compared to microstrip patch
antenna using conventional substrates. Simulation results show
improvements in the return loss, gain, directivity, front to back
ratio and the radiation efficiency. Further reduction in mutual
coupling between different antenna elements is also realized.
II.
THE EBG STRUCTURE AND THEIR
PROPERTIES
EBG structure as shown in fig1 consist of a two dimensional
lattice of metal plates conductively connected to a ground
plane by metal-plated vias [4-7].
Keywords— Electromagnetic Band Gap(EBG), Microstrip
Patch Antenna (MPA),Microstrip antenna array,Mutual
coupling,Surface waves,Dispersion diagram
I.
INTRODUCTION
An efficient, wide band small size Microstrip Patch Antennas
(MPA) design is a major challenge. Such antennas due to
some of the advantages such as low-profile, conformability,
low-cost in fabrication and ease of integration with feeding
networks have found extensive usage in wireless
communication systems. MPA suffers from one of the major
disadvantage of narrow bandwidth (<5%), in addition to low
gain (~6dbi) primarily due to propagation of surface waves
[1]. Optimizing the antenna characteristics i.e. broad-banding,
better accuracy & performance, lighter weight, etc can lead to
considerable improvements in the overall system performance.
Surface wave propagation is a serious problem in microstrip
antenna. Surface waves reduce antenna efficiency and gain.
Importantly it limits bandwidth, increase end-fire radiations,
and cross-polarization levels. Also it limits the applicable
frequency range of microstrip antennas. One of the techniques
minimizes surface waves using micromachining technology.
In this technique part of the substrate beneath the radiating
element of the antenna is removed thereby realizing a low
effective dielectric constant environment. The others are the
use of Electromagnetic Band Gap (EBG) substrate or Defected
Ground Structure (DGS) [2]. In all techniques the power loss
through surface waves excitation is reduced and coupling of
978-1-4799-6761-2/15/$31.00 ©2015 IEEE
(a) Top view
(b) Side view
(c) Equivalent Model
Fig 1: Geometry of EBG structures.
L= 0.2
h [ln (
C=
116
) - 0.75]
(1)
(2)
In the equations (1) and (2), w = width of EBG patch; h and r
are the height and radius of the via respectively. For the
values w=4.8mm, h=1.27mm, r = 0.12mm, = 10.2,the values
of L and C obtained are 1.64 pf and 0.58nH respectively. An
EBG structure is designed at a resonant frequency
corresponding to 5 GHz.The reflection phase is defined as the
phase of the reflected electric field when plane waves
normally incident on EBG structure. Although it is not 180°
for a perfect electrical conductor surface and 0° for a perfect
magnetic conductor surface. Within the radiation bandwidth
the reflection coefficient falls between +90° and -90° and the
image current is more in phase rather than out of phase [8-13].
To realize the same, simulations are carried out using Ansoft
HFSS [14]. The substrate under consideration is Rogers
RO3010(tm) with dielectric constant 10.2 and height 1.27 mm.
The incident plane wave on unit cell of EBG structure is
shown in fig. 2. It is observed as shown in fig 3 that it has a
continuously varying phase with value 0o at resonant
frequency.
antennas can easily be solved by replacing their ground plane
by Artificial Magnetic Conductor (AMC) provided the
antenna work in band-gap of AMC or EBG.
Z=
(4)
The impedance of a parallel resonant circuit vide equation (4)
is shown in fig 4.It is a very high value at resonant frequency
.The real component of
( ) for EBG substrate where =
√
Z is due to spacing between the EBG unit cells. Hence these
EBG structures are also known as High Impedance Surface
(HIS).HIS can block the propagation of surface waves, reduce
mutual coupling between different antenna elements, gives
real impedance at resonant frequency re(Z) > 377Ω at resonant
frequency and reduce the thickness of low-profile wire
antennas.
Fig:4 Variation of Impedance with freq.
Fig 2 Simulation set-up for unit cell EBG
Conventional ground planes are generally surface of good
conductor which allows electric charges to flow freely on
them. These electric conductors also support propagating TM
surface waves[15-16].Practically for EBG structures closer to
the resonant frequency there is a stop band for surface waves
(in the region where the propagation factor for the TM wave is
very large but TE waves still cannot propagate). However to
fully characterize the mushroom structure and to conform its
CRLH (combined right/left handed) (fig. (5)), a dispersion
diagram as shown in fig. 6 [3] is utilized.
Fig:3 Variation of reflection phase with freq.
B.W. = (
°-
°)
/
°
(3)
Where
° represents frequency at which reflection phase
is 90°,
° represents frequency at which reflection phase
is 90° and ° is the frequency at which the frequency is 0°.
The approximate bandwidth of the EBG structures (fig (3)
refers) can be defined as shown in equation (3).The
performance of low-profile wire antenna can be enhanced if
their conventional ground plane is replaced by EBG structures
and hence best return loss can be achieved. The problem of
multipath destructive interference caused in low-profile wire
Fig:5 Representation of Reciprocal space with physical Space,
HFSS model to plot dispersion diagram.
In particular a two-dimensional dispersion diagram is desired
in order to observe the unit-cell propagation constants for
different angles of propagation. HFSS’s Eigen mode solver is
used to plot dispersion diagram as shown in fig (5), no source
117
excitations are required for this approach. To generate the
dispersion diagram one has to move along the path given
below of irreducible Brillouin zone shown in fig 5. The path
traced is as given below
Γ-X (px=0°, py=0°→180°)
X-M (px=0°→180°,py=180°)
M-Γ (px,py:0°→180°)
Where, px=phase offset in x-direction, py=phase offset in ydirection. In the figure 6 it is shown that the approximate band
gap of EBG substrate in which no surface wave propagation is
a possible lie between 4-6 GHz.
point at 1.6 mm from the centre. The E and H-plane radiation
pattern realized is shown in fig. 8.The improvement in both
return loss and bandwidth is shown in fig 9 when antenna is
designed with EBG substrate.
Fig 8: E-plane and H-plane radiation pattern with EBG
substrate.
Fig 6: Dispersion Brillouin diagram of mushroom type
EBG structure obtained by HFSS with periodic boundary
conditions.
III.
MPA WITH EBG SUBSTRATE
Fig:9. Variation of S11(dB) with frequency.
The antenna is designed for WLAN (Wireless Local Area
Network) applications at 5.2 GHz (IEEE 802.11a and
HYPERLAN/2).High dielectric constant is used so as to make
antenna compact but at the same time problem of surface
waves is exacerbated. It becomes easy to show clearly the
effect of EBG substrate in suppressing surface waves. Small
bare substrate around the patch is left so as to avoid the shift in
resonant frequency as shown in fig 7.
Table1:Comparision of various antenna parameters for both
conventional and EBG substrate.
S.no.
Antenna
Parameters
1.
2.
Max U
Peak
Directivity(dB)
Peak Gain(dB)
Peak Realized
Gain(dB)
Radiated
power
Accepted
power
Incident Power
Radiation
Efficiency
Front to Back
Ratio
3.
4.
5.
6.
7.
8.
9.
Fig:7 Patch antenna with EBG Substrate
IV.
The parameters of the patch are: Length L=8.2 mm, width
W=12.18 mm, a finite ground plane size of 50 50 mm.
Dielectric material considered for the design is Rogers
RO3010(tm) with =10.2, loss tangent tand=0.0023Np/m,
substrate height h=1.27 mm. A co-axial feed is used with feed
Patch with
Conventional
substrate
0.26051 W/Sr
4.0344
Patch with
EBG
Substrate
0.36352 W/Sr
5.1049
3.4535
3.2737
4.5814
4.5682
0.81145 W
0.89486 W
0.94792 W
0.99712 W
1W
0.85603
1W
0.89744
74.143
243.06
2 2 MPA ARRAY WITH EBG
SUBSTRATE
The improvement in return loss and radiation pattern is
reported here with 2 2 antenna array[1,17-21] which is
designed to operate at different frequency 5.6 Ghz but within
118
the Band gap of EBG substrate. The effect of Mutual coupling
between the array elements is also taken into consideration.
The patch impedance is calculated from the knowledge of Rin.
The edge impedance with mutual coupling with E-Plane
separation of 25 mm and H-Plane separation of 20 mm is
calculated to be as 231.52 ohm. The magnitudes of G , G ,G12
(E-plane),G12 (H-plane) given in eqn (6) to eqn (10) are calculated as
0.0017 mho,3.86 10 mho,1.92 10 mho, 0.0015 mho
respectively. Here self-conductance of Patch and conductance
due to mutual coupling in E-plane and H-plane are considered
to calculate edge impedance. The conductance due to
diagonally opposite patches is avoided which if considered the
edge impedance can further be reduced. The individual
patches are connected using Edge feed and finally co-axial
feed connection is used as shown in fig 10 and 11. The
impedance is calculated with the aid of following formulas.
(5)
Rin =
The patch impedance is calculated from the knowledge of Rin.
Where G and G are self and mutual conductance.
G =
(6)
J
G
L
=
(7)
Z=
The parameters of the patch are: Length L=8.07 mm, width
W=11.13 mm, a finite ground plane size of 50 50 mm.
Dielectric material considered for the design is Rogers
RO3010(tm) with =10.2, loss tangent tan δ = 0.0023Np/m,
substrate height h=1.27 mm.The Edge impedance is calculated
to be as 586.60 ohm using eqn (5) with values of G and G
from eqn (6) and (7) is calculated as 0.0017 mho and 3.86
10 mho respectively.
.
.
(11)
The impedance is calculated with the aid of formula shown
and
above. Where
Where ∇W
log 4
(12)
.
G12 (E-plane) =
The parameter A and B are written as:
.
A=
(8)
G12 (H-plane) =
Rin =
G
G
E
G
H
G
4
and
B=
Standard 50 Ω coaxial connector is used to match with strip
line of width 0.233 mm and length 20.05 mm.The quarter
wave transformer is used for impedance matching between
Patch and 50Ω feed with its impedance √50 231.52
107.59 Ω.The width of strip line used as quarter wave
transformer is calculated as 0.020 mm which is again verified
with ADS momentum LineCalc.
(9)
(10)
Fig 11: Layout of a 2 2 Microstrip Patch Antenna Array
with EBG Substrate.
Fig 10: Layout of a 2 2 Microstrip Patch Antenna Array.
119
V.
REDUCTION OF MUTUAL COUPLING
The factors causing mutual coupling are substrate dielectric
constant, substrate thickness, and distance between patches.
Mutual coupling [22-30] occurs both in terms of surface
waves (the first mode TM0 has zero cutoff frequency and is
always present and its effect is more prominent in E-plane
than H-plane) and in terms of space waves. Hence efforts are
made here to show reduction in mutual coupling in E-plane
using EBG substrate. Surface waves become comparatively
larger than space waves when high dielectric constant and
high thickness of substrate is used. In antenna array with
thicker substrates large mutual coupling severely deteriorates
the antenna performance including side lobe level,
deformation of main lobe, input impedance mismatch and scan
blindness and thus must be reduced.
Fig:12. Variation of S11(dB) with frequency.
Fig 13: E-plane and H-plane radiation pattern with
Conventional substrate.
Fig 15: Mutual coupling with conventional substrate.
The antenna is designed to operate within the band gap of
EBG substrate at 5.2 GHz (IEEE 802.11a) so as to minimize
surface waves. The parameters of the patch are: Length L=8.2
mm, width W=12.18 mm, a finite ground plane size of 70 70
mm. Dielectric material considered for the design is Rogers
RO3010(tm) with =10.2, loss tangent tand=0.0023Np/m,
substrate height h=1.27 mm. A co-axial feed is used with feed
point at 1.6 mm from the centre The E-plane and H-plane
distance is kept constant at 20 mm and improvement in both
return loss and mutual coupling is observed.
Fig 14: E-plane and H-plane radiation pattern with EBG
substrate.
As seen in Fig 12, 13 and 14, a 3dB improvement in return
loss and minimization of back lobes is achieved with EBG
substrate.
Fig 16: Mutual coupling reduction with EBG substrate.
120
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Fig:17 Smn(dB) variation with convenntional and EBG
substrate.
It can be observed from fig 17 that 15 dB improvement
(approximately) in return loss and 2 dB reduction in mutual
coupling at resonant frequency.
VI.
CONCLUSION
a explored and
In this paper basic properties of EBG are
bandgap of unit cell EBG was determinedd using dispersion
diagram .Furthermore, the patch antenna waas designed within
the band gap of EBG so as to suppress the prropagating surface
waves and improvement in return loss was obtained. A 2 2
microstrip patch antenna array was desiigned considering
mutual coupling between individual antennaa elements in both
E-plane and H-plane. Then the improvementt in return loss and
minimization of back lobes was shown witth EBG substrate.
Finally reduction in mutual coupling beetween individual
antenna elements was reported.
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