Science and Engineering Applications 1(2) (2016) 20-25 ISSN-2456-2793(Online)

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Science and Engineering Applications

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SAEA

Aperture Coupled Stacked Microstrip Antenna Array

Ayushi Agarwal, Amanpreet Kaur*
Department of ECE, Thapar University, Patiala, India
*Email: amanpreet.kaur@thapar.edu

ABSTRACT
This paper presents a novel design of stacked microstrip antenna array for MIMO systems. An aperture coupled
feeding technique with rectangular slots has been used to ensure equal coupling to each radiating slots. The
antenna system is a layered structure consisting of two driven patches and two parasitic patches. The proposed
antenna uses FR4 substrate with dielectric constant 4.4 and height 1.57 mm. The designed structure covers A
BROAD wireless band from 5.59 GHz to 5.86 GHz with impedance bandwidth of 270 MHz. Antenna shows gain
of 6.828dBi and CORE;ETOION COEFF of 0.35. These characteristics make this antenna suitable for wireless
MIMO communications. To deploy the antenna in MIMO system S11, diversity gain, envelope correlation
coefficient (ECC) and capacity analysis is also carried out.
Keywords: Stacked, aperture coupled, MIMO, S11, diversity gain, ECC, capacity, CST MWS’14, MATLAB r’11.
Received on: 8/7/2016 Published online on: 1/8/2016

With stacking, aperture coupling is used to further improve the

1. INTRODUCTION performanceof the antenna array system [2, 3].
Antenna is one of the most vital components of any With the increasing demand of data rate in wireless
wireless communication system. They behave as transducers and communication field, it is necessary to design small as well as
convert the data embedded on a high frequency carrier signal efficient transmitters and receivers.The feed network in aperture
from electrical to electromagnetic waves and discharge those in coupling makes little influence on radiating elements and is
free air for transmission and vice versa for reception [1]. Since suitable as feed points of antenna array [4]. Another advantage
mirostrip antennas have advantage over the other types of of it is the elimination of the probe penetrating through the
antenna of being small in size, ease of fabrication, substrate layer [5].
conformability to planar and non-planar surfaces, and ease of
integration with RF front end circuits,These are preferred
choices for most of the wireless applications. The basic In the aperture coupler adjusting the size of the aperture can
configuration of microstrip antenna consists of a metallic patch control the impedance of the array system [4, 5].
printed on a thin ground dielectric substrate. The radiating
The system, in which the multiple antennas are
element and the feedlines are photo etched on the dielectric
employed at the input and the output, is known as multiple
substrate [1]. The microstrip patch is designed so that by
inputmultiple output (MIMO) system. Multiple antennas
excitation beneath the patch its pattern maximum is normalto the
increase the capacity proportionally [6].
plane of the antenna.
The current work focuses on for employing antenna in
Apart from the advantages provided by the microstrip
MIMO system by using an array of MSA to designed and
antenna, one disadvantage to be overcome is low bandwidth.
simulated for parameters like diversity gain, and ECC. In context
Thus to provide greater bandwidth to support a good data rate,
to the same, in this paper the design for an aperture coupled
there are different techniques available like U-slot technique [2],
antenna is described and simulated results for the S11, radiation
probe-feeding, slit loading etc. in this paper bandwidth was
pattern, ECC, diversity gain, and capacity of the antenna array
improved by stacking parasitic patches over the driven patches.
system are presented.

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 Ayushi Agarwal and Amanpreet Kaur, Science and Engineering Applications 1(2) (2016) 20-25

The paper is organised as follows section II gives the The basic design of the proposed structure is an array
brief about the MIMO system model. Section III describes the of two antennaswith stacked patches and fed using aperture
antenna geometry and specifications. Next sections are dedicated coupling feeding technique.The antenna array consists of three
layers as shown in figure 2. The length and width of ground plane
to the discussion of the simulated results and the conclusions.
and substrate is 7.6cm X 3.6cm. All the parametric details of the
antenna are mentioned in table 1 for reference.
On the top and middle layers array of two rectangular
2. SYSTEM MODEL
patches is photoetched. The array of top patches act as radiating
This array system is designed to deploy at the receiver patches. Both the patches are excited respectively with two
side. A single input multiple output system is considered. The feedlines that are energised with the separate ports 1 and 2 as
channel is assumed to be quasi-static Raleigh flat fading between shown in figure 2. The dimensions of the patches were optimised
the transmitter and the receiver. We also assume that the transmit so that all the four patches resonate at a centre frequency of 5.8
antenna has no state information of the channel i.e. CSI and only GHz. Values of the various parameters are calculated using
the receiver knows about the actual realization of the channel. transmission line equations [1]. All the calculated parameters are
Thus, uniform power allocation among the transmitter is presented in table 2. The overall dimension of the antenna is 7.6
reasonable and deployed. X 3.6 X 4.78 cm3.

Diagrammatically system can be represented as:

Fig.2. Side View of the Proposed Antenna Array

Two rectangular shaped defected ground slots (DGS)

Fig.1. 1X2 System Model are etched on the ground plane for back radiation reduction [2],
which is sandwiched between the lower and middle substrate.
For a narrowband MIMO channel, when CSI is not The antenna is fed using aperture coupling feed method in which
known at the transmitter, the capacity is given by [7]: the feedline is etched on the lower part of the lowest
substrate.The next section presents the simulated results of the
𝐻𝐻 + antenna in terms of its bandwidth gain, diversity gain, radiation
𝐶 = 𝐸 [𝑙𝑜𝑔2 𝑑𝑒𝑡 (𝐼𝑛𝑟 + 𝜌 )] (1) pattern and correlation coefficient.
𝑁

Where 𝜌 is the average SNR at receiver and H is the

NtXNrchannel matrix. Given by:

H= [h11 h12]

N=min(Nt,Nr). Ntand Nrare the number of transmitting and

receiving antennas respectively. More often each element of H
is taken to be i.i.d complex Gaussian distributed random variable
signifying that each pair of transmitters and receivers
experiences independent fading. But, this is not true in practical
Fig.3. Front View of the Ground-plane
situations. Because of spacing and mutual coupling between the
elements, independent fading is not a valid assumption. This TABLE 1: Patch Antenna Specifications
kind of system is acceptable in many applications of C-band
MIMO system like WLAN, WIFI. To allow the antenna to be
easily embedded inside the devices used for communication, a Parameter Value
MSA is a good choice. The proposed antenna array for such Substrate Material FR4
systems is discussed in the next sections.
Dielectric constant 4.4

3. ANTENNA GEOMETRY AND Loss Tangent 0.009

SPECIFICATION Substrate Thickness 1.57mm

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 Ayushi Agarwal and Amanpreet Kaur, Science and Engineering Applications 1(2) (2016) 20-25

Substrate Width 76 mm

Substrate Length 36 mm

Lower patches width 17 mm

Lower patches length 11 mm

Upper patches width 17 mm

Upper patches length 11 mm

Aperture width 12 mm

Aperture length 5 mm

Feedline Length 20 mm

Feedline width 2 mm

Fig.5. S11 of Various Iterations done on the Driven Patch

4. RESULTS AND DISCUSSIONS When the patches were optimised the slot was iterated. The
results of iterating the slot is given in figure 6.
All the simulations related to antenna array designs
were carried out using CST MWS’14. All the four patches were
optimized for the desired results. The results are presented in this
section.

4.1. S11(dB):

The S11 graph of an antenna represents its impedance

bandwidth with respect to a return loss of less than -10dB. Figure
4 shows the return loss of a stacked antenna.It covers an
impedance bandwidth of 132.4 MHz around 5.8GHz with S 11 of
-15dB.

Fig.6. S11 of Various Iterations done on the Aperture

The figure 7 represents the plot of simulated array’s S11

on the Y-axis with the frequency on the X-axis. The antenna
resonates at 5.8 GHz with the S11 of -29 dB covering the
frequency range of WLAN. Due to the presence of many
elements when their resonances are close they produce a larger
bandwidth. The structure shows an impedance bandwidth of 270
MHz. Bandwidth and capacity is related to each other according
Shannon theorem. As antenna array is offering greater
Fig.4. S11 of Single Antenna Element bandwidth, hence supports a greater capacity.

Various antenna parameters were optimized for the desired

results. The iterations done on the driven patch are shown in
figure 5.

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 Ayushi Agarwal and Amanpreet Kaur, Science and Engineering Applications 1(2) (2016) 20-25

Fig.9. Radiation Pattern at Excitation 1

Fig.7. S11 of Stacked Antenna Array

When the two ports are excited simultaneously the

energy from one port is transferred to other port, the parameter
representing this value is known as transmission coefficient.
When then power is transfer from port to port 2 the parameter is
S21. For any MIMO system array this value should be less than -
15 dB that shows an ECC less than 0.5. Figure 8 shows the graph
of S21 of the proposed antenna array. The antenna has S21 of -24
Db with desirable ECC.

Fig.10. Radiation Pattern at Excitation 2

The approximated value of this coefficient is based on

Fig.8. S21 of Stacked Antenna Array a simple closed-form equation and also varies from 0 to 1. Quite
ideal performance for MIMO applications is attained when this
4.2. Radiation Pattern: parameter approximates to zero. For the proposed structure the
approximate value is 0.035. For the desirable result this value
The antenna structure uses two feedlines to excite the
should be less than 0.4 in any MIMO system.
patches simultaneously. When the antenna is energised it
radiates and the 3D plot of the radiation characteristics can be
inferred as the radiation pattern. A radiation pattern defines the
variation of the power radiated by an antenna as a function of the
direction away from the antenna. Fig 9 shows the gain of 6.8dBi
at port 1. And from graph 10 it can be inferred that proposed
antenna shows the gain of 6.8 dBi from exciting port 2.

4.3. Envelope Correlation Coefficient:

In a MIMO antenna array system envelope correlation

coefficient (ECC) represents the influence of different
propagation paths of the RF signals reaching the antenna
elements. When antennas are oriented with less than half
wavelength distance between them then mutual coupling takes Fig.11. Envelope Correlation Coefficient
place which affect the envelope correlation coefficient [13].
For two antenna elements equation of ECC can be
written as [14]:

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 Ayushi Agarwal and Amanpreet Kaur, Science and Engineering Applications 1(2) (2016) 20-25

∗ 𝑆 +𝑆 ∗ 𝑆 |2
|𝑆11 B. Capacity vs Bandwidth:
12 21 22
𝜌 = (1−|𝑆 2 2 2 2
(2)
11 | −|𝑆21 | )(1−|𝑆22 | −|𝑆12 | )
As discussed above power cannot be increased indefinitely so
the next quantity that can be worked upon is bandwidth. The
figure 14 shows the graph of capacity VS bandwidth. Graph
4.4. Diversity Gain: shows that firstly there is an exponential increase in capacity,
after a limit the increase is linear. More bandwidth means more
In a MIMO system, diversity gain gives gain of number of transmissions per second, hence increase in the
transmitted signals that is attained by employing multiple capacity. Maximum achievable capacity by increasing
antennas at transmitter and/or receiver, with respect to single bandwidth is 1.44 times the P/No value.
antennas at both the ends. Figure 12 shows the diversity gain
achieved in the proposed array.

Fig.12. Diversity Gain Fig.14. Capacity vs Bandwidth

5. Capacity of the Proposed Antenna Array System 6. CONCLUSION

5.1. Capacity vs Power: A stacked antenna array is designed and simulated using CST
MWS. To ensure its usage in MIMO system, capacity of the
Increase in the signal power infers splitting the signal level into array system is also evaluated using MATLAB r’11. The array
large number of levels while ensuring low probability of error.
was designed using an aperture coupling feed. The proposed
Hence increase signal power will lead to more capacity. Figure
13 shows the plot of capacity vs power. Equation (3) is used for structure can be used for WLAN wireless application from 5.59
plotting the graph. to 5.86 GHz. Antenna offers a bandwidth of 270 MHz The
dimensions of the patch, and length and width of the aperture
𝑃
𝐶 = 𝐵.∗ 𝑙𝑜𝑔2 (1 + 𝑁 ) (3) was optimized for the desired results. The compact size of the
∗𝐵
𝑜 array makes it desirable for wireless receiver circuits. Antenna
has a gain of 6.828 dBi making it suitable for indoor wireless
applications

REFERENCES
[1] C.A. Balanis, Antenna Theory: Analysis and Design, 2nd ed. United
States of America, John Wiley&Sons Inc., 1997.
[2] R.S Kushwaha, D.K Srivastava, S. Dhupkariya, J.P saini and K.
Prasad, ‘Slot loaded Electromagnetically Coupled Microstrip Line
Fed Microstrip Patch Antenna for Wideband Applications’,IEEE
Conference on Communications and Signal Processing (ICCSP),
2013, 1082-1086.
[3] Serra, A.A.; Nepa, P.; Manara, G.; Tribellini, G. and Cioci, S.: ‘A
wideband dual-polarized stacked patch antenna’. IEEE Antennas
Wireless Propag. Letter, 6 (2007), 141–143.
Fig.13. Capacity vs Power [4] Grag, R.; Bhartia, P.; Bahl, I. and Ittipiboon, A.: Microstrip Antenna
Design Handbook. Artech House, 2001.
[5] I. Park and R. Mittra, ‘Aperture-coupled small microstrip antenna’,
From the graph it can be inferred that capacity is exponentially Electron. Lett., vol. 32, pp.1741 - 1742 , 1996.
increasing with the increase in power. As an ideal case scenario [6] Alper Ocalan, AsumanSavascihabes, Ibrahim Gorgec, OzgurErtug
in this case noise and bandwidth were taken to be fixed. The and ErdemYazgan: ‘Compact Space-Multimode Diversity Stacked
graph was then plotted for capacity VS power (dBm). But the Circular Microstrip Antenna Array for 802.11n MIMO-OFDM
WLANs’, Loughborough Antennas and Propagation Conference,
power cannot be increased beyond certain limits. November 16-17 2009.

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 Ayushi Agarwal and Amanpreet Kaur, Science and Engineering Applications 1(2) (2016) 20-25

[7] Xingjin Lin, Ming Luo, Yang Yang, Kama Huang ‘High Gain
Aperture Coupled Antenna Array’, IEEE, 2012.
[8] H. Alias, M.T. Ali, S. Subahir, N. Ya’acob and M.A. Sulaiman,
‘Aperture Coupled Microstrip Antenna Array Integrated with DGS
and Parasitic Elements’, IEEE Symposium on Wireless Technology
and Applications (ISWTA),September 22-25,2013.
[9] Daniele Pinchera, Fulvio Schettino, ‘A Dual-Polarized Parasitic
Patch Antenna for MIMO Systems’, 39th EuropeanMicrowave
Conference, 2009.
[10] Hassan Elesawy, W. Swelam, AbdalmonemFouda, Ismail M. Hafez,
‘Design of Microstrip Array Antenna for Wireless Communication
Application’, IOSR Journal of Engineering (IOSRJEN), Vol. 3, Issue
12, December, 2013.
[11] Eduardo Rodriguez A, EzdeenElghannai, Roberto G. Rojas and
Roberto Bustamante, ‘Design and Evaluation of an Antenna Array
forMIMO Systems and Handset Wireless Applications’, IEEE, 2015.
[12] Chandan Kumar Ghosh, Susanta Kumar Parui, ‘Design of Triple
Frequency Compact Microstrip Patch Antenna Array for MIMO
Application’, IEEE, 2009.
[13] Matilde Sanchez-Fernandez, Eva Rajo-Iglesias, Oscar Quevedo-
Teruel, M. Luz Pablo-Gonzalez. 2008 Spectral Efficiency in MIMO
Systems Using Space and Pattern Diversities Under Compactness
Constraints. IEEE T 1637-1645.
[14] R. G. Vaughan and J. B. Andersen, “Antenna Diversity in Mobile
Communications,” IEEE Transactions on Vehicular Technology,
Vol. 36, 1987, pp. 149-172.
[15] R. G. Vaughan, “Signals in Mobile Communications,” IEEE
Transactions on Vehicular Technology, Vol. 35, 1986, pp. 133-145.

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