Leonardo Electronic Journal of Practices and Technologies
ISSN 1583-1078
Issue 25, July-December 2014
p. 72-83
Design and analysis of microstrip slot array antenna configuration for
bandwidth enhancement
Duvvuri Sri RAMKIRAN1, Boddapati Taraka Phani MADHAV1, Nimmagadda HARITHA2,
Ravuri Sree RAMYA2, Kalyani M. VINDHYA2, Sai P. ABHISHEK2 1Associate Professor, Dept of ECE, K L University, Guntur DT, AP, India
2Project Students, Dept of ECE, K L University, Guntur DT, AP, India Emails: [email protected], [email protected],
[email protected], [email protected], [email protected]
Abstract
A bandwidth-enhanced Microstrip slotted array antenna is fabricated on an
electrically-thin substrate to provide an economical solution for
communication applications. To improve the impedance bandwidth of a
rectangular micro strip patch without having any impact on its radiation
characteristics, we perform etching of straight slot which is parallel and near
to top side of the patch. For demonstrating the usefulness of the suggested
technique, a planar array of dimension 6X6 with a side-lobe of 25 dB in both
the E and H-planes are studied and fabricated on the FR4 substrate with 0.787
mm thickness. Simulation studies will be carried out on Method of Moments
based EM Tool and analysis of the model is done by changing different
operational parameters like the width of the slot, array element spacing, and
array pattern. The antenna output parameters will be studied by changing
substrate materials, εr ranging from 2 to 4.4 and complete analysis is
presented in a detailed manner.
Keywords
Array antenna; Bandwidth enhancement; Impedance bandwidth; Method of
moments
72 http://lejpt.academicdirect.org
Design and analysis of microstrip slot array antenna configuration for bandwidth enhancement
Duvvuri S. RAMKIRAN, Boddapati T. P. MADHAV, Nimmagadda HARITHA, Ravuri S. RAMYA, Kalyani M. VINDHYA, Sai P. ABHISHEK
73
Introduction
Microstrip antennas are very useful candidates in communication systems because of
their simple geometry and are relatively inexpensive to design and manufacture [1]. A single
layered FR4 is used as substrate in designing a micro strip antenna array, which is having
dielectric constant of 4.4 and thickness 0.787 mm. We implemented the current models on a
thin substrate. The thick substrate has two disadvantages. One is more energy dissipation by
antenna and other is unwanted surface wave propagation, so which will decrease energy,
efficiency and deteriorate the energy pattern. By increasing thickness of the substrate we can
attain the bandwidth improvement, but there is some limitation on the height of the substrate,
over which the antenna performance will be degraded [2].
Impedance matching will play a vital role in the antenna performance, especially for
the bandwidth improvement. Choosing proper impedance matching network, while
connecting array configuration is very much needed for good radiation mechanism. Because
of the variations, nature of the inherent narrow bandwidth on conventional microstrip patch
antenna is observed. Except for single-feed circularly polarized elements, the resonant
behaviour of the input impedance has an effect on bandwidth limitation and not due to the
radiation pattern or gain variations [3]. To increase the bandwidth by a factor of at least 3.9,
we use an optimally designed impedance-matching network. Therefore, a broadband
impedance matching is proposed to increase the bandwidth [4].
In thick microstrip antennas, probe inductance prevents matching of the patch
impedance to the input connector. The probe inductance can be tuned out with a capacitive
gap. To maintain simplified construction the gap will be etched on the patch surface. The use
of a single probe-compensated feed results in distortion of radiation pattern, high cross
polarization and low efficiency because of higher-order modes and surface-wave generation
[5]. Two-probe feeding is used to overcome these problems and to produce a wide-band
antenna with good radiation pattern control and high efficiency [6].
To double the bandwidth of rectangular micro strip patch antennas, capacitive excited
short circuit parasitic elements are located at their radiating edges. It is shown that the
bandwidth improvement is independent of the coupling capacitance. To produce multiple
resonances we arrange the right angle slot of the long arm parallel to non-radiating edges. The
length of the long arm is more than ninety percent of the patch and short is forty percent.
Leonardo Electronic Journal of Practices and Technologies
ISSN 1583-1078
Issue 25, July-December 2014
p. 72-83
There are many methods to improve Uni-polar bandwidth. The impedance bandwidth can be
raised by producing TM at the basic TM10 mode. Moreover, these slots will disturb the
fundamental mode current division particularly at high frequency [7].
Novel bandwidth of micro strip antennas can be enhanced by loading a U-shaped slot
and a pair of right angled slots. Dimensions required for the U shaped slot and the right-
angled slots can be determined experimentally. The obtained antenna bandwidth when
compared with an un-slotted rectangular micro strip antenna will be as large as about 2.4. The
other simplest technique to increase the impedance bandwidth in this communication is to use
two parasitic straight slots etched parallel and near to its non-radiating edge [8]. This is simple
because manufacturing intricacy, price and time can be reduced. Depending on the slot
configuration, a 6x6 array is used in antenna for communication. These methods is used in
communication sensors and also in inter vehicle communication system. The benefits are light
gravity, low shape, an easy fabrication and low expenses of large scale manufacturing. Planar
arrays afford more symmetrical patterns with lower side lobes and the ability to scan the main
beam toward any point in space. These arrays are mainly used in the communication systems
to transmit the signals with high gain.
Material and method
All the models are designed and simulated using Method of Moments based
electromagnetic solver IE3D tool. FR4 substrate material with dielectric constant 4.4 with
thickness 1.6 mm is used in these designs. Quarter wave transformers are incorporated in the
array models for impedance matching. Basically starting with single element patch,
subsequent models are designed with 2x2, 4x4 and 6X6 array configurations.
Antenna geometry
The array factor for linear array is:
∑=
++−=M
ma dmjkIU
10m )]coscossinsincoscossin(cos)1(exp[)/(I),( θδφθβφθαφθ (1)
Io represents the magnitude of the excitation current at the centre element of the array so
all other Im currents are normalized to the centre element's current.
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Design and analysis of microstrip slot array antenna configuration for bandwidth enhancement
Duvvuri S. RAMKIRAN, Boddapati T. P. MADHAV, Nimmagadda HARITHA, Ravuri S. RAMYA, Kalyani M. VINDHYA, Sai P. ABHISHEK
75
If we have M elements along the x-axis the array factor is given by the following equation
∑=
+−=M
mxxxna kdmjIIU
10 )]cos(sin)1(exp[)/(),( βφθφθ (2)
where Iox is the current of the centre element if an odd number of elements or center
elements, if an even number of elements and we are removing the restriction that the currents
have equal amplitudes. Now, if we have N elements along the y axis with an inter element
spacing of dy and a progressive phase shift βy.
∑=
+−=N
nyyyna kdnjIIU
10 )]cos(sin)1(exp[)/(),( βφθφθ (3)
where Ioy is the current of the centre element if an odd number of elements or centre
elements, if an even number of elements and α=γ=90° and β=0° in equation (1).
For a large array, with its maximum near broad side, the elevation plane half-power beam
width is
]))sincos[cos/(1( 022
0022
002
φφθ −− Θ+Θ=Θ yxh (4)
where Θx0 and Θy0 indicates the half power beam-width of M element and N element
broadside linear array respectively.
Here
U(θ, Ф) = Power density from the planar array, P watt/m2
θ o = Direction of the maximum beam in the elevation plane
Фo = Direction of the maximum beam in the horizontal plane
(θ o, Фo) = direction of the maximum beam scanning in the phase Imn current amplitude in the
mnth element
α = Progressive current phase shift
β = constant phase shift 2 θ / λ
M = Total elements in X-direction
N = Total elements in Y-direction
dx, dy =spacing between adjacent elements in x and y direction
AF(θ,Ф) = Array factor
Leonardo Electronic Journal of Practices and Technologies
ISSN 1583-1078
Issue 25, July-December 2014
p. 72-83
Figure 1a. Single element patch model, 1b. 2x2 Array model
Figure 2. (a) 4X4 array model (b) 6x6 array model
A single element patch is designed by using the method of moments based EM tool
IE3D and after that the array models are constructed for 2x2, 4x4 and 6x6. Quarter wave
transformers are used in the design of arrays for impedance matching. To simulate single
element, 2x2 and a 4x4 array model the computational facilities that are available in our
university is sufficient with the RAM availability, but when the number of elements are
increased we need higher computational facilities to get the result in time. In this method, an
equivalent circuit network was built up by combining the equivalent circuit model of the path
and that of the feeding network. Meanwhile, the mutual couplings among patches are also
considered while incorporating their equivalent circuits. Figures 1 and 2 are showing all the 4
models with their dimensional characteristics.
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Design and analysis of microstrip slot array antenna configuration for bandwidth enhancement
Duvvuri S. RAMKIRAN, Boddapati T. P. MADHAV, Nimmagadda HARITHA, Ravuri S. RAMYA, Kalyani M. VINDHYA, Sai P. ABHISHEK
77
Results and discussion
An antenna should be perfect radiator, rather than perfect absorber. The amount of
radiated power returned back through the port can be calculated for finding return loss at that
resonating frequency. For the resonant frequencies the return loss should be less than -10dB
i.e. S11<-10 dB. Figure 4 shows the return loss curve for all the 4 models and it is observed
that for a 6x6 array model an impedance bandwidth of 16% is attained.
Figure 4. Return loss Vs frequency
VSWR is a function of reflection coefficient, which describes the power reflected
from the antenna. Figure 5 shows the VSWR curve for 4 models and it is observed that all the
models are maintaining 2:1 ratio of VSWR at the resonating frequency.
Figure 5. VSWR Vs frequency
Leonardo Electronic Journal of Practices and Technologies
ISSN 1583-1078
Issue 25, July-December 2014
p. 72-83
Antenna impedance is presented as the ratio of voltage to the current at the antenna’s
terminals. Figure 6 shows the impedance characteristics of the model and it is noted that
almost 50ohms is obtained at the desired frequency.
Figure 6. Z-Parameters for all the models
Antenna gain describes how much power is transmitted in the direction of peak
radiation to that of an isotropic source. By increasing the order of elements it is noted that the
gain of 3, 8, 12 and14dB obtained from the models. A 6x6 array is giving maximum gain of
14dB at the resonant frequency and it can be observed from Figure 7.
Figure 7. Gain Vs frequency
The radiation performance of the entire array is evaluated by the array factor technique
and the array factor is calculated from the scattering parameters of our equivalent circuit
network. Calculated results are presented in Figure 8 for a single element, 2x2, 4x4 and 6x6.
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Design and analysis of microstrip slot array antenna configuration for bandwidth enhancement
Duvvuri S. RAMKIRAN, Boddapati T. P. MADHAV, Nimmagadda HARITHA, Ravuri S. RAMYA, Kalyani M. VINDHYA, Sai P. ABHISHEK
79
Figure 8. Radiation characteristics of the models
Table 1 showing all the antenna parameters like resonant frequency, directivity,
radiation efficiency, gain and 3dB beam width. The efficiency of the antenna is more for 6X6
array model with high gain of more than 12 dB.
Table 1. Antenna parameters for single, 2x2, 4x4 and 6x6 models S.NO Parameter Single element 2 x 2 4 x 4 6 x 6 1 Frequency
(GHz) 1.836 1.84 1.828 1.842
2 Incident Power 0.01 W 0.01 W 0.01 W 0.01 W 3 Radiated
Power 0.00688558 W 0.00597487 W 0.0025073 W 0.00210908W
4 Directivity 6.20227 dBi 12.1549 dBi 17.811 dBi 17.968 dBi 5 Radiation
Efficiency 71.3948% 69.9562% 71.6808% 71.676%
6 Antenna Efficiency
68.8558% 69.7487% 65.073% 71.09%
7 Gain 4.58167 dBi 9.9182 dBi 11.80 dBi 12.02 dBi 8 3db Beam
Width (84.6649, 170.267) deg.
(46.2137, 50.2583) deg.
(22.9598,24.1022)deg. (15.033, 30.8845) deg.
Figure 9 shows the current distribution over the surface of the antenna with its
intensity readings. On the surface of the antenna the orientation of current elements at a
particular frequency is presented with colour scaling. With respect to the current scaling we
can identify the mode of propagation in the current models.
Leonardo Electronic Journal of Practices and Technologies
ISSN 1583-1078
Issue 25, July-December 2014
p. 72-83
Figure 9. Current distribution at resonant frequency
The basic 6x6 array model is modified by placing slots on top edge as shown in the
Figure 10(a) and slot at the centre point on the patch as shown in Figure10(b).
Figure 10. (a) 6x6 array open slot, (b) closed slot model
By placing these slots we observed the change in resonant frequencies of the antenna.
For an open slot model shown in Figure 11 antenna is resonating at 1.6,1.68 and 1.98Ghz with
return loss of -20,-32 and -14dB respectively. From the VSWR curve of Figure 12, 2:1 ratio is
attained at the desired bands for both the models. Gain of 9dB for an open slot model 10dB
for a closed slot model is observed from Figure 13.
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Design and analysis of microstrip slot array antenna configuration for bandwidth enhancement
Duvvuri S. RAMKIRAN, Boddapati T. P. MADHAV, Nimmagadda HARITHA, Ravuri S. RAMYA, Kalyani M. VINDHYA, Sai P. ABHISHEK
81
Figure 11. (a) Return loss Vs frequency of closed slot and open slot models (b) VSWR Vs
frequency of closed slot and open slot models
Figure 12. Frequency Vs gain of 6X6 closed (measured) and open slot (simulated) array
models
Figure 13. Radiation pattern Azimuth cut in 2D and polar planes at 2.1 GHz
Leonardo Electronic Journal of Practices and Technologies
ISSN 1583-1078
Issue 25, July-December 2014
p. 72-83
Figure 14. Radiation pattern Azimuth cut in 2D and polar planes at 1.9 GHz
All the antenna parameters for the 6x6 open and closed slots are tabulated in Table 2.
Table 2. Antenna parameters S NO Parameters 6 x 6 closed slot 6 x 6 open slot 1 Frequency 1.832 ghz 1.672ghz 2 Incident power 0.01 W 0.01W 3 Radiated power 0.00171666 W 5.20438e-005 W 4 Directivity 12.9667 dBi 17.2814 dBi 5 Radiation efficiency 17.678% 0.521003% 6 Antenna efficiency 17.1666% 0.520438% 7 Gain 9.3135 dBi 10.5549 dBi 8 3 db beam width (15.3368, 29.442) deg. (16.7832, 31.6625) deg.
The simulation results are giving the performance characteristics of the model in
virtual environment. Once by attaining radiation characteristics of the model based on
element spacing, then we can optimize the model and fabricate the model.
Conclusions
The designed models are showing excellent gain and directivity at the resonating
frequencies with high radiation efficiency. Bandwidth enhancement of more than 2-3% is
attained when compared with basic models. The proposed models are giving excellent
radiation characteristics with minimum return loss at desired frequencies. Some special
observations from the current study are
We observed from the fig 13 and 14, that the inter element spacing between the
elements for an N element array is increased then the beam width will be decreased.
From this study we observed that for the uniformly spaced arrays the maximum space
length is a half-length to avoid grating lobes.
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Design and analysis of microstrip slot array antenna configuration for bandwidth enhancement
Duvvuri S. RAMKIRAN, Boddapati T. P. MADHAV, Nimmagadda HARITHA, Ravuri S. RAMYA, Kalyani M. VINDHYA, Sai P. ABHISHEK
83
We also observed that the array element spacing is non uniform then aliasing can be
avoided. Particularly if the spacing is not the multiples of each other.
Acknowledgments:
Authors would like to express their deep gratitude towards the ECE department and the
management of K L University for their support and encouragement during this work.
Authors also like to express their thanks to the department of science and technology through
SR/FST/ETI-316/2012 FIST program.
References
1. Constantine A., Balanis, Antenna theory, analysis and design, John Wiley & Sons,
Inc., Hoboken, New Jersey, 2005.
2. Liang H. and Ke W., 24-GHz Bandwidth-enhanced microstrip array printed on a
single-layer electrically-thin substrate for automotive, applications”, IEEE
Transactions on antennas and propagation, 2012, 60(5).
3. Pozar D.M., and Schaubert D.H., Microstrip antennas: the analysis and design of
microstrip antennas and arrays, New York: IEEE Press, 1995.
4. Sze J.-Y. and Wong K.-L., Slotted rectangular microstrip antenna for bandwidth
enhancement,” IEEE Trans. antennas propag., 2000,48(8), p. 1149–1152.
5. IE3D Package14.1 ed. Fremont, CA, Zeland Software, 2009.
6. Mohammad T. I., Mohammed N.S., Norbahiah M., and Baharudin Y., Analysis of
broadband slotted microstrip patch antenna, IEEE Trans, 2008, AP-1-4244-2136.
7. Madhav B.T.P., Pisipati V.G.K.M., H. Khan, Prasad V.G.N.S., K. Kumar P., Bhavani
K.V.L, and Datta Prasad P.V., Microstrip 2x2 square patch array antenna on K15
liquid crystal substrate, International journal of applied engineering research, 2011,
6(9), p. 1099-1104.
8. Wen-Chung L., Chao-Ming W., and Yen-Jui T., Parasitically loaded CPW-Fed
monopole antenna for broadband operation, IEEE Transactions on antennas and
popagation, 2011, 59(6).