Design and development of low profile, dual band microstrip antenna with enhanced bandwidth, gain, frequency ratio and low cross polarization

InternationalJournal of Electronics and Communication Engineering & Technology (IJECET),
International Journal of Electronics and Communication
Engineering 6464(Print), ISSN 0976 – 6472(Online) Volume 1, Number 1, Sep - Oct (2010), © IAEME
ISSN 0976 –
& Technology (IJECET) IJECET
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online)
Volume 1, Number 1, Sep - Oct (2010), pp. 88-98 ©IAEME
© IAEME, http://www.iaeme.com/ijecet.html

DESIGN AND DEVELOPMENT OF LOW PROFILE, DUAL
BAND MICROSTRIP ANTENNA WITH ENHANCED
BANDWIDTH, GAIN, FREQUENCY RATIO AND LOW
CROSS POLARIZATION
Suryakanth B
Department of PG Studies and Research in Applied Electronics
Gulbarga University, Gulbarga
E-Mail: surya_recblk@yahoo.co.in

Shivasharanappa N Mulgi
Department of PG Studies and Research in Applied Electronics
Gulbarga University, Gulbarga
E-Mail: s.mulgi@rediffmail.com

ABSTRACT
This paper presents the experimental investigations carried out for obtaining dual
band operation of an antenna by placing two short circuited stubs along the non-radiating
boundaries of the conventional rectangular microstrip antenna. The frequency ratio is
found to be 1.23. Further, by embedding two parallel slots in the patch and vertical slots
in the stubs, the antenna shows the property of virtual size reduction without changing the
frequency ratio. However by placing slot loaded stub along the radiating edge of the
patch the upper operating bandwidth can be enhanced to 21.13% and frequency ratio to
1.43. This technique also enhances the gain to 12.13 dB and minimizes the cross polar
power level to -20 dB down with respect to co-polar. The enhancement of bandwidth,
gain, frequency ratio and reduction of cross-polar power level does not affect the nature
of broadside radiation characteristics. The design concepts of antennas are presented and
experimental results are discussed.
Keywords: microstrip antenna, dual band, stubs, slots.

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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 1, Number 1, Sep - Oct (2010), © IAEME

1. INTRODUCTION
The microstrip antennas (MSAs) are widely used for the last few years due to
their attractive features such as light weight, low volume, ease in fabrication and low cost
[1]. However, two major disadvantages associated with MSAs are low gain and narrow
bandwidth. The traditional MSAs have typical gain of about 6 dB and bandwidth nearly
2 to 5% [1-2], which restricts their many useful applications. Number of studies has been
reported in the literature for enhancing the bandwidth [3-6] and gain [7-8]. Further in
modern communication systems, such as satellite links or radar communications, dual
band MSAs are more attractive as they avoid use of two separate antennas for
transmit/receive applications. The dual band antennas are realized by many methods
such as by using shorting pins on the patch [9-10], using aperture coupled parallel
resonators [11], reactively loaded patch [12] etc. However, the antennas adopted these
designs have narrow operating bandwidths, usually in the order of 2% or less than that.
But in this presentation enhanced dual band antenna is realized by using short circuited
stubs along the non radiating edges of the conventional rectangular patch. Further the
proposed antennas are also capable for the enhancement of frequency ratio, gain and
reduction of cross polar power level by placing rectangular slots in the patch and stubs
and by loading slots in the stub connected along the radiating edge of the patch, without
affecting the nature of broadside radiation characteristics.
2. DESCRIPTION OF ANTENNA GEOMETRY
The art work of proposed antennas are developed using computer software
AutoCAD-2006 and are fabricated on low cost glass epoxy substrate material of
thickness h=1.4 mm and permittivity εr=4.4. The conventional rectangular microstrip
antenna (CRMA) has been designed using the equations available in the literature [1, 13].
Figure 1 show the geometry of conventional rectangular microstrip antenna which is
designed for the resonant frequency of 9.4 GHz. The antenna is fed by using
microstripline feeding. This feeding has been selected because of its simplicity and it can
be simultaneously fabricated along with the antenna element. Figure 1 consists of a
radiating patch of length L and width W, quarter wave transformer of length Lt and width
Wt, used between the patch and 50Ω microstripline feed of length Lf and width Wf. At

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the tip of microstripline feed, a 50Ω coaxial SMA connector is used for feeding the
microwave power. Figure 2 shows the geometry of dual stub rectangular microstrip
antenna (DSRMA). The two short circuited stubs of length L1 and width W1 are placed
along the centre axis of the non radiating boundaries of the patch. The dimensions of the
stubs are taken in terms of λ0, where λ0 is the free space wavelength in cm corresponding
to the designed frequency of 9.4 GHz. The feed arrangement of Figure 2 remains same as
that of feed arrangement of Figure 1. Figure 3 shows the geometry of dual stub slot
loaded rectangular microstrip antenna (DSSRMA). In this antenna two parallel slots of
length L2 and width W2 are embedded on the patch at a distance of 1 mm from the non-
radiating edges of the patch. Also a slot of length L3 and width W3 is embedded on both
the stubs. The slot in the stub is placed at a distance of 1 mm from the non-radiating edge
(L) of the patch. The feed geometry of this antenna remains same as that of Figure 1.
Figure 4 is the extension of Figure 3. In this antenna a slot loaded stub used in
Figure 3 along the length (L) of patch is also connected along the width (W) of the patch.
The slot in this stub is placed vertically at a distance of 1 mm from upper radiating edge
of the patch. This antenna is named as triple stub slot loaded rectangular microstrip
antenna (TSSRMA). The feed geometry of this antenna remains same as that of Figure 1.
The proposed antennas are fabricated using photolithography process. Table 1 shows the
list of designed parameters of the proposed antennas. The substrate area of the all the
antennas is A=M×N.

Figure 1 Geometry of CRMA Figure 2 Geometry of DSRM

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Figure 3 Geometry of DSSRMA Figure 4 Geometry of TSSRMA

Table 1 Design Parameters of Proposed Antennas
Antenna Dimensions Antenna Dimensions Antenna Dimensions
Parameters in mm Parameters in mm Parameters in mm
L 7.06 mm W 9.89 mm Lt 3.16 mm
Lf 4.10 mm L1 3.99 mm Wt 4.18 mm
Wf 3.16 mm W1 3.16 mm L2 5.06 mm
M 25 mm L3 2.16 mm W2 1.00 mm
N 25 mm W3 1.00 mm

3. EXPERIMENTAL RESULTS
The bandwidth over return loss less than -10 dB for the proposed antennas is
measured. The measurement is taken on Vector Network Analyzer (Rhode & Schwarz,
Germany Make ZVK model 1127.8651). Figure 5 shows the variation of return loss
versus frequency of CRMA. From this figure it is seen that the antenna resonates very
close to its designed frequency of 9.4 GHz. This validates the design concept of CRMA.
From Figure 5 the bandwidth is calculated by using the equation,
 fH − fL 
Bandwidth =  × 100 % (1)
 fC 

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Figure 5 Variation of return loss versus frequency of CRMA
Where, fH and fL are the upper and lower cut-off frequency of the band
respectively when its return loss becomes -10 dB and fc is the center frequency between
fH and fL. Hence by using equation (1) the bandwidth BW1 of CRMA as shown in Figure
1 is found to be 4.4%. The theoretical impedance bandwidth of this antenna is calculated
using [7].

 A× h  W (2)
Bandwidth ( % ) =  ×
λ 0 εr
 
 L

Where, A is the correction factor, which is found to be 180 as per [7]. The
theoretical bandwidth of CRMA is found to be 4.42 % which is in good agreement with
the experimental value.
For the calculation of the gain of antenna under test (AUT), the power transmitted
‘Pt’ by pyramidal horn antenna and power received ‘Pr’ by AUT are measured
independently. The gain G in dB is given by [14],
 Pr   λ0 
(G ) dB = 10 log   - (Gt ) dB - 20 log   dB (3)
 Pt   4π R 

Where, λ0 is the operating wavelength in cm, R is the distance between the
transmitting and receiving antenna and Gt is the gain of the pyramidal horn antenna. With

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the help of these experimental data, the maximum gain G (dB) of CRMA measured in
BW1 using the equation (3) and is found to be 5.26 dB.
Figure 6 shows the variation of return loss versus frequency of DSRMA. From
this figure it is seen that the antenna resonates at two frequencies FL=9.28 GHz and
FH=11.37 GHz. The bandwidth BW2 and BW3 as shown in Figure 6 are found to be
3.54% and 3.52% respectively. The BW2 is due to the fundamental mode of the patch and
BW3 is due to the use of stubs in DSRMA. Hence the use of stubs is effective in getting
dual band operation. The ratio of two resonance frequencies FH/FL is 1.23. Figure 7 shows
the variation of return loss versus frequency of DSSRMA. From this figure it is seen that
the antenna again resonates for dual bands BW4 and BW5 with an impedance bandwidth
of 0.5% and 6.4% respectively. The resonant frequency of BW4 and BW5 are 7.27 GHz
(FL) and 8.98 GHz (FH) respectively. It is clear from this figure that the use of slots in the
patch and stubs does not affect the dual band property of antenna but enhances the upper
band BW5 from 3.52% to 6.4% and decreases the lower band BW4 from 3.54% to 0.5%,
when compared to BW3 and BW2 of Figure 6 respectively. However, the ratio FH/FL
remains same as that of DSRMA. Further from Figure 7 it is seen that the DSSRMA
shifts the resonant frequency FL from 9.28 to 7.27 GHz and FH from 11.37 GHz to 8.98
GHz respectively, when compared to the resonant frequency of DSRMA as shown in
Figure 6. This is one of the useful property of virtual size reduction of DSSRMA.

Figure 6 Variation of return loss versus frequency of DSRMA

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Figure 7 Variation of return loss versus frequency of DSSRMA

Figure 8 Variation of return loss versus frequency of TSRMA
Figure 8 shows the variation of return loss versus frequency of TSSRMA. From
this figure it is seen that the antenna resonates for dual bands. The magnitude of
bandwidth of BW6 and BW7 is found to be 2.69% and 21.13% respectively. From this
figure it is seen that the slot loaded stub used along the radiating edge of the antenna does
not affect much the resonant frequency FL in BW6 but enhances the BW6 from 0.5% to
2.69% and BW7 from 6.4% to 21.13% when compared to BW4 and BW5 as shown in
Figure 7 respectively. The ratio FH/FL is also increases from 1.23 to 1.43. This isolation
ratio is 16.08% better when compared to the literature value [12]. The gain of DSRMA,

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DSSRMA and TSSRMA are measured in their operating bands using the equation 3 in a
similar manner as explained for the measurement of gain of CRMA. The obtained values
are shown in Table 2. From this table, it is seen that the maximum gain of 9.51dB in BW6
and 12.13dB in BW7 is achieved respectively in case of TSSRMA. Hence, TSSRMA is
quite effective in enhancing the gain of antenna when compared to the gain of other
antennas mentioned in Table 2. The various antenna parameters of proposed antennas are
also given in Table 2 for the sake of comparison.
Table 2 Experimental results of proposed Antennas
Antenna Number of Resonant Maximum Bandwidth FL/ FH
bands frequency Gain (dB) (%)
(GHz)
CRMA 1 Fr = 9.11 5.26 4.40
DSRMA 2 FL=9.28 3.72 3.54 1.23
FH=11.37 5.62 3.52
DSSRMA 2 FL= 7.27 4.33 0.5 1.23
FH=8.98 6.16 6.4
DSSRMA 2 FL= 7.27 4.33 0.5 1.23
FH=8.98 6.16 6.4
TSSRMA 2 FL= 7.81 9.51 2.69 1.43
FH=11.17 12.13 21.13
Figures 9-12 show the typical co-polar and cross-polar radiation patterns of
CRMA, DSRMA, DSSRMA and TSSRMA respectively measured at their operating
bands. From these figures, it can be observed that the patterns are broadsided and linearly
polarized. The TSSRMA gives the cross polar power level of -20 dB down when
compared to its co-polar power level, which is minimum among the proposed antennas.

Figure 9 Co-polar and cross polar radiation patterns of CRMA measured at 9.11GHz

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Figure 10 Co-polar and cross polar radiation patterns of DSRMA measured at 11.38
GHz

Figure 11 Co-polar and cross polar radiation patterns of DSSRMA measured at 8.98
GHz

Figure 12 Co-polar and cross polar radiation patterns of TSRMA measured at 7.81
GHz

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4. CONCLUSION
From the detailed experimental study it is concluded that by placing the stubs
along the non-radiating edges of CRMA results into dual band operation with frequency
ratio of 1.23. Further by embedding parallel slots on the patch and slots in the stubs,
antenna gives dual bands with same frequency ratio but shows the property of virtual size
reduction. However by placing slot loaded stub along the radiating edge i.e. TSSRMA the
antenna resonates for two bands with frequency ratio of 1.43. This technique enhances
the bandwidth to 21.13 % and gain 12.13 dB and reduces the cross polar power to -20 dB
down with respect to its co-polar power level. The enhancement of impedance bandwidth
and gain does not change the nature of broadside radiation characteristics. The proposed
antennas are simple in their design and fabrication and they use low cost substrate
material. These antennas may find application in microwave communication systems
particularly in synthetic aperture radar (SAR), where dual bands are required.
ACKNOWLEDGEMENTS
The authors would like to thank Dept. of Science & Technology (DST), Govt. of
India, New Delhi, for sanctioning Vector Network Analyzer to this Department under
FIST project.
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6. Kumar G and Gupta K. C. (2003), “Broad-band microstrip Antennas using additional
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