Indian Journal of Radio & Space Physics
Vol 40, June 2011, pp 166-170
Design and development of complementary-symmetry microstrip antenna for
broadband and high gain operation
N M Sameena$ & S N Mulgi#,*
Department of PG Studies and Research in Applied Electronics, Gulbarga University, Gulbarga 585 106 (Karnataka), India
E-mail: $sameena.nm@rediffmail.com, #s.mulgi@rediffmail.com
Received 6 October 2010; revised 15 April 2011; accepted 25 April 2011
A novel design of complementary-symmetry microstrip antenna (CSM) is presented for triple band operation. The triple
bands are merged together by embedding a H-shaped slot on the ground plane of CSM which gives maximum impedance
bandwidth of 57.66% and a gain of 6.53 dB. The design concept of antennas and experimental results are presented. The
proposed antenna may find applications in WLAN systems.
Keywords: Microstrip antenna, Complementary-symmetry antenna, Slot antenna, Impedance bandwidth
PACS No.: 84.40.Ba
1 Introduction
In the recent wireless local area network (WLAN),
the microstrip antennas (MSAs) have gained
importance because of their significant merits such as
small size, light weight, planar, low cost, etc. However,
the major limitations of MSAs are their narrow
impedance bandwidth and low gain. Several promising
methods are reported in the literature for enhancing the
impedance bandwidth1-6 and gain7. But these methods
either increase the area or thickness of the substrate.
Further, dual or triple band antennas with small
physical size are an oncoming challenge to meet the
needs of integration, cost and efficiency of the
emerging wireless world. Dual or triple band antennas
avoid the use of two separate antennas for transmit/
receive applications. Various techniques are available
to obtain dual and triple band operation8-9. But, these
techniques use non-planar geometries. In view of this,
an effort has been made to have number of resonant
modes, wide impedance bandwidth and gain of
microstrip antenna (MSA) retaining the area, planar
geometry and thickness of the substrate using
complementary structure.
2 Design of antenna geometry
The proposed antennas are designed using low cost
glass epoxy substrate material having thickness (h) of
1.66 mm, area of A×B and permittivity, εr = 4.2. The
artwork of the antennas is sketched using computer
software Auto-CAD 2006 to achieve better accuracy. The
antennas are fabricated using photolithography process.
Figure 1 shows the top view geometry of
complementary-symmetry microstrip antenna (CSM)
with tight ground plane shielding. The CSM is
designed for a resonant frequency of 4 GHz using the
equations available in the literature for designing
rectangular MSA10. The length and width of
rectangular patch are L and W, respectively. The self
complementary-symmetry geometry is embedded on
the rectangular patch. The CSM structure is antisymmetrical along its centre axis. The metallic part of
CSM is drawn by black shade as shown in Fig. 1. The
complementary C-slots are placed at the centre along
the length of rectangular patch. The dimensions of Cslot are taken in terms of λ0; where, λ0 is the free
space wavelength in cm corresponding to the
designed frequency of 4 GHz. The thickness of C-slot
and complementary C-slot is Tc with vertical length
Vc and horizontal width Hc. The feed arrangement
consists of quarter wave matching network of length
Lt and width Wt between radiating patch and
microstrip feedline of length Lf and width Wf.
An H-shaped slot is placed in the ground plane of
CSM as shown in Fig. 2 and this antenna is named as
complementary-symmetry slotted ground plane
microstrip antenna (CSSGM). The length and width
of middle arm of H-slot are Lm and Wm, respectively.
The length and width of side arms are Ls and Ws,
respectively. The H-slot is fabricated by taking
thickness, Ws = Lm = 0.5 mm. The slot less than
0.5 mm thickness is difficult to fabricate. Hence, this
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SAMEENA & MULGI: COMPLIMENTARY-SYMMETRY MICROSTRIP ANTENNA − DESIGN
slot is treated as minimal optimum slot. However, it
can be designed for any dimension greater than
0.5 mm but widening the slot in the ground plane may
cause more back lobes in the radiation characteristics.
The H-slot is placed exactly with its centre below the
centre of the complementary rectangular patch shown
in Fig. 1. The dimensions of H-slot are taken in terms
of λ0. The various design parameters of the proposed
antennas have been listed in Table 1.
3 Results and Discussion
For the proposed antennas, the experimentally
measured results are presented. The impedance
bandwidth over return loss less than −10 dB is
measured on Vector Network Analyzer (Rohde and
Schwarz, Germany make ZVK model 1127.8651).
The variation of return loss versus frequency of CSM
is shown in Fig. 3. From this figure, it is seen that the
antenna resonates at three mode of frequencies: f1 =
3.72 GHz, f2 = 5.12 GHz and f3 = 6.61 GHz with the
corresponding impedance bandwidths of BW1 =
2.54%, BW2 = 3.46% and BW3 = 2.3%, respectively.
The three bands are due to independent resonance of
rectangular patch and two complementary C-slots11.
The first resonance f1 is the fundamental resonance of
the complementary rectangular patch. The second
resonance f2 and third resonance f3 are due to current
along the edges of self-complementary C-slots of
CSM. When the H-slot is placed in the ground plane
of CSM, i.e. CSSGM, the antenna resonates for a
single band BW4. The magnitude of impedance
bandwidth BW4 is found to be 57.66% as shown in
Fig. 4. By inserting H-slot on the ground plane, the
slot couples the energy to the complementary slots
which introduces an additional resonance in between
BW1 to BW3 that in turn merges all the three bands
Fig. 1—Top view geometry of complementary-symmetry
microstrip antenna (CSM)
Fig. 3—Variation of return loss vs frequency of complementarysymmetry microstrip antenna (CSM)
Table 1—Design parameters of proposed antennas
Antenna parameters Dimensions,
mm
Fig. 2—Bottom view geometry of complementary-symmetry
slotted ground plane microstrip antenna (CSSGM)
A
B
h
L
W
Tc
Vc
Hc
50
50
1.66
16.8
23.2
λ0/7.5
λ0/0.85
λ0/1.87
Antenna
parameters
Dimensions,
mm
Lt
Wt
Lf
Wf
Lm
Wm
Ls
Ws
9.6
0.5
9.6
3.2
λ0/15
λ0/7.5
λ0/3
λ0/15
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INDIAN J RADIO & SPACE PHYS, JUNE 2011
shown in Fig. 3 into a single band BW4 as shown in
Fig. 4 (ref. 12).
The far field co-polar and cross-polar radiation
patterns of the proposed antennas are measured in their
operating bands. The typical radiation patterns of CSM
measured at f1, f2 and f3, i.e. at 3.72, 5.12 and 6.61 GHz
of frequencies are as shown in Figs 5, 6 and 7,
respectively. From these figures, it is clear that the
patterns are broadsided and linearly polarized. The
radiation pattern of CSSGM measured at f4, i.e. 4.52
GHz frequency is as shown in Fig. 8. From this figure,
it is clear that the pattern is again broadsided in nature.
The cross-polar power level, in all the cases, remains
below −10 dB when compared to corresponding copolar power levels.
Fig. 4—Variation of return loss vs frequency of complementarysymmetry slotted ground plane microstrip antenna (CSSGM)
Fig. 6—Radiation pattern of CSM measured at 5.12 GHz
Fig. 5—Radiation pattern of CSM measured at 3.72 GHz
Fig. 7—Radiation pattern of CSM measured at 6.61 GHz
SAMEENA & MULGI: COMPLIMENTARY-SYMMETRY MICROSTRIP ANTENNA − DESIGN
169
Fig. 9—Variation of gain versus frequency of complementarysymmetry slotted ground plane microstrip antenna (CSSGM)
Fig. 8—Radiation pattern of CSSGM measured at 4.52 GHz
The gain G in dB of proposed antennas is measured
by absolute gain method13 using the formula:
P 
 λ 
(G) dB=10 log  r  - (G t ) dB - 20log  0  dB ,
P 
 4πR 
 t
where, Pt, is the power transmitted by pyramidal horn
antenna; Pr, the power received by antenna under test
(AUT); Gt, the gain of pyramidal horn antenna; and R,
the distance between transmitting antenna and AUT.
The experimental gain of CSM is found to be 2.55 dB
measured at 3.72 GHz and that of CSSGM is 6.53 dB
measured at 4.52 GHz. Hence, CSSGM gives larger
gain.
Figure 9 shows the variation of gain versus
frequency of CSSGM in its operating band. Stable
gain is obtained for the entire operating band with
maximum value at its resonant frequency (4.52 GHz).
4 Conclusions
From the detailed experimental study, it is
concluded that the proposed antennas use low cost
substrate material and are simple in their design and
fabrication. The triple bands are obtained by
constructing the antenna in the form of CSM which
may find applications in communication systems such
as the global positioning system, vehicular
communication, mobile satellite, 802.11a, HiperLAN,
HiperPAN, cordless phone, 802.15.3, fixed wireless,
etc. Further, these triple bands are merged into a single
band by embedding H-slot in the ground plane of
CSM, i.e. CSSGM which gives maximum impedance
bandwidth of 57.66% and gain of 6.53 dB. The
wideband CSSGM operating from 3.3 to 6.1 GHz of
frequency certainly covers the WLAN frequency band
(2.4 to 5.2 GHz). Therefore, it can be used for almost
all WLAN applications.
Acknowledgements
The authors thank the Department of Science &
Technology (DST), Govt. of India, New Delhi, for
sanctioning Network Analyzer under FIST project to
the Department of Applied Electronics, Gulbarga
University, Gulbarga.
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