This document summarizes research on the design of octagon shaped slot loaded rectangular microstrip monopole antennas. The antennas are designed to operate across multiple frequency bands from 1-16 GHz and provide virtual size reduction. Experimental results show the proposed antenna operates over 8 bands with a maximum virtual size reduction of 62%. Modifying the antenna design by changing the radius of a circular slot inside the octagon allows operation over 5 bands while further increasing bandwidth and gain. The antennas demonstrate omnidirectional radiation patterns and have potential applications in microwave communication systems.

1. INTERNATIONAL JOURNAL OF ELECTRONICS AND
International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 4, Issue 2, March – April, 2013, pp. 158-164
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OCTAGON SHAPED SLOT LOADED RECTANGULAR MICROSTRIP
MONOPOLE ANTENNAS FOR MULTI-BAND OPERATION AND
VIRTUAL SIZE REDUCTION
M. Veereshappa1 and Dr.S.N Mulgi2
1
Department of Electronics, L.V.D.College, Raichur: 584 101, Karnataka, India
2
Department of PG Studies and Research in Applied Electronics, Gulbarga University,
Gulbarga 585 106, Karnataka, India
ABSTRACT
This paper presents the design and development of octagon shaped slot loaded
rectangular microstrip monopole antenna for multi-band operation and virtual size reduction.
The antenna operates for eight bands of frequencies in the frequency range of 1 to 16 GHz. If
the radius of complimentary circular slot inside the octagonal is changed from 0.6 cm to 0.5
cm the antenna operates for five bands of frequencies without changing the nature of
monopole radiation characteristics. The antenna gives maximum virtual side reduction of 62
% and highest gain of 11.86 dB. The proposed antennas are investigated experimentally and
may find application in microwave communication systems.
Keywords: microstip antenna, monopole, octagonal slot, ominidirectional, virtual size
1. INTRODUCTION
Emerging trends in microwave communication systems often require antennas with
compact size, simple in design, low manufacturing cost and capable of operating more than
one band of frequencies. Owing to its thin profile, light weight, low cost, planar configuration
and easy fabrication, the microstrip antenna is the better choice for these requirements.
Number of investigations have been reported in the literature for dual, triple, and multiband
operation [3-6]. Design and analysis of octagon shaped hybrid coupled microstrip antenna
for multiband operation [7], octagonal microstrip antenna for RADAR and spacecraft
applications [8], CPW- feed octagon shaped slot antenna for UWB application [9], bandwidth
enhancement of wide slot antenna fed by CPW and microstripline [10] etc. The designs of
single feed equilateral triangular microstip antennas with a virtual size reduction up to about
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2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
22 % by embedding cross slots on radiating patch [11], a square-ring microstrip antenna with
truncated corners shows 19 % virtual size reduction [12], double C-slot microstip antenna is
designed and simulated to have a gain of 6.46 dBi and gives a virtual size reduction of 37 %
[13], slotted rectangular microstip antenna has been designed to achieve maximum virtual
size reduction around 50 % [14] etc have been found in the literature. In this paper a simple
technique has been demonstrated to construct the monopole antennas for multi-band
operation, large virtual size reduction and high gain by loading octagon shaped slot on the
patch and changing the radius of circle inside the octagonal slot.
2. DESIGN OF ANTENNA GEOMETRY
The art work of the proposed antenna is sketched by using computer software Auto-
CAD to achieve better accuracy and is fabricated on low cost FR4-epoxy substrate material
of thickness of h = 0.16 cm and permittivity εr = 4.4.
Figure 1 shows the top view geometry of octagonal shaped slot loaded rectangular
microstrip antenna (OSLRMA). In Fig.1 the area of the substrate is L × W cm. On the top
surface of the substrate a ground plane of height which is equal to the length of microstripline
feed Lf is used on either sides of the microstripline with a gap of 0.1 cm. On the bottom of the
substrate a continuous ground copper layer of height Lf is used below the microstripline. The
OSLRMA is designed for 3 GHz of frequency using the equations available for the design of
conventional rectangular microstrip antenna in the literature [2]. The length and width of the
rectangular patch are Lp and Wp respectively. The feed arrangement consists of quarter wave
transformer of length Lt and width Wt which is connected as a matching network between the
patch and the microstripline feed of length Lf and width Wf. A semi miniature-A (SMA)
connector is used at the tip of the microstripline feed for feeding the microwave power. In
Fig.1 octagon shaped slot is loaded on rectangular patch of vertices X. Further a circle of
radius R is loaded inside octagon slot.
Fig: 1 Top view geometry of OSLRMA
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3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
Figure 2 shows the geometry of modified octagonal shaped slot loaded rectangular microstrip
antenna (MOSLRMA). In this figure the radius of circle inside the octagon and is taken as 0.5
cm.
Fig: 2 Top view geometry of MOSLRMA
The other geometry of Fig. 2 remains same as that of Fig.1. The design parameters of the
proposed antenna is shown in Table 1
Table 1
Designe parameters of proposed antenna
Antenna L W Lp Wp Lf Wf Lt Wt X R
parameter
Dimensions 8.0 5.0 2.34 3.04 2.48 0.3 1.24 0.05 0.714 0.6
in cm
3. EXPERIMENTAL RESULTS
The antenna bandwidth over return loss less than -10 dB is tested experimentally on
Vector Network Analyzer (Rohde & Schwarz, Germany make ZVK model 1127.8651). The
variation of return loss verses frequency of OSLRMA is as shown in Fig. 4. From this graph
the experimental bandwidth (BW) is calculated using the equations,
f −f 
BW =  2 1  ×100 % (1)
 fc 
were, f1 and f2 are the lower and upper cut of frequencies of the band respectively when its
return loss reaches – 10 dB and fc is the center frequency of the operating band. From this
figure, it is found that, the antenna operates between 1 to 16 GHz and gives eight resonant
modes at f1 to f8, i.e. at 1.12, 1.29, 2.01, 4.89, 6.29, 7.41, 8.99, and 15.53 GHz. The magnitude
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4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
of experimental -10 dB bandwidth measured for BW1 to BW8 by using the equation (1) is
found to be 50 MHz (4.48 %), 50 MHz (3.90 %), 50 MHz (2.51 %), 80 MHz (1.63 %), 80
MHz (1.26 %), 330 MHz (4.46 %), 1.62 GHz (17.25 %), and 5.32 GHz (40.06 %)
respectively.
The resonant mode at 1.12 GHz is due to the fundamental resonant frequency of the
patch and others modes are due to the novel geometry of OSLRMA. The multi mode
response obtained is due to different surface currents on the patch. The fundamental resonant
frequency mode shifts from 3 GHz designed frequency to 1.12 GHz due to the coupling
effect of microstripline feed and top ground plane of OSLRMA. This shift in fundamental
frequency gives a virtual size reduction of 62.66 % which is 12.66 % large compared to the
literature value [14].
Fig: 3 Variation of return loss versus frequency of OSLRMA
Figure 4 shows the variation of return loss verses frequency of MOSLRMA. It is
seen that, the antenna operates for five bands of frequencies BW9 to BW13. The magnitude of
these operating bands measured at BW9 to BW13 is found to be 210 MHz (17.28 %), 140
MHz (2.90 %), 410 MHz (5.56 %), 1.76 GHz (18.96 %), and 5.40 GHz (40.60 %)
respectively. The resonating modes f1, f2, and f3 of BW1, BW2, and BW3 of Fig.3 are merged
together into single band BW9 as shown in Fig.4. Further from Fig.4 it is clear that, the
MOSLRMA is capable of widening its operating bands when compared to the operating
bands of OSLRMA. The resonant mode at 1.12 GHz is slightly shifts towards higher
frequency side at 1.14 GHz resulting a virtual size reduction of 62 %.
Fig: 4 Variation of return loss versus frequency of MOSLRMA
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5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
The gain of the proposed antennas is measured by absolute gain method. The power
transmitted ‘Pt’ by pyramidal horn antenna and power received ‘Pr’ by antenna under test
(AUT) are measured independently. With the help of these experimental data, the gain (G)
dB of AUT is calculated by using the formula,
P   λ 
(G) dB=10 log  r  - (G t ) dB - 20log  0  dB (2)
 Pt   4πR 
Where, Gt is the gain of the pyramidal horn antenna and R is the distance between the
transmitting antenna and the AUT. Using equation (2), the maximum gain of OSLRMA and
MOSLRMA measured in their operating bands is found to be 9.57, 11.86 dB respectively. It
is evident that, the MOSLRMA is capable of giving lager gain when compared to the gain of
OSLRMA.
The co-polar and cross-polar radiation pattern of OSLRMA and MOSLRMA is
measured in their operating bands. The typical radiation patterns measured at 4.83 GHz and
7.27 GHz are as shown in Fig 5 to 6 respectively. The obtained patterns are ominidirectional
in nature.
Fig: 5 typical radiation pattern of OSLRMA measured at 4.83 GHz
Fig: 6 typical radiation pattern MOSLRMA measured at 7.27 GHz
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6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
4. CONCLUSION
From the detailed experimental study, it is concluded that, the OSLRMA excited
through microstripline feed has been designed for multi-band operation. The antenna operates
for eight bands of frequencies in the frequency range of 1 to 16 GHz and gives virtual size
reduction of 62.66 %. If radius of circle inside the octagonal slot is varied from 0.6 cm to 0.5
cm the antenna operates for five bands of frequencies in which magnitude of each operating
bands are enhanced compared to operating bands of OSLRMA. The MOSLRMA also
enhances the gain when compared to the gain of OSLRMA. In both the cases the antenna
gives ominidirectional radiation characteristics. The proposed antennas are simple in their
design and fabrication and they use low cost FR4 substrate material. With these features the
proposed antennas may find application in microwave communication systems operating in
the frequency range of 1 to 16 GHz.
ACKNOWLEDGEMENTS
The authors would like to thank Dept. of Sc. & Tech. (DST), Govt. of India. New
Delhi, for sanctioning Vector Network Analyzer to this Department under FIST project. The
authors also would like to thank the authorities of Aeronautical Development Establishment
(ADE), DRDO Bangalore for providing their laboratory facility to make antenna
measurements on Vector Network Analyzer.
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