Appendix A COMPACT DUAL FREQUENCY MICROSTRIP ANTENNA A.1 INTRODUCTION Compact dual frequency microstrip antennas are getting more and more attention due to the fast developments in the area of communication. A microstrip antenna could be made compact through different methods. Some of the methods involve the use of a shorting pin [110, 119, 120, 121] and some others involve the geometrical modification [97, 115, 143]. In this appendix, we present a dual frequency microstrip antenna by adding a shorting pin to a compact drum- shaped microstrip antenna. The shorting pin provided dual frequency operation along with further reduction of the resonant frequency. The range of frequency ratio of the antenna can be varied by changing the aspect ratio.
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Appendix A
COMPACT DUAL FREQUENCYMICROSTRIP ANTENNA
A.1 INTRODUCTION
Compact dual frequency microstrip antennas are getting more and more attention due to the fast
developments in the area of communication. A microstrip antenna could be made compact
through different methods. Some of the methods involve the use of a shorting pin [110, 119,
120, 121] and some others involve the geometrical modification [97, 115, 143]. In this appendix,
we present a dual frequency microstrip antenna by adding a shorting pin to a compact drum-
shaped microstrip antenna. The shorting pin provided dual frequency operation along with
further reduction of the resonant frequency. The range of frequency ratio of the antenna can
be varied by changing the aspect ratio.
142
A.2 DESIGN DETAILS AND EXPERIMENTAL SETUP
The schematic diagram of the proposed miniature dual frequency microstrip antenna is shown
in Figure A. I. The configuration consists of a drum-shaped patch [143] etched on a substrate of
thick ness h and dielectric constant cr_L is the length, B is the width and W is the central width
of the antenna. The feed point and shorting pin positions are specified in tenus ofcoordinates (x,
TB
1
L
T (XI, y~ (x . y.)W ~ - - - - -.~ - ~SHORTJNG PlN
i- ~PROBEFEED
DRU~DPATCH
FIGURE A.1Geometry of the proposed miniature dual frequency drum-shaped microstrip antenna
The different characteristics of the antenna like resonance frequency, input impedance,
radiat ion pattern, etc., are measured as explained in Chapter 3.
A.3 EXPERIMENTAL RESULTS AND DISCUSSION
It is noted that the maximum reduction in size of the antenna is achieved if the shorting pin is
143
placed at the centre of the radiating edge. With a prudent selection ofthe feed point along the Y-
axis, matching without the excitation of TM IOmode can be achieved. Here, the antenna is found
resonating with the first two frequencies having the same polarisation by eliminating TMlOmode.
N:I:::e>=ozw::>o 750wa::LLwoo:!:...In
508.00
- 1atMODE•• -0.. 2nd MODE
.p'"..........
•...
W/B
N:J:~
>=ozw::>
2250 0wCl:LLWoo:!:"Cc::
'"2000
1.10
FIGURE A.2 Variation offirst and second mode resonant frequencies with central width
Figure A.2 shows the variation of first and second resonance frequencies with central
width W for a typical drum-shaped antenna with length L = 3.8 cm, width B = 2.53 cm, fabricated
on a substrate of Er = 4.5 and h = 0.16 cm. From the graph, it is observed that the first resonance
frequency increases from 675 MHz to 743 MHz and the second resonance frequency decreases
from 2275 MHz to 2043 MHz with decreasing WIB. i.e., the frequency ratio varies from 3.37 to
2.75. The variation of frequency ratio with W is maximum when LIB < 1 and decreases as we
increase the LIB ratio. Figure A.3 shows the frequency ratio variation with respect to WIB, for
144
different LIB values. From the observations it is found that a frequency ratio of about 5 is
achieved when LIB is 0.5. Hence the frequency ratio can be varied by trimming LIB and/or WIB.
ot=<a:>ozw::>owa:u,
5
4
3
6.00
-'.-0-.
.- .....--0-"
LIB =2.0LIB =1.5LIB =1.0L/B=O.S
0.55
W/B
•
1.10
FIGURE A.3 Variation offrequency ratio with central width for different aspect ratios
In a particular drum-shaped antenna configuration, there exists a feed point along the
central line at which both the resonance frequencies can be excited with good matching. When
W= 0.7 B, for the typical antenna mentioned above, the feed point is found at (1.75 cm, 0 cm)
145
when the shorting pin is at (L/2, 0 cm). Figure A.4 shows the return loss variation ofthe antenna.
The antenna is resonating at 701.8 MHz and 2201 MHz. The percentage bandwidths are 1.19%
and 1.59% respectively. Figure A.5 shows the E- and H-plane eo and cross polar patterns at the
central frequencies of the two bands. The gain of the antenna has been studied by using
rectangular microstrip antennas fabricated on the same substrate and resonating at the same
frequencies. For the second resonance, no deterioration in antenna gain is observed till the central
width reaches 0.58 and beyond that the gain decreases. For the first mode, the gain in all the
cases is found less than that of the corresponding rectangular patch antennas. Here the reduced
gain may be compensated by integrating amplifiers on the substrate or by superstrate technique