A Novel Design of Gap-Coupled Sect oral Patch
Antenna for X-band High Frequency Applications
Nagendra Prasad Yadav
Nanjing University of Science and Technology
School of optical and electronics
Nanjing, China
nagendra1nagendra@gmail.com
Abstract— Using gap-coupling between the elements, antenna of
four sectors by cutting the slots through and through from a
circular patch is designed for application in X-band. The
bandwidth enhancement along with reduced side lobes is
obtained by marginally or fully moving one of the feeding
sectoral patches diagonally away. The results are analyzed for
different positions of the slot. A bandwidth of 1.2 GHz in X-band
along with a reduced side lobe level of -15.0 dB and a gain of 12.0
dBi is obtained. An analysis of mutual coupling and parasitic
patch effect is also presented and discussed in this work.
Index Terms— Gap coupling, impedance, bandwidth, antenna
gain.
I. INTRODUCTION
Narrow bandwidth with low gain and high side lobe level has
been a major concern for the optimum use of the microstrip
antennas. For circularly polarized antennas, axial ratio
bandwidth also needs to be catered for in addition to
impedance bandwidth. A number of techniques have been
proposed in order to improve the characteristics of antennas
such as to enhance gain or widen the impedance bandwidth.
Some of them resort the material side by using a thicker
substrate or bi-layered substrate [1-3] or reducing the
dielectric constant [4]. Gap coupling is one of the techniques
being used by the researchers throughout the world [5-9].
Different types of gap coupled antennas have been proposed
by the contributors. In some applications, multi frequency
operations are required; this can be achieved by gap-coupling
[10]. In [11-12] the experiment is performed on gap-coupled
circular microstrip antennas and mutual coupling is measured.
In contrast to the corporate feed network, gap-coupled method
is a good replacement to achieve wide bandwidth with
increased gain and reduced side lobe level. Using gap coupled
method, the return loss of a mismatched microstrip fed
antenna can be decreased significantly [13]. This work
analyzes if the gap coupled technique may serve in those cases
where the coaxial fed method leads to a low gain patch. There
are other methods applying slots in order to improve the
performance of a single layer microstrip patch antenna provide
motivation for this work. Approaches like rectangular or
circular slots take an undisturbed TM10 or higher as an
essential starting point [14-15]. Gap coupling in broadband
microstrip antennas is therefore discussed in [16] which
describe mostly probe fed constellations. Different multiple
Abhishek Kumar
Dept. of Electronics and Communication
Simla, India
different resonance frequencies lead to wider impedance
bandwidth and improved gain when losses are low with
reduced side lobe level. However, precise general design
procedures are not given due to the complex behavior of the
parameters.
In this paper, we have introduced a novel form of gap coupled
antenna in which we divided the circular patch into four
sectors through gap coupling. Then, exciting one of the sectors
by applying coaxial feeding with increasing gap in between
the feeding patch and rest of the other three sectoral patches.
This will introduce the effect of coupling in between the three
patches. This will result in the increasing bandwidth as we
move the feeding patch away from the other three sectoral
patches. The simulations are performed with the simulation
software CST Studio Suite and for experimental purpose
Vector Network Analyzer is used. The simulated and
experimentally measured results are in good agreement with
each other. A significant comparison is also shown for the
excited sectoral patch at a distance of 2 mm up to a distance of
20 mm. The radiation pattern measured is also presented with
improved gain and reduced side lobe level. It is clearly shown
in the measured results that how the radiation be affected with
the introduction of different gap in between the sectoral
patches.
II. THEORY
Bandwidth enhancement can be achieved by using closely
spaced elements or using parasitic patches. Different
approaches have been implemented in order to improve the
bandwidth of the antenna such as multilayered structures,
single layer multi-resonant designs, aperture coupled designs,
etc. The coupling between the elements is a function of the
patch size and gap between the patches [17-18]. For a fixed
unit cell size, the coupling between the patch elements
increases by reducing the spacing between them. However if
the size of the unit cell is reduced, a closer spacing between
the patch elements is required so as to achieve the same level
of coupling for larger unit cell sizes. In this paper, the parasitic
patches improve the impedance bandwidth of the proposed
antenna design. Only one sectoral patch is excited and the
other three patches acts as parasitic patches. These three non
fed sectoral patches lead to an increase in the resulting
impedance bandwidth. The impedance bandwidth is found to
increase up to a distance of 20 mm from the center of the
circular patch. At a distance less than 20 mm, there are two
resonant modes. But when the gap is maintained at a distance
of 20 mm, the two resonant modes combine and therefore
results in the enhancement of the impedance bandwidth. A
wideband is obtained at this particular gap, after which the
bandwidth starts reducing as the two resonant modes again
separates.
An analysis of mutual coupling is also presented in this
section. In recent times, one of the topics of investigation is
how the mutual coupling relates to the antenna radiation
properties such antenna gain and side lobe level. The large
mutual coupling leads to the reduction in the antenna radiation
efficiency, decrease in antenna effective gain, etc but
adjacently also creates an effective pattern. Theoretical
research has shown that the radiation properties of the printed
antennas [19] and mutual coupling between printed dipoles are
affected by the radiation fields and the type and the number of
surface wave modes propagating within antenna substrate. In
order to enhance the radiation properties the antenna structure
is divided into four sectoral patches using gap coupling. As
the gap between one of the sectoral patches (excited patch)
and the other three parasitic patches is increased, the mutual
coupling starts reducing. This coupling effect results in the
enhancement of the antenna gain, directivity and reduction in
the respective side lobe level which is very good for better
antenna radiation and thus for wide band applications. And
reached a maximum value after which the radiation properties
again start degrading due to a large separation between the
sectoral patches.
III. ANTENNA CONFIGURATION AND DESIGN
The geometry of the antenna proposed is shown in figure (1)
which comprises of a gap coupled sectoral patch antennas.
The proposed gap coupled antenna is printed on an FR4
substrate with permittivity of 4.1 and a loss tangent of 0.024.
The dimension of substrate when four sectoral patches are
unmoved is (50 x 50) mm2 and for moving sectoral patch
antenna is (70 x 70) mm2 as shown in figure 2(a). The
thickness of the dielectric substrate used is h = 1.59 mm. The
circular patch antenna is of radius r = 23.5 mm and is fed
coaxially at 9 mm in order to achieve 50 Ω characteristic
impedance. For the first case, a double gap of 2 mm each is
introduced both vertically as well as horizontally as shown in
the geometry. The circular patch after introducing gap both
horizontally as well as vertically form four regions namely
region-I, II, III and IV. Then the feeding sectoral patch is
moved slowly away by keeping the other three sectors
unmoved. This feeding sector is moved up to a distance of x =
y = 20 mm from the centre at which antenna shows a
maximum impedance bandwidth with maximum gain. The
prototype of the antenna designed is shown in figure (2).
Figure 2(a) shows the top view and 2(b) shows the bottom
view of the proposed antenna design.
IV. RESULTS AND DISCUSSIN
In order to show the improved performance of the antenna
with changing gap between the sectoral patches, it will be
compared through simulations and measurements. The
proposed antenna structure is simulated using a simulation
software CST Studio Suite and experimentally tested using a
Vector Network Analyzer at different gaps. As discussed in
the earlier section, due to the coupling effect in between the
sectoral patches, the impedance bandwidth increases with the
increase in antenna gain as we move the feeding sectoral patch
away from the other unmoved three sectoral patches up to a
certain distance. After which due to larger separation the
antenna characteristics again starts degrading.
A. Return loss measurement at different gaps
B. Input impedance measurements at different gaps
C. Radiation pattern at different gaps
A. Return loss measurement at different gaps
The variation of return loss with the frequency is shown in
figure (3). Figure (3) shows the return loss variation for the
first case when the circular patch antenna is divided into four
sectoral patches. The gap introduced in between the four
sectoral patches is 2 mm each. One of the sectoral patches is
excited by employing a coaxial feed at a distance of 9 mm
from the center of the circular patch. At this first step, all the
four sectoral patches are unmoved. The antenna resonates in
the X-band in the frequency range 9 GHz to 11 GHz. Figure
3(a) shows the simulated return loss variation with frequency
and 3(b) shows the experimentally measured variation. The
simulated resonating frequencies are 9.4 GHz, 9.7 GHz and
10.4 GHz. At frequency 10.4 GHz, the antenna shows a
maximum return loss of -28 dB. The experimentally measured
resonating frequencies are 9.7 GHz and 10.4 GHz. At
frequency 10.4 GHz, the antenna shows a maximum return
loss of -31 dB. But the antenna does not show wideband
characteristics. It gives a minor impedance bandwidth of about
1.6 %. As we have given feed to just one of four sectoral
patches, the other three acts as parasitic sectoral patches. Due
to coupling between the four sectoral patches, it acts as one
antenna as a whole. For the next step, in figure 4(a), the
excited patch is moved in a direction both horizontally as well
as vertically with x = 2 mm and y = 2 mm at an angle of 45
degree. Adjacently the coaxial feed also moved to a distance
of 11 mm. Now, the frequency range changes to 9.3 GHz to
10.3 GHz and the antenna shows a dual resonance. The
resonating frequencies are 9.4 GHz, 9.7 GHz and 10.3 GHz. It
gives an increase in the impedance bandwidth from 1.6 % to
4.2 %. The antenna shows a dual resonance with resonating
frequencies at 9.4 GHz and 9.7 GHz with a maximum return
loss of -36 dB at frequency 9.7 GHz.
When the excited sector is moved further at an angle of 45
degree from the center, the impedance bandwidth goes on
increasing up to a gap of 20 mm of the excited sectoral patch
with feed at 29 mm. With a gap of 4 mm of the sectoral patch
and feed at 13 mm, the resonating frequencies are 9.5 GHz,
9.7 GHz and 10.2 GHz as shown in figure 4(b). The antenna
shows a dual resonance with impedance bandwidth increased
to 5.1 % with a maximum return loss of -45 dB at the
resonating frequency 9.5 GHz. Similarly moving to 15 mm
gap of the sectoral patch with feed at 24 mm in figure 4(c), the
antenna shows a dual resonance at the two frequencies 9.5
GHz and 9.7 GHz with an equal return loss of -23 dB at both
resonating frequencies. The increased impedance bandwidth
reaches up to 6.3 %.
The maximum impedance bandwidth is reached when gap of
the sectoral patch is maintained at a distance of 20 mm from
the center with the coaxial feed at 29 mm. Figure 5(a) shows
the simulated resonating frequencies are 9.5 GHz, 9.7 GHz
and 10.0 GHz. At these frequencies antenna designed shows a
triple resonance with a maximum return loss of -17 dB at the
resonating frequency of 9.7 GHz. The experimentally
measured results as shown in figure 5(b) are also in good
agreement with the simulated results showing a wide band at
the same resonant frequency. At this gap the antenna shows a
wideband characteristic with the impedance bandwidth
increased up to 12.3 %. Thus the results show an increase in
the impedance bandwidth 1.6 % to 12.3 % giving a wideband
at the excited sectoral patch gap of 20 mm. The designed
antenna can be very useful for wideband applications
especially at the higher frequencies.
By further increasing the gap of the excited sectoral patch
from the center of the circular patch antenna in a direction 45
degree, the impedance bandwidth starts reducing. The
frequency range is still the same as for other lesser gaps
introduced but the triple resonance again changes to dual
resonance with the reduction in return loss. The impedance
bandwidth reduces to 8.1 % from 12.3 % as shown in figure
4(d). This is because of the coupling effect which results in
bandwidth reduction due to larger separation of the excited
sectoral patch. Gap in between the patches results in different
resonant frequencies in the frequency range 8-12 GHz. At a
distance of 20 mm, the antenna shows a wide impedance band,
higher gain and low side lobe level. At this particular gap, the
different resonant frequencies combine and therefore results in
a wide band in the same frequency range in X-band.
B. Input impedance measurements at different gaps
The input impedance curves for different gaps are shown in
figure (6). Figure 6(a) shows simulated input Impedance (in
ohms) variation with frequency (in GHz) when the four
sectoral patches are unmoved and figure 6(b) shows the
experimentally measured input Impedance (in ohms) variation
with frequency (in GHz). Figure (7) shows simulated input
impedance (in dB) variation with frequency (in GHz) when
the excited sectoral patch is moving for different gaps. Figure
8(a) and 8(b) shows the simulated and experimental input
Impedance (in ohms) variation with frequency (in GHz) for
the excited sectoral patch at a gap of 20mm respectively. The
results show a good impedance matching at different
resonating frequencies in the frequency range from 9 GHz to
11 GHz. The coaxial feed is given at a calculated distance in
order to achieve 50 ohms characteristic impedance. When the
four sectoral patches are unmoved, at frequency 9.7 GHz and
10.4 GHz, good impedance matching is obtained as shown in
the figure below. At the resonating frequency of maximum
impedance bandwidth with a gap of 20 mm of the excited
sectoral patch from the center, a good impedance match of 50
ohms is obtained at the resonating frequencies of 9.7 GHz and
10.0 GHz. The simulated results are in good agreement with
the experimentally measured ones. The impedance matching
at different frequencies for different gaps is therefore
presented.
C. Radiation pattern at different gaps
The radiation patterns for different gaps at the resonant
frequency are shown in figure (9). The polar plots at the
central resonant frequency of 10 GHz are therefore plotted at
different gaps. Figure 9 (a) shows polar plot at frequency 10
GHz for the antenna structure when the four sectoral patches
are unmoved. The plot is shown for Directivity Abs (Phi = 0)
between Theta degree vs. dBi. At frequency 10 GHz in the
main lobe direction 25.0 degree, the antenna gain is 7.8 dBi.
Within 3 dB angular width in the direction 23.4 degree, the
side lobe level is -7.8 dB.
When the gap of the excited sectoral patch is increased by 2
mm that is x = y = 2 mm, the radiation properties starts
improving with the increasing impedance bandwidth as shown
in figure 9(b). The side lobe level reduces to a good extent. In
the main lobe direction 12.0 degree, the antenna gain is 7.1
dBi within 3 dB angular width in the direction 35.6 degree.
The side lobe level is reduced to -9.1 dB which shows
improvement in the radiation characteristics.
In figure 9(c), for the gap of the excited sectoral patch
increased to 4 mm that is x = y = 4 mm, the radiation
properties again improve with the increasing impedance
bandwidth. The side lobe level reduces to a good extent. In the
main lobe direction 7.0 degree, the antenna gain is 8.6 dBi
within 3 dB angular width in the direction 41.5 degree. The
side lobe level is reduced to -15.2 dB which shows
improvement in the radiation characteristics.
Similarly, when the gaping is increased to 15 mm, the
radiation properties again show good response. The antenna
gain in the main lobe direction 1.0 degree increased to 10.3
dBi. And for 3 dB angular width, the side lobe level is further
reduced to -14.4 dB in the direction 40.7 degree as in figure
9(d).
The maximum antenna gain is obtained when the excited
sectoral patch is placed at a distance of 20 mm from the center
of the circular patch antenna that is for x = y = 20 mm. At this
distance, the antenna shows maximum antenna gain for the
resonant frequency 10 GHz. In figure 9(e), in the main lobe
direction 0.0 degree, the antenna gain obtained is 12.0 dBi.
The side lobe level is reduced to a good extent that is, for 3 dB
angular width in the direction 35.0 degree, it is reduced to 15.0 dB.
Further increasing the gap of the excited sectoral patch from
the other three sectoral patches leads to a degradation in the
radiation properties of the antenna design proposed. The
antenna gain reduces to 9.7 dBi and the side lobe level
increased to -11.1 dB at the resonant frequency of 10 GHz as
can be seen in figure 9(f).
Therefore the above discussion of the results verify that by
introducing gap in between the circular patch antenna, the
antenna characteristics improved nicely to a good extent as the
gap leads to coupling effect in between the sectoral patches.
This coupling between the parasitic patches results in the
enhancement of the impedance bandwidth of the antenna.
Further in the proposed antenna design, as the gaping goes on
increasing the antenna characteristics further improve. With
the increase in the impedance bandwidth, the radiation
characteristics such as antenna gain increases and the side lobe
level decreases. For better antenna radiation, the antenna gain
should be good enough with the side lobe level as low as
possible. As the patch separation increases the coupling
becomes much smaller as compared to conventional circular
patches. So the antenna characteristics improve up to a
particular distance. But after that, due to large separation the
impedance bandwidth as well as the antenna gain starts
reducing and the side lobe level starts increasing which is not
good for better antenna radiation. For large separation or
thicker substrates, the surface waves would become more
important, and then a lowering of the mutual coupling would
indeed occur through a reduction in the surface-wave
excitation.
The gap in the sectoral patches results in different resonant
modes with a low impedance bandwidth. As the gaping
increases, these different resonant modes start combining with
the improvement of antenna characteristics. At a certain gap of
20 mm, as discussed above in the results, the resonant modes
match and combine to provide a wide impedance bandwidth.
At this particular gap, the other antenna characteristics such as
antenna gain, side lobe level and back radiation improve and
give better results. Further increase in the gap leads to
impedance bandwidth reduction because the resonant modes
again start separating into different resonant modes.
Subsequently the radiation properties of the proposed antenna
design also degrades with the lowering of antenna gain and
increase in the side lobes.
V. CONCLUSION
In this paper, a novel form of gap coupled sectoral patch
antenna based on moving excited patch has been presented.
The excited sectoral patch is moved slowly away from the
other three sectoral patches and the variation in the antenna
characteristics is therefore shown in the results. Due to
coupling in between the sectoral patches, the impedance
bandwidth increases up to a certain distance and the antenna
proposed results in the wideband characteristics with perfect
impedance matching. With this the radiation characteristics
are also improved with the enhancement of antenna gain and
subsequent reduction in the side lobe level. The gap coupled
antenna proposed is very helpful in high frequency wideband
applications. This antenna can also be very handful in
pervasive wireless applications.
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List of figures:
Figure-1: Geometry of antenna design
Figure-2: Prototype of antenna designed
(a) Front view
(b) Bottom view
(a) Simulated return loss vs. frequency
(b) Experimental return loss vs. frequency
Figure-9: Radiation Pattern Directivity Abs (Phi = 0) between
Theta degree vs. dBi
(a) When the excited sector is unmoved
(b) For gap = 2mm
(c) For gap = 4mm
(d) For gap = 15mm
(e) For gap = 20mm
(f) For gap = 22mm
Figure-1
Figure-3: Return Loss (in dB) variation with frequency (in
GHz) when the four sectoral patches are unmoved
(a) Simulated return loss vs. frequency
(b) Experimental return loss vs. frequency
Figure-4: Simulated return Loss (in dB) variation with
frequency (in GHz) when the excited sectoral patch is moving
(a) For gap = 2mm
(b) For gap = 4mm
(c) For gap = 15mm
(d) For gap = 22mm
Figure-5: Return Loss (in dB) variation with frequency (in
GHz) for the excited sectoral patch at a gap of 20mm
(a) Simulated return loss vs. frequency
Figure-2
(b) Experimental return loss vs. frequency
Figure-6: Input Impedance (in ohms) variation with frequency
(in GHz) when the four sectoral patches are unmoved
(a) Simulated return loss vs. frequency
(b) Experimental return loss vs. frequency
Figure-7: Simulated input impedance (in dB) variation with
frequency (in GHz) when the excited sectoral patch is moving
(a) For gap = 2mm
(b) For gap = 4mm
(c) For gap = 15mm
(d) For gap = 22mm
Figure-8: Input Impedance (in ohms) variation with
frequency (in GHz) for the excited sectoral patch at a gap of
20mm
(a)
0
-5
Return Loss (dB)
-10
-15
-20
-25
-30
Gap = 2mm
-35
-40
8
9
10
11
12
Frequency (GHz)
0
(b)
Return Loss (dB)
-10
-20
-30
-40
Gap = 4mm
0
-50
8
9
10
11
12
Frequency (GHz)
-5
Return Loss (dB)
-10
-15
-20
0
-25
-5
Simulated
(For Unmoved sectors)
-35
8
9
10
11
12
Return Loss (dB)
-30
-10
-15
Frequency (GHz)
-20
Gap = 15mm
-25
8
9
10
Figure. (a)
11
12
Frequency (GHz)
0
-2
0
-4
-5
Return Loss (dB)
-6
Return Loss (dB)
-10
-15
-20
-8
-10
-12
-14
-16
Gap = 22mm
-18
-25
-20
8
9
10
Frequency (GHz)
-30
Experimental
(For unmoved sectors)
-35
8
9
10
11
12
Frequency (GHz)
(c)
(b)
Figure-4
Figure-5
11
12
0
350
Experimental
(For unmoved sectors)
300
Input Impedance (ohms)
Return Loss (dB)
-5
-10
-15
Simulated
Gap = 20mm
250
200
150
100
50
-20
8
9
10
11
0
12
8
Frequency (GHz)
9
10
11
12
Frequency (GHz)
(a)
(b)
Figure.7
0
Return Loss (dB)
-5
350
-10
Gap = 2mm
300
Experimental
Gap = 20mm
-20
8
9
10
11
12
Frequency (GHz)
(b)
Input Impedance (ohms)
-15
250
200
150
100
50
Figure-6
0
8
9
10
350
300
12
Simulated
(For unmoved sectors)
250
200
350
150
300
100
250
50
0
8
9
10
Frequency (GHz)
11
12
Input Impedance (ohms)
Input Impedance (ohms)
11
Frequency (GHz)
Gap = 4mm
200
150
100
50
(a)
0
8
9
10
Frequency (GHz)
11
12
(a)
350
Simulated
Gap= 20mm
Input Impedance (ohms)
300
350
Gap = 15mm
Input Impedance (ohms)
300
250
200
150
100
250
50
200
0
8
150
9
10
11
12
Frequency (GHz)
100
50
(a)
0
8
9
10
11
12
Frequency (GHz)
150
350
Input Impedance (ohms)
Experimental
Gap = 20 mm
Gap = 22mm
Input Impedance (ohms)
300
250
200
100
50
150
0
100
8
9
10
11
Frequency (GHz)
50
0
8
9
10
11
(b)
12
Frequency (GHz)
Figure-9
For unmoved sectors
0
330
10
30
5
0
300
-5
Figure-8
60
-10
-15
-20
-25
270
90
-20
-15
-10
-5
240
120
0
5
10
210
150
180
Theta \ degree vs. dBi
(a)
12
Gap = 2mm
Gap = 20mm
0
10
330
0
30
15
5
330
30
10
0
300
5
60
-5
0
-10
-5
300
60
-10
-15
270
90
-15
-15
-20
-10
-20
-5
240
270
90
-15
120
0
-10
-5
5
210
10
150
0
180
5
Theta \ degree vs. dBi
10
15
240
120
210
150
180
Theta / degree vs. dBi
(e)
Gap = 4mm
0
330
10
30
5
G = 22mm
0
300
-5
0
60
-10
330
10
-15
30
5
-20
-25
270
90
-20
300
0
60
-5
-15
-10
-10
-5
240
120
0
-15
270
90
-10
5
10
210
-5
150
0
180
240
120
5
Theta \ degree vs. dBi
10
210
150
180
(c)
Theta / degree vs. dBi
Gap = 15mm
0
330
10
30
5
0
300
60
-5
-10
-15
270
90
-15
-10
-5
0
240
120
5
10
210
150
180
Theta / degree vs. dBi
(f)