Post on 07-Dec-2021
A Mechanically Tuned Via-Patch Loaded
Compact ET-PIFA
Anirban Sarkar*, Sourav Pal and Monojit Mitra
Department of Electronics and Telecommunication Engineering
Indian Institute of Engineering Science and Technology, Shibpur, Howrah-711103, W.B, India.
E-mail: anirban.sonarpur@gmail.com
Abstract- An idea of a compact tunable 30° sectoral
equilateral triangular planar inverted-F antenna
(ET-PIFA) is introduced and fabricated. In the
proposed approach, the conventional ET-patch
geometry is reduced to 30° sectoral ET-PIFA by
via-loading the zero voltage plane and miniaturized
by incorporating two slits and capacitive loading to
lower the operating frequency. The proposed
antenna size is reduced by 93.37 % in comparison
with the conventional ET-patch size and it exhibits
a 10 dB return loss bandwidth of 1.1 % suitable for
2.4 GHz ISM band applications. By simply
replacing the via from via patch with a screw and
adjusting the height of the via patch by turning the
screw, a tuning range of 2.4 GHz from 3.35 GHz
can be achieved which is suitable for mobile
WIMAX application. Both simulation and
experimental results are presented together with
parametric studies on proposed ETMSA structure,
which shows a good agreement between two
results.
Index Terms- Planar inverted-F antenna (PIFA),
slit loading, capacitive loading, size reduction,
mechanical tuning.
I. INTRODUCTION
Antennas are one of the most important
components for the transmission and reception of
electromagnetic waves in wireless
communications. Over the past decade, many
techniques for compactness of patch geometry
have been proposed, but in our structure we have
utilized via-loading, meandering and capacitive
loading technique to reduce the ET-PIFA [1]
geometry. We have chosen the PIFA [2] structure
because of its ease of design, low profile and low
cost. Conventional PIFA with either shorting wall
or shorting pin at one end of the radiating
element gives its length reduction of about a
quarter of a wavelength (λ0/4) at the center of the
operating frequency band. Moreover, designing a
small antenna (smaller than a quarter
wavelength) is always a challenging task. Studies
on microstrip antenna with equilateral triangular
patch geometry reveal the versatility of the same
for compactness and miniaturization of physical
dimensions with respect to the other common
shapes like circular or rectangular geometry and
thereby reducing the area and weight of the
antenna configuration. Space requirement for an
antenna installation in any device always a
constraint, therefore the aim of this paper is to
design an antenna geometry that takes a very
small space in comparison with its conventional
shape for its mounting. Recently it is found that
meandering technique [3],[4] and capacitive
loading technique are used to the PIFA structure
for further reduction in the antenna operating
frequency.
In this paper the alternative patch geometry is
chosen as ET-PIFA in comparison with other
geometries [5],[6]. In the proposed antenna, an
extra plate in between patch and ground plane
does the job of capacitive loading [7]. The via
patch is connected to the ground through via
while the coaxial feed is directly connected to the
radiating element. Also use of meandering
technique, i.e. cutting multiple slits in the patch
geometry increases the electrical path length that
reduces the operating frequency of the antenna.
With parametric studies on the location of the
vias, position of the slits on the patch and
location of the via-patch, structure of this paper
provides a design guideline to the proposed ET-
PIFA. The proposed design can provide a very
compact frequency tunable ET-PIFA through a
simple mechanical tuning to cover mobile
WIMAX as well as ISM band applications.
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II. CONVERSION OF CONVENTIONAL ET-
PATCH STRUCTURE TO 30º SECTORAL ET-
PIFA STRUCTURE
In general, microstrip antennas are half-
wavelength structures and are operated at the
fundamental resonant mode TM01 or TM10. Based
on the cavity model approximation, the
fundamental or first resonant frequency of the
triangular patch [8],[9-10] is given by
12 2 22 (m mn n )
3 e
mnl mn
e
cf f
S ε= =
+ + (1)
where Se is the effective side length, εe is the
effective permittivity of the dielectric i.e air and
the integers m, n, l satisfy the condition m + n + l
= 0. Instead of using m, n, and l , only m, n has
been used for simplicity, it is implied that l = -
(m + n), where eS is
4
e
eh
S Sε
= + (2)
where S is the original side length of the ET-
patch and h is the height of the radiating patch
from ground plane. We would like to
demonstrate the degree of size reduction obtained
by shorting the zero voltage plane [11].
In our antenna design, We have started with
conventional ET patch geometry with side length
70mm and it resonates at 2.4 GHz. After that we
choose the side length as 40 mm and by posting
vias in proper locations, geometry is changed to
basic ET-PIFA which is resonating at 3.73GHz.
We have shorted along the zero voltage line
which is 2/3rd
of height from the triangle tip to
the bottom edge of the triangle, by several vias
and separate the whole structure by two parts.
This 600
sectoral ET-PIFA whose electrical
length is λ/4, will resonate at the same resonant
frequency as the basic ET-PIFA. By applying the
same shorting technique the 600
sectoral ET-
PIFA is converted to the smallest 300 sectoral ET-
PIFA structure which resonates almost at the
same frequency which has shown in Fig. 1 and
table 1.
Fig. 1. Conversion of Conventional ET-patch
Structure to 30° Sectoral ET-PIFA Structure.
(a)
(b)
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(c)
Fig. 2. Configuration of the proposed PIFA with slits
and via-patch loading (units: mm). (a) Top view. (b)
3-D view. (c) Side view
Table 1. Conversion to 30º sectoral ET-PIFA structure
III. PIFA WITH SLIT LOADING
In this part, antenna radiating frequency is
reduced by meandering [12] the excited patch
surface current path by loading several narrow
slits at the patch’s non-radiating edge. In our
antenna geometry two slits are cut at the same
non-radiating edge. The upper slit and the lower
slit has the length 12.68 mm and 7.54 mm
respectively. The width of both of these slits is
0.5 mm. This slit loading reduces the resonating
frequency as well as bandwidth from 3.73 to 3.18
GHz and 3.1% to 2.26% respectively. The
corresponding simulated S11 plot in comparison
with 30° sectoral ET-PIFA return loss plot is
shown in Fig. 3.
IV. PIFA WITH VIA-PATCH LOADING
A. Via-patch Loading
Further reduction in the operating frequency of
the proposed PIFA can be done by capacitive
loading. The via- patch is 0.4 mm separated from
the radiating patch to provide the capacitive
effect [13],[14]. In fact, the capacitive load could
be adjusted by changing the size and height of the
via-patch, and this will not increase the overall
dimensions of the original structure. Both the
radiating patch and via-patch are made of 0.02
mm thick copper plate and they are 3.4 mm and
3.0 mm in height above the ground plane
Fig. 3. Comparison of resonant frequency between
30° sectoral ET-PIFA and slit loaded 30° sectoral ET-
PIFA.
respectively. The substrate of the whole antenna
is air and a 1.2 mm diameter coaxial probe is
directly fed to the radiating patch as depicted in
Fig. 2. The radiating element and via-patch are
Fig. 4. Comparison of resonant frequency between
slit loaded 30° sectoral ET-PIFA and slit and via-
patch loaded 30° sectoral ET-PIFA.
placed at the middle of the ground plane
(100×100 mm2). There are three pins in total in
the proposed antenna and they function
differently. First, the two 1.2 mm diameter
shorting pins are connected to the main upper
Type of the
structure
Resonant
frequency(fr) in
GHz
S11 (dB)
Basic ET-PIFA 3.73 -36
60° sectoral ET-PIFA 3.7 -24.52
30° sectoral ET-PIFA 3.74 -25
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Table 2. Variation of resonant frequency and
capacitance with separation between two patches
Table 3. Variation of resonant frequency by varying
via-patch area
radiating patch and they also support the patch in
the air. Next, another 1.2 mm diameter pin is
built in via-patch, which supports the lower patch
in air and provides a capacitive load eventually
making that radiating. Putting a cylindrical via on
a patch is easier in fabrication than a shorting
wall, and we can adjust the resonance of the
antenna by varying the via location. Capacitive
loading tunes the operating frequency from 3.18
GHz to 2.4 GHz as shown in Fig. 4 but
impedance bandwidth reduces to 1.1%.
B. Parametric Studies
In Fig. 5 and Fig. 6, different resonant
frequencies of antennas are obtained by
simulation for different separation between via
patch and radiating patch and area of the via-
patch respectively (All dimension values shown
in Fig. 5 and Fig. 6 are relative to the antenna
model described in Fig. 2). In table 2 and 3, it is
seen that the resonant frequency is more sensitive
to the separation than the area of the via-patch.
As capacitance increases, the resonant frequency
decreases and this is shown in Fig. 7. On the
other hand, in Fig. 8, an equation can be obtained
from 3 sets of data. When the actual separation of
0.4 mm, 1.4 mm and 2.4 mm and their
corresponding resonant frequencies are put into
(3), the parameters a, b and c could be computed
and the calculated values are 3.84, -20 and 26.3,
respectively. The simulation result
Fig. 5. S11 vs resonant frequency plot with varying
the separation between the via-patch and radiating
patch by keeping both of areas constant.
Fig. 6. Variation in resonant frequeny by varying the
via-patch effective area.
( ) ( ) ( )2
. . 3separation a freq b freq c= + +
shows that the separation between the two
patches and the resonant frequency of the antenna
Separation between radiating patch and via-patch
(mm)
Capacitance (pF)
Resonant frequency
(GHz)
Bandwidh (%)
2.4 0.047 3.35 2.18
1.9 0.06 3.27 2.07
1.4 0.08 3.15 1.87
0.9 0.125 2.91 1.33
0.4 0.28 2.4 1.1
Variation of area of the via-patch (mm
2)
Resonant frequency
(GHz) S11(dB)
Capacitance (pF)
7.91 2.68 -18 0.17
9.42 2.59 -23.2 0.20
12.82 2.4 -38.5 0.28
15.92 2.27 -24 0.35
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Fig. 7. Resonant frequency versus capacitance values.
can be represented by a quadratic equation. The
units of (3) for separation and frequency are mm
and GHz, respectively. Faster changes in the
resonant frequency are observed when the
separation is getting smaller. Therefore, the via-
patch should be placed close to the radiating
element in order to obtain an optimum antenna
size reduction. In fact, enlarging the via-patch
size or placing the via-patch closer to the
radiating element could further reduce the size of
the radiating element. The capability of lowering
the resonant frequency by loading a via- patch
under the main radiating patch has been
demonstrated above. However, we found that the
degree of size reduction of the antenna is also
closely related to the location of the via. Fig.
10(a) shows five via locations with different
distance, away from the right edge of the via-
patch. Actually, the antenna described in Fig. 2 is
the one with x equals to 1.34 mm. Referring to
Fig. 10(b), we noticed that the larger the value of
x, the higher the resonant frequency of the
antenna will be. This could not be explained by
the introduction of a capacitive load to the PIFA
only. The simulated surface current distribution
of two extreme cases with x equals to 1.34 mm.
and 6.14 mm are shown in Fig. 11(a) and (b). It is
clearly seen that the surface current is flowing
from the radiating patch to the ground plane
along the shorting pins, and then flowing up to
the via-patch through the via. By moving the via
to the right hand side (smaller value in x), longer
current paths are provided on the ground plane.
This will help in lowering the resonant frequency
of the antenna. Although this will provide the
freedom in miniaturizing the antenna, but reduces
the impedance bandwidth. The top view of the
fabricated antenna structure are shown in
Fig 12(a).
Fig. 8. Quadratic curve fit between the separation and
resonant frequency.
Fig. 9. Comparison of resonant frequency between
simulated result in IE3D simulation software [15] and
measured result in Agilent Technologies N5230A, 10
MHz - 20 GHz , PNA- L Network Analyzer of the
fabricated PIFA.
C. Frequency Tuning Capability
In principle, a ET-PIFA is not allowed to change
the resonant frequency because the resonance is
mainly governed by the current paths of the
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radiating element. Once the length of the
radiating patch is estimated, the resonant
frequency is fixed. However, the resonant
frequency of our proposed PIFA also depends on
the amount of capacitance introduced by the via-
patch and the radiating patch, and the capacitance
in turn depends on the separation
(a)
(b)
Fig. 10. (a) Via locations with different x. (b) Return
losses of via-patch loaded PIFAs with different via
locations.
Fig. 11. Simulated surface current distribution (a) with
via placing on x equals to 1.34 mm at 2.27 GHz and
(b) with via placing on x equals to 6.14 mm at 2.5
GHz.
between them and the area of the via-patch. The
effect on tuning of resonating frequency is more
for separation between the two patches than the
area variation of the via-patch. Therefore, the ET-
PIFA becomes frequency tunable simply by
adjusting the height of the via-patch. This can be
easily realized by replacing the via with a screw
as depicted in Fig. 12(b).
(a)
(b)
Fig. 12. (a) Top view of Fabricated proposed PIFA
(left side) & (b) Configuration of proposed PIFA with
a screw as via(right side).
V. CONCLUSION
In this paper we have presented a new approach
for a ET-PIFA with slit and capacitive loading.
The proposed antenna gain and directivity
obtained are 3.29 dbi and 4.31 dbi respectively.
We also found that meandered current paths on
the radiating patch and the capacitive load are the
main reasons for antenna size reduction. As
triangular geometry is chosen and modified to
30º sectoral geometry, more compactness is
achieved compared to other geometry like
rectangular or circular and it is possible to creat
more compact structure by converting the
proposed antenna to 15º sectoral geometry with
an electrical length of λ0/16. Measured and
simulated results show that the proposed ET-
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PIFA covers the 2.4 GHz ISM and mobile
WIMAX bands with an electrical length of λ0/8.
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