M. ARULAALAN et al.: DESIGN OF A TRIPLE-BANDPASS FILTER USING A MODIFIED T-SHAPED ...
309–316
DESIGN OF A TRIPLE-BANDPASS FILTER USING A MODIFIED
T-SHAPED RECTANGULAR COUPLED WITH A STEPPED
IMPEDANCE RESONATOR FOR SMART PORTABLE
COMMUNICATION DEVICE APPLICATIONS
DIZAJN TRIPASOVNEGA FILTRA Z UPORABO MODIFICIRANEGA
TRIPASOVNEGA PRA VOKOTIKA T-OBLIKE ZDRU@ENEGA S KORA^NIM
IMPEDAN^NIM RESONATORJEM V PAMETNIH PRENOSNIH NAPRA V AH
ZA KOMUNIKACIJSKE APLIKACIJE
M. Arulaalan
1*
, Viswanathan Ramasamy
2
, R. Saravanakumar
3
, S. Maheswari
4
1
Department of ECE, CK College of Engineering and Technology, Cuddalore, Tamil Nadu, India
2
Department of CSE, Koneru Lakshmaiah Education Foundation, Vaddeswaram, AP, India
3
Department of Wireless Communication, Institute of ECE, Saveetha School of Engineering, Savertha Institute of Medical
and Technical Science, Chennai 602105, India
4
Department of ECE, Panimalar Engineering College, Chennai, India
Prejem rokopisa – received: 2023-04-03; sprejem za objavo – accepted for publication: 2023-05-11
doi:10.17222/mit.2023.846
This paper presents a ground-breaking triple-bandpass filter design utilizing a modified T-shape rectangular coupled with a
stepped impedance resonator (MTSR-CSIR) for smart portable communication device applications. The MTSR-CSIR filter op-
erates at (2.2, 3.62 and 4.6) GHz, providing wide passbands in three operating modes. To achieve the optimal performance, the
filter design is executed on a multi-layered liquid-crystal polymer (LCP) substrate with a thickness of 50 μm, dielectric constant
of 2.9 and loss tangent of 0.002. Simulation results for the MTSR-CSIR filter demonstrate a high level of accuracy and consis-
tency with the measurement results for the fabricated filter. The filter exhibits an excellent stopband rejection, low loss and com-
pact size while maintaining high-performance levels. Its performance parameters, such as insertion loss, return loss and group
delay, are considered to evaluate the filter’s performance. The results highlight the applicability of the MTSR-CSIR filter for
smart portable communication devices.
Keywords: triple-bandpass filter, modified T-shape rectangular coupled with stepped impedance resonator, smart portable com-
munication device applications, liquid-crystal polymer substrate, performance evaluation
V ~lanku avtorji predstavljajo oblikovanje (dizajn) novega naprednega tripasovnega filtra z uporabo modificiranega pravo-
kotnika T-oblike zdru`enega s kora~nim impedan~nim resonatorjem (MTSR-CSIR; angl.: Modified T-Shape Rectangular Cou-
pled Stepped Impedance Resonator) v pametnih prenosnih napravah za komunikacijske aplikacije. MTSR-CSIR filter obratuje
pri frekvencah 2,2 GHz, 3,62 GHz in 4,6 GHz in pri tem omogo~a {iroko pasovno obratovanje na tri razli~ne na~ine. Da bi
dosegli optimalne lastnosti naprave so avtorji uporabili substrat (podlago) iz ve~plastnega polimera izdelanega na osnovi teko~ih
kristalov (LCP; angl.: liquid crystal polymer) debeline 50 μm, z dielektri~no konstanto 2,9 in tangensom izgub 0,002. Rezultati
simulacij in meritev MTSR-CSIR filtra so pokazali njegovo veliko natan~nost in konsistenco. Filter ponuja odli~no zavrnitev
(angl.: stopband rejection), majhne izgube in kompaktno velikost pri ohranitvi nivoja visoke kakovosti. Avtorji si izvedli
evaluacijo njegovih performan~nih parametrov kot so vztopne in povratne izgube ter zamuda skupine. Rezultati predstavljeni v
pri~ujo~em ~lanku osvetljujejo uporabnost novega dizajna MTSR-CSIR filtra v pametnih napravah za komunikacijske
aplikacije.
Klju~ne besede: tripasovni filter, modificiran pravokotnik T-oblike zdru`en s kora~nim impedan~nim resonatorjem, pametne
prenosne komunikacijske naprave, substrat iz teko~ega kristalnega polimera, ovrednotenje lastnosti.
1 INTRODUCTION
Today’s demand for high-quality and reliable com-
munication services has increased due to the extended
wireless and ultra-wideband networks. A microwave fil-
ter is an essential component that operates the entire sys-
tem and determines the range of frequency that can be
allowed or restricted, depending on the application.
1
The
next generation of 5G technology utilizes low-band,
mid-band and millimeter wave band frequencies of
700 MHz, 3.5 GHz and 26 GHz. Bandpass filters are
primarily used in these frequency ranges and they are de-
signed to use resonators and various micro-elements that
represent the properties of the filter. The miniaturization
of bandpass filters is done to reduce parasitic elements
with various planar miniaturization methods. Several re-
search scholars have presented differently structured res-
onators, including the stepped impedance resonator, stub
impedance resonator, stub-loaded stepped impedance
resonator (SLSIR) and parallel-coupled microstrip lines.
2
A tunable resonator filter has been designed for
multi-wideband operations that operate at various modu-
lation frequencies to meet the demands of various appli-
Materiali in tehnologije / Materials and technology 57 (2023) 4, 309–316 309
UDK 004.382.73/.77:535.417.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(4)309(2023)
*Corresponding author's e-mail:
arulaalan@gmail.com

cations. Different filters have been designed to satisfy in-
dustrial demands for a triple passband. It is crucial that a
passband can be adjusted to make the filter applicable to
different requirements. A passband adjustment is made
by adjusting the values to the expected values, but the
bandwidth control may be challenging. In the case of a
stepped impedance resonator (SIR), the frequency re-
sponse is tuned by varying the stub, namely the open
stub and short stub.
3–5
A filter is a device that processes
frequency-dependent signals. The size of a filter depends
on the resonators, which are the fundamental elements of
the filter. Therefore, the resonator design used for a filter
fabrication determines the size of the filter.
6
The filter
size plays a significant role in many compact and ad-
vanced devices, especially in the case of compact wire-
less devices where a small and thin filter is required for
integration. The miniaturization of filters is essential for
reducing parasitic elements and improving the perfor-
mance. Various planar miniaturization methods have
been utilized to achieve this goal. These methods include
using fractals, meander lines and interdigital struc -
tures.
7–9
In recent years, there has been a growing demand for
compact and cost-effective wireless devices that operate
in multiple modes to meet various communication re-
quirements. To achieve this, the size of microwave fil-
ters, essential components of the wireless communica-
tion system, must be reduced. This can be achieved by
optimizing the size of the resonator that is the filter’s ba-
sic element. Traditionally, the waveguide technology has
been used to enhance the Q-value of the resonator. How-
ever, the bulky nature of waveguides limits their use in
compact devices. Coaxial cavity resonators have been
proposed as an alternative to waveguides for the com-
pact-filter design to overcome this limitation.
10,11
This study proposes a multi-mode resonator, a modi-
fied T-shaped rectangular coupled with a stepped imped-
ance resonator (MTSR-CSIR), which operates at three
resonant frequencies, by modifying the resonator struc-
ture. The width of the resonator is optimized to achieve
the compact filter size. The MTSR-CSIR design exhibits
significant advantages, including cost-effectiveness, sim-
plicity of the structure, and high performance in different
modes, making it a promising solution for future wire-
less communication devices.
The major contributions of the proposed MTSR-
CSIR filter are as follows:
• Tri-passband filter: The modified T-shaped resonator
is designed to pass the three bands at different reso-
nant frequencies. The pass can be adjusted by varying
the length of the resonator. As a result, the proposed
filter covers frequency ranges of 2–2.5 GHz,
3.1–3.76 GHz, and 4.32–5.2 GHz, respectively.
• Enhanced bandwidth: The bandwidth of the passband
is enhanced by varying the d3 width of the resonator.
The isolation of three passbands is observed in the
simulation results due to the adjustment of the reso-
nator width. While improving the width of the reso-
nator, the bandwidth is enhanced. Thereby, the
Q-factor of the filter will also be improved.
• Rejection of undesired frequency: The filter design
has an additional rectangular resonator to eliminate
the harmonic frequency beyond the passband fre-
quency range. The rejection of undesired frequency
enhances the performance of the TRCSIR filter.
The structure of this paper is as follows: Section 2
presents a review of previous work related to designing
ultra-wideband filters for wireless applications operating
in a multi-mode system. Section 3 presents the design of
the proposed modified T-structured resonator with a rect-
angular coupled with a stepped impedance resonator. A
resonator analysis is described in detail, followed by the
MTSR-CSIR filter design. Simulation results for the pro-
posed filter regarding the insertion and return loss are
presented in Section 4. Finally, Section 5 concludes the
paper by summarizing the MTSR-CSIR filter design and
evaluating its performance.
2 RELATED WORKS
This literature survey explores the current research of
the microwave filter design and its applications in wire-
less communication systems. We have discussed the im-
portance of bandpass filters for meeting the demand for
extended wireless and ultra-wideband networks. The use
of tunable resonator filters for multi-band operations has
been highlighted. We have seen how the filter size can be
reduced by optimizing the resonator width and utilizing
compact resonator structures.
In recent years, there has been a significant focus on
designing and developing compact and efficient filters
for wireless communication applications. Research stud-
ies have proposed various resonators and filter structures
to achieve this goal. Dong-Sheng et al.
12
presented the
design of a UWB bandpass filter that operates between
3.1 GHz and 10.6 GHz. The filter consists of a WLAN
notch combined with a BPF at 5.8 GHz and a microstrip
structure, which provides a 108 % bandwidth. The filter
achieved a return loss of approximately 1.0 dB at
6.85 GHz, indicating that it can effectively pass the
broad passband frequency while rejecting the high
stopband suppression. The filter is suitable for integra-
tion into small devices and antennas due to its compact
size.
Wang et al.
13
proposed a small bandpass Wilkinson
power divider with UWB harmonic reduction. The pro-
posed model suppresses the DC component and third-,
fifth- and seventh-order harmonic components. The fre-
quency-selecting coupling structure unit is the central
component for harmonic suppression. The dimensions of
the WPD are 14.65 mm × 19.64 mm, and it has a return
loss above -3.4 dB at the operating frequency, with a
bandwidth of -20 dB, or 1.74 GHz to 3.71 GHz. The tun-
ing of the WPD was accomplished by modifying the res-
onators’ length and breadth. The bandpass WPD com-
M. ARULAALAN et al.: DESIGN OF A TRIPLE-BANDPASS FILTER USING A MODIFIED T-SHAPED ...
310 Materiali in tehnologije / Materials and technology 57 (2023) 4, 309–316

pletely rejects the harmonic components. This design is
an efficient solution for UWB systems, requiring a small
size and high performance.
Zheng et. al.
14
proposed a defected structure of a
microstrip to create a bandpass filter with incorporated
switches in the defected T-shaped structure to adjust the
frequency. The filter has a dual-band gap and is suitable
for a cognitive radio with minimal distortion. Passband
frequencies were achieved using a 0.25 mm wide hole
and gap. A wideband antenna with dimensions of 65 mm
by 40 mm was designed, and a switch was incorporated
to change the slot height, thereby tuning the frequency as
required.
Al-Shalaby et al.
15
designed a novel band-stop filter
by utilizing a defected ground structure in the form of a
dumbbell and a U-shaped cavity on microstrip lines. The
proposed filter achieved a remarkable notch depth of
-39.13 dB for the 1.4 GHz resonant frequency, surpass-
ing the performance of conventional dumbbell filters
with a notch depth of -32.21 dB for the 2.6 GHz resonant
frequency. The spiral dumbbell shape contributed to a
high Q-value of 8.823, demonstrating the proposed de-
sign’s effectiveness in suppressing unwanted frequen-
cies.
Tantiviwat et al.
16
proposed the multi-mode SLR to
design a compact tri-band BPF, but it suffered from a
narrow frequency range and high insertion loss. To ad-
dress these issues, the interaction between two multi-
mode resonators, such as stepped-impedance resonators
(SIRs) in tri-band and dual-band BPFs and SLRs in
tri-band and dual-band BPFs, was utilized to achieve
passband contributions from both odd and even bands.
Ren et.al.
17
proposed a modified approach where two
SIRs were coupled to create a compact dual-band band-
pass filter. However, this resulted in a significant inser-
tion loss and narrow passbands. Another approach in-
volved dividing a passband into multiple passbands and
adding structures with stopbands or transmission zeros
(TZs) to BPFs or low-pass filters.
Wang et al.
18,19
presented a novel dual-band graphene
microstrip filter for 5G applications, but its adoption is
limited due to a high insertion loss. However, there is a
lack of research on tri-band BPFs that can meet the re-
quirements for small-size, low-loss and wide passbands
for 5G mobile communication systems. The above high-
lights the need for further investigation and development
of tri-band BPFs that can effectively address the chal-
lenges of 5G communication systems.
Hou et al.
20
proposed the development of a dual-
wideband and a tri-wideband BPF for 5G mobile com-
munications. The dual-wideband BPF is created using
two folded open-loop stepped-impedance resonators
(FOLSIRs). In comparison, the tri-wideband BPF is
achieved by inserting two folded uniform impedance res-
onators (FUIRs) into the dual-wideband BPF. A unique
structural deformation of SIR produces a FOLSIR in the
filters. However, the proposed filters may still have phys-
ical-dimension and insertion-loss limitations. In response
to this review, we propose a three-passband filter utiliz-
ing the modified T-structured resonator and a rectangular
slot-coupled filter to reject undesired frequencies.
3 EXPERIMENTAL PART
3.1 Analysis of the modified T-resonator
The MTSR-CSIR filter incorporates the modified
T-resonator in its design, which is analyzed as follows.
The modified T-resonator comprises a low- and a
high-impedance section, both on either side of a /2 SIR.
The length of the low-impedance section is /4, while the
length of the high-impedance section is  /8. Figure 1
shows the equivalent circuit of the resonator’s impedance
circuit. The resonator’s characteristic impedance and
electrical length are denoted by Z
1
and Z
2
, respectively.
The electrical length of the low-impedance and high-im-
pedance sections are represented by deg 2 and  ,r e -
spectively. The following expressions relate to the reso-
nant frequency, characteristic impedance and electrical
length:
iZ
X = arctan (1)
f
f
X
d
Z
7
0
2
=
π /
arctan
'
(2)
M. ARULAALAN et al.: DESIGN OF A TRIPLE-BANDPASS FILTER USING A MODIFIED T-SHAPED ...
Materiali in tehnologije / Materials and technology 57 (2023) 4, 309–316 311
Figure 1: Equivalent impedance diagram for the modified T-SIR

Here, f
0
denotes the resonant frequency of the modi-
fied T-SIR, i
is the instant electrical length of the reso-
nator, and i is the ratio of low and high characteristic im-
pedance parts. The characteristic impedance of an
individual region is given by
Z
w
h
w
h
0
1
1393 0667 144
=⋅
++ +
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ π
  r
r
r
r
..l n.
(3)
 =+ +
⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎛ ⎝ ⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ −
r+1 r
r
2
11
12
1
2
h
w
(4)
The physical dimensions of the modified T-resonator
are analyzed with respect to Equations 1 and 4, consider-
ing the substrate material of the filter with the width w
r
,
thickness of the ground plane h
r
and dielectric constant
 r
. The resonant and second harmonic frequencies f
d7
are
calculated based on these parameters. The modified
T-SIR’s physical length is reduced to improve the trans-
mission zeros in the passband. The difference in the
transmission line impedance is balanced by varying d3.
The simulation response of S21 for varying lengths of
the resonators is analyzed and presented in Figure 2.
The input impedance of the individual impedance
part is expressed as
ZdZ
zj zx x
zj zx x
x
x
in
()
tan
tan
=
+
+
0
0
0
(5)
ZZ
zj z
zj z
x
x
in
=
+
+
0
0
0
(6)
The resonant frequency of the modified T-resonator
is determined with Equation (6) where Z
0
represents the
characteristic impedance, Z
x
represents the load imped-
ance, and deg is denoted as x. The parameters of the
substrate and resonator, such as the electrical length, fre-
quency and length, are obtained with the following ex-
pressions after determining the substrate parameters:
qx
i
=∝ (7)
∝=
π
 g
'
/2
(8)
    g
g
'
= (9)
Theoretical analysis of the proposed filter is based on
Equations (4), (7) and (9). The effect of varying distance
d7 on the filter performance is studied by changing its
value from 3.5 mm to 5.5 mm. The simulation results of
the filter response for different values of d7 are shown in
Figure 2a, and the filter’s frequency response is shown
in Figure 2b based on the simulation analysis.
To meet the requirements of industrial applications,
the physical dimensions of the filter are adjusted. To im-
prove the S21 response, the d7 length is varied from 5.5
mm to 3.5 mm. The S21 response is enhanced by in-
creasing the length of the resonator. The frequency sup-
pression after 4.8 GHz is demonstrated for d7 = 5.2 mm
and 5.5 mm. The length of 5.5 mm is found to achieve
the desired cutoff frequency of the passband region.
3.2 Design of the triple-bandpass filter
In the previous section, we discussed the theoretical
analysis of the resonator needed for designing a tri-
ple-bandpass filter. Now, we present the physical dimen-
sions of the proposed MTSR-CSIR filter, as shown in
Figure 3. The input is fed through a 50-ohm transmis-
sion line, and the coupling distance is reduced to en-
hance the quality factor of the filter. We analyze the re-
sponse of the proposed filter using an EM simulator and
evaluate the insertion loss and return loss through simu-
lations. We vary the length of d3 to improve the
passband width.
M. ARULAALAN et al.: DESIGN OF A TRIPLE-BANDPASS FILTER USING A MODIFIED T-SHAPED ...
312 Materiali in tehnologije / Materials and technology 57 (2023) 4, 309–316
Figure 2: Simulation result of S21: a) analysis of various d3 lengths, b) response of resonant frequency f
0
and f
d7

The transmission zeros in the passband of the
TRCSIR filter help suppress the resonant frequency and
improve the passband bandwidth. The physical length of
the coupling section, d2, is responsible for turning the
passband frequency. During the design process, the
widths of d3 and d7 are varied to obtain the desired re-
sults for the frequency range of 1–5 GHz, as shown in
Figure 3. Furthermore, the spacing between the bent
T-structure, d5, is reduced and adjusted to achieve the
desired response. The coupling effect between the
microstrip lines of the MTSR-CSIR filter enhances the
overall quality factor of the triple-bandpass filter.
The width of the d3 resonator is a significant factor
affecting the filter’s passband. The modified T-resonator
in the middle section of the proposed design enables the
filter to achieve triple passbands that cover the frequency
range of 1–5 GHz. A rectangular slot is added to the
modified T-structure to reject undesired frequencies be-
yond the 5 GHz frequency range. The transmission zeros
of the passbands are improved by varying the d3 width
of the MTSR-CSIR filter. To achieve the desired fre-
quency range, the d3 dimension is reduced from 1 mm to
0.6 mm. The variation in the d3 width and its impact on
the passband response is shown with the simulation re-
sults presented in Figure 4.
The d12 length is varied to achieve the desired tuning
of the three-pass band in the designed filter. The d7
width is also adjusted to place the resonant frequency of
the three passbands to the desired frequency range. The
d3 dimension is optimized to enhance the transmission
zeros of the passband. The simulation analysis of S21,
adjusting d7 was discussed in the previous section. Once
the physical dimensions of the filter are optimized, the
designed filter is fabricated on a multi-layered LCP sub-
strate with a thickness of 50 μm, a dielectric constant of
2.9 and a loss tangent of 0.002. The filter’s performance
is evaluated using the EM simulation results, including
the insertion loss, return loss and group delay. The re-
sulting filter covers the frequency range of 1–5 GHz,
with three passbands at (2.2, 3.62 and 4.6) GHz, respec-
tively, and exhibits excellent performance.
4 RESULTS AND DISCUSSION
A novel triple-bandpass filter is proposed in this
work, utilizing a modified T-shaped resonator to design
an MTSR-CSIR filter. The physical dimensions of the
filter are carefully adjusted to achieve the desired fre-
quency range. The bent/modified T-structure is the main
component of the MTSR-CSIR filter, which enables the
filter to have the desired passband frequency response.
The resonant frequency response is derived theoretically,
and the simulation results are presented in Figure 2b and
discussed in Section 3. Additionally, a rectangular cou-
pled with a resonator is incorporated to improve the fil-
ter’s performance further. The undesired frequency is
suppressed by adjusting the d7 dimension of the filter.
The three-pass band is adjusted by varying the d12
length, while the transmission zeros of the passband are
improved by adjusting the d3 dimension. The d7 width is
also varied and analyzed to place the resonant frequency
of the three passbands within the desired frequency
range. The simulation analysis of S21 for adjusting d7 is
shown in Figure 2a and discussed in Section 3. After the
optimization of the physical dimensions of the filter, the
designed filter with unique physical dimensions is fabri-
cated on a multi-layered LCP substrate with a thickness
of 50 μm, dielectric constant of 2.9 and loss tangent of
0.002. The EM simulation results, including perfor-
mance parameters, such as the insertion loss, return loss
and group delay, are considered to evaluate the perfor-
mance of the MTSR-CSIR filter.
The EM simulation results are used to evaluate the
performance of the MTSR-CSIR filter, considering the
performance parameters, such as the insertion loss, re-
turn loss and group delay. The resultant filter covers a
frequency range of 1–5 GHz, with three passbands at
2.2, 3.62 and 4.6 GHz, respectively. The simulated re-
sponse effectively suppresses the transmission zeros at
each passband frequency range.
In this proposal, the d3 width of the modified
T-shaped resonator is varied to enhance the passband re-
sponse of the triple-bandpass filter. The d3 width is a
M. ARULAALAN et al.: DESIGN OF A TRIPLE-BANDPASS FILTER USING A MODIFIED T-SHAPED ...
Materiali in tehnologije / Materials and technology 57 (2023) 4, 309–316 313
Figure 3: Design of modified T-shaped rectangular coupled with triple-bandpass filter. The dimensions: d1 = 6 mm, d2 = 7 mm, d3 = 0.6 mm,
d4 = 2 mm, d5 = 0.5 mm, d6 = 1 mm, d7 = 5.5 mm, d8 = 2.1 mm, d9 = 6.5 mm, d10 = 2.2 mm, d11 = 0.2 mm, d12 = 5.5 mm

critical parameter that influences the bandwidth of the
passband response. The effect of varying the d3 value on
the passband response is analyzed through the simula-
tion.
The simulation results, as shown in Figure 4, indi-
cate that increasing the d3 value improves the passband
response. If the width is too narrow, like 0.6 mm, the
passband response is very narrow, which is unsuitable
for practical applications. In contrast, if the width is in-
creased to 1 mm, the bandwidth of the passband re-
sponse is enhanced significantly, which is desirable for
practical applications.
Therefore, the d3 width is set to 1 mm to achieve the
desired passband response. This enables the passband to
shift to the expected frequency region while increasing
the bandwidth of the passband response. Overall, varying
the d3 value significantly impacts the passband response
of the triple-bandpass filter, and proper optimization of
this parameter is crucial for achieving the optimal filter
performance.
The EM simulator simulates the entire filter design
after optimizing all the physical dimensions. Figure 5
displays the simulation results for the return loss S11 and
insertion loss S21 parameters for the proposed modified
MTSR-CSIR filter design.
The simulation results of the MTSR-CSIR filter de-
sign indicate that it achieved high performance in terms
of both return loss and insertion loss. The IS
11
I filter re-
sponse demonstrates a high return loss, with values of
-65 dB at 2.2 GHz, -46 dB at 3.62 GHz and -40 dB at
4.6 GHz for the three passbands of the filter. This im-
plies that the filter has a very low reflection loss, which
means that most of the input signal is transmitted to the
output port with a minimal power loss.
Furthermore, the simulation response of the IS
21
Ip a -
rameter shows that the designed filter achieves a low in-
sertion loss at the cutoff frequencies. It achieves low in-
sertion losses of 0.62 dB at 2.2 GHz, 0.65 dB at
3.62 GHz, and -0.71 dB at 4.6 GHz for the three pass-
bands of the filter. This indicates that the filter effec-
tively passes signals in the desired frequency range while
suppressing the signals in the unwanted frequency range.
The MTSR-CSIR filter operates at three simulta-
neous passbands covering the frequency range of
2–2.5 GHz, 3.1–3.76 GHz and 4.32–5.2 GHz. The band-
width of the passbands is also wide enough to allow the
passage of the required signals.
The designed triple-passband filter exhibits effective
signal-selection performance for the propagated signal,
as demonstrated by the simulation of the electric-field
distribution. The filter was fabricated and analyzed using
a network analyzer, and it was applied to wireless com-
munications. The simulated results closely match the
measured results, as seen in Figure 6. The fractional
bandwidth (3 dB) at the passband frequencies of (2.2,
3.62 and 4.6) GHz is (16, 14.3 and 11.52) %, respec-
tively. The high return loss and low insertion loss
achieved by the designed filter make it an efficient signal
selector in the frequency range of 2–5 GHz.
The filter suits various wireless communication ap-
plications like WLAN, Bluetooth and WiMax. The fil-
ter’s ability to simultaneously pass three different fre-
quency bands allows it to be used in multi-band wireless
communication systems. The fabricated filter also dem-
onstrates good stability, making it a suitable choice for
harsh environments. The proposed MTSR-CSIR filter
design is an effective and efficient solution for wireless
communication systems requiring a triple-passband filter
with high-performance parameters.
The transmission zeros of the three passbands are ob-
served at (1.5, 2.72 and 4.6) GHz, and the filter can ef-
fectively isolate the three passbands. The rectangular slot
eliminates the undesired frequencies after 5.2 GHz.
A comparative analysis of different triple-bandpass
filters includes the following data: center frequencies ab-
breviated as CF (GHz), defected ground structure abbre-
viated as DGS, fractional bandwidth abbreviated as
M. ARULAALAN et al.: DESIGN OF A TRIPLE-BANDPASS FILTER USING A MODIFIED T-SHAPED ...
314 Materiali in tehnologije / Materials and technology 57 (2023) 4, 309–316
Figure 4: S21 response analysis for different dimensions of d3
Figure 5: Comparative analysis of simulated S11 and S21 response of
the proposed MTSR-CSIR filter

FBW (%), insertion loss abbreviated as IL in (dB), and
return loss abbreviated as RL in (dB).
Table 1 presents a comparative analysis of the pro-
posed MTSR-CSIR filter and various other tri-
ple-passband filters reported in the literature. The results
show that the proposed filter performs better than the
FUIR, MTM, MW-BPF, coupling MBF and UIR filters.
The MTSR-CSIR filter achieves an enhanced bandwidth
and low insertion loss with a compact physical size.
The MTSR-CSIR filter has a simple structure with a
low fabrication complexity, making it a cost-effective
and practical solution for various wireless communica-
tion applications. Center frequencies of 2.2/3.62/4.6 GHz
relating to the three passbands are achieved by the
MTSR-CSIR filter. Furthermore, fractional bandwidths
of about 16/14.3/11.52 %, respectively, are achieved with
the MTSR-CSIR filter, which is higher than with the
other filters.
The insertion loss obtained by the MTSR-CSIR filter
is 0.62/0.65/0.71, respectively, lower than that of the
other filters. The return loss of the proposed filter is also
superior, achieving values of 65/46/40, respectively. A
defected ground-structure filter is also considered in the
comparison, and the MTSR-CSIR filter outperforms it in
terms of its simple structure and superior performance.
The comparative analysis shows that the proposed
MTSR-CSIR filter performs better than the other filters.
The filter has a compact physical size, low insertion and
high return loss, making it an ideal candidate for wireless
communication applications. The simple structure of the
MTSR-CSIR filter also makes it easy to fabricate and
implement.
5 CONCLUSION
The proposed MTSR-CSIR filter is compact and sim-
ple, without any defected structure in the ground plane.
The modified T-shaped resonator includes a highly appli-
cable triple-bandpass filter. The physical dimensions of
the filter are adjusted to tune the desired frequency. The
proposed MTSR-CSIR filter is compared with the exist-
ing tri-BPFs, such as the FUIR filter, MTM filter,
MW-BPF filter, coupling MBF filter and UIR filter. The
simulation results show the center frequencies of
2.2/3.62/4.6 GHz for the three passes. The fractional
bandwidth achieved by the MTSR-CSIR is about
16/14.3/11.52 %, respectively. The insertion loss ob-
tained by the filter is 0.62/0.65/0.71, respectively, and
the return loss of the proposed BPF is about 65/46/40,
respectively. The triple-passband BPF exhibited an effec-
tive signal-selection performance for the propagated sig-
nal, as demonstrated by the simulation of the elec-
tric-field distribution. The results show that the proposed
MTSR-CSIR applies to 5G mobile communications. The
proposed MTSR-CSIR filter outperforms the existing
tri-BPFs regarding bandwidth, insertion loss and physi-
cal size. The MTSR-CSIR filter has a simple structure
with low fabrication complexity, making it a cost-effec-
tive solution for 5G communication systems. The pro-
posed MTSR-CSIR filter is a compact, simple, cost-ef-
fective solution for tri-band filtering applications. The
filter achieves the desired center frequency and fractional
bandwidth with a low insertion loss and high return loss.
The filter’s simple structure and improved performance
make it an excellent candidate for 5G mobile communi-
cation systems.
Declaration of competing interest
The authors declare that there are no known compet-
ing financial interests or personal relationships that could
have influenced the work reported in this paper.
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Materiali in tehnologije / Materials and technology 57 (2023) 4, 309–316 315
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