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International Journal of Electronics Engineering Research.
ISSN 0975-6450 Volume 9, Number 1 (2017) pp. 37-55
© Research India Publications
http://www.ripublication.com
Design of BPF using Pseudo-Interdigital Structure for
Impedance Bandwidth enhancement of Monopole
Antenna for UWB Application
Yatindra Gaurav1 and R.K. Chauhan2
1, 2 Department of Electronics and Communication Engineering, Madan Mohan
Malaviya University of Technology, Gorakhpur, Uttar Pradesh, India.
Abstract
This paper presents a design of combine structure of multimode resonator as
bandpass filter with monopole antenna aimed at enhancing the impedance
bandwidth keeping the radiation pattern almost same as of monopole antenna
in E-field and H-field. Pseudo-interdigitl structure is used as multimode
resonator showing passband characteristics for ultra wide band application. To
improve the return loss, Insertion loss and to remove the ripples in the
passband the traditional Interdigital structure is modified to pseudo-interdigital
structure. The BPF is first designed and simulated showing lower and higher
cut off frequency as 3.1 GHz and 11.8 GHz with minimum Insertion loss as
0.22 dB and return loss as more than 13 dB. Secondly the UWB monopole
antenna is designed and simulated covering a range of 3.2 GHz to 10.85 GHZ
with return loss more than 13.8 dB. The integration of designed UWB
bandpass filter with UWB monopole antenna is presented and results in
advantage over impedance bandwidth of overall system. The lower and higher
cutoff frequency of filter – antenna is found to be 2.7 GHz and 10.6 GHz with
return loss more than 11 dB. The resulting parasitic element is engineered to
produce its fundamental resonant mode in such a manner that overall compact
design is realized illustrating an enhancement in 10 dB rejection fractional
bandwidth of 0.5 GHz at lower frequency side is obtained while maintaining
impedance matching without any significant changes to the original design
parameters with sharp frequency cuttoff at both the edges of UWB band. The
proposed integrated filter-antenna is designed and simulated in Agilent
Advance Design System.
Keywords: Pseudo-interdigital structure, ultra wideband, bandpass filter,
UWB monopole antenna and Impedance bandwidth.
38 Yatindra Gaurav and R.K. Chauhan
INTRODUCTION
UWB devices and circuits became more popular and efficient in microwave
communication systems when Federal Communication Commission (FCC) has
commercialized the unlicensed ultra wide band (UWB) frequency spectrum ranging
from 3.1 to 10.6 GHz [1]. The planar configuration of UWB devices and systems is of
valuable interest because of its superior s-parameter performance characteristics
leading to low cost, easy fabricable and integration with monolithic RF circuits with
compact size and simple structure [2]. Planar design causes radiation pattern
degradation as in high accuracy positioning system, portable devices and cognitive
radios, to overcome these drawbacks the system should have stable radiation pattern
over the bandwidth of ultra wide band spectrum [3, 4]. Several techniques that have
been used for stable radiation pattern, one of them are loading of parasitic elements
[4-6] and modification of the radiating patches such as rectangular notch embedded
patch, epitrochoid-shaped patch, and miniaturized patch [7-9], another method is by
modifying ground planes (ex. Fractal ground) and by improving feeding element such
as differential feeding techniques [10-16]. These design strategies made the radiation
pattern stable but with certain drawbacks such as increases in physical size, multilobe
problems and impedence mismatch defects may arise.
In this paper, a designed planar and compact UWB BPF is integrated at the feeding
point of UWB monopole antenna to enhance the impedance bandwidth of filter-
antenna. Step impedance radiating patch with defected ground structure is used for
improving impedance matching of antenna. The integration is demonstrated by
simulation, designed by UWB BPF using pseudo-interdigital structure. The
individually design of antenna, filter and combine structure is done on substrate
FR4_epoxy of permittivity 4.4 with dielectric thickness of 1.6 mm.
2. ANALYSIS AND DESIGN OF PROPOSED UWB BPF
The proposed UWB BPF structure consists of pseudo-interdigital structure loaded
with rectangular slots at both the ports, see Fig. 1. This pseudo-interdigital structure is
a combline structure with coupled finger lines. As compared to interdigital structure
the effect of inductance and capacitance is generated by the length of fingers and the
space between them respectively. The Capacitors, CP1 and CP2 generated due to
dielectric material of the filter, are shown in the equivalent circuit, Fig. 2 and Fig. 3.
Figure 1. Configuration of the proposed filter.
Design of BPF using Pseudo-Interdigital Structure for Impedance Bandwidth… 39
Figure 2. Interdigital Structure.
Figure 3. Equivalent circuit of interdigital structure.
Equivalent circuit and frequency response symbolizes the nature of interdigital
structure as bandpass filter with ripples. These ripples are generated due to different
coupling of capacitors and inductors. To eliminate these ripples from the passband,
couplings of fingers is modified known as pseudo interdigital structure, see Fig. 4 and
Fig. 5.
Figure 4. Pseudo-Interdigital Structure.
40 Yatindra Gaurav and R.K. Chauhan
Figure 5. S-Parameter of Interdigital Structure and Pseudo-Interdigital structure.
The length of fingers L1 and L2 in the proposed UWB BPF structure controls the
higher cut off frequency of the filter. On increasing any of the length either L1 or L2
and keeping the other as constant, the inductive effect of structures increases and
because of this, the higher cut off frequency of filter decreases, see Fig. 6 and Fig. 7.
Figure 6. S-Parameter of Pseudo-Interdigital Structure when L2 is kept constant and L1 is
varied.
Design of BPF using Pseudo-Interdigital Structure for Impedance Bandwidth… 41
Figure 7. S-Parameter of Pseudo-Interdigital Structure when L1 is kept constant and L2 is
varied.
For further improvement in S-parameter of passband, a rectangular slot is introduced
at the interface of pseudo-interdigital structure at its input/output ports, see Fig 1. The
impact of these additional structures can easily be seen from simulated response,
shown in Fig. 8. The higher cutoff frequency reduces by 0.37 whereas lower cutoff
frequency remains almost constant keeping insertion loss constant with slight change
in return loss. For selectivity a transmission zeros can be created near the cut off
frequencies of filter by loading stubs on pseudo interdigital structure but due to
radiation loss the stubs can’t be loaded as calculated from expression 1 −(|𝑆11|
2 + |𝑆21|2) [25-28]. By optimizing the dimensions of fingers and rectangular
slots, the pass band range of the filter is achieved between 3.1 GHz and 11.8 GHz,
with 117% as 3dB fractional bandwidth. The minimum insertion loss achieved is 0.22
dB whereas return loss is more than 13 dB, see Fig. 9. The optimized dimensions of
the design are as follows: L1 = L2 = 5.3 mm, W1 = 0.15 mm, W2 = 2.87 mm, W3
=1.65 mm, S1 = 0.15 mm, S2 = 0.2mm. The overall size of the filter is 6.15 mm x
1.65 mm.
42 Yatindra Gaurav and R.K. Chauhan
Figure 8. Frequency response of Pseudo-Interdigital structure with and without rectangular
slot.
Figure 9. Frequency response of proposed UWB bandpass filter.
ANALYSIS AND DESIGN OF PROPOSED UWB MONOPOLE ANTENNA.
To design an ultra wide band antenna two or more resonant parts operating at its
frequency is required and overlapping of these multiple resonance frequencies on each
other results in multiband performances. On one side of the dielectric substrate of
proposed antenna, it comprises of step impedance radiating rectangular patch having
microstrip feeding and on other side, it comprises of defected ground structure, see
Fig. 10 and Fig. 11. The overall design generates three or four resonant bands
Design of BPF using Pseudo-Interdigital Structure for Impedance Bandwidth… 43
achieving ultra wide band bandwidth, see Fig 12. The defected ground structure near
the feeding is used to improve the return loss and bandwidth of the proposed antenna.
The ground is extended upward around the radiating patch achieving V-shaped
structure, to increase the bandwidth of antenna. A good impedance matching is
obtained over the entire operating band with four resonant bands.
Figure10. Configuration of the Ground structure of proposed Antenna
Figure 11. Configuration of the radiating patch of proposed Antenna.
44 Yatindra Gaurav and R.K. Chauhan
Figure 12. 3D structure of the proposed monopole antenna.
The proposed antenna is designed with dimensions as follows: L3 = 22, L4 = 11, L5 =
3.18, L6 = 2.79, L7 = 2.7182, L8 = 0.1501, L9 = 1.05, L10 = 0.5, L11 = 4.98, L12 =
2.41, L13 = 5.25, L14 = 2.71, L15 = 2.7922, L16 = 3.92, W2 = 2.8683, W4 = 4, W5 =
7.4494, W6 = 2.79, W7 = 0.81, W8 = 0.75, W9 = 1, W10 = 1, W11 = 1, W12 = 0.8,
W13 = 1.65, W14 = 4.43, W15 = 0.585, W16 = 0.39, W17 = 24, W18 = 11.3, W19 =
4.99, W20 = 1.9895 (all dimension are in mm). Total area of proposed antenna is 24
x 22 x 1.6 mm3 with patch and ground on either side of dielectric substrate. Total
dimension of step impedance radiating patch is 14 x 11 mm2 for introducing different
resonance and for improve the return loss. Microstrip feeding technique is used in the
proposed antenna with length 5.25 mm and width of 2.87 mm.
Simulated results of proposed antenna is defined by S11 parameter i.e. Return Loss of
proposed antenna. The proposed antenna covers the impedance bandwidth from 3.2
GHz to 10.85 GHz with four resonance points on 3.6 GHz, 5.854 GHz, 8.120 GHz
and 10.06 GHz frequency. Return Loss at their respective resonance frequencies are
11 dB, 28.73 dB, 21.321 dB and 27.365 dB, see Fig. 13.
Figure13. Frequency response of proposed monopole antenna.
Design of BPF using Pseudo-Interdigital Structure for Impedance Bandwidth… 45
The 2D far–field Radiation pattern of proposed antenna is measured at different
frequency of 3.67 GHz, 5.33 GHz and 9.78 GHz, see Fig. 14. A uniform, stable and
good radiation pattern is obtained for the antenna proposed. Table 1 shows the E-field
and H-field simulated performance in decibel of the proposed antenna. Fig. 15 shows
the 3D radiation pattern of omnidirectional monopole antenna. Fig. 16 shows the
surface current distribution on the patch of the proposed antenna.
Table 1. E-field and H-field simulated characteristics of the proposed Antenna.
S. No. Properties Antenna
E-Field H-Field
3.67
GHz
5.33
GHz
9.78
GHz
3.67
GHz
5.33
GHz
9.78
GHz
1. Radiated Power
(Decibel)
-35.26 -35.2 -32.79 -53.95 -56.1 -59.4
2. Directivity
(Decibel)
4.1 4.3 7.18 -14.56 -18.9 -19.6
3. Gain (Max)
(Decibel)
2.27 1.9 4.25 -16.4 -16.6 -22.2
4. Efficiency
(%)
65.96 56.97 50.913 65.97 56.97 50.91
E- Field H-Field
(a)
46 Yatindra Gaurav and R.K. Chauhan
E- Field H-Field
(b)
E- Field H-Field
(c)
Figure 14. Simulated radiation Pattern of proposed antenna at frequencies (a) 3.67
GHz (b) 5.33 GHz (c) 9.78 GHz.
Figure 15. 3D radiation pattern of proposed UWB monopole antenna.
Design of BPF using Pseudo-Interdigital Structure for Impedance Bandwidth… 47
Figure 16. Current distribution of proposed antenna.
An antenna with microstrip feeding is proposed for UWB application. The simulated
result shows stable radiation pattern overall bandwidth of UWB. The good impedance
matching characteristics and omni-directional radiation patterns over the UWB
bandwidth is obtained.
INTEGRTION OF PROPOSED UWB BPF WITH PROPOSED UWB
MONOPOLE ANTENNA.
Next, the proposed pseudo-interdigital structure as multimode resonator for UWB
BPF is integrated to the proposed UWB monopole antenna, the filter was connected
directly to the microstrip feed line, see Fig. 17 and Fig. 18. It is clear from the
frequency response that the integration of proposed filter with the proposed monopole
antenna shows remarkable improvement in 10 dB rejection fractional bandwidth at
lowe frequency side, The integrated structure bandwidth extends from 2.7 GHz to
10.6 GHz, see Fig. 19. Fig. 20 shows the gain performance of the filter-antenna and
antenna for UWB application. The integrated filter antenna has a good
omnidirectional radiation performance and shows almost same radiation pattern
considering cross polarization, see Fig. 21. The optimize structural overall size is 32.9
X 22 mm2
48 Yatindra Gaurav and R.K. Chauhan
Figure17. Configuration of the integrated filter-antenna radiating patch.
Figure18. Configuration of the integrated filter-antenna ground structure.
Design of BPF using Pseudo-Interdigital Structure for Impedance Bandwidth… 49
Figure 19. Frequency response of the integrated filter-antenna radiating patch.
Figure 20. Gain performance of proposed integrated filter-antenna.
The simulated performance characteristics of the integrated filter-antenna are given by
the magnitude of the reflection coefficients over the UWB frequency range. The
simulated 10 dB rejection fractional bandwidth of the integrated filter-antenna is from
2.7 GHz to 10.6 GHz is 118.8 %. Comparing the frequency response of the antenna
and the filter-antenna, it is seen that the lower cutoff frequency is downshifted by 0.5
GHz whereas the higher cutoff frequency is downshifted by 0.25 GHz. This small
difference is due to the presence of filter since it is well known (diagnosed) that the
presence of filter reduces the error associated with long coaxial cable on the measured
impedance performance as reported earlier [32 and 33]. Table 2 shows the E-field and
H-field simulated performance in magnitude and decibel of the proposed filter-
antenna.
50 Yatindra Gaurav and R.K. Chauhan
Table 2. E-field and H-field simulated characteristics of the proposed Filter-Antenna.
S.No. Properties Filter-Antenna
E-Field H-Field
3.67 GHz 5.33 GHz 9.78 GHz 3.67 GHz 5.33 GHz 9.78 GHz
1. Radiated Power
(Decibel)
-34.97 -36.3 -34.22 -53.81 -36.1 -58.8
2. Directivity
(Decibel)
4.15 4.125 6.22 -14.69 -15.7 -18.2
3. Gain (Max)
(Decibel)
2.06 0.82 2.815 -16.78 -18.9 -21.6
4. Efficiency
(%)
61.89 46.77 45.6 61.89 46.77 45.6
E- Field H-Field
(a)
E- Field H-Field
(b)
Design of BPF using Pseudo-Interdigital Structure for Impedance Bandwidth… 51
E- Field H-Field
(c)
Figure 21. Simulated radiation Pattern of filter-antenna at frequencies (a) 3.67 GHz
(b) 5.33 GHz (c) 9.78 GHz.
Figure 22. 3D radiation pattern of proposed filter-antenna.
52 Yatindra Gaurav and R.K. Chauhan
Figure 23. Current distribution of proposed filter-antenna.
On comparing the gain of antenna and filter-antenna, it is found to be almost same,
see Fig. 20. The co-polarization and cross polarization of filter-antenna is found to be
almost same as in antenna, see Fig. 14 and Fig. 21. The impedance bandwidth is
found to be improved at both higher and lower frequency side, see Fig. 13 and Fig.
19. Fig. 22 shows a good omnidirectional radiation pattern of filter-antenna. Fig. 23
shows the current distribution of filter-antenna. Current distribution of antenna and
filter-antenna clears that the integration of filter with antenna changes the current
distribution at the feed point due to which current is enhanced on the patch at
microstrip feedline improving the impedance bandwidth.
CONCLUSIONS
A compact and planar pseudo-interdigital UWB BPF structure is proposed. Insertion
loss and return loss of the filter is good. The filter has small size of 6.15 mm x 1.65
mm with 3 dB fractional bandwidth of 119.5 %. An omnidirectional Monopole
antenna is also proposed for UWB application with overall size of 24 mm x 22 mm.
The simulated and measured result shows stable omnidirectional radiation pattern on
overall bandwidth of UWB. A good impedance matching is achieved in the proposed
antenna. 10 dB rejection fractional bandwidth of the proposed antenna is 109 %.
When this proposed UWB monopole antenna is integrated with the proposed UWB
BPF, then an enhancement in UWB bandwidth is seen that is 10 dB rejection
fractional bandwidth from 109 % to 118.8 %. The integration of proposed filter to
proposed antenna shows the down shifting of lower and higher cutoff frequency in s-
parameter performance by 0.5 GHz and 0.25 GHz keeping the gain performance and
Design of BPF using Pseudo-Interdigital Structure for Impedance Bandwidth… 53
cross polarization in both E plane and H plane nearly same to as of proposed antenna.
A good omnidirectional radiation pattern is obtained in both antenna and filter-
antenna. The overall size of the integrated filter-antenna structure is 32.9 x 22 mm2.
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56 Yatindra Gaurav and R.K. Chauhan
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