design of bpf using pseudo-interdigital structure for ... · the bpf is first designed and...

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.


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.


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.


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


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.


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.


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)


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.


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


Figure17. Configuration of the integrated filter-antenna radiating patch.

Figure18. Configuration of the integrated filter-antenna ground structure.


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.


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)


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.


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


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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