A Simple Wideband Microstrip Bandstop Filter for WLAN and WiMAX Band Rejection Purpose

Lakhindar Murmu1* and Sushrut Das 2 1, 2 Department of Electronics Engineering Indian School of Mines, Dhanbad, India.

*lakhndar.kgec25@gmail.com

Abstract— In this paper, a simple hexagonal wideband microstrip bandstop filter with an open-end stepped impedance stub has been proposed for the rejection of WLAN and WiMAX bands. Controlling the dimensions of the shunt open-end stepped impedance stub, the rejection bandwidth and the level of rejection can be controlled. The structure is very simple and is also easy to fabricate. The final filter structure provides a stop band that extends from 3.19 GHz to 5.36 GHz within and more than -20dB rejection and hence it can reject the 3.5 GHz WiMAX bands and 5.2 GHz WLAN frequency bands. The simulated response has been validated after comparing it with the measured data.

Index Terms—Open loop hexagonal filter, bandstop filter (BSF), stepped impedance resonator (SIR).

I. INTRODUCTION With rapid growth of communication technologies, WLAN

and WiMAX communications have been developed a lot. These technologies have become so popular that in present days it has become almost a part of our life and almost every corner of the world is flooded with the signals from WLAN and WiMAX. These high speed WLAN and WiMAX signals propagate rapidly through the infrastructure of office and home environments and couple with other RF systems. These coupled signals propagate along the transmission line and circuits of the RF system and may result in an undesired behaviour of the system. Therefore to protect the RF systems from these signals bandstop filters can be used.

Recently, numerous methods and structures have been proposed for realizing the bandstop filters [1-2]. Based on the formulae of SIR, bandstop filters with high skirt selectivity as well as wide rejection bandwidth and miniature circuit configuration were presented in [1,2]. Divyabramham et al. [2] presented a design technique of sharp-rejection wideband bandstop filters by using a lossless transmission line model. In the year 2008, a new compact bandstop microstrip filter using two cascaded ring resonators was presented and the equivalent circuit model was developed to characterize the proposed bandstop structure [3]. One year later, in [4] a technique to design a dual-band bandstop filter (DBBSF) was presented with a dual-mode loop resonator.

The hexagonal geometry promises better confinement in the microstrip circuits due to their large interior angles (as compared to rectangle and square counterparts).Open loop hexagonal multiplexer for communication system were proposed by R. Kumar and G.A. Edae [5]. In that paper they have presented the tri-band band multiplexer topology based on the hexagonal close loop resonators of different size which were capacitive coupled from a single input. In [6] a novel microstrip bandstop filter was proposed, where signal

interference technology was applied for achieving the bandstop behaviour. Two open loop resonators were introduced to the conventional transmission line with larger electrical length. Then, two transmission zeros were obtained in the stopband. Meanwhile, several transmission poles were used to improve the flatness of the passband. In comparison with the traditional bandstop filters using signal interference technique, the proposed filter has wider stopband and flat passband. In the same year, a new and miniature circuit configuration for wideband bandstop filter (BSF) operating at X-band range was presented [7]. The presented planar microstrip filter consists of a three-section stepped impedance resonator (SIR) paralleled with three open-circuited stepped impedance stubs (SISs).

In this paper the authors have proposed a compact open loop hexagonal resonator filter with open-end stepped impedance stub which has been designed for the rejection of 3.5 GHz WiMAX band and 5.2 GHz WLAN frequency band. Commercial IE3D simulator has been used to design the structure.

II. OPEN LOOP HEXAGONAL FILTER DESIGN An open loop hexagonal loop filter with direct coupled

tapered ports is shown in Fig.1. The length L (total loop length, L= [(6×a) –g]) determines the centre frequency of filter. A Pair of 50-Ω collinear tapered lines feed with the resonator. RT/Duroid 6010 with a thickness of 1.27 mm and a relative dielectric constant of 10.2 has been used as the substrate. The simulated response i.e. insertion loss and return loss versus frequency is shown in Fig.2. The figure reveals that it has a band pass response from DC to 2.2 GHz (up to 2.2 GHz) and a band stop response from 2.2GHz to 5.4 GHz.

Fig.1. Hexagonal open loop bandstop filter (Filter A).

Inspired by the band stop behaviour of a simple hexagonal

open loop, a second hexagonal bandstop filter, but this time loaded with open ended stepped impedance stub, has been

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developed as shown in Fig.3. The filter structure has been studied by varying the dimensions of the stubs of the hexagonal resonator. The frequency response of the proposed filter is shown in Fig.4 for different parameter values. The respective filter parameters are tabulated in table 1.

Fig.2. Simulated response of hexagonal open loop bandstop filter (Filter A).

Fig.3. Hexagonal resonator bandstop filter with open-end stubs (Filter B).

Fig.4. Simulated frequency response of the proposed (Filter B).

Table 1: Geometrical dimensions of the filters A-B

Filter A B a 6mm 6mm b -- 3mm d -- 4mm f 2.05mm 2.05mm g 1mm 2mm w1 0.85mm 0.2mm w2 -- 3mm w3 -- 1mm

III. FABRICATION AND MEASUREMENT RESULTS All Based on the simulated results obtained from IE3D, the

filter has been fabricated with the proposed dimensions: a=6mm, b=3mm, d=4mm, w1=0.2mm, w2=3mm, w3=1mm,

g=2mm (Filter B) as shown in Fig. 5 and then measured for frequency response. The measurement was carried out using an Agilent E5071C Vector Network Analyzer and the measured response is shown in Fig.6. Simulated frequency response of the proposed filter is also shown in same figure. The measure frequency response shows the 3dB cut-off frequencies of 2.62 GHz and 5.63 GHz. The highest insertion loss in the low and in the high frequency pass-band is better than 0.3 dB and 1.5 dB, respectively. The highest return loss in the stop-band is better than 0.4 dB. The measured result provides a stop band that extends from 3.03 GHz to 5.28 GHz within and more than -20dB rejection and hence it can reject the 3.5 GHz WiMAX bands and 5.2 GHz WLAN frequency bands. The measured result shows a good agreement with the simulated data. The result shows a passband return loss of about 20dB & stopband return loss about 0.4 dB.

Fig.5. Photograph of bandstop filter (Filter B).

Fig.6. Measured and simulated frequency response of the proposed filter

(Filter B). The stop band insertion loss is better than 30dB. Fig. 6 also

shows that at the rejection band plenty of ripples are present whereas the simulation is smooth. The ripples are due to the connector's flanges and screws that are missing in the simulation model.

IV. CONCLUSIONS A Compact open loop hexagonal resonator bandstop filters

with and without a loaded open stub have been presented in this paper. The bandwidth of the filter with a loaded open stub has been observed to have a better performance. The optimization of the various parameters of the filter has been accomplished to achieve a good response. The proposed filter is compact, easy to fabricate and easy to integrate with MIC/MMIC microwave circuits. The final filter structure provides a stop band that extends from about 3.19 GHz to 5.36 GHz and hence it can reject the 3.5 GHz WiMAX and

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5.2 GHz WLAN frequency bands. The simulated response has been validated after comparing it with the measured data. In the measured results, the centre frequency of the fabricated filter has been shifted towards a lower frequency. This is due to the fabrication tolerance.

REFERENCES [1] Wang Y.-Z. and M.-L. Her, “Compact microstrip bandstop filters using

stepped-impedance resonator (SIR) and spur-line sections,” IEE Proc.-Microw. Antennas Propag. Vol. 153, No. 5, pp. 435-440, 2006.

[2] Divyabramham, K., M.K. Mandal and S. Sanyal, “Sharp-Rejection Wideband Bandstop Filters,” IEEE Microwave and Wireless Components Letters, Vol. 18, No. 10, pp. 662-664, 2008.

[3] A. Boutejdar, A. Batmanov, A. Omar and E. Burte, “A new method to improve the reject band of a 5.6 GHz bandstop filter using λ/2 open-loop ring microstrip resonators,” Asia-Pacific Microwave Conference, APMC,pp.1-4, 2008.

[4] H.-K. Chiou and C.-F. Tai, “Dual-band microstrip bandstop filter using dual-mode loop resonator,” Electronics Letters, Vol.45, No.10, pp.507-509, 2009.

[5] Raj Kumar and Girma Alemu Edae, “On the Design of Close and Open Loop Hexagonal Multiplexer for Communication System,” 2nd International Conference and workshop on Emerging Trends in Technology (ICWET), Proceedings published by International Journal of Computer Applications® (IJCA), pp.7-14, 2011.

[6] Hao-Ran Zhu , Wei Shen and Jun-Fa Mao, “A novel bandstop filter with wide stopband using signal interference technology”, International Workshop on Microwave and Millimeter Wave Circuits and System Technology (MMWCST), pp.1-4, 2012.

[7] Hui Chen, Hong-sheng Zhong, Zhi-qin Zhao and Yu-xing Zhang, “A compact X-band microstrip bandstop filter using stepped impedance line with open-end stepped impedance stubs,” International Conference on Microwave and Millimeter Wave Technology (ICMMT), Vol.2, pp.1-3, 2012.

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