A Simple Wideband Microstrip Bandstop Filter for WLAN and WiMAX Band Rejection Purpose Lakhindar Murmu 1* and Sushrut Das 2 1, 2 Department of Electronics Engineering Indian School of Mines, Dhanbad, India. * [email protected] 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 8th International Conference on Electrical and Computer Engineering 20-22 December, 2014, Dhaka, Bangladesh 978-1-4799-4166-7/14/$31.00 ©2014 IEEE 47