Narrow Gap Coupled L-Shaped Microstrip Filter Design For WIMAX Applications 1 Srishti Singh 2 Anupma Marwaha M.Tech Research Scholar 1 , Associate Professor 2 ECE Deptt. SLIET Longowal, Punjab-148106, India Abstract In modern microwave communication systems high performance bandpass filters having low insertion loss and high selectivity together with linear phase in the passband are required. L-shaped coupled microstrip lines with controlled gap coupling are utilized in this paper to obtain narrowband filter characteristics. The bandpass filter is designed on Rogers substrate material with permittivity of 3.38 and achieves bandwidth of 163.02 MHz (4.52% fractional bandwidth) at center frequency of 3.65 GHz while maintaining narrow gap between coupled resonators. The simulative analysis of microstrip bandpass filter is performed using FEM based COMSOL Multiphysics software. Keywords Finite Element Method, Insertion Loss, Microstrip Line, Microstrip Filter, Return Loss, WIMAX 1. Introduction Bandpass filter is a passive component which is able to select signals inside a specific bandwidth at a certain center frequency known as pass band and reject signals in another frequency region, especially in frequency regions, known as stop band. At microwave frequency, the bandpass filter with low insertion loss and compact size plays an important role in the microwave communication systems especially in the transmitting and receiving systems to identify and transmit the desired signals [1]. Filters with planar nature are particularly attractive because of their easy fabrication, simple synthesis procedure, good repetition, compact size, and low cost [2]. Besides their evident application of providing frequency selectivity, filters are necessary for matching networks which form an integral part of multiplexers and amplifiers. Microstrip transmission lines are one of the most popular types of planar transmission lines, used in microwave integrated circuits (MIC) and monolithic microwave integrated circuits (MMIC), primarily because of its relative ease of fabrication and its simple integration with other passive and active microwave devices. It has many advantages, which include small size, low cost, no critical machining, no cutoff frequency, ease of active device integration, use of photolithographic method for circuit production, good reproducibility, repeatability and ease of mass production [3]. There are four different types of bandpass filters at microwave frequency: the combined filters, interdigital filters, parallel coupled filters and the hair-pin line filters. In modern RF/microwave communication systems, the size of the planar microwave filters is one of the major concerns, especially when these filters are applied in the monolithic microwave integrated circuits (MMIC) [4]. Larger gaps between coupled resonators is desirable to achieve narrow bandwidth and high selectivity demanding large quality factor, Q, which however will lead to increased filter size. The present day applications in wireless communication near 3 GHz demand for small size and low cost designs while achieving narrow FBW and high value of Q. 2. Design Methodology The filter design is based on the coupling of microstrip lines through a common ground plane. A coupled microstrip line configuration consists of two transmission lines placed parallel to each other and in close proximity. In this configuration there is a continuous coupling of the electromagnetic fields between the two lines and cascading the stripline elements gives rise to bandpass and bandstop filter structures that are most easily designed with the aid of RF circuit simulation packages. A simple modelling approach of coupled microstrip line interaction is established considering the geometry depicted in Figure1. 957 International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 www.ijert.org Vol. 2 Issue 6, June - 2013 IJERTV2IS60419