1 FRACTAL SHAPED MICROSTRIP COUPLED LINE BAND PASS FILTERS FOR SUPPRESSION OF 2 ND HARMONIC Il Kwon Kim, Nickolas Kingsley, Matt Morton, Ramanan Bairavasubramanian, John Papapolymerou, Manos M. Tentzeris, and Jong-Gwan Yook ABSTRACT — In this paper, microstrip coupled line band pass filters using Koch fractal curves are proposed for the first time. These filters are fabricated on Liquid Crystal Polymer (LCP) substrate for Ku Band. Conventional microstrip coupled line filters are very popular for RF front ends because these can be made easily. However, their large 2 nd harmonic causes the shape of the pass band to be asymmetric in the upper band and it makes the skirt properties worse. By proper design, the 2 nd harmonic of fractal filters can be significantly suppressed through the use of fractal shapes. In LCP, the maximum harmonic suppression is almost 42 dB. Index Terms — Koch Fractal geometry, Fractal shape band pass microstrip coupled line filter, Liquid Crystal Polymer (LCP), 2 nd harmonic suppression I. INTRODUCTION Traditionally, microstrip coupled line filters have been used to achieve narrow fractional bandwidth band pass filters due to their relatively weak coupling [1]. However, a parasitic second harmonic contributes to an asymmetric pass-band shape and degrades upper band skirt properties. In addition, a large 2 nd harmonic signal can degrade the performance of system components, such as mixers. Due to the large difference between the even and odd mode effective dielectric constants of microstrip coupled lines, the phase velocity between two modes is significantly different. This problem is more pronounced when filters are fabricated on high dielectric constant materials, such as silicon or GaAs [2]. To overcome this problem, in this paper, Koch fractal geometry has been applied to the coupled sections of the filter. I.K. Kim and J.G. Yook are with Department of Electric and Electronic Engineering, Yonsei University, Seoul, Korea. N. Kingsley, M. Morton, R. Bairavasubramanian, J. Papapolymerou and M. Tentzeris are with the Department of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA. Several fractal geometries (Koch curve, Sierpinski gasket, and Hilbert curve, etc.) have been widely studied to develop various microwave devices, such as antennas, frequency selective surfaces (FSS) [3], and photonic band gap (PBG) devices [4]. All of these fractal shape devices have several advantages, including reduced resonant frequencies and broad bandwidth. These characteristics give the fractal shape two unique properties: space filling and self similarity. A fractal shape can be filled on a limited area as the order increases and occupies the same area regardless of the order. This is due to the space filling property. By self similarity, a portion of the fractal geometry always looks the same as that of the entire structure. Predominantly, fractal research in microwave engineering is concentrated on antennas because the above properties enable the development of miniaturized, multi-band antennas [5-8]. Conventionally, there are two methods to solve the second harmonic problem in microstrip coupled line structures: making the phase velocity of even and odd modes the same or compensating different electrical length of both modes. To date, researchers have further added reactive components such as photonic band gap (PBG) and defect ground structures (DGS) [9]. However, in these cases, the components become complicated and have a leaky wave problem due to discontinuities in the ground plane. The second configuration involves making optimum line structures by inserting periodic shapes, such as grooved, wiggly and inter-digitized lines into conventional coupled lines [10-12]. These periodic structures can be used to create Bragg reflections to suppress the second harmonic. In this paper, Koch fractal shaped microstrip band pass filters are proposed for the first time. These fractal shape filters are fabricated on 8-mil liquid crystal polymer (LCP) substrate. The center frequency of the designed fractal shape filter is approximately 13 GHz. It has been found that by applying the Koch fractal geometry into a coupled line microstrip band pass filter (BPF), the second harmonic can be greatly suppressed.