Fractal-shaped microstrip coupled-line bandpass filters for suppression of second harmonic

In this paper, microstrip coupled-line bandpass filters using a Koch fractal shape are proposed for the first time. These filters are fabricated on a liquid crystal polymer (LCP) substrate for Ku-band. Conventional microstrip coupled-line filters are very popular for RF front ends because they can be fabricated easily. However, their large second harmonic causes the shape of the passband to be asymmetric in the upper band and it worsens the skirt properties. By proper design, the second harmonic of fractal filters can be significantly suppressed through the use of fractal shapes. In this paper, using LCP, the maximum harmonic suppression was almost 42 dB. This type of filter can be used to suppress the second harmonic without any additional devices and regardless of the substrate.