A Miniature Hilbert Fractal-based Bandpass Filter Design with Tuning Stub and 2nd Harmonic Suppression

A new compact microstrip bandpass filter design is introduced in this paper as a candidate for use in modern wireless communication systems. The proposed filter structure is composed of two fractal-based microstrip resonators. The structure of each resonator is in the form of the Hilbert fractal curve geometry. Two microstrip single-mode resonators with structures based on the 3rd and 4th iteration Hilbert fractal-shaped geometries have been modeled at a design frequency of 2.4 GHz. The resulting filter structures based on these resonators, show considerable size reduction compared with the other microstrip bandpass filters based on other space-filling geometries designed at the same frequency. A second set of bandpass filter designs based on the same resonators but with a tuning stub has been also presented, in an attempt to provide practically useful means to tune the filter to the specified performance with a considerable tuning range. The performance of the resulting filter structures has been evaluated using a method of moments (MoM) based software package, Microwave Office 2007, from Advanced Wave Research Inc. Results show that the proposed filter structures possess good return loss and transmission responses besides the size reduction gained, making them suitable for use in a wide variety of wireless communication applications. Furthermore, performance responses show that the second set of filters, based on Hilbert shaped resonators with stubs has less tendency to support the 2nd harmonic.