Compact microstrip band-pass filters based on semi-lumped resonators

A new design approach for compact microstrip band-pass filter based on semi-lumped resonators are proposed. The resonators, which are coupled through quarter wavelength meander lines acting as admittance inverters, are shunt connected to the line. They consist of parallel combination of a grounded (inductive) stub and a narrow metallic strip followed by a capacitive patch to ground. With this topology, the necessary degree of flexibility to design narrow and broad-band-pass filters with compact dimensions and good out-of-band performance was obtained. Another key advantage of the devices, as compared to previous lumped or semi-lumped element- based structures reported by the authors, is the absence of ground plane etching. To illustrate the potentiality of the proposed approach, a third-order (30% fractional bandwidth) and a nineth-order (35% fractional bandwidth) Chebyshev band-pass filters have been designed and fabricated. The measured frequency responses are very symmetric and exhibit low in-band losses as well as good out-of-band rejection up to approximately 3f_o. Filter dimensions are as small as 0.40lambda times 0.12lambda (third-order prototype) and 0.62lambda times 0.16lambda (nineth-order prototype), lambda being the guided wavelength at f_o. With these dimensions and performance, and the possibility to synthesise microstrip filters with controllable bandwidth over a wide margin, it is believed that the reported approach can be of actual interest for the design of planar filters at microwave frequencies.