Miniaturized wideband ring-type bandpass filters with upper stopband characteristic

A new class of wideband ring-type microstrip bandpass filter is proposed under smaller size of three quarters waveguide length section. One via hole is placed at perpendicular position in a squared ring, whereas two short-circuited sections are formed in the ring-type microstrip bandpass filter similar to a dual-mode ring filter in shape, thereby making up a three quarters waveguide length ring-type microstrip bandpass filter. By adjusting the short-circuited sections, the bandwidth of the center frequency can be controlled easily. As a pair of open-circuited stubs is placed between the two ports, two extra resonances can be used to improve the out-of-band performance. Afterwards, a novel microwave microstrip filter has been successfully fabricated with the lower insertion loss S ₂₁ of 0.48 dB, return loss S ₁₁ of 30 dB, 3 dB bandwidth of 80%, and central frequency of 2.4 GHz. Simulated and measured results show good wideband filtering performance with widened upper stopband outside the wide passband.