Miniaturised UWB filter with improved lower stopband performance

Recently a design technique was proposed for realizing miniaturized high-order UWB bandpass filters using a direct cascade of E-shape microstrip structures. This class of filters has one transmission zero at the upper stopband. In order to increase the lower stopband attenuation, this paper presents an alternative design technique to generate a further transmission zero in the lower stopband, leading to a new class of miniaturised UWB filters using the E-shape microstrip structure. In the new design method, second-order E-shape microstrip structures are electromagnetically coupled through a λ/4 microstrip line patterned on an extra dielectric layer added to the conventional substrate. Based on this technique, a fourth-order UWB bandpass filter is designed, simulated, fabricated and measured. The filter size is about 2.6 mm × 3 mm × 0.5 mm excluding the feeding ports. The results from the simulation and measurement show good agreement.