A Highly Selective Bandpass Filter Based on Suspended Substrate Resonators with a Two-Sided Stripline Pattern
New resonator design employing a hairpin stripline conductor with a stub situated on one side of a dielectric substrate and regular stripline conductors (connected to a screen) on the other side. Eigenfrequencies of the first three oscillation modes of this resonator can be made closer to each other, so that the resonances of two modes are involved in the formation of the passband while the third mode resonance forms a minimum of the transmission coefficient adjacent to the passband. A structure comprising four resonators of this type has the characteristic of an eighth-order bandpass filter arranged in a case with 45 × 16 × 6.25-mm internal dimensions possessing f0 = 0.52 GHz central frequency with a 14% relative bandwidth. The filter is highly selective due to the attenuation poles being close to the pass band and a wide high-frequency stopband extending above a fivefold f0 value at a level of –100 dB.