Features of Characteristics of Second-Order Bandpass Filters Based on Half-Wave and Quarter-Wave Microstrip Resonators

A comparison is made of the characteristics of bandpass filters consisting of two interacting resonators, which are either half-wave or quarter-wave segments of microstrip transmission lines with the same widths of strip conductors. Plates with a thickness of 0.5 mm made of TBNS high-frequency ceramics with a relative permittivity ε = 80 were used as substrates for microstrip structures. The structures having a maximum coupling region of the resonators and the maximum value of their coupling to external ports with a wave impedance of 50 Ω were studied. The filters were tuned by selecting the gap between the strip conductors so that the maximum reflection level in the passband was equal to –20 dB. Electromagnetic simulation of 3D models showed that the fractional bandwidth of filters based on quarter-wave resonators is almost 2 times wider than the bandwidth of filters based on half-wave resonators, and this fact is confirmed by the measurement results. The discovered effect is explained by twice the characteristic impedance of a quarter-wave resonator relative to a half-wave one.