Miniaturized high selectivity bandpass filter based on hairpin two-conductor stripline resonators

Abstract

A resonator based bandpass filter miniature design formed by two U-shaped metal strip conductors located at different sides of a suspended dielectric substrate is developed. In each conductor one end is grounded to the screen and the other is free. A resonators are connected in pairs with overlapping sections of strip conductors, it leads not only to the reduction in the dimensions of the device, but also to the increase in its selectivity due to transmission zeros near the passband. The prospects of the proposed design are proved by the measured characteristics of the 6th-order filter made on a substrate with the relative permittivity er = 80 (TBNS-ceramic) and the thickness of 0.5 mm. The filter passband central frequency f0 = 150 MHz, and the fractional bandwidth Δf/f0 = 25 %, while the size of the device is only 42 × 20 × 8.25 мм3 (0.02λ0 × 0.01λ0 × 0.005λ0, where λ0 is the wavelength at the center frequency f0). The filter has high slope steepness and a wide stopband, which extends up to the frequency of 3f0 at the level of –50 dB.