High-power VHF tunable filters with high selectivity

A tunable helical bandpass filter with high selectivity designed for 30-88 MHz operation is presented. The two-pole tunable filter is discretely tuned by connecting PIN switched capacitor banks at the terminal ends of the helix. The filter schematic provides two finite transmission zeros located in the lower and upper stopbands, respectively. The experimental filter has an insertion loss between 1.65 and 2.75 dB with its return loss better than 13 dB. The 3-dB fractional bandwidth of the filter is 3.3%-3.7% and the shape factor (BW_{30 dB}/BW_{3 dB}) is smaller than 5.5 over the entire tuning range. The input power handling capability is greater than 45 dBm. The measured results show good agreement with the simulated ones.