The BP-1A kit is a 3 to 30MHz continuous bandpass filter. It is intended to improve the front stage in analog receivers or SDR (software defined receivers), experiments in radio laboratories, measuring instruments etc. where a filter is required to select any frequency segment within of 3 to 30 MHz range. In particular, medium and low cost SDR receivers need bandpass filters when receiving HF (OC) radio signals.
The BP-1A allows you to tune and select the frequencies of any amateur radio or HF broadcasting band. The tuning is done manually with a potentiometer that allows to be located at any point in the coverage range. The BP-1A incorporates three relay-switched bandpass filters covering from 3 to 6MHz, 6 to 12MHz and 12 to 30MHz.
Nice video from F6CRP https://www.youtube.com/watch?v=8A-0swuuzkc