I have added this simple preselector to my RX. (from LF today - RSGB publications)

Advantages : parts are easy to find  - it is easy to build - verry efficient to pick up little signal in a verry small bandpass.

Inconvenients :if you want monitoring a large bandpass with your SDR, the signals outside de BP of the filter are excluded : the best way is then to include a bypass position (use then a 6 position rotary swich), first bypass the whole preselector, pick the wanted signal , and then tune it using the other postions of the rotary switch.

Table of positions of the rotary switch :

(*) use only 1 X 500 uF section for the 210-600 Khz position

The RF parts are to find at Mouser.com - see table below for the references.