Over the years this has been a torture of armonics and interferences for me and for other I supposse. But I could live really well after I bought an ICE - BCB Interference Filter, exactly the 401 model from 500 to 1.800 kHz.
This work for me really well, but I like and want to receive the NDB's and LW or VLF signals too and to get a better response of this filters. So I add on the same box a notch calculated for that frequency and made it with standard parts and compossed only with one fixed bobin and two capacitors that gives me the posibilities to get what I want to hear, permiting to me decode time signals as DCF77 or MSF from over 2000 km on 77.5 or 60 kHz respectivelly.
I have the RSP-1A but this work the same good before with my RTL-SDR + Spyverter...
Reason: No reason