I enjoy listening to non directional beacons (NDB) used for aircraft navigation. They can be heard in the 200 Khz to 500 Khz range.
One problem I faced with my Icom IC-7600 was hash from stations in the AM broadcast band. (550-1750 Khz). I was hearing voice & music intermixed with the NDB's. In some cases the hash covered up NDB's that I wanted to listen for.
I looked around and found plans for an inexpensive low pass filter. The filter is designed to block out broadcast band stations and everything above. Here is the link: 500kHz Low Pass Filter
It took forever to build, because I had to order the ferrite toroids and find a source for magnet wire. I changed the design slightly and mounted it in a cast aluminum case that I had in my parts box.
http://ve4cy.net/misc-images/lowpass.jpg
I connected it up and was surprised to find the thing works pretty well. One thing however. The low pass doesn't kick in until about 700 Khz. (I found another link to where someone built this and they found the same to be true). I scanned it with my AA-600 antenna analyzer. (From 10 khz to 1 Mhz with 505 khz in the middle). You can see the SWR shoot up and go off the scale at the 700 Khz mark. At 800 Khz, the SWR is 24:1 and it goes up from there.
http://ve4cy.net/misc-images/lowpass-sweep.jpg
Finally, to demonstrate how it works, here's a quick (and crude) YouTube video I made. I tune from an NDB at 275 Khz to 300 Khz where I always hear BCB interference. I do it once without the filter and again with the filter in place. You can clearly see the difference. https://youtu.be/tnQEzxYYJmw
One problem I faced with my Icom IC-7600 was hash from stations in the AM broadcast band. (550-1750 Khz). I was hearing voice & music intermixed with the NDB's. In some cases the hash covered up NDB's that I wanted to listen for.
I looked around and found plans for an inexpensive low pass filter. The filter is designed to block out broadcast band stations and everything above. Here is the link: 500kHz Low Pass Filter
It took forever to build, because I had to order the ferrite toroids and find a source for magnet wire. I changed the design slightly and mounted it in a cast aluminum case that I had in my parts box.
http://ve4cy.net/misc-images/lowpass.jpg
I connected it up and was surprised to find the thing works pretty well. One thing however. The low pass doesn't kick in until about 700 Khz. (I found another link to where someone built this and they found the same to be true). I scanned it with my AA-600 antenna analyzer. (From 10 khz to 1 Mhz with 505 khz in the middle). You can see the SWR shoot up and go off the scale at the 700 Khz mark. At 800 Khz, the SWR is 24:1 and it goes up from there.
http://ve4cy.net/misc-images/lowpass-sweep.jpg
Finally, to demonstrate how it works, here's a quick (and crude) YouTube video I made. I tune from an NDB at 275 Khz to 300 Khz where I always hear BCB interference. I do it once without the filter and again with the filter in place. You can clearly see the difference. https://youtu.be/tnQEzxYYJmw
