Using a FM notch filter

Earlier I mentioned that I’d gotten an FM notch filter via eBay  from iseeabluewhale. That filter is now up on the roof in a metal box, wrapped with aluminum foil. The discone has a 6′ pigtail of LMR-240 coax, that then goes into the FM notch filter, the output of the notch filter goes into the LMR-240 that runs down into the shack. It’s not a super installation, but let’s see what it looks like from the receiver’s point of view.


Looks bad, but still better than the original (red, below) but significantly worse than the terminated version (green, below).


So, the filter does seem to do a fairly good job of cleaning things up.

Next, I removed the cast enclosure from the mix and just wrapped aluminum foil around the filter assembly and over the coaxial cables connected to the filter.


Even better than before. Filter blow-by is a real issue here. We are talking a filter that advertises 85 dB rejection, after all.


As well, I realize that the 25 cm RG-178 coaxial jumpers that I’m using to get from the LMR-240 to the filter are partially exposed to the world, so I’ll go do something about that right now.


More shielding is better, so it appears! I’ve wrapped the Al foil from LMR-240 connector to the other, covering all the RG-178. Perhaps I’ll fix that next. I believe I’ve a small die-cast box in the garage that I could use.

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