Here's a simple experiment that you can do that requires absolutely no special equipment. Take the antennas and grounds off the radios, and see if you can still hear that flamethrower, especially away from its nominal frequency. If you can (and I suspect that you will - I had a similar issue years ago), then you must find and resolve (if you can) the source of the RF coming in. This is the tough part - there are so many possible sources of reradiation that it will no doubt be a lot of work to nail them all. I've even heard of rust between joints or nails on aluminum gutters becoming crude diodes and rectifying a MW signal. If you should get this nailed down, then- and only then- should you consider a filter on your antenna.
I ended up putting toroids and wrap around transformers on every lead to and from my receivers (including speakers), a power line filter on every cable, and even then I didn't get rid of it all (I was living in an apartment, and had no way to address outdoor issues). This was a good 15-20 years ago. What a mess! I even had to put a transformer on my phone line because it was coming through that, too (Barbara Streisand and Mel Torme 24X7 surely a nightmare on Elm street, hi)
Mike