Reddit reviews SO-239 TO 3/8" x 24 THREADED STUD ANTENNA MOUNT ADAPTER
We found 2 Reddit comments about SO-239 TO 3/8" x 24 THREADED STUD ANTENNA MOUNT ADAPTER. Here are the top ones, ranked by their Reddit score.
SO-239 to 3/8" X 24 Threaded Antenna Mount AdapterAll Brass SO-239 Antenna Stud3/8" X 24 Threaded Stud FittingChrome Plated
Assuming your coax has a PL-259 connector, you will need something like this. It's an adapter with female 3/8-24 and SO-239 jacks on it. There are others like it on Amazon and other sites, so may want to look for the best deal.
If you need a way to attach the whole thing to the flagpole, you could use something like this mount.
You might even be able to find a mount like that which comes with the SO-239 / 3/8-24 adapter.
Hopefully that helps.
Get one of these: SO-239 to 3/8", drill the appropriate hole in the bucket, and screw it on.