When I searched for Merwyn online, Google took me to Brown County Airport in Georgetown, OH. No idea why…but that seems to be the “Merwyn RCAG” area.
I’m not too up on my geography…but back in the day, the FAA had a Flight Service Station in Dayton, OH…and of course, there are major airports (DAY, CVG, etc.)
Most likely the FAA has the antennas co-located at FAA locations, like Dayton FSS back in the day, or a major airport, navigational aid, radar site, etc. The FAA’s radio network of Center and AFSS frequencies are probably all in the same old places. I’m surprised they are not listed on the charts…maybe someone with an older sectional chart might be able to find the info. Or, call Indy Center and ask…what the heck?
I just looked for a frequency box at Seligman, AZ, which has an FAA Radar and when I worked in AFSS through the mid-90’s we had a Flight Watch transmitter/receiver site there, along with some other center frequencies.
The other symbol on the charts you might look for are Remote Communications Outlets (RCO).