An ATSC TUNER, antenna-in coax, outputs HDMI to TV.
If you use an external tuner or set top box, then its output CAN be fed to an HDMI input. It will cut the internal TV tuner out of the loop, nd you wouldn't need the coax connection. You still can't use a HDMI input for RF signals to the internal TV tuner.
set top box, then its output CAN be fed to an HDMI input.
OP says he wants ANTENNA, not cable service.
The first part of the quoted sentence acknowledged that fact if you had left it in. In either case, the point was that the OP needed some sort of external box to convert RF signals from an antenna to a format that could be input via HDMI.