syseng68k
HF Antenna: The state of this would be reflected in the vswr reading, which essentially says how well the transmitter is matched to the antenna. If the antenna is missing, or cable open circuit or shorted, the transmitter autotune would detect it and either run at reduced power or shutdown completely.
This may depend on the transmitter being keyed to detect it. On receive there is generally not enough power available anywhere to run a VSWR bridge to detect the mismatch from a disconnected antenna.
These days a missing AMU, antenna matching unit, would probably be noticed. The transmitter would probably maintain communications with the AMU as a health check. In the bad old days I've had my fingers upon there was no such knowledge in the transmitter until you tried to use the AMU, as when the transmitter keyed.
{^_^}