The jack toward the inside is for Antenna 1 and the one toward the
outside is Antenna 2. Pressing and holding the third button down on
the left switches between the two. There are two beeps for Antenna 2
and one beep for Antenna 1. The fourth button down on the left, when
held down for a second, operates the antenna tuner. Be careful not
to do what I did. I accidentally held down the third button instead
of the fourth button when trying to tune the antenna and my TS-480
couldn't pick up any signals. I thought my antenna had stopped
working, but eventually realized I had accidentally switched to
Antenna 2 and, like you, I didn't have anything attached to that jack!
73 de W7OQ, Dave