Hooking up an FM antenna?

I recently got a new power amplifier/tuner from my dad's buddy.
On the back, where you hook up the antennas, there are 4 terminals: 2 for FM and 2 For AM.
One on the AM side is for the antenna, and the other is ground.
This make sense to me, because an AM loop antenna has two wires which need to be hooked up.
On the FM side though, it also has a terminal for ground... From what I understand, FM antennas are composed of a single "wire", like those coming out of radios or the ones on cars.
What's the ground on the FM side for?