My Aerial plug won't fit at the back of my CD Player

I just got a new Alpine head unit for my 91 civic hatch. The problem is, the aerial connector is too big to atach to the back of my head unit, this means i get no reception for listening to the radio. Can anyone help?

if you get a converter it will convert the cable to fit in the back. i have seen them at walkmart for like 2 bucks.