The head unit has one connection for an antenna to be connected. If you are getting an HD radio tuner, it needs it's own (antenna connection that is). Therefore, even though the HD tuner COMES with its own antenna cord, you will need an "antenna splitter" to be able to connect yours car's antenna connection to both of them at the same time.
Some model cars (I had an Infiniti) need what is called an "antenna adapter" that first connects to the cars connection and then to the head unit. This is due to some manufacturers antenna connections not matching to Pioneer head units.
I hope this helps as I had the antenna adapter ordered and ready when I did my install, but was unaware of needing the antenna splitter, and had to wait a day or two until I could locate one.
Cheers,
Kev