My 2009 radio occasionally sounds as if the antenna is not connected. I've looked for a replacement and found so many radios which specify other years. They all look the same, is there anything particular about the 2009 radio?

 

No same as the 2006 thru 2011

 

There are minor differences between year, some years have USB, some years have a audio jack, some don't have either. That is the difference in the factory installed, replacement radios need a digital interface to work properly. If you know what you are doing you don't need the interface. If you have to ask "how" you need the interface.
Your problem could be a loose connection. the antenna is amplified with power from the pink wire from the radio

 

The antenna that screws into the piece on the roof might be damaged and need replacement.

 

My experience with replacing GM audio with another is tougher than it appears. It's not R&R and good to go. I was foiled by the anti-theft features and locked out trying to change to a cd/cassette from a different model to my van (not hhr)
So I'd add that for you to consider.
If your happy otherwise. I'd advise fix first maybe save some efforts.

 

It might be an issue with the tuner. The other day I was listening to one station when the radio went to noise, but I could barely hear music from the last station. I switched back to that station and got a clear reception.

 

I may have my radio that I removed from my 2011 if you need one...