What Does An Antenna Do For A Radio Install An XM Satellite Radio On My 2008 Honda Civic DX And Need To Find The Antenna. Not Visible?

Install an XM Satellite Radio on my 2008 Honda Civic DX and need to find the antenna. Not visible? - what does an antenna do for a radio

I checked the manual and my next step is to call to the dealer. Does anybody know where the radio antenna?