I'm using the antenna that came with it also, I'm down south in southern texas so I get a really good signal all the time. never been less than 70% even in the garage. I know of an antenna called the Blitzsafe Micro. From what I've read from others who use it, it's one of the best for mobile XM. Check this out.
Not terribly expensive either. I was going to buy it just because it's small.