They won't solve the problem, but they will lessen it significantly. XM reception is line-of-sight to the satellite, so under very heavy tree cover you'll be SOL either way. But I've been an XM subscriber for a lot of years and have used a lot of receivers and antennas and the one in my bike is the poorest quality of them all. I may look into the antenna that Fireman linked to. I have a similar unit on my truck and it really pulls in the signal.

 

I had sirius for 7 years in my pick up my big rig and my street glide and I get the best signal on the bike. All three have differant antennas the one on the bike being the smallest.

 

The stock (wristwatch-size) antenna is about as good as any, but I have found a mounting solution that is a bit better than the factory mounting suggestion. Instead of mounting the antenna on the small metal plate beneath the fairing, I mounted it between the 3 windshield bags on the metal baseplate for the bags. The antenna wire is so thin that it easily goes between the windshield and fairing, and this places the antenna higher and with fewer obstructions (the fairing being the primary obstruction). The antenna is so small that it is hardly noticeable between the windshield bags. I haven't tried the other handlebar-mounted antennas, as I don't like the look of stuff hanging off the bars...I prefer a cleaner look. No antenna will completely overcome the line-of-sight issues with the satellites or the terrestrial repeaters.