Out of curiosity, where do you live? I believe XM and Sirius both use terrestrial repeaters in metro areas, because with all the buildings, it's easy to lose the sat signal. Is there any chance with your XM that when you moved it away from the window, it picked up the terrestrial signal, while near the window it tried to pull in the satellite signal? Just a thought, thinking about the physics of it. Satellite systems use microwaves, and it doesn't take much to block microwaves - think about even sold C-band satellite dishes - we had a mesh one, which would reflect the signal towards the horn. So honestly, I'm hard-pressed to believe that you pick up a solid legitimate microwave satellite signal with solid objects in the way.
This is going back to 2003-2004. Before I made the switch to Sirius because I felt, and still feel, they have the better programming. But everyone is different, as the above poster mentioned.
Anyway, if you are familiar with both units(i had a SkyFi 1 for XM, and currently have an Audiovox PNP 2 model for Sirius), and message boards, MOST(not all obviously) people who have had both will tell you that XM's indoor reception is FAR SUPERIOR to Sirius's becasue XM's birds(satelites Rock and Roll) are GEOSTATIONARY in the Southern sky(from my point of view in Rural NW Pennsylvania. Amish country), while Sirius birds move in a GEO SYCHRONUS orbit above the U.S.(a Figure 8 pattern). So while people will get STRONG indoor reception for XM, even though it may not be pointed out any window, but may be looking at a wall, Sirius's indoor reception leaves little to be desired. But they have a little better coverage in the car because their birds are moving, and not stuck in one place like XM's. XM's car reception is not bad by any stretch of the imagination, but you will be more prone to droputs if you are on the wrong side of a mountain/obstruction while driving, if the XM birds will be on the opposite side of it.
Since I have no repeaters near me(closest are in Pittsburgh at over 55-60 miles away), and the Skyfi 1 differentiates between a SATELLITE SIGNAL and a TERRESTRIAL SIGNAL(ground repeaters), I know it was a satellite signal I was receiving while pointing at a wall.
Others confirm this with the indoor antenna on XM. Many have shared there results in forums that even though pointed at a wall(not a brick, or steel wall. But you normal/everyday sheetrock wall) there reception was vastly improved for some reason over a window sill type of set-up.
For those with XM, try it out. You may be a little surprised or stunned to find out that the near a window, or on the window sill, is not the best place to for the antenna to be pointing at.
No harm in trying.
thanx for the great responses on the indoor satellite TV antenna mount pointing at a wall or the like.
Seems like Science(Physics) just cannot aloow it to be that way at this point in time, or maybe never.
Unless, again, pointed out a cheap single paned window, or a place where there is not window, or the window is opened.