Interesting. I don't recall what the manual said when I installed it. But the Wikipedia site John listed says "the antenna should be mounted to a wall which faces the center of the continental United States". And the map of the satellite orbit shows it being partially over the center of the US, Canada, Central & western South America.
If I drive on the north side of anything, a building, big tree, cliff, etc. I don't get squat till I move away from it. Diving downtown Seattle or Spokane is terrible reception.