There’s a great post at wireless.gumph.org explaining how long range wireless links can be made without exceeding power limitations (hat tip RootPrompt.org):

Some wireless devices however, let you reduce the transmit power, which allows us to use a more powerful antenna. This does not increase the overall strength of the signal , but does increase the receive sensitivity. If we do this at both ends of the link, we can increase the range, while staying within legal limits.