Wacko is right- those gps geek antennas will do the job but they'll also draw down your batteries. the gpsaccessories antenna draws less than half as much power, and gives 33+dba of boost.
Garmin factory antennas are outdated and overpriced. Trimble has pretty good OEM antennas, otherwise aftermarket is the way to go.
GG- I've been testing all sorts of amplified antennas for 7 years in lots of situations and never seen anything like what you describe. I suppose it might be possible for ONE signal from ONE sat to bounce off a distant object, but your reciever uses more than that one signal, it'll see several birds through the trees and off the horizon in other directions. In a poor coverage area with a decent amp antenna you should expect strong signal from atleast 5-6 sats at any given time. Those stronger, consistent signals will be used to factor your location.
The closest thing you should see in real life is if you are in a cave or tunnel, might pick up stray signals from both ends of the tunnel, and with no other info it'll factor a location that may be a few hundred feet off, but that isn't really a bad thing if you understand that a non amplified gps in the same situation would loose position entirely.
-Ed