Well, you don't really give us a lot of information about what you're using right now. Radio Shack makes any number of UHF antennas.
You say you're 11 miles away and you're using a preamp? At this range, you might very well be overloading your receiver with signal. Have you tried reception without a preamp to compare? Even without a preamp, depending on your antenna, you may *still* be getting too much signal.
Unless you have extreme mountainous terrain, which could make reception difficult under any and all conditions, you might want to try Radio Shack's single and double-bow-tie antennas at ground level. Since you can return them for any reason, there's no harm in trying them out.
Finally, what's your analog UHF reception look like? Is it good/fair/poor?