Just get the biggest you can, with antennas mate, it's all about size. The more elements the more gain. Which normally equates to, the longer it is the better it works.
Pop into a local TV store, not a chain or dept store, but a small specialist shop. They'll know the best antenna for your area, and probably give you some tips on the best place/way to mount it.
Have a look here.....
http://www.tvaerials.com/shop.aspx?categoryid=3
Hope that helps
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