Ingo's absolutely right, get your existing antenna as high as is possible, and that'll extend your range more than a fancy new twig! I'm situated on top of cliffs in sw Cornwall, with a standard supplied (thoroughly waterproofed) antenna stuck on an old Peugeot 405 brake disc on top of a flat roof, ht. 210' asl, and can pick up an aircraft at FL400 at 270nm distance due west. You can't get a lot better than that, but it's totally due to the uninterrupted line of sight looking west! Height is might!!