It has to be a good sender and receiver! The high powered ones are 500mW, but not sure if they're legal everywhere. I think generally they're 100mW to 200mW? Anyways, what you need is a good receiver coupled with a high gain antenna. You can buy quite powerful external antennae for certain receivers that may do the trick! I'm not suggesting a yagi (common tv antenna style) type, which is directional, just an omni-directional rubber insulated antenna.