Legally 5.8GHz you are good. But it doesn't punch through tree's etc very well unless you use a helical antenna tuned to 5.8GHz. Some people are using 1.3GHz, not sure if its legal though. The thing to consider is if it will conflict with emergency services frequencies. But the laws are very behind the times here about FPV. You will probably get away with a more powerful TX like 500mw, and I can't remember reading any info about the laws regarding power, but considering the law that you are not suppose to fly beyond line of sight, you might guess that something like 20mw might do about this distance. I use 5.8GHz 500mw and have even been pulled over by the police with my whole ground station in the back seat and they asked me what it was etc, I told them and it didn't concern them at all. Anyway hope this is a help.