Handheld Radio Relay?

I have a problem in the area that I hunt in. We hunt on a mountain and even though I have a longer range radio (ie. 20+ mile range), I still can not contact anyone on the opposite side of the mountain. I was trying to look and see if there is any kind of radio relay station or reciever that I can set up like on my car or something so I can relay my signal over a larger distance. I was looking at radio repeaters but I don't know if they are legal or if there are any rules/ laws against using them. Any advice on this situation would be greatly appreciated. Thanks in advance.