Hello,
I'm new with phantom 4 pro. I flew about 15 times and I have never flew more than 300 m in distance. Today i wanted to fly further. I went to a country side, where was no houses, no big trees, no mountains and the terrain was plain. So I reached altitude about 60 metters and flew forward about 500 metters and in that distance I just lost my signal and aircraft disconnected.  There was no obstacles between me and my drone. Thanks god that after disconnect, drone flew back to home. But is it normal that I lost signal in 500 metters? What could be wrong? What did i do wrong?  

It can depend on the area your flying interference etc, but as you say you were out the country, so what position your antennas are in, and what frequency are you on there are two, also try a little higher flight.