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Hi. Sorry , but it sound like this when I translate directly from my language.You have to connect the Arduino ground to radio ground not directly but with diode. Cathode to radio and anode to Arduino..
I supose my answer come very late but I solve the problem with the comunication . The problem come when the ground from atmega is connected to ground to radio. I connect true a diode and now it's ok. Thank to author for this project.I made it and work very well.
look for stainless steel wire used for welding and use terminal board for conexion