Atmosphere does play a part but just freespace losses are going to be massive, probably at least 250dB just for the distance voyager is at. Atmosphere could add another 60-100dB. Voyager's antenna has about 40dB of gain but the DSN network can array multiple antennas to make up for the losses. They have up to 2 70m dishes and several 34m dishes that can point at Voyager, quite a massive antenna gain.