I have been using an FT-857D in my car with an ATAS 120A and remote head without problems.? The rig is in the boot and the head is on the dash board.? The cables are as supplied in the Yaesu? YSK-857 separation? kit.
I recently tried 80M with a loaded whip mobile antenna and found the radio was shutting down even at 10W or 5W on that band.?? After some tests it seems there was some RF pickup in the control cable.? Putting a ferrite ring the rig end of the control cable cleared the problem.? I put ten turns of the cable on the ferrite ring and? can now run up to full power without any problems.
Just in case anyone else has problems with their FT-857D and remote head shutting down on transmit
Dick G4BBH