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Putting things like diodes and switches on the ground return back to the power supply
is a bad idea.? There are many other ways that the ground return could be completed.

For example, some power supplies will have the 12v ground return connected to the?
electrical safety ground of the three wire power plug into the wall socket.?
And your antenna coax outer braid should have a good solid earth ground as well.
So there's always a completed ground return, and the rig is always on.

A connection from a PC USB port into the Raduino could complete the ground return,
putting 3+ amps through those wires might even damage the PC motherboard.
?
Perhaps you have it powered from an automotive electrical system.
Having the rig chassis touch the any metal in the vehicle would turn the rig on.

The diode in the ground return would probably offer adequate reverse polarity
protection for the uBitx.? ?But I wouldn't do it.
For one, my brain doesn't work that way.

For another, all of the power supply return current might be going through your
antenna coax braid or through your computer's USB cable, since that may well
have a lower voltage drop than the 0.7 volts across a forward biased silicon diode.
Could cause trouble.? Better to spend an extra $0.02 on a second diode.

Jerry, KE7ER