I was wondering why the ring had to be grounded in a stereo plug when the shell
of a mono plug would do the same thing. From that site is a real answer:

How do I disable the internal MW antenna?

Using a 1/8" stereo connector, ground the ring terminal to the sleeve. Or
else use a 1/8" mono connector. The FAQ author much prefers the former method
as this can be controlled by a SPST switch to disable and reenable the internal
antenna, thus saving the antenna jack inside the radio from wear.

The external antenna jumper that came with my DX-392 has a mono plug. I can't
use the MW loop inside my metal mobilehome. So the plug just stays plugged in.

Bill, K5BY