This may have been posted, if so sorry.

A friend asked a question over diner tonight and I thought of this map
I fouond severa years ago.

It shows the local nadir or antipode for every location on the earth.

Scroll down to the well diggers map.
http://www.wendycarlos.com/maps.html

For a radio station at your antipode the great circle or shortest path
is every compas heading.

Sadly for me there aren't likely to be all that many stations in the
middle of the ocean.

Terry