Best I can come up with is a wave propagating through a vacuum, but of course there's no such thing as a perfect vacuum, which means the medium the wave is propagating through is not perfectly uniform. Furthermore, gravity will influence the wave's propagation, meaning that not only do you need a uniform medium, you need uniform gravity from every direction.

It's a lot, lot harder than you might think to make a perfect circle. I'm curious if mojo has any suggestions.