Has it been tested in a vacuum?
Because the first observation I'll make is that it will heat up if microwaves are being bounced around inside it, and if operated in an atmosphere, the larger end will heat more air than the smaller end. Boyle's law can take over from there, producing a small thrust.
In other words, the reaction mass would be the air it heats up; this effect wouldn't work in a vacuum any more than a rowboat would.
Biting the hand that feeds IT
