Ron, I did not see the following explanation in the comments, but it could be there... especially to anyone interested in chirality :-)
RF radiation from an oven is generally circularly polarized. There are advantages to polarization in efficiency of absorption. Amana held the patent for this - but it expired in 2005. Circular polarization would create a rotational current in a conductor, which then creates its own magnetic field, which then interacts with the magnetic field of the magnetron. There could be pseudo loss or gain depending on the chirality. This could be tested by reversing the polarity. That would actually not be easy with an oven but it could be done with a specialized circuit. The apparent loss would show up as gain. Jones
