Attenuators in Waveguides
But why does an orthogonal field keep undisturbed? I'd say that it would induce an orthogonal current inside the card, which also dissipates power.


Remember the conductive part of the pad is just a thin film coated onto the glass plate.

It has to be thin presumably to have enough resistance to not disturb the field significantly. If it did, instead of absorbing the signal it would reflect it back towards the source. This would still reduce the forward travelling signal, but might be detrimental to the operation of the source.

Since the film is so thin, there's nowhere for a significant current to flow if you apply the electric field orthogonal to its surface.


  Is that card a good conductor, a very good conductor, or a bad conductor?'


It has to be only moderately conductive.

If it were a very good or ideal conductor, it would produce reflections rather than absorption when placed in the waveguide.