A waveguide antenna assembly for a magnetic resonance tomography apparatus, particularly a waveguide antenna assembly that can be detuned for an operation with local coils, is composed of multiple metal surfaces connected by diodes respectively located between the metal surfaces. Only when the diodes are through-connected between these metal surfaces are the surfaces connected with one another in terms of RF signals via these diodes, so the assembly operates as a waveguide antenna. When the diodes are disabled between these metal surfaces, the surfaces are unconnected with one another for RF signals.