Abstract: An E-plane waveguide is provided comprising a housing having opposed walls defining a waveguide channel. The waveguide includes at least two elements spaced apart in a direction along the waveguide channel and disposed between and spaced from the opposed walls, and which define a resonant cavity therebetween with the waveguide channel. At least one of the walls has first and second protrusions which are spaced apart in a direction along the waveguide channel and protrude into the waveguide channel. The spacing between the protrusions is less than half a wavelength of the resonant frequency of the resonant cavity.