A mode converter for a level radar includes an input line introducing a high-frequency signal into the mode converter; a helix structure into which a conductor of the input line projects and which includes a plurality of helical electrical conductors; and a waveguide which encloses the helix structure. The helix structure is configured to convert the high-frequency signal from the input line into an H01 mode in the waveguide.

A waveguide coupler comprising: a first coupling portion arranged to couple with a first waveguide and a second coupling portion arranged to couple with a second waveguide so as to connect the first waveguide and the second waveguide together; wherein arranged between the first and second coupling portions is a wave modulation portion arranged to modulate wave signals transferred between the first waveguide and the second waveguide.