Abstract: A waveguide coupler for coupling energy between two different waveguides through a coupling of their magnetic fields. Each of the waveguides, between which coupling is to take place, is capable of supporting electromagnetic waves having different phase velocities. The first waveguide has a circular cross section and is shaped and dimensioned so as to support a TE.sub.mn or a TM.sub.mn mode of wave propagation. A further length of waveguide, such as one having a rectangular cross section, is capable of supporting a different order mode of wave propagation. The wave supported within the rectangular waveguide has a higher phase velocity than that supported within the circular waveguide. The rectangular waveguide is helically wrapped around the outer surface of the circular waveguide in such a manner that the longitudinal fields of the electromagnetic waves are coupled between the two waveguides through a plurality of apertures which are provided within both waveguides along their contacting helical path.