Rectangular waveguides were one of the earliest types of transmission lines used to transport microwave signals and are still used in many applications today. High-power systems, millimeter wave systems and precision test applications are just a few areas where these waveguides are used.
Transverse electric (TE) and transverse magnetic (TM) modes are supported (and are thus able to propagate) in hollow rectangular waveguide, while transverse electromagnetic (TEM) waves are not able to propagate, since only one conductor is present. The TE and TM modes have cutoff frequencies, below which propagation is not possible [Pozar, D.M].
The stepped binomial waveguide transition is designed for the fundamental TE10 mode of operation, in air-filled waveguide. The transition would be used to connect rectangular waveguides of different dimensions with one another, whilst maintaining a low reflection level and the correct operational mode (TE10).