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 rectangular to circular waveguide transition is designed for the fundamental TE10 mode of operation in the rectangular waveguide and the TE11 mode of operation in the circular waveguide. The entire structure is design using air-filled sections. This transition is used to connect a rectangular waveguide to a circular waveguide, whilst maintaining low levels of reflection and transforming the operational mode from TE10 (rectangular) to TE11 (circular). As an application example, the transition might be used in a horn and reflector antenna setup, where a circular waveguide feed requires a standard rectangular waveguide for the connection to the rest of the microwave system.