Rectangular waveguides were one of the earliest types of transmission lines used to transport microwave signals. Despite the recent trend towards miniaturization and integration, where microwave circuitry is often implemented using planar transmission lines such as microstrip and stripline, waveguides still play an important role in certain applications, such as high-power systems, millimeter wave systems and precision test equipment. [Pozar]
In many microwave systems using waveguides, it is often necessary to interface to other type of transmission mediums, for example, a transition from a coaxial line to a rectangular waveguide.
In the more common version of this transition, the center conductor of the coaxial feed is extended into the waveguide as a probe. An advanced option, employed in this implementation, uses a shaped probe to improve the bandwidth of the structure.