Waveguides and coaxial assemblies are the most common types of RF interconnect used outside of planar circuit technologies. Both waveguides and coaxial assemblies have their advantages and drawbacks, which is why waveguide to coaxial adapters, as these factors can often lead to both waveguide and coax being used in the same application.
Where waveguides are banded, coax operates from DC to a maximum cut-off frequency. Hence, it is important to remember that a waveguide to coax adapters are also banded by the maximum and minimum cut-off frequencies of the waveguide, or the maximum cutoff frequency of the coax. There are also power limitations in these transitions, which are typically limited by the coaxial interface. This is because waveguides generally have much higher power handling than coaxial transmission lines at the same frequencies of operation.
Waveguide to coaxial transitions are used whenever it is necessary to bridge between a coaxial transmission line environment and a waveguide environment. As waveguides have lower loss than coax, there is often a need to use waveguides where the loss of the interconnect may limit the dynamic range of a sensing or communication system. This is a common situation in test and measurement, radar, and long-range communication systems. These types of transitions are also common when it is advantageous to use waveguide antennas, such as gain horns for antenna testing or for high directivity communications at millimeter-wave frequencies. Other use cases may be based on mechanical or space constraints. Waveguide interconnects are generally larger than coaxial interconnects for the same frequencies, and hence it is often easier to route coaxial interconnects than waveguide. Though flexible waveguides do exist, they exhibit poorer performance than rigid waveguides, and are still larger than coaxial interconnects. This is why coaxial interconnects are often preferred for complex and compact routing scenarios.
However, when high power or low loss is the priority, waveguide interconnects are still preferred. This is why waveguide interconnects will sometimes be used to feed coaxial-based antennas, and Coaxial to waveguide adapters are necessary to enable that transition. There are some cases where legacy equipment may have a waveguide interface, but updated or retrofitted technology up-stream or down-stream to that component may be coaxial. This scenario and other scenarios where it may be preferable to use a waveguide component/device in a coaxial interconnected system or vice versa is another instance where waveguide to coax transitions are useful.