Full-wave design of coaxial to waveguide transitions and waveguide mode transducers

Abstract

The purpose of this work was to provide an approach to design, simulate, optimize and analyze several coaxial to waveguide transitions with different output modes and waveguide mode transducers (both rectangular and circular types). Both transitions and mode transducers are devices of the utmost importance and widely used in the radio-frequency field, and nowadays the existence of CAD and computational electromagnetics techniques allows for very efficient design procedures dealing with unprecedentedly demanding specifications, leaving behind the traditional heuristic and laboratory adjustment methods. First, a traditional TEM1 coaxial mode to TE2 10 rectangular waveguide mode transition was designed by making use of a metallic protuberance to improve the matching properties without the need of any tuning screw providing a very good electric response with a simple geometry throughout the complete WR375 usable band. Secondly, two designs for a non-conventional TEM coaxial mode to TE20 rectangular waveguide mode transition were developed by achieving the electrical geometry of the output mode with two different feeding configurations; the usage of tuning screws and an appropriate feeding structure allowed for a response with a 3.43% of band. After the coaxial to waveguide transitions, the design of several waveguide mode transducers was conducted: a TE10 rectangular waveguide mode to TM4 01 circular waveguide mode transducer was designed by using a proposal and a second one by exploring new geometries with an own design achieving a 15.38% of band with a central frequency f0 13.25 GHz; an easily manufacturable TE10 rectangular waveguide mode to TE20 rectangular waveguide mode transducer with a symmetric iris to achieve the latter mode field configuration; a narrow-band TE20 rectangular waveguide mode to TE01 circular waveguide mode transducer by the design of intermediate sections to adapt progressively the field configuration to the one of the desired mode (flared-type); and a TE10 rectangular waveguide mode to TE01 circular waveguide mode transducer based on the Marié-type mode converter with an outstanding usable bandwidth (the complete Q-band, 41% of band) for a return loss over 20 dB. Finally, two prototypes of the latter device were manufactured via Additive Manufacturing (SLS5) and measured in back-to-back configuration to further validate the obtained theoretical results; not only did these measurements validate the simulated results by showing a high degree of correlation between the measured and simulated values but also proved the suitability of this manufacturing technique for this sort of flared-type structures, providing in this measurements a worst measured value of conversion efficiency of 83% in the whole Q-band, an excellent result which reveals the fact that even at such high frequencies, where traditionally only machining manufacturing processes were used, Additive Manufacturing has an extraordinary potential.