Design of rectangular to circular waveguide converter for S-band frequency

Due to the problem of circular waveguide in the excitation process, in this paper, a rectangular to circular waveguide converter is proposed as an alternative method to excite the circular waveguide from rectangular transducer. The converter is designed for S-band frequency to produce a dominant mode of circular waveguide. A WR248 type waveguide transducer that has working frequency of 2.60-3.95GHz for TE10 mode is used as the wave exciter to be converted to a WC248 type of circular waveguide. Physical parameters of converter including length of rectangular segment, length of transition segment and length of circular segment are numerically investigated to obtain the optimum design. Hence, the return loss of converter is used as indicator in the design criteria. From the investigation, it is found that the length of transition segment affects to the return loss of converter and the smoothness of dominant mode transition. It shows that a minimum length of transition segment to produce TE11 mode of circular waveguide smoothly has to be more than twice of waveguide wavelength. From the result, therefore the length of transition segment is chosen to be 275mm as it shows better return loss compared to other lengths almost in the designated working frequency. Furthermore, some discussion related to the field distribution inside of converter is also included in the analysis.