Mode purity evaluation of high-power millimeter wave transmission line by 3-port directional coupler

In high-power millimeter wave systems, transmission lines based on the circular corrugated waveguide are widely used to realize efficient millimeter wave transmission. The mode purity is a very important parameter for the high-power millimeter wave transmission lines. The main mode in the corrugated waveguide is usually the LP01 mode, while the main spurious modes include the LP02 mode, the LP11 even mode, the LP11 odd mode, and so on. For the mode purity analysis, we designed a 3-port directional coupler, considering 3 major spurious modes. A calculation code was developed for the mode purity investigation. In addition, the error caused by ignoring the minor spurious modes was analyzed with some calculation examples. Finally, high-power experi-ments were carried out with a 105 GHz millimeter wave system. The mode purity was evaluated with the data measured by the 3-port directional coupler. The results indicated that the content of the main mode LP01 is up to 90.38%, which aligns with the parameters derived from the factory test results of 105 GHz high-power millimeter wave source. It shows that the 3-port directional coupler can be used as an optional method for mode purity estimation of the high-power millimeter wave systems.