Analysis of guided modes in vertical strip line embedded in an NRD guide and its application to design of LSE mode suppressor

Guided modes in a vertical strip line embedded in an NRD guide were analyzed based on a 2-dimensional finite element method in this paper. The dominant mode in this guiding structure is TEM wave, and besides, it is evident that a quasi-LSM₁₁ mode occurs using the wide metal strip, while a quasi-LSE₀₁ mode generates employing the narrow one from the theoretical investigation. To verify this fact, the propagation constants were found out by measuring the resonant frequency for each mode. Agreement between the theory and measurement is quite satisfactory, and thus, the existence of such parasitic modes was confirmed. Based on the calculated results, we clarified a condition to suppress the parasitic modes and exhibited a design chart of the LSE₀₁ mode suppressor, which is a powerful tool to create high performance NRD guide integrated circuits. This design chart is indispensable for avoiding an error in design of the LSE₀₁ mode suppressor.