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This study discusses the characteristics of a mode converter to generate microwaves in the TE01 mode for the long-range inspection of pipes. Numerical analysis was conducted to evaluate the effect of the pipe diameter and the number of coaxial cables inserted into the pipe on the efficiency of converting microwaves from the coaxial TEM mode to the circular TE01 mode. The results reveal that the most efficient frequency range for TE01 mode propagation strongly depends on the number of cables and pipe diameter. Subsequent experiments were performed using brass pipes with a length of 5 m and diameters of 39.0 and 74.0 mm. The results of these experiments are consistent with the results of the numerical analysis: the pipe with a diameter of 39.0 mm showed a clear signal from an axial slit 1.5 m away from the mode converter and the pipe with a diameter of 74.0 mm showed almost no difference between signals with or without the slit. [ABSTRACT FROM AUTHOR]