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Partial discharge (PD) detection using a UHF band signal is a well known advanced insulation diagnosis method in a gas insulated switchgear (GIS) and has been actively studied. Detailed investigation of detection principles and electromagnetic wave propagation inside the GIS tank is required for significant improvement in sensitivity and accuracy. As the wavelength of UHF-band signals is comparable to the GIS tank size, higher-order modes also appear in electromagnetic waves propagating inside at frequencies at or above a certain value, creating complicating phenomena. This study investigated these phenomena, focusing on experiments with a UHF sensor inside a coaxial pipe as a simplified model of GIS structure, while varying conditions such as sensor shape and installation position (both circumferential and longitudinal), the input method of a simulated PD signal, and the presence or absence of an insulating spacer. On the low frequency side (TEM mode only), the sensor output can be described by an equivalent electric circuit determined by the coupling capacitance between the sensor surface, the center conductor and the tank. At the cutoff frequency of the TE11 mode or above, resonance phenomena appeared, which were related to the pipe length. Further, it was found that installing an insulating spacer inside the coaxial pipe shifted the resonant frequency in a manner closely connected with the spacer position's relationship to the electric field distribution inside the tank.