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In recent years, the ultra high frequency (UHF) method is employed to reduce and avoid the noises effects on partial discharge (PD) detection for the power transformer on-site. In order to know the UHF signal radiation from the partial discharge in oil impregnated insulation and its propagation characteristics in a transformer tank, the experiments have been carried out in high voltage laboratory. The current impulse waveforms of PD and its UHF signals are measured through the partial discharge detecting circuit and a coupler, respectively. The results show that the electromagnetic wave can be excited from the partial discharge in oil-impregnated insulation with the higher frequency than that in air. The propagation and attenuation characteristics of UHF signals in a metal tank are different from that in opening air. The experimental results also show that the UHF signal can be radiated from the inside of a metal tank through a wire, which makes it possible to measure partial discharge for the power transformer with the coupler fixed at the outside of the tank.