Analysis of the transmission and leakage characteristics of partial discharge (PD) signal in Gas Insulated Switchgear (GIS) is of great importance for PD detection. Traditional connector of two GIS tanks is a disc insulator, so the leakage of PD signal is easily detected. Recently, to improve the performance of GIS, the disc insulator is often sealed by a metal belt and an aperture is made in the metal belt for PD detection. The leakage of PD is weak and the PD detection is delicate. This paper is devoted to the theoretical and experimental study of the transmission and leakage characteristics of the PD signals in such GIS. The electric field in the GIS and the characteristics of the leaked signals are simulated with software Ansoft HFSS. The PD source is assimilated by two simple models: "line feed" and "waveguide feed". The simulation results show that the frequency band and amplitude of the leaked signals depend on the size of the aperture and the distance of the detector from the GIS. A novel ultra wideband planar antenna is developed and a full experiment setup is built to validate the simulation and to test the characteristics of the leaked PD signal. The transmission coefficient S 21 has been measured with a vector network analyzer. The experimental results correspond better to the simulation data with "line feed" than "waveguide feed". But the prediction of the frequency characteristics by the dipole model is very different from the experiment results in the high frequency domain. Furthermore, we show by the experiments that the amplitude and frequency profile of the transmission coefficient S 21 is sensible to the position of the PD source in the GIS.Index Terms -Electromagnetic leakage, gas insulated switchgear (GIS), partial discharge (PD), transmission coefficient.