The internal ultra-high frequency（UHF） couplers are getting more and more practical application for the on-line partial discharge monitoring in gas insulated switchgear（GIS）. Because of this

the internal UHF couplers with axially symmetric structure are experimentally investigated on its partial discharge detection sensitivity. A GIS test model is established based on the spectrum analyzer and a swept frequency signal generator

and the influence of the dimensions of the coupler on the sensitivity is analyzed by comparing the insert loss of the couplers with different dimensions. The results show that the diameter of the coupler should match that of the hand-hole. A relatively larger-diameter hand-hole with a smaller-diameter coupler will improve the sensitivity

while opposite effect will be obtained if the diameter of the coupler is close to that of the hand-hole. Besides

a remarkable decline will be seen on the sensitivity if the electrode and its insulation support is fixed on the cover of the hand-hole using metal bolts.