The online partial discharge (PD) diagnosis has been recently accepted as an effective technique for the evaluation of the insulation condition of stator windings in high-voltage rotating machines. There are several types of sensors used for detecting PD in rotating machines. The so-called capacitive coupler, which is connected to the conductor, has been used most widely. Another sensor, the so-called stator slot coupler (SSC), is used by inserting it between the wedge and the top stator coil. However, these are not economically efficient for small-size rotating machines due to their high installation costs. Also, it takes a long time for insertion to take place and it a difficult process, if the SSC is inserted into small-size rotating machines. In this paper, a novel sensor called the partial discharge locating coupler (PDLC) was developed, which is easily mounted into the small-size rotating machines for the PD measurement, and tested to characterize its detectable frequency band and sensitivity. As a result, it was found that the PDLC has a detection frequency band up to several tens of megahertz, and it detects the induced signals with reasonable sensitivity. The PDLC combined with the digital filtering technique enables us to locate the PD activities. Also, it was found that the radiation or end-winding coupling of the PD signals is initiated > 13 MHz.