Detecting partial discharge (PD) caused by the degradation of insulating materials is needed to prevent the electric breakdown. We have studied radio remote sensing to detect PD. Therefore, it is necessary to remove the electromagnetic (EM) waves reflected on the power equipment when PD source is localized. In this paper, we consider to estimate the direction-of-arrival (DOA) of EM waves emitted from PD in a free space. The signal subspace method, which uses an eigenstructure of the correlation matrix obtained from the received signals, is approved useful to estimate DOA. The finite difference time domain (FDTD) method is used to simulate the propagation of EM waves emitted from PD sources. The wideband signal subspace methods, e.g. coherent signal subspace method (CSSM), test of orthogonality of projected subspace (TOPS), test of orthogonality of frequency (TOFS), and improved CSSM (I-CSSM) are applied to estimate the DOA of EM waves.