Abstract: When the signals received by any two array elements are not fully correlated, the performance of DOA estimation methods might degrade if the plane wave model assumption is still used. For narrowband equal-spaced linear array, by utilizing the property of Toeplitz matrix, it is proved that the received signals can be expressed as the sum of several plane wave signals, which can be viewed as the "splitting" parts of the true sources. The "splitting" of sources makes it difficult to estimate the true number of sources. As a result, fake peaks might occur in the spatial spectrum when subspace-based high-resolution spatial spectrum estimation technique is used.