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• A robust WLB for non-circular signals in ULAs with unknown mutual coupling is designed. • The decoupled AINCM is reconstructed by decoupling all of the related parameters. • The ESV of SOI is obtained by estimating the steering vector mismatch and ESV mismatch. • The proposed WLB is robust against mutual coupling mismatch at all SNRs. The performance of widely linear beamformer (WLB) is seriously degraded by any uncalibrated mutual coupling between antenna elements. In this paper, we propose an architecture to mitigate the unknown mutual coupling mismatch in uniform linear arrays and design a robust WLB for non-circular signals. We exploit the fourth-order cumulants method and the steering vector property of a uniform linear array to estimate the directions of incident signals as well as the mutual coupling coefficients. We decouple the received signals by using the mutual coupling matrix so that the powers and non-circularity coefficients of incident signals can be estimated appropriately. This allows us to reconstruct a so-called decoupled augmented interference-plus-noise covariance matrix (AINCM). We minimize the variance of beamformer with the subspace constraint to estimate the steering vector mismatch. Meanwhile, the extended steering vector (ESV) mismatch is estimated by minimizing the variance of WLB with the extended subspace constraint. Therefore, the ESV of signal-of-interest can be estimated accurately. Numerical simulations illustrate that the proposed WLB is robust against the unknown mutual coupling mismatch and significantly outperforms some of the state-of-the-art WLBs. [ABSTRACT FROM AUTHOR]