This paper proposes a beam-forming method for semi-anechoic chamber (SAC) evaluation by utilizing a synthetic cylindrical array. A study of the influence of the antenna elements position-error on the beam pattern of the array is presented. The analysis is accomplished for the narrowband signals of 1 to 6 GHz where the required side-lobe level of -40 dB and the maximum physical dimension of the array of 2 meters in height and 1 meter in width are presumed for the purpose of SAC evaluation. Dolph-Chebyshev algorithm was used for beam-forming because of its optimal beam-width for a predefined side-lobe level. Monte-Carlo simulations reveal the sensitivity of the beam-pattern maximum side-lobe level to the elements position error.