Abstract: Based on the theory of induced current rate, a method of calculating the relative shading rate combined the shielding area with the aperture field distribution is proposed. The problem of the inhomogeneity of the incident field in the traditional algorithm is modified. And the scattering of the metal bar in the electromagnetic window is analyzed. The effect of the grid shading rate on the antenna transmission characteristics under the aperture distribution of a specific antenna is discussed. The insertion loss of the planar array is calculated. The results are compared with the method of moments(MOM) simulation results using the FEKO software.The results show that the relative blocking algorithm improves the accuracy greatly with the maximum error less than 0.1 dB compared with the traditional algorithm. The insertion loss of the electromagnetic window is not only affected by the shielding area and the induced current rate, but also by the aperture field energy flow density distribution. Reducing the relative blocking rate is an effective way to design the low-loss radome.