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Fig. 4.23. Surface efficiency he as function of the ratio rms error to wavelength.

Now follows a set of curves, representing Eq. (4.44) [Mat.4.26], of the level of the error pattern fE (0) with respect to that of the (weakened) main beam fD (0) as function of e/1 and for a number of values of the ratio c* = c / d. From Fig. 4.24 one sees that for an rms error of less than 1/20 and a correlation length smaller than d/6 the error pattern level stays below approximately -20 dB, i.e. comparable with the normal sidelobe level of the main diffraction pattern. In conclusion, the relation of Eq. (4.46) [Mat.4.27], showing the ratio of the power in the error pattern to that in the main beam is depicted in Fig. 4.25. When the rms surface error approaches one twentieth of a wavelength, about thirty percent of the received power is distributed to the error pattern.

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