A method for calculating the measurement error of the directivity characteristics for a two-dimensional antenna placed in a Fresnel zone is presented. The calculation method is based on replacement of a planar-area field with rays converging to an observation point, by a convex-area field with parallel rays. The method is different from the well-know method. It is easier to understand. It permits more exactly defining the magnitude of an error by more rigorous calculation of the dimensions of an equivalent radiative surface, and by taking into consideration the effects on the amplitude of the received signal of different distances between separate antenna points and the observation point.