ANSI C63.7-2005 pdf download American National Standard Guide for Construction of Open-Area Test Sites for Performing Radiated Emission Measurements
6.1 Ground plane Size and shape of ground plane The size and shape of the reflecting surface, or ground plane, are dependent upon the measurement geometry and whether or not the EUT can be rotated (see 6.3 for ground plane material discussion). In general, the reflecting surface is contained wholly within the obstruction-free area. It is prudent, however, to construct the ground plane so that it may be extended in all directions if needed. For a site equipped with a turntable, the theoretically minimum ground plane size and shape is that of the first Fresnel ellipse. See Annex A for a discussion of Fresnel ellipses, the note associated with Figure A.1 for a descriptive picture, and Table 1 for dimensions of several representative ellipses. Practical experience indicates that rectangular ground planes having length and width dimensions substantially less than the Fresnel ellipse dimensions shown in Table 1 may be satisfactory. For example, [B4] shows a 6 m by 9 m ground plane for making 3 m measurements. Table 1 indicates that at 30 MHz the ellipse is 9.9 m by 9.5 m, which for certain areas of the ellipse is larger than the rectangular 6 m by 9 m size (which is generally adequate for measurements). The 6 m by 9 m ground-plane size may not be adequate for vertical polarization, particularly if the ground plane is raised. In this case, it should be expanded to the dimensions in Table 1. The Fresnel ellipse dimensions should, therefore, be used only as a guide, and the adequacy of an actual ground plane should be determined using the site characterization procedure and acceptability criterion given in ANSI C63.4-2003. Since the first Fresnel ellipse encloses the area on the ground plane from which the major part of the ground-reflected energy comes, it may be prudent to have the capability to extend the rectangular or trapezoidal dimensions of the ground plane up to or larger than the dimensions of the Fresnel ellipse. This is especially important for ground planes that are elevated above the surrounding surface.