ANSI C63.7:2005 pdf download.Guide for Construction of Open-Area Test Sites for Performing Radiated Emission Measurements.
For large EUTs. the obstruction-free area must be expanded so that the obstruction clearance distances exist from the perimeter of the EIJT. Since the first Fresnel ellipse encloses the area on the ground plane from which the major part of the ground-reflected energy comes, it follows logically that there should be no obstructions in this area. For a 3 m site, the first Fresnel ellipse (see Table I) is larger than the obstruction- free area ellipse shown in Figure 1. If the site fails to meet the normalized site attenuation criteria in ANSI C63.4-2003, the area that is outside of the construction-free ellipsc but enclosed within the first Fresnel ellipse should he investigated for scatterers and obstructions. The effects of poor test-site construction including inadequate obstruction-free area, improper ground-plane construction and sizing, and other site defects arc discussed in 1B31 and (BlO). lithe site is not equipped with a turntable. that is. if the EUT is stationary, the recommended obstruction-free area is a circular area such that the radial distance from the boundary of the EUT to the boundary of the area is equal to the measurement distance multiplied by 1.5 (sec Figure 2). Tn this case, the receiving antenna will bc moved about the EUT and a clear area of 1.5 times the measurement separation distance will provide the same prevention of undesired reflections as the boundary shown in Figure I.
The terrain within the obstruction-free area should be smooth, but does not have to be tiat. Small slopes needed for adequate drainage may be acceptable as long as the normalized site attenuation measurement is not significantly affected. (See 6.2 for a discussion of smoothness requirements.)
6.3 Ground-plane material
As stated in ANSI C63.4-2003, section 5.4.1 and section 5.4.3. a conducting ground plane is required. Metals such as copper. steel, tin, and aluminum or alloys of these metals as long as electrical conductivity is no lower than steel arc recommended as (he ground-plane material for measurement distances between 3 m and 30 m. Some examples include solid metal sheets. metal foil, perforated metal, expanded metal. vire cloth, wire screen, and metal grating. Metals should be protected to the largest extent practical from weather effects if no protective structure covers the test site. e.g.. steel should be galvaniied to prevent rust. In any case, periodic nonnalized site attenuation measurements may be needed to determine any deleterious effects due to the ground plane aging and other eflects such as the structure itself being exposed to the weather (if a structure is used). The ground plane should have no voids or gaps with linear dimensions that are greatcr than 1/10 of a wavelength at the highcst frequency of measurement (about 3 cm at 1000 MHz). Material comprised of individual sheets, rolls, or pieces should be soldered or welded at the seams, preferably continuously, but in no case with gaps longer than the 1/10 wavelength criterion. Afler installation and especially if the above criterion is not adhered to. the adequacy of the ground-plane material and interconnecting mechanism and fastening techniques should be determined by using the site- characterization procedure and acceptability criterion in ANSI C63.4-2003. If the ground plane is exposed to the weather, adequate precautions should be taken to ensure that the material and interconnecting mechanism do not oxidize or corrode to the poin that the conductive properties of the entire test area ground plane are degraded. Repeat site-attenuation measurements are clearly needed for all sites (a minimum of every six months is recommended) and more frequently for sites exposed to the weather. In fac, some test laboratories perform representative attenuation measurements at the start of each major test program.
