Effect of flush mounted turntables on site attenuation

When performing various electromagnetic emissions tests, rotation of the equipment under test is often accomplished by the use of turntables. The increasing use of flush mounted metal turntables, instead of above-ground nonmetallic turntables, leads to the problem of how to characterize the interface between the metallic turntable and the ground plane. Several different approaches are used to address this problem: ignoring it, single-point grounding, capacitive coupling, and massively parallel grounding. Site attenuation measurements were made at an open-area test site that has a 12 m by 42 m ground screen and an 8 ft diameter aluminum turntable. The graphs shown include the cable loss (which is constant throughout this test) and thus do not represent true site attenuation, but the anomalies measured translate directly to site attenuation curves. Both horizontally and vertically swept site response measurements were made from 20 MHz to 1000 MHz using biconical and log-periodic broadband antennas. A best case was determined by covering the gap between the turntable and ground screen with copper foil tape. This became the reference with which all other measurements were compared. The impedance across the gap between the turntable and the ground plane was measured, using a vector impedance analyzer, to determine any correlation to site anomalies