Effect of cross-polar coupling on open area test site measurement correlation and repeatability

Radiated emission measurements taken on open area test sites (OATSs) show repeatability problems when comparing measurements of the same equipment under test (EUT) on different sites. Significant levels of cross-polar coupling have been observed in the authors' measurements of insertion loss on an OATS. It is demonstrated that cross-polar coupling is a source of repeatability problems when emissions measurements are performed on an arbitrarily polarised EUT. Cross-polar coupling also leads to problems when trying to correlate measurements taken on different OATSs. Simple OATS modelling, using the method of moments code NEC, is presented. From the results, the authors isolate some mechanisms which cause the cross-polar coupling observed in their measurements and assess their relative importance. It is shown that small disruptions to the geometry of OATS equipment can cause cross-polar coupling of the same size as that observed in measurements, and that it is very hard to remove cross-polar coupling effects from any OATS measurement.