Preliminary Interpretation of Near-Field Effects on Measurement Accuracy in Shielded Enclosures

It has been well documented that measurements made in shielded enclosures can differ significantly from corresponding measurements made in the open-field. At frequencies above one or two hundred MHz these differences can be attributed to the existence of standing waves in shielded enclosures caused by multipath reflections from enclosure walls, ceilings and floors. However, large differences in corresponding open-field and shielded enclosure measurements also have been observed at frequencies well below 100 MHz, where the dimensions of most conventional enclosures are small enough compared to wavelength to preclude any significant conventional multipath interference effects. This paper discusses the results of an investigation into the causes and effects of these observed measurement inaccuracies in shielded enclosures at frequencies below 100 MHz. A preliminary theory is postulated which accounts for the observed inaccuracies in terms of a near-field measurement effect which is present in a shielded enclosure environment and absent in the corresponding open-field environment. All shielded enclosure and open-field measurement data collected to date has been found to be consistent with the postulated theory. A shielded enclosure measurement technique which is based on minimizing this near-field enclosure effect is discussed. Shielded enclosure measurement results obtained using this technique have been found to differ only slightly from corresponding open-field results.