Experience of correlating measurements in the frequency range 30 MHz to 1 GHz

This paper describes how the results of EMC conformance tests in screened rooms can be made comparable to the results of tests on open area test sites (OATS). In the frequency range of 30 MHz to 1000 MHz a typical unlined screened room acts like a cavity with resonances which can give measurement uncertainties of up to ±30 dB. By placing RF absorbing material (RAM) in the room it is possible to considerably reduce the Q-factor of the room. Sufficient strategically placed absorber enabled the measurements of absolute field strength from an equipment under test (EUT) to be made to within ±6 dB of the free-space value at frequencies above 70 MHz. Below 70 MHz an excessive amount of absorber was required to meet this specification, which was relaxed to ±9 dB. One aim of the project was to keep the amount of RAM used to a minimum in order to achieve the target of ±6 dB at an acceptable cost. Measurements using the same antennas and EUT were made in 7 different screened rooms and the results largely agreed within the target specifications. (6 pages)