Antenna related uncertainties in screened room emission measurements

Discusses the measurement of electric field strength of radiated emissions in screened rooms in the frequency range 30 MHz to 1 GHz. The use of screened rooms rather than open area test sites (OATS) is specifically called for in the American standard MIL-STD461 and the UK standard Def.-Stan. 5941. The main differences between the two types of facility are: a) the wall reflections in undamped screened rooms enormously increase the measurement uncertainty, b) the military specification limits are lower than the civil ones which leads to c) the requirement for testing at a distance of 1 m from the equipment under test (EUT) in contrast to 10 m on OATS. Besides the uncertainty caused by wall reflections, there are antenna related uncertainties. The uncertainty in the measurement of field strength is not simply the uncertainty of the antenna factor given in the certificate of calibration for the antenna. There are four additional sources of uncertainty, namely mutual coupling, phase centre, directivity and size. The case is presented for increasing the distance used in military testing from 1 m to 2 m or 3 m. As a consequence of this: a) measurement uncertainties are reduced and b) the measured field is more absolute in the sense that it is less influenced by near-field effects and there is a convergence with methods and results pertaining to civil testing