Study on anechoic chamber for EMI measurement. 2. Measurement and evaluation of chamber characteristics

For pt.1 see IEEE 1989 Int. Symp. on EMC, p.149-53 (1989). It is pointed out that site attenuation of a semi-anechoic chamber must be measured in the same manner as for an open-field test site. Absorbers generate strong reflections at some frequencies, necessitating swept frequency measurements. It is noted, however, that reliable reference values of site attenuation for wideband biconical or log-periodic antennas cannot be obtained either by comparison with open-field sites or by theoretical calculation. The authors propose the use of shortened dipole antennas from which calculated theoretical values can be obtained. Results of actual measurements in a full-anechoic chamber using a shortened dipole antenna are shown. Chamber defects caused by the influence of reflections from the chamber wall appear much more clearly when measurements are made with the antenna height fixed than when antenna height is varied to obtain minimum site attenuation. Thus, fixed height is more suitable for strict chamber evaluation