An unbiased data sheet
Anechoic linings inside shielded chambers have only one purpose, that is, to increase radiated measurement accuracy.
It is generally considered that an unlined (reflective) chamber with have emissions measurement accuracy of +/-20dB
For radiated immunity there will be massive field strength differentials over very small frequency changes.
Clearly it is impossible to claim any credible radiated measurements in an unlined shielded chamber.
To add anechoic lining incrementally may suit financial affordability and could avoid large budget spending at one time.
Partially lined chambers will always be better than reflective chambers and as the lining is closer to full coverage the measurement accuracy will increase.
The wavelength of the radio wave/microwave will change with frequency and so the reflective characteristics from any remaining surfaces will cause measurement distortions at some frequencies and therefore overall claims of measurement accuracy should be moderated.
It is important that the EMC engineer not expect compliant or near compliant radiated EMC measurement accuracy until the anechoic lining is completed in its entirety or more than say 90% completed.
History and experience have proved the above to be the case.
