Anechoic chambers are a vital since they simulate a quiet open-space of infinite dimension, which is useful when exterior influences would otherwise give false results.
[*] Ref: http://en.wikipedia.org/wiki/Anechoic_chamber
I used to work for a electronics company who manufactured circuit boards for satellites where we tested for "free field" conditions (free-field meaning that there are no reflected signals)
The company we used, which I recommend highly is, http://www.emscan.com. They've been around for quite some time and done similar work with Nasa and other aviation / electronic manufacturers.
Hope this helps.