What is quiet zone in anechoic chamber?

Answer 1

"The Quite Zone of an anechoic chamber is a defined volume within the chamber where a Device Under Test (DUT) is to be placed for evaluation. Quiet Zone Probing is accomplished by moving a probe antenna through the Quiet Zone to determine the level of extraneous signals entering the volume." (from Sirles, C.W., et al., "Anechoic Chamber Performance Characterization Using Spherical Near-Field Imaging Techniques")

In other words, a quite zone is a specific volume inside the anechoic chamber, wherein the level of RF power does not exceed a certain limit. The "quiteness" of the zone is a function of signal frequency, i.e. different frequencies will correspond to different RF levels within the quite zone volume.