Switch Network Analyzer Figure 12.3 Drawing (left) and photo (right) of the Bluetest standard reverbera- tion chamber, shown with open door. The drawing shows the setup for measuring MIMO system capacity, and the photo shows the setup for measuring radiated power of a mobile phone close to the head phantom. The chamber size shown allows for use down to 850 MHz with 25 MHz frequency stirring. The reverberation chamber emulates such an isotropic environment. The isotropic reference environment has no counterpart in reality, but still it could be representa- tive because any environment will appear isotropic if the mobile terminal is used with arbitrary orientation in the environment. The basic measurement setup in a reverberation chamber that is used for cali- bration as well as for measuring passive antenna performance is shown in the left part of Figure 12.3. The reverberation chamber is large in terms of the wavelength at the frequency of operation, which means that several modes are excited. It can be shown that each mode in a rectangular cavity can be expressed as eight plane waves [15] and that the directions of arrival of these plane waves are uniformly distributed over the unit sphere, if enough modes are excited. By stirring the modes, the set of incoming plane waves seen by an antenna in the chamber will change, as will how they com- bine to the received signal. The result is a signal that is fading. When the stirring is done properly, the resulting environment in the chamber will represent an isotropic multipath environment (i.e., the desired reference environment mentioned earlier). The key is the mode stirring, which can be done in different ways. The chamber shown in Figure 12.3 has the following stirring capabilities: Mechanical stirring is done by means of two metal plates that can be moved along a complete wall and along the ceiling by step motors. The larger the volume is that the stirrers cover, the better. In platform stirring [16], the antenna or mobile terminal under test is placed on a rotata