Electromagnetic reverberation chamber
An electromagnetic reverberation chamber (also known as a reverb chamber (RVC) or mode-stirred chamber (MSC)) is an environment for electromagnetic compatibility (EMC) testing and other electromagnetic investigations.
[1] A reverberation chamber is screened room with a minimum of absorption of electromagnetic energy.
Due to the low absorption, very high field strength can be achieved with moderate input power.
A reverberation chamber is a cavity resonator with a high Q factor.
Thus, the spatial distribution of the electrical and magnetic field strengths is strongly inhomogeneous (standing waves).
To reduce this inhomogeneity, one or more tuners (stirrers) are used.
A tuner is a construction with large metallic reflectors that can be moved to different orientations in order to achieve different boundary conditions.
The Lowest Usable Frequency (LUF) of a reverberation chamber depends on the size of the chamber and the design of the tuner.
The concept of a reverberation chamber is comparable to a microwave oven.
[2] For statistic quantities like mean and maximal values, a more explicit notation is used in order to emphasize the used domain.
) means that quantities are taken for different chamber positions, and ensemble domain (subscript
) refers to different boundary or excitation conditions (e.g. tuner positions).
A reverberation chamber is cavity resonator—usually a screened room—that is operated in the overmoded region.
To understand what that means we have to investigate cavity resonators briefly.
are the cavity's length, width and height, and
With that equation, the number of modes with an eigenfrequency less than a given limit
In principle, two modes—a transversal electric mode
are given by Due to the boundary conditions for the E- and H field, some modes do not exist.
The leading term is proportional to the chamber volume and to the third power of the frequency.
This term is identical to Weyl's formula.
An important quantity is the number of modes in a certain frequency interval
The factor Q of the TE and TM modes can be calculated from the fields.
For a rectangular cavity follows[4] Using the Q values of the individual modes, an averaged Composite Quality Factor
includes only losses due to the finite conductivity of the chamber walls and is therefore an upper limit.
For the lower frequency range the dominant loss is due to the antenna used to couple energy to the room (transmitting antenna, Tx) and to monitor the fields in the chamber (receiving antenna, Rx).
The quality factor including all losses is the harmonic sum of the factors for all single loss processes:
Resulting from the finite quality factor the eigenmodes are broaden in frequency, i.e. a mode can be excited even if the operating frequency does not exactly match the eigenfrequency.
Therefore, more eigenmodes are exited for a given frequency at the same time.
is a measure of the frequency bandwidth over which the modes in a reverberation chamber are correlated.
That is the time constant of the free energy relaxation of the chamber's field (exponential decay) if the input power is switched off.
