What is a Reverberation Chamber?

A Reverberation chamber is a room with walls that reflect sound. EMC tests and measurements can be done by using a TEM cell, or an anechoic chamber. A reverberation chamber (MSRC) is simply a metallic cavity resonator (with high-quality factor) fitted with a mode stirrer. It looks like a screened room, but the difference is that there are metal reflectors and stirrers in the reverberation chamber. A (large) structure of aluminium plates form a stirrer system that is stepwise turned during the EMC testing.

The objective of the reverberation chamber is to create high EM fields with high voltage standing wave ratios. The stirrers modify the electromagnetic field distribution within the cavity by changing resonances of the cavity and thus create a statistically isotropic and homogeneous field environment during EMC tests and measurements. 

With the help of these reflectors and stirrers, it is possible to create high-intensity EM fields, up to several hundred or thousands of V/m, with optimized input power. The minimum usable frequency of a reverberation chamber is based upon the room sizes as well as reflector/stirrer design.

 Advantages of reverberation chamber:

1. High field strength possible with optimum input power

2. Isotropic illumination of EUT (from all directions and polarizations)

3. No need for EM absorbers hence, lower cost when compared to an anechoic chamber.

4. Easy to upgrade, dismantle, rebuild, and move.

Reverberation chambers are useful for performing immunity & emission tests in compliance with all international EMC standards on a wide range of frequencies (typically from a few MHz to 18/40 GHz). The application areas of reverberation chambers are aeronautics (RTCA DO 160), automotive (IEC EN61000-4-21), military (MIL-STD-461), antenna measurement, and channel modeling of mobile communication.

 Operating principle:

Figure 3: Test set-up

Inside a reverberation chamber that has perfectly conducting walls, the electromagnetic field distribution is characterized by a standing-wave pattern because of the reflection from the walls. As per the electromagnetic theory, electromagnetic fields have different modes. The modes of the electromagnetic field will change with the boundary of the cavity, thus changing the standing-wave pattern within the cavity boundary. 

Figure 4: Field distribution

When the mode stirrer rotates, it will change the boundary of the chamber inside a reverberation chamber that results in modification of electromagnetic field distribution inside the cavity (Figure 4). The stirrer system is rotated electronically or mechanically (by a metallic paddle). Hence, by mode-stirring, a statistically isotropic and homogeneous field environment (as per EMC standards) is created inside the chamber. Since MSRC (mode-stirred reverberation chamber) is a multi-mode resonant cavity; so it allows to obtain high field strength.

 Key Parameters of a Reverberation Chamber:

Frequency range: Represents the useable frequency range of the reverberation chamber. It is usually in the range of MHz to GHz range. 

Typical shielding effectiveness: Represents the maximum shielding effectiveness/attenuation (in dB) provided by the reverberation chamber at a given frequency range (MHz to GHz).  

Dimension overall: Represents the overall dimension of the reverberation chamber in L (m) × W (m) × H (m).

Working volume: Represents the inside working volume of the reverberation chamber.

Recommended max. EUT size: Represents the maximum allowable size of EUT that can be handled by the chamber.

Door size: Represents the size of the door in length and height parameters.

Material of the chamber walls: Type material used in the chamber walls.

Stirrer system: It can be an oscillating wall stirrer/ vertical Z-Fold pedal stirrer/ asymmetric triple pedal stirrer.

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