How reverb chambers work

Reverberation chambers have become increasingly popular among engineers who need to test the susceptibility of electronics equipment to radiated RF. As evidence, consider last year's IEEE symposium on Electromagnetic Compatibility (August 19–23, 2002, Minneapolis, MN), at which seven papers and one workshop covered how to use this technology.

How do reverberation chambers work? Basically, a reverberation chamber is a shielded enclosure with a stirrer/tuner. An antenna radiates RF energy into one corner of the chamber, and most of the waves reflect off the wall (Figure 1). Without the stirrer/tuner, the energy would quickly set up standing waves in the chamber. As a result, the field strength would not be uniform, and testing using this field would not be effective.

The stirrer/tuner has motors that rotates its arms. The revolving arms change the path lengths for the injected RF energy as well as the number of waves that impinge on a specific point. The result is that the field strength magnitude at any point in the chamber differs from the field strength magnitude at other points, and it changes as the stirrer/tuner rotates.

In addition, an effective stirrer/tuner perturbs the field such that it yields a statistically isotropic, randomly polarized, uniform field across a large portion of the chamber volume. The field strength continuously varies, but the average strength over time remains constant. The maximum field strength will be 7 to 8 dB higher than the average field strength in a properly operating chamber.

There are several good reasons for using reverberation chambers instead of other test methods:

• They provide high field strengths for moderate input power, achieving field strengths higher than 25 V/m for inputs of around 1 W.
• Reverberation chambers provide large test volumes—up to 50% of the chamber volume.
• Testing in a reverberation chamber is a better simulation of a complex field in a closed environment, as the field acts like a random array of plane waves coming from all directions. You needn't run multiple tests beaming the source at the unit under test at different angles.
• You can use reverberation chambers for both immunity testing and emissions testing.

Using a reverberation chamber does have drawbacks though. For example, because the field is random, the test does not provide any information about the direction or polarization of radiation that may cause a test to fail. Also, when doing radiated emissions tests, you can only measure the total radiated power of a UUT, not the electric field at a specified distance, as required by many test standards. Nevertheless, I expect the advantages to outweigh these disadvantages, and the use of reverb chambers to increase in the future.

Goldsmith, Kevin, "Reverberation Chambers—What Are They?" IEEE EMC Society Newsletter Online, Fall 1999. http://www.ieee.org/organizations/pubs/newsletters/emcs/fall99/contents.htm.