What are Anechoic chambers?
Editorial Team - EMC Directory
An Anechoic Chamber is a shielded room with walls, ceiling, and floor covered with radiation-absorbent material (RAM) or RF absorbers. The word anechoic means “non-reflective or without echo”. The purpose of the anechoic chamber is to provide an interference-free, free-field environment (i.e., reflections-free environment) for conducting EMC testing. The chamber provides a perfectly controlled electromagnetic environment with no unwanted electromagnetic waves inside the chamber, ideal for conducting reliable EMC emission and immunity tests.
Figure 1: Anechoic Chamber (Full anechoic chamber (FAC))
EMC (Electromagnetic compatibility) testing is conducted to check whether an electrical and electronic product complies with the relevant EMC standards. The EMC testing process involves emission measurement and immunity tests, which are performed on products. These tests ensure that the product or device under test (DUT) will function satisfactorily in its indented electromagnetic environment without disturbing other product(s) in the same environment. These tests are conducted as per EMC standards.
The anechoic chamber is extensively used to conduct EMC pre-compliance and full-compliance testing of a wide range of electrical and electronic products used in various industries, including domestic, commercial, aeronautical, defense, automotive, telecommunications, and RF measurements. The chamber can be used to perform full compliance testing for ANSI C63.4, IEC 61000-4-3, EN61004-3, CISPR25, ISO 11452-2, MIL-STD 461, and other EMC standards.
How does the anechoic chamber establish an interference-free, free-field environment?
Figure 2: Pyramidal RF absorber
The anechoic chamber is usually built into a metallic shielding room, and the walls and ceiling of the chamber are covered by RF absorbers (usually in a pyramidal shape, figure 2). The RF absorber is a lossy material. It transforms the incident electromagnetic energy into thermal energy and dissipates it into the surrounding air. Thereby, the RF absorbers stop the reflection of Electromagnetic waves inside the chamber, providing a free-field or reflection-free environment. The metallic shielding of the chamber acts as a Faraday cage and prevents the entering of external EMI and other noises into the chamber, offering an interference-free environment.
In this way, the anechoic chamber provides an interference-free, free-field environment (i.e., no reflections environment), ideal for conducting reliable and error-free EMC testing. This chamber creates an accurate, stable, and repeatable testing environment for EMC testing and RF (wireless) measurements.
Types of Anechoic Chamber:
Two types of anechoic chamber are available: the fully anechoic chamber (FAC) (Figure 1) and the semi-anechoic chamber (SAC) (Figure 3). In a fully anechoic chamber, all four sides of the walls, the ceiling, and the floor are covered with absorbing materials (RF absorbers). The fully anechoic chamber simulates interference-free, free field conditions/environment, which is suitable for conducting EMC testing as well as some antenna testing.
Figure 3: Semi anechoic chamber (SAC), Reflective ground plane.
In a semi-anechoic chamber (SAC), a portion of the chamber is made reflective; i.e., all four sides of the walls and the ceiling are covered with absorbing materials (RF absorber), and the ground plane is reflective sheet metal. The reason for keeping the floor reflective and not loaded with RF absorbers is to simulate the real-world environment (i.e., open area test site). The purpose of SAC is to simulate a real-world environment condition (i.e., Open Area Test Site). It makes the semi-anechoic chamber (SAC) suitable for EMC testing of heavy vehicles. The solid floor area is used as a work surface for supporting heavy items.
Other key information:
The most familiar RF absorber material used in the anechoic chamber is the carbon-loaded foam pyramidal-shaped type. The pyramidal-shaped RF absorbers come in blue and other colors, including white. The RF absorbers come in other types also that include Ferrite tiles absorbers, Hybrid Absorbers, and Ecosorb Absorber Sheets.
Anechoic chambers are available from small compartments the size of household microwave ovens to large aircraft hangars. The size of the chamber is decided by the size of the objects and frequency ranges being tested. These chambers are available to support 3m, 5m, and 10m test distances, and customized chambers are also available to perform EMC testing. Most of the anechoic chamber manufacturers supply the chamber with additional accessories such as a turntable, antenna positioner, power line filters, video camera, and other accessories. The CISPR16-1-4 standard explains all the requirements required to be satisfied by anechoic chambers.