A semi-anechoic chamber (SAC) is a shielded room in which all four sides of the walls and the ceiling are covered with RF absorbing materials (RF absorbers); however, the ground plane is left flat with no RF absorber materials. In this chamber, the ground plane is a reflective surface made of reflective material, usually a sheet metal ground plane. The purpose of this chamber is to simulate real-world environment conditions (i.e., Open Area Test Site), making the semi-anechoic chamber (SAC) ideal for EMC testing of heavy vehicles or heavier items such as industrial machinery. The solid floor area acts as a work surface for supporting heavy items. The chamber is shielded from outside interference and designed to provide a free space condition above the reflective plane. 

Electromagnetic compatibility (EMC) testing is performed to check whether an electrical and electronic product complies with the relevant EMC standards. EMC testing involves emission and immunity tests which are performed on products. The EMC testing ensures that the product or equipment under test (EUT) will function satisfactorily in its indented electromagnetic environment without disturbing other product(s) in the same environment. The EMC tests are conducted as per EMC standards.  

Figure: Semi-anechoic chamber  

The semi-anechoic chamber provides an interference-free controlled electromagnetic environment with reduced reflections to perform EMC testing. The EMC testing of products is conducted according to various published EMC standards. The SAC provides an ideal environment for performing EMC emission and immunity tests of electrical and electronics products used in various fields, including automotive, aerospace, military, medical, consumer electronics, telecommunications, research and development, and others. The chamber can be used to perform full compliance testing for ANSI C63.4, FCC Parts 15 and 18, CISPR 16/EN55016, EN 50147-2, CISPR 11/EN 55011, CISPR 22/EN55022, CISPR 32/EN55032, VCCI V-3/2003.04, and other EMC standards. 

How does the semi-anechoic chamber establish the interference-free, free-space environment?
The semi-anechoic chamber is built into a metallic shielding room, and the walls and ceiling are covered by RF absorbers. The reflective ground plane is made of reflective materials such as metal.

The RF absorber is a lossy material that 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 above the reflective ground plane of the chamber. Also, the metallic shielding of the chamber acts as a Faraday cage and prevents the entering of external electromagnetic interference, unnecessary sound, or noises into the chamber, offering an interference-free environment. The shielding, RF absorbers, and reflective ground plane setup create a real-world free space environment with reduced reflections within the chamber. It makes the chamber ideal for performing EMC testing. The chamber creates an accurate, stable, and repeatable testing environment for EMC testing.  

Other key information:
The semi-anechoic chamber uses RF absorber materials, such as carbon-loaded foam (usually pyramidal-shaped type), Ferrite F006, H600, and H450. The pyramidal-shaped RF absorbers are available in blue and other colors, including white. Also, other types of RF absorbers are available, including Ferrite tiles absorbers, Hybrid Absorbers, and Ecosorb Absorber Sheets.

The SACs are available in various sizes. The SAC size is decided by the EUT size and frequency ranges being tested for. These chambers support 3m, 5m, and 10m test distances, and can be customized for specific conditions of EMC testing. Most of the manufacturers provide the semi-anechoic chamber with additional accessories such as antenna positioner, turntable, power line filters, video camera, and other accessories. The CISPR16-1-4 standard explains all the requirements for semi-anechoic chambers. 

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