The anechoic chamber and semi-anechoic chamber are the shielded testing rooms with RF absorbers. They are designed to provide an optimal environment suitable for performing EMC testing. However, both chambers can be used in EMC testing; there are some differences between these two chambers. This article discusses the differences between the anechoic chamber and the semi-anechoic chamber.

What is an Anechoic chamber?

Figure: Anechoic chamber An anechoic chamber is a shielded room with walls, ceiling, and ground plane covered with radiation-absorbent material (RAM) or RF absorbers. Since all the surfaces are covered by absorbers, this chamber is also called a fully anechoic chamber (FAC). The meaning of the 'anechoic' word is “non-reflective or without echo”. 

The RF absorber is a lossy material. The incident electromagnetic energy on the RF absorber will be converted into thermal energy and dissipated into the surrounding air. Thereby, the RF absorbers absorb the electromagnetic waves from all directions of the chamber, resulting in completely stopping electromagnetic wave reflections within the chamber. The metallic shielding of the chamber acts as a Faraday cage and prevents the entry of external interference or noises, offering an interference-free environment.

 In this way, the anechoic chamber establishes/creates an interference-free, free-field environment, which is suitable for performing EMC testing.   This chamber is ideal for EMC testing of electrical and electronic devices used in various areas, such as telecommunications, consumer electronics, aerospace, and defense, and is also used for antenna gain, efficiency, and radiation pattern measurements.

What is a Semi-anechoic chamber (SAC)?

A semi-anechoic chamber (SAC) is a shielded room with walls and a ceiling covered with radiation-absorbent material (RAM) or RF absorbers; however, the ground plane is left as a flat reflective surface with NO absorber materials. The ground plane of the SAC is made of reflective material, usually a sheet metal ground plane. The metallic shielding of the chamber acts as a Faraday cage and prevents the entry of external interference or noises, offering an interference-free environment. 

Figure: Semi-anechoic chamber (SAC), Has a Reflective ground plane. 

 Since the ground floor is made as the reflective surface, this chamber partially absorbs the electromagnetic wave inside the chamber. Hence, the name semi-anechoic chamber. This chamber's purpose is to simulate the real-world environment conditions (i.e., Open Area Test Site). The chamber creates a free-field environment above the ground plane. The semi-anechoic chamber (SAC) is particularly suitable for performing EMC testing of heavy vehicles or heavier items such as industrial machinery. The reflective solid floor area of the chamber is used as a work surface for supporting heavy items. 

The objective of both the anechoic chamber and semi-anechoic chamber is to provide a controlled optimal environment for performing EMC testing. Both these chambers establish an accurate, stable, and repeatable testing environment for performing the EMC testing. The choice between the anechoic chamber and semi-anechoic chamber for EMC testing depends mainly on the tested products, testing requirements, and the EMC test standards adopted. Some EMC standards require testing in the anechoic chamber, and some standards require testing in the semi-anechoic chamber.

The following table provides a quick overview of the key differences between anechoic and semi-anechoic chambers in various aspects.

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