>> A radio frequency (RF) anechoic chamber is a room which is designed to suppress the electromagnetic waves.
The internal appearance of the radio frequency (RF) anechoic chamber is covered with radiation absorbent material (RAM). The RF anechoic chamber is typically used to house the equipment for performing measurements of antenna radiation patterns, electromagnetic compatibility (EMC) and radar cross section measurements. Many RF anechoic chambers use pyramidal absorbers.
The performance quality of an RF anechoic chamber is determined by its lowest test frequency of operation, at which measured reflections from the internal surfaces will be the most significant compared to higher frequencies.
An RF anechoic chamber is usually built into a Faraday cage.
This is because most of the RF tests that require an anechoic chamber to minimize reflections from the inner surfaces also require the properties of a Faraday cage to attenuate unwanted signals penetrating inwards and causing interference to the equipment under test and prevent leakage from tests penetrating outside.
The radiation absorbent material (RAM) is designed and shaped to absorb incident RF radiation (also known as non-ionising radiation), as effectively as possible, from as many incident directions as possible. The more effective the RAM is the less will be the level of reflected RF radiation. Many measurements in electromagnetic compatibility (EMC) and antenna radiation patterns require that spurious signals arising from the test setup, including reflections, are negligible to avoid the risk of causing measurement errors and ambiguities.
One of the most effective types of RAM comprises arrays of pyramid shaped pieces, each of which is constructed from a suitably lossy material. To work effectively, all internal surfaces of the anechoic chamber must be entirely covered with RAM. Sections of RAM may be temporarily removed to install equipment but they must be replaced before performing any tests. To be sufficiently lossy, RAM can neither be a good electrical conductor nor a good electrical insulator as neither type actually absorbs any power. It has to be an intermediate grade of material which absorbs power gradually in a controlled way as the incident wave penetrates it. Typically pyramidal RAM will comprise a rubberized foam material impregnated with controlled mixtures of carbon.
An alternative type of RAM comprises flat plates of ferrite material, in the form of flat tiles fixed to all interior surfaces of the chamber. This type has a smaller effective frequency range than the pyramidal absorber and is designed to be fixed to good conductive surfaces. It is generally more durable than the pyramidal type RAM but is less effective at higher frequencies. Its performance might however be quite adequate if tests are limited to lower frequencies (ferrite plates have a damping curve that makes them most effective between 30-1000MHz).
There is also a hybrid type, a combination of ferrite tiles and pyramidal foam absorbers. Containing the advantages of both technologies, the frequency range can be maximized while the pyramid remains small (45 cm).
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