Anechoic chamber

Full anechoic chambers aim to absorb energy in all directions. To do this, all surfaces, including the floor, need to be covered in correctly shaped wedges. A mesh grille is usually installed above the floor to provide a surface to walk on and place equipment. This mesh floor is typically placed at the same floor level as the rest of the building, meaning the chamber itself extends below floor level. This mesh floor is damped and floating on absorbent buffers to isolate it from outside vibration or electromagnetic signals.

In contrast, semi-anechoic or hemi-anechoic chambers have a solid floor that acts as a work surface for supporting heavy items, such as cars, washing machines, or industrial machinery, which could not be supported by the mesh grille in a full anechoic chamber. Recording studios are often semi-anechoic.

The distinction between "semi-anechoic" and "hemi-anechoic" is unclear. In some uses they are synonyms, or only one term is used.[7] Other uses distinguish one as having an ideally reflective floor (creating free-field conditions with a single reflective surface) and the other as simply having a flat untreated floor.[8][9] Still other uses distinguish them by size and performance, with one being likely an existing room retrofitted with acoustic treatment, and the other a purpose-built room which is likely larger and has better anechoic performance.[10]

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, the lower the resulting 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.

Close-up of a pyramidal RAM

Waves of higher frequencies have shorter wavelengths and are higher in energy, while waves of lower frequencies have longer wavelengths and are lower in energy, according to the relationship ${\displaystyle \lambda =v/f}$ where lambda represents wavelength, v is phase velocity of wave, and ${\displaystyle f}$ is frequency. To shield for a specific wavelength, the cone must be of appropriate size to absorb that wavelength. 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. Pyramidal RAM is at its most absorptive when the incident wave is at normal incidence to the internal chamber surface and the pyramid height is approximately equal to ${\displaystyle \lambda /4}$, where ${\displaystyle \lambda }$ is the free space wavelength. Accordingly, increasing the pyramid height of the RAM for the same (square) base size improves the effectiveness of the chamber at low frequencies but results in increased cost and a reduced unobstructed working volume that is available inside a chamber of defined size.

An RF anechoic chamber is usually built into a screened room, designed using the Faraday cage principle. 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 screened room to attenuate unwanted signals penetrating inwards and causing interference to the equipment under test and prevent leakage from tests penetrating outside.

At lower radiated frequencies, far-field measurement can require a large and expensive chamber. Sometimes, for example for radar cross-section measurements, it is possible to scale down the object under test and reduce the chamber size, provided that the wavelength of the test frequency is scaled down in direct proportion by testing at a higher frequency.

RF anechoic chambers are normally designed to meet the electrical requirements of one or more accredited standards. For example, the aircraft industry may test equipment for aircraft according to company specifications or military specifications such as MIL-STD 461E. Once built, acceptance tests are performed during commissioning to verify that the standard(s) are in fact met. Provided they are, a certificate will be issued to that effect. The chamber will need to be periodically retested.

Test and supporting equipment configurations to be used within anechoic chambers must expose as few metallic (conductive) surfaces as possible, as these risk causing unwanted reflections. Often this is achieved by using non-conductive plastic or wooden structures for supporting the equipment under test. Where metallic surfaces are unavoidable, they may be covered with pieces of RAM after setting up to minimize such reflection as far as possible.

A careful assessment may be required as to whether the test equipment (as opposed to the equipment under test) should be placed inside or outside the chamber. Typically most of it is located in a separate screened room attached to the main test chamber, in order to shield it from both external interference and from the radiation within the chamber. Mains power and test signal cabling into the test chamber require high quality filtering.

Fiber optic cables are sometimes used for the signal cabling, as they are immune to ordinary RFI and also cause little reflection inside the chamber.

The following health and safety risks are associated with RF anechoic chambers:

• RF radiation hazard
• Fire hazard
• Trapped personnel

Personnel are not normally permitted inside the chamber during a measurement as this not only can cause unwanted reflections from the human body but may also be a radiation hazard to the personnel concerned if tests are being performed at high RF powers. Such risks are from RF or non-ionizing radiation and not from the higher energy ionizing radiation.

As RAM is highly absorptive of RF radiation, incident radiation will generate heat within the RAM. If this cannot be dissipated adequately there is a risk that hot spots may develop and the RAM temperature may rise to the point of combustion. This can be a risk if a transmitting antenna inadvertently gets too close to the RAM. Even for quite modest transmitting power levels, high gain antennas can concentrate the power sufficiently to cause high power flux near their apertures. Although recently manufactured RAM is normally treated with a fire retardant to reduce such risks, they are difficult to eliminate. Safety regulations normally require the installation of a gaseous fire suppression system including smoke detectors.