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An anechoic chamber is a room in which there are no echoes. This description was originally used in the context of acoustic (sound) echoes caused by reflections from the internal surfaces of the room but more recently the same description has been adopted for the radio frequency (RF) anechoic chamber. An RF anechoic chamber is designed to suppress the electromagnetic wave analogy of echoes: reflected electromagnetic waves, again from the internal surfaces. Both types of chamber are normally constructed, not only with echo suppression features, but also with effective isolation from the acoustic or RF noise present in the external environment. In a well designed acoustic or RF anechoic chamber, the equipment under test will only receive signals (whether acoustic or RF) which were emitted directly from the signal source, and not reflected from another part of the chamber.
Acoustic anechoic chambersEdit
Anechoic chambers are commonly used in acoustics to perform experiments in nominally free field conditions. This means that all sound energy will be traveling away from the source with almost none being reflected back. Common anechoic chamber experiments include measuring the transfer function of a loudspeaker or the directivity of noise radiation from industrial machinery. Anechoic chambers are also generally very quiet chambers, with typical noise levels in the 10-20 dBA range. According to Guinness World Records, 2005, Orfield Laboratories NIST certifield anechoic chamber is "The quietest place on earth" measured at (minus) -9.4 dBA.
Radio-frequency anechoic chambersEdit
The internal appearance of the radio frequency (RF) anechoic chamber is sometimes similar to that of an acoustic anechoic chamber, however, the interior surfaces of the RF anechoic chamber are covered with radiation absorbent material (RAM) instead of acoustically absorbent material . 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. Testing can be conducted on full-scale objects, including aircraft, or on scale models where the wavelength of the measuring radiation is scaled in direct proportion to the target size. Coincidentally, many RF anechoic chambers which use pyramidal RAM also exhibit some of the properties of an acoustic anechoic chamber.
Radiation absorbent materialEdit
The 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 and iron.
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 is more narrow band than the pyramidal RAM and is designed to be fixed to good conductive surfaces. It is generally cheaper, easier to fit and more durable than the pyramidal type RAM but is less effective at lower frequencies. Its performance might however be quite adequate if tests are limited to the higher RF or microwave frequencies.
Effectiveness over frequencyEdit
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 when the pyramid height is approximately equal to $ \lambda/4 $, where $ \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.
Installation into a screened roomEdit
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.
Chamber size and commissioningEdit
The actual test setups usually require extra room than that required to simply house the test equipment, the hardware under test and associated cables. For example, the far field criteria sets a minimum distance between the transmitting antenna and the receiving antenna to be observed when measuring antenna radiation patterns. Allowing for this and the extra space that may be required for the pyramidal RAM means that a substantial capital investment is required into even a modestly dimensioned chamber. For most companies a such an investment in a large RF anechoic chamber is not justifiable unless it is likely to be used continuously or perhaps rented out. Sometimes for radar cross section measurements it is possible to scale down the objects under test and reduce the chamber size provided that the wavelength of the test frequency is scaled down in direct proportion.
RF anechoic chambers are normally designed to meet the electrical requirements of one or more accredited standards. 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, valid for a limited period.
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 of whether to place the test equipment (as opposed to the equipment under test) on the interior or exterior of the chamber is required. Normally this may be located outside of the chamber provided it is not susceptible to interference from exterior fields which, otherwise, would not be present inside the chamber. This has the advantage of reducing reflection surfaces inside but it requires extra cables and particularly good filtering. Unnecessary cables and/or poor filtering can collect interference on the outside and conduct them to the inside. A good compromise may be to install human interface equipment (such as PCs), electrically noisy and high power equipment on the outside and sensitive equipment on the inside.
One useful application of fiber optic cables is to provide the communications links to carry signals within the chamber. Fiber optic cables are non-conductive and of small cross-section and therefore cause negligible reflections in most applications.
It is normal to filter electrical power supplies for use within the anechoic chamber as unfiltered supplies present a risk of unwanted signals being conducted into and out of the chamber along the power cables.
Health and safety risks associated with RF anechoic chamberEdit
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 completely eliminate. Addition of a retractable telescoping sprinkler head may reduce fire risk.
Safety regulations normally require the installation of a gaseous fire suppression system including smoke detectors. Gaseous fire suppression avoids damage caused by the extinguishing agent which would otherwise worsen damage caused by the fire itself. A common gaseous fire suppression agent is carbon dioxide. Normally the fire detection system is linked into the power supply to the chamber arranged to disconnect it if smoke or a fire is detected.
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