It is often necessary to provide some type of shielding in order to perform RF/wireless testing with an acceptable amount of external noise. It is also often necessary to perform testing where the radiation from a given device-under-test (DUT) or system-under-test (SUT) doesn’t reflect back from a shielded chamber and create interference. In other cases, there is a need to perform testing over a long range in a relatively wide and open space, provided the space doesn’t have a significant amount of external interference.

Without RF shielded chambers and ranges, it would be very difficult to perform RF testing on devices and systems in a way that external interference and self-generated interference wouldn’t interfere with the test data. Moreover, without electromagnetic compatibility (EMC) chambers made to a precise standard, it would also be difficult to provide an apples-to-apples comparison of the performance of DUTs/SUTs to ensure that they meet emissions and radiation standards for certification.

A brief introduction of RF test chambers and ranges

Figure 1 RF chambers are designed to support a wide range of radiated test applications. Source: Rohde & Schwarz

Hence, a wide range of the types and variations of RF test chambers and ranges are used to test various types of components, devices, and systems.

Common types of RF test chambers and ranges

  • Electromagnetic interference (EMI)/EMC test range
  • EMI/EMC pre-compliance test range
  • Semi-anechoic chamber (SAC)
  • Fully anechoic chamber
  • RF shielded test chamber (could be a box, small chamber, or full room)
  • Gigahertz transmissive electromagnetic cell (GTEM)
  • Electromagnetic reverberation chamber (RVC) or mode-stirred chamber (MSC)
  • Near-field antenna test range
  • Compact antenna test range
  • Indoor far-field antenna test range
  • Outdoor far-field antenna test range
  • Open area test site (OATS)

RF test chambers

The simplest of RF test chambers is a simple RF shielded chamber. A shielded chamber can be a small box or enclosure, such as a Faraday cage, or a full room that is surrounded with conductive walling or structural material such that the room is shielded from external RF radiation.

RF shielding is typically a necessary step as radiation from natural and artificial sources are essentially omnipresent now, and a lack of shielding can dramatically throw off test results or even cause testing to fail due to unacceptably high levels of noise. There are also some types of device testing that could result in damaged electronics if external interference is amplified/transmitted by a device or received by a highly sensitive device.

Another reason for shielding is that to avoid violating communications regulations, some level of shielding may be necessary. However, when a device is within a shielding chamber, radiated energy results in reflections. As these reflections are often undesirable, creating their own interference, the internal walling or structure of RF shielded chambers are often coated or otherwise covered in RF absorbing materials.

These RF absorbers are either dielectric/magnetic absorber tiles or dielectric absorbing conical/pyramidal structures. These shielded- and absorber-covered RF test chambers are known as either semi-anechoic or fully anechoic test chambers. A semi-anechoic test chamber isn’t completely shielded or surrounded by absorbers on all slides, whereas a fully anechoic chamber is completely surrounded by shielding and absorbers.

A brief introduction of RF test chambers and ranges

Figure 2 An anechoic chamber is a shielded enclosure or room that has radio-wave absorbing material applied to the walls, ceiling, and floor. Source: Antenna Test Lab

It is important to remember that no level of RF shielding or absorbers are perfect, which is why anechoic chambers contain a quiet zone (QZ) that is tested and determined to be quiet to a certain amount.

There are other types of RF test chambers, such as GTEMs and reverb chambers, though RF shielded chambers and anechoic chambers are more common for universal testing purposes. GTEMs, reverbs, and semi-anechoic EMI/EMC chambers are more commonly used for EMC pre-compliance and compliance testing or certain limited RF device testing.

There are also certain anechoic chambers or types of chambers specifically designed for antenna testing. These are near-field, compact, and far-field antenna test ranges. A compact antenna test range uses a reflector that is large enough to act as a range extender which effectively increases the chamber test range without the need for physically increasing the chamber or test range size.

RF test ranges

Open area test sites (OATS) are typically located in places with very minimal external RF generators. These sites can only be used when the weather is permitting and there aren’t significant interference effects from natural RF interference sources such as space weather and lightning. A benefit of OATS is that these ranges are typically clear for a substantial distance around the test site, so there are only reflections from the ground plane.

A brief introduction of RF test chambers and ranges

Figure 3 The OATS setups are commonly used for EMC and emissions testing for large machines. Source: Keysight

OATS are used for all types of RF, wireless, and EMI/EMC emissions testing. Given that OATS testing inevitably results in some external interference being picked up by the test apparatus, it often requires retesting and operator experience to ensure that interference isn’t being included in the reported results of a given test. Hence, RF testing done in shielded chambers can often go faster and is more easily scheduled and planned than with OATS testing.

Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.

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