What is an Anechoic Chamber?

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Editorial Team - everything RF

Jan 10, 2021

An RF Anechoic Chamber (an-echoic meaning "non-reflective, non-echoing, echo-free") is a shielded room that is designed to provide an environment with no unwanted electromagnetic waves. It is used to test RF components like antennas and radars, in the absence of any noise or interference.

The idea of a perfect anechoic chamber is that a device under test (DUT) should witness no unwanted signal when placed inside the chamber. The are two main sources of unwanted signals:

  1. Waves originating from devices located outside the chamber (external waves) and 
  2. Reflections of the signals transmitted by the DUT. Anechoic chambers can come in different sizes ranging from “mini-chambers” that can be placed on a lab bench to ones as large as aircraft hangars.

A fighter jet in a large anechoic chamber

Anechoic Chambers are usually designed by implementing a Faraday cage i.e., the entire chamber will be covered by a metallic or highly conductive layer. This prevents external waves from entering the chamber and causing interference which can result in errors while testing a device. A Faraday cage operates on the principle that an external electrical field causes the electric charges within the cage's conducting material to be distributed so that they cancel the field's effect in the cage's interior. This is the same principle why passengers sitting inside a car are not affected when lightning strikes the vehicle. This handles signal interference from outside the chamber.

To eliminate reflections of the signal transmitted by DUT, the interior of an anechoic chamber is covered with radiation absorbent material (RAM). The purpose of RAM is to absorb incident RF signal as effectively as possible from any direction. A highly effective RAM will result in reduced internal reflections, which is the desired outcome. Generally, RAM used in anechoic chambers are pyramid (or cone) shaped because sharp tips keep RF waves from bouncing off and allowing the wave to be slowly absorbed.

Pyramidal RAM

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 λ / 4, where λ is the free space wavelength. Increasing the pyramid height usually improves the effectives of RAM at lower frequencies but also results in increased cost. The amount of radio waves that bounces off from anechoic chamber walls is often 0.1% to 1% (-30 to -20 dB) of the original wave. This prevents the reflected waves from causing interference in the transceiver of the DUT and various RF parameters such as gain, efficiency, and radiation patterns can be measured accurately.

Absorbing of incident signal on pyramidal RAM
Credits: COMSOL

The performance quality of an RF anechoic chamber is determined by its lowest frequency of operation, at which measured reflections from the internal surfaces will be most significant. Anechoic chamber designs are getting ever more complex with single devices, like smartphones, incorporating multiple technologies such as cellular, Wi-Fi, Bluetooth, LTE, MIMO, RFID and GPS.

Glenn Robb - Antenna Test Lab Co

Jun 21, 2017

Anechoic means without echoes. Simply put, it is a chamber (a box or room) without reflections from the walls, ceiling, or floor. It may be an acoustic anechoic chamber, where the walls are treated with fiberglass sound absorbers to suppress echoes. In our RF world, we are talking about a shielded room, where the inner surfaces have been treated with radio wave absorbers. Typical absorbers may be foam pyramids loaded with carbon (for frequencies above 500 MHz) , or ferrite tiles (for frequencies below 500 MHz).

 

Anechoic Chamber At Antenna Test Lab

 

In RF engineering, the anechoic chamber is used for “Over The Air” (OTA) measurements, as opposed to “conducted” (in coax) measurements. The RF anechoic chamber environment allows antenna patterning and radiated measurements from antennas or devices with embedded antennas … without reflections or ambient radio signals. For more information, you may view an anechoic chamber description here.

 

 Pyramidal Absorber Close Up


 

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