An anechoic chamber is a room that is shielded and has radio-wave absorption material attached to the ceiling, walls and the floor. Anechoic chambers could be table-top sizes, but generally, they are room-sized spaces where engineers are able to enter and work. The absorbers inside the surface are typically pyramidal which gives the room a distinctive “science fiction” look. You might have heard that they’re expensive. Why would anyone want to go to this effort? There are many advantages of testing antennas!
The Flashlight Comparison
We have found that often the ability to compare radio waves and antennas to light aids in our customers’ understanding of the basic concepts. We often demonstrate the functions of the Anechoic Chambers using this flashlight-to-light comparison.
Closeup of Pyramidal RF Absorber
Imagine that you were asked to “pattern” a flashlight. That is, you have to determine and draw a diagram of how the flashlight beams light, and where it doesn’t, and how much light is directed where. It could be intuitive since you’re a visual person and you are able to intuitively glance through your flashlight. We’re used to “seeing” light and it’s patterns, which is why it’s easy to see the patterns of a flashlight.
We cannot, however, observe radio waves in the direct light, so we need to measure it using antennas and radio receivers. The propagation of radio waves is quite similar, so keep imagining light coming that comes from the flashlight. Return to your job … creating patterns with the flashlight. Imagine you’re blind, however, you are armed with the light-meter. It can be used to determine the output of the flashlight in various directions, to form an image of beam. It is possible to use an antenna and a receiver to accomplish similar things for the patterning of an antenna.
You begin your work and quickly realize that the readings of your light meter are being affected due to the sunlight. In the sun on a bright day your light meter is detecting many rays of sunshine, and conflating it with the light of the flashlight. You’ll need “shielding”.
Then you head indoors into a room without windows, with the lights off. Within that “dark room”, all of the visible light comes coming from the flashlight you are testing. The room is secluded from external light, similar to the walls made of metal in an anechoic chamber. It shields our equipment from outside radio signals. This is the ideal solution, but not for one thing… reflections!
Imagine that the dark space you’ve chosen was filled with mirrors. The room is still “dark room” and it is completely insulated from light coming in from the outside. But, the mirrored ceiling/wall/floor reflections can cause confusion with the measurements of your flashlight. What options are available?
You decide to cover the mirrors using a flat black paint which absorbs a lot of light. You now can have an anechoic optical room. No more reflections, nor echos. Ground run-up enclosures are the word that means without echos. The flashlight you are testing seems to reside in space. There isn’t any ambient light and the walls aren’t evident.
Absorbers in Anechoic Chambers
When world reflections of walls in the shielded area are made “invisible” or “anechoic” by covering them with absorption materials. The most commonly used absorber is the carbon-loaded foam pyramid. Sharp edges on absorbers stop radio waves from scattering off, allowing radio waves to slowly shift from the air to the losing carbon in the foam. The gradual transition from the tip to the core is the key to absorption of RF. Amount of radiofrequency bounced off the walls of an echo chamber typically 0.1 percent to 0.1% (-30 to 20 decibels) of the initial wave. The small amount of reflections inside our chamber allow us to measure precisely the efficiency, gain of your antenna and patterns of radiation.
Ground Run-Up Enclosures
The ceiling, walls and floors of our room are adorned with more than 2000 absorbent pyramids! The absorbers measure between 12 to 18” in length. We also have an absorber in our chamber. It is with 12,000 pounds worth of ferrite tiles. The tiles trap at frequencies that are too small for foam pyramids and also extend the Ground run-up enclosure to frequencies that are far lower than those of other test chambers for antenna testing. We test antennas regularly up to 300 MHz and also product emissions as low as 30 MHz.
More About Anechoic Chamber
An anechoic chamber can be described as an enclosed space that is to block out sound from outside and electromagnetic radiation. It was originally designed to block out the acoustic (sound) echos that result from reflections inside rooms Anechoic chambers are now used to create a safe space to shield radio frequencies (RF) as well as microwave.
The presence of people is not permitted in the chamber during measurement In part, because they might interfere with transmission, but also because they may be exposed to radiation.
Risks to health and safety related to RF anechoic chambers include:
- RF radiation hazard
- Fire hazard
- Personnel trapped
Based what frequency band that is being examined or the size of objects being tested Ground run-up enclosure vary in size, from as large as an aircraft hangar to smaller than a microwave oven.