What is an Anechoic Chamber?

What is an Anechoic Chamber?

The Basics

An anechoic chamber is a shielded room that has radio-wave absorbing material applied to the walls, ceiling, and floor. Chambers may be table top sized enclosures, but are normally room sized enclosures where engineers can walk in and work. The absorbers on the inside surfaces are often pyramidal shape, and give the room an unique “science fiction” look. You may have also heard that they are very expensive. So why would anyone go to all of this trouble? Because there are many benefits to antenna testing!

The Flashlight Comparison

At Antenna Test Lab Co, we often find that comparing antennas and radio waves to light is helpful in our customer’s understanding of basic concepts. We often illustrate the anechoic chamber’s functionality with the following flashlight comparison.

Imagine your task was to “pattern” a flashlight. In other words, your job is to measure and create a diagram of where the device shines light, where it does not, and how much light goes where. It may seem intuitive, because you have eyes and you can intuitively look at your flashlight. We are used to simply “seeing” light and it’s patterns, so it is easy to visualize a flashlight’s pattern.

However, we cannot see radio wave energy directly, so we have to measure it with radio receivers and antennas. Luckily, the propagation is very similar, so just continue to visualize light from the flashlight. Now back to your task … patterning the flashlight. Imagine that you are blind, but you have a light-meter. You can use it to measure the flashlight’s output in various directions to create the pattern of the flashlight beam. We would be using an antenna and a receiver to do the same for patterning an antenna.

You start the job and quickly discover that your light meter readings are being interfered with by the sun. Outdoors on a sunny day, your light meter is picking up lots of sunlight, and confusing that with the flashlight’s light. You need “shielding”.

Shielding

So you go indoors, to a windowless room, with the lights turned off. In this “dark room”, all of the observed light is from your flashlight under test. The room is shielded against outside light, just like the metal walls of an anechoic chamber shield our equipment from the outside radio signals. This would be the perfect solution, if it were not for one problem… reflections !

Reflections

Imagine the dark room you just chose was lined with mirrors. It is still a “dark room” and it is still fully shielded from outside light. However, the mirrored wall/ceiling/floor reflections will cause chaos with your flashlight measurements. What can be done?

You decide to paint over the mirrors with a flat black paint, one that absorbs lots of light. Now you have an optical anechoic chamber. No more reflections, or echoes. The word anechoic means without echoes. Now your test flashlight appears to be in free-space. There is no ambient light, and the walls are not visible.

In the RF world, the reflective walls of the metallic shielded room are rendered “invisible” or “anechoic” by covering them with special RF absorbing materials. The most common absorber is the carbon loaded foam pyramid. Sharp tips on the absorbers keep RF waves from bouncing off, allowing the radio waves to slowly transition from air to the lossy carbon inside of the foam. This gradual taper from tip to core is the secret to RF absorbers. The amount of RF that bounces off of anechoic chamber walls is often 0.1% to 1% (-30 to -20 dB) of the original wave. The low level of reflections in our chamber allow us to accurately measure your antenna’s gain, efficiency, and radiation patterns. Find out more about how we do it in this educational article.

Our Anechoic Chamber

The walls, ceiling, and floor of our chamber are treated with over 2000 absorbing pyramids! The absorbers are between 12″ and 18″ long. We have an additional absorber in our chamber, 12,000 pounds of ferrite tile. The ferrite tiles absorb RF at frequencies too low for the foam pyramids, and extend the anechoic ability of our chamber down to frequencies far below many antenna testing chambers. We routinely test antennas down to 300 MHz, and product emissions down to 30 MHz.

See It For Yourself

Drop us a note, and see how we can help get your antenna project and costs on track ! Many successful companies partner with an antenna testing service, and reap the benefits.