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An anechoic chamber is a space designed specifically to absorb sound or electromagnetic waves. The surfaces of the space are covered in pyramids of radar absorbing material, and any radiation generated inside the room gets absorbed by the walls. Anechoic chambers are typically used for measuring radiation patterns of antennas, or radar cross-section measurements. It's also where antenna designers go for peace and quiet. Did you ever notice that they lock the door?
Types of chambers
Anechoic chambers can be any size, from a smallish box to an aircraft hanger. There are a variety of absorbing materials that can line the inside of the box, depending on what you're measuring, but most RF chambers use pyramids made of foam embedded with conductive carbon black. The pyramidal structures must be sized according to wavelength; if you want to do 100 MHz measurements you will need pyramids several feet long. Often, the height required for a chamber is surprising, up to 30 feet or more.
Long pyramids tend to sag and tear off the walls as the material is heavier than it looks. If there is ever a fire in an anechoic chamber you need to get out quickly, the smoke is toxic and you need a fire protection plan. This is something you need to carefully consider if you plan on performing high-power tests.
When planning for a chamber, you might have to build a new building to house it. Alternatively, you can dig a big hole under an existing building...
Calibrating the chamber
Calibration is done using standard horn antennas that have gain that is known with great accuracy by analysis. The device under test's data is compared to the calibration to determine gain characteristics
Positioner
The positioner is where you will mount your device under test. It provides elevation, azimuth and rotation control. Some masts fold down for ease of mounting your sample, some will require a ladder.
check out Cal Poly's chamber description!