Anechoic chamber, RAM, and faradays cage

[Nabla_101]

Hi,

I was reading about anechoic chambers for performing tests of RF equipment, and read that they are usually screened rooms, built as a Faradays cage (Grounded, closed metal structure) in order to block radiation from the outside, and stop radiation escaping.

But also they coat the inside of the chamber with rubberised pyramidal structures of Radiation Absorbing Material (RAM) in order to absorb radiation emitted by the equipment, and reduce/eliminate reflections/echoes.

What I was wondering is if the room is a faradays cage, then why use the pyramidal RAM structure to line the walls? Wouldn't a faradys cage just absorb all the radation, and route it to ground anyway? (And be cheaper and easier to build)?

 

[the_emi_guy]

A screen room certainly is certainly cheaper and easier to build than an anechoic chamber. Problem with screen room is that RF is not absorbed by the conductive walls, but reflected. We use anechoic chambers when we want to mimic free space. The reflections from the walls cause multipath interference which spoils our measurements. There are cases were we want to mimic free space over a ground plane (radiated emissions testing). In these cases we use RF absorbers on all surfaces except the floor. This is called a semi-anechoic chamber.

 

[AlephZero]

What the-emi-guy said.

An example of this is a microwave oven. That is basically a Faraday Cage to keep the microwave radiation inside, but the radiation doesn't heat up the walls of the oven. Most of it keeps being reflected until it is absorbed by the food.