You are new to your organization and your employer asks you to give a tour of the test facilities, including the newly installed electromagnetic compatibility (EMC) anechoic test chamber. Suddenly you realize you do not have your speech together and are unable to respond in a succinct manner should anyone in the tour group happen to ask about the strange looking pyramidal-shaped blue foam material mounted to the walls, ceiling, and floor of the chamber. In preparation for these and other questions, you quickly jot down some notes concerning what you know about radio-frequency (RF) absorber material. Here is the information you end up with.
Purpose
RF absorber materials are sometimes referred to as Radar Absorber Material (RAM) or Microwave Absorber. RF absorber is a key component of the anechoic chambers used for EMC testing. The walls of the chamber are made of metal and any RF energy radiated inside the chamber, emanating either from the transmit antenna for radiated immunity testing, or from the equipment under test (EUT) for radiated emissions testing, could reflect off the walls and interfere with the desired signal of interest. Since typical RF absorber material is lossy and specially shaped to allow for incoming RF energy to enter and then be transformed into heat, reflections within the chamber are minimized with its use.
Absorber Types
The typical RF absorber material familiar to most is the carbon loaded foam pyramidal shaped type. These are typically blue in color but can now be made to order in other colors including white. Note: The white color will help brighten up what would otherwise be a rather dark room.
Also available are flat ferrite tiles or the hybrid type of absorber which is a combination of both the pyramidal shaped type and ferrite tiles. Once the types of absorbers are understood, the performance differences of each one is usually the next subject of concern.
Performance
Pyramidal foam: The pyramidal type absorber material works best for high frequencies and worse for low frequencies. The larger the pyramids, the lower the frequency in which the absorber is best at attenuating the RF signal at. Smaller pyramids work best at attenuating higher frequencies.
Ferrite tiles: Ferrite tiles work well for mid-frequencies (30 MHz to 1000 MHz), and sometimes as high as 1.5 GHz with special construction and mounting techniques. Hybrid: The hybrid type works well over the largest frequency span since it is a combination of the pyramidal and ferrite tile types of absorber. In addition to knowing about performance, knowing the advantages and disadvantages of each type of absorber is also important.
Advantages
Pyramidal foam: The advantage to using pyramidal foam absorbers within an anechoic chamber is that they are not as heavy as ferrite tiles and therefore do not require extra support to keep them from falling off the walls and ceiling (although they occasionally still do). Should this type of absorber fall off the chamber wall or ceiling they will probably not harm anyone and they are relatively easy to reinstall.
Ferrite tiles: The advantage to using ferrite tiles as RF absorber in anechoic chamber is that they do not take up as much real estate within the chamber as the pyramid or hybrid types do. They are also more rugged and can take more of the abuse associated with everyday use of the anechoic chamber without getting damaged (although some chipping is still possible). They are also less susceptible to fire, humidity and chemicals.
Hybrid: In addition to working over a wider frequency range than either of the two other types of absorber material, the main advantage with the use of the hybrid type of RF absorber is that they do not take up as much space as the pyramid absorber type. However, they do take up a little more room than ferrite tile does.
Disadvantages
The biggest disadvantage with each type of absorber is cost. Buying and then properly installing RF absorber material is not cheap. However, trying to perform radiated immunity or emissions inside a non-absorber lined chamber would be impractical and there is usually no way to avoid the costs associated with their purchase.
Pyramidal foam: Next to cost, the biggest disadvantages with the pyramidal foam type of absorber is the significant loss of chamber volume associated with their use. The typical nominal size of Pyramidal RF Absorber ranges between 4 inches (105mm) and 72 inches (1830 mm) depending on the frequencies of concern.
Also, the tips of pyramidal foam absorbers can easily get damaged during everyday use and after some time they will eventually start to droop and look ragged. Truncated-tip versions of this type of absorber are available to preclude these issues from occurring.
The low-density polyurethane foam that is impregnated with a dielectric “lossy” solution (which the pyramid absorber material is constructed of) can become fuel for a fire, should one occur within the chamber. Depending on local rules, special fire suppression techniques may be required in chambers which use this type of absorber. Consult your local fire marshal for further information.
Ferrite tiles: The main disadvantage associated with ferrite tiles is the weight. Ferrite absorber is produced as tiles of size 4 in. x 4 in. (100 mm x 100 mm) and thickness available between 0.24 in. to 0.25 in. (6 mm to 6.7 mm). For ease of installation the ferrite tiles are factory glued to wood or steel panels of size 23.62 in. by 23.62 in. (600 mm x 600 mm). These panels are heavy and the ceiling of the chamber requires additional reinforcement if they are to be installed there. A ferrite tile falling on someone could seriously cause them harm.
Another disadvantage with ferrite tiles is that tile to tile gaps can cause performance issues. The gaps must be carefully controlled and minimized to maintain performance. This requires precise machining of each of the six surfaces of the tile to ensure a tight fit during installation inside the chamber.
Hybrid: While the hybrid type of absorber includes many of the advantages found in both the ferrite tile and pyramid absorber types, it also includes many of the disadvantages of each. These include some reduction in chamber space, additional weight (also requires additional ceiling support), potential for damage to the tips, reduced performance if gaps in tiles are present, and finally the issues with flammability.
Floor Treatment
Finally, depending on the type of tests to be conducted inside the chamber (RF emissions, RF immunity, or both) will dictate placement (or non-placement) of RF absorber on the floor of the chamber. RF immunity standards typically have some form of field uniformity requirement, and this is best accomplished by placing absorber on the floor to suppress the wave that is reflected off the floor that was radiated from the transmit antenna. It also prevents constructive and destructive interference from affecting operation of the EUT (prevents over-testing or under-testing).
RF emissions standards require no absorber on the floor. To find the maximization RF signal emanating from the EUT, the scan height of the antenna is varied from 1 m to 4 m thereby detecting the maximum reflected signal.
If both RF immunity and emissions tests are performed in the same chamber then the absorber used on the floor will have to constantly be set down and removed depending on the test that needs to be performed. If the chamber is only used for precompliance emissions testing then you may get away with simply leaving the absorber on the floor for both emissions and immunity tests. Some experimentation to determine if this practice will work for your specific situation is required.
For now, I think you have the RF absorber information you need. Good luck with your tour!
References
- Williams, T., EMC For Product Designers Fifth Edition, Newnes, 2017.
- Ferrite Tile Absorbers for EMC Test Chamber Applications, Fair-Rite Products Corp., www.fair-rite.com, Wallkill, NY
- RF Absorber Materials, Panashield – A Braden Shielding Systems Company, http://www.panashield.com/emc_absorbers.asp, Tulsa, OK
