You understand how RF absorber works, appreciate why its pyramidal shape influences performance, can describe how it’s specified for performance, know how to select the correct length and shape for your application, and understand how it is manufactured and how the manufacturing process affects absorber quality. These are all great attributes of RF absorber that you must take into consideration when properly outfitting an anechoic chamber for EMC testing purposes. But have you also considered safety attributes, including the maximum power handling capability the absorber will need to meet in your application and also its flammability rating? The safety performance specifications of RF absorbers are just as important as the other attributes of RF absorber performance. Let’s talk about them next.
Normal Power Handling Capability (Polyurethane Based Absorbers)
Polyurethane based absorbers withstand a maximum power of 0.5W/in2 continuous wave (CW) illumination in a normal temperature environment with no additional forced air movement. To give some perspective, for the plane wave/far electric field (E-field) condition, this amounts to roughly 540 V/m of E-field. These types of RF absorbers can safely handle up to 90°C temperature without permanent damage. If a greater field strength than 540V/m is expected, then a more porous foam-based absorber may be required for higher power handling capability. These types of absorbers are known as filtered foam absorbers. We’ll look at these in the next paragraph.
High Power Handling Capability (Filtered Foam Absorbers)
This type of RF absorber is known as filtered foam, and it is usually positioned over air-vents and the like in EMC chambers. Because this type of foam is porous, it can handle heat more efficiently than the non-porous types of absorber. Without forced air, a power handle rating of 1W/in2 is possible. Higher levels of power handling capabilities are achievable within an anechoic chamber outfitted with forced air ventilation.
Really High Power Handling Capability (Honeycomb Substrate, Phenolic-based Absorbers)
If an even higher power handling capability then previously mentioned is required, then consider an absorber containing a honeycomb substrate phenolic-based material. If constructed properly, this type of absorber can withstand a maximum power of 2W/in2 CW and up to 250°C temperature without permanent damage. This is accomplished by coating the walls of the honeycomb with a lossy film that effectively absorbs incident electromagnetic waves. Air flows more efficiently through the honeycomb structure and the phenolic material withstands higher temperatures. Add forced-air into the chamber and this type of absorber can develop an even higher power handling rating.
A properly setup anechoic chamber can contain a large amount of RF absorber material. Should a fire occur in the chamber, it’s crucial that the absorber does not become fuel for the fire. There are several specifications to look for that indicate the absorber contains combustion limiting/flame retardant properties. These specifications are:
- DIN 4102 Class B-2
- MIT Lincoln Laboratory Specification MS-8-21 (Tests I, II, and III)
- NRL Report 8093 (Tests 1, 2, and 3)
- Raytheon Drawing No. 2693066 (latest revision)
- UL 94 HBF
- UL 94-5VA and UL 94-5VB
If absorber is used within its power handling ratings as described above, is correctly installed, correctly specified, isn’t damaged during normal use (tips not broken off, isn’t stepped on/smashed, doesn’t sustain water damage, etc.) then there should be no reason to replace it after a certain amount of time. Properly specified and undamaged RF absorber should last for the life of the chamber or very close to it. The only reason to change out RF absorber early or some other time during the life of the chamber is because better absorber, with better specifications, has become available. With the new/enhanced absorber replacing the old absorber, superior chamber performance is achievable.
WARNING: The above information is only a guideline into a very complicated subject on absorber RF power handling capability. As with most things in EMC, there are a lot of variables to consider. These variables include frequency range of testing, near or far-field conditions, ambient temperatures, and air flows (or lack thereof). Before selecting any particular RF absorber solution, it would be wise to conduct an extensive power handling study using simulation tools that produce results that are backed up by real-life test data. Consult the references for more information.