Absorbing
Materials
RF Absorber Power Handling and Safety Considerations
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ou 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.
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Flammability 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
Absorber Life
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 reference for more information.

Reference and Further Reading
ETS-Lindgren, Top 10 Anechoic Absorber Considerations for RF and Microwave Applications apparatus. Retrieved from http://www.ets-lindgren.com/sites/etsauthor/General_Brochures/Top%2010%20Anechoic%20Absorber%20Considerations.pdf
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Tips for Selecting
Absorbers
  1. Know how anechoic absorber works. Absorber is designed to reduce reflections from impinging electromagnetic (EM) waves, and “absorb” incident EM energy.
  2. Match absorber design to application. Absorber works by dissipating the EM energy inside the absorber and turning it into heat. In most RF, microwave and antenna applications, the lossy dielectric works best because of superior EM absorption at higher frequencies.
  3. Pyramidal shape influences absorber performance. Broadband absorbers work by avoiding “sudden” changes in impedance for the impinging wave and gradually terminate and absorb the EM wave.
  4. Specify absorber for RF performance. Absorber is specified for reflectivity as a function of frequency at normal incident angle. Understand that reflectivity degrades at larger incident angles
  5. Match absorber length and shape for your application. Pyramidal absorber length determines how well it can work at the lowest frequencies.
  1. Use commonly available absorber lengths. Save money, speed delivery by choosing absorber in typical, off the shelf lengths.
  2. Determine maximum power handling capability of absorber. Polyurethane absorbers are specified to withstand a maximum power of W/in2 of continuous wave (CW) incident field under room temperature with no additional forced airflow.
  3. Manufacturing process impacts absorber quality. Find a computer controlled manufacturing process that begins with homogeneous blocks of polyurethane foam.
  4. Use non-hygroscopic absorber. Maintain a stable absorber electrical property and RF reflectivity by reducing moisture, which affects electrical conductivity significantly.
  5. Confirm RF performance verification testing. Ask how your supplier tests the absorber to verify performance reflectivity before shipment. Is each piece of absorber tested or sample tested one piece per batch? Is each piece serialized? Ask these questions protect your investment!
These tips are presented by
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