RF & Microwave Absorbers (Selection Guide)
Lining or re-fitting an anechoic or EMC chamber requires the right absorber, depending on your frequency range, chamber type and where it sits. This guide compares the ETS-Lindgren absorber range of pyramidal, high-power, wedge and walkway, so you can match type to requirement and specify the exact part. APC supplies the full range, with technical support to size it to your chamber.
How to choose an RF absorber
Five criteria decide the absorber you need:
- Frequency range - Low frequencies require more material, taller pyramids, wedges or hybrid/ferrite layers; smaller pyramids handle high frequencies. The ETS-Lindgren EHP range covers 30 MHz to 100 GHz
- Chamber type - A fully anechoic chamber (FAC) lines every surface, including the floor. A semi-anechoic chamber (SAC) keeps a metallic ground plane on the floor, so no floor absorber is needed
- Placement - Back walls, end walls near antennas, corners and floors each have different reflection demands. See 'Absorbers by chamber type' below
- Reflectivity target - Performance is measured as reflectivity in dB - the lower (more negative), the better. Taller absorbers give better low-frequency reflectivity
- Physical and fire constraints - Load-bearing floors need walkway absorbers; high-power or high-temperature work needs the HP range, every absorber must meet your fire-rating requirement
Absorber Comparison Guide
|
Absorber type |
Frequency range |
Typical reflectivity |
Best for |
Key property |
Available from APC |
|
EHP Pyramidal |
80 MHz - 40 GHz¹ |
to -50 dB (size-dependent) |
Broadband EMC & antenna chambers, walls, floors, ceilings |
Carbon-loaded urethanefoam, 1.0W/in², non-hygroscopic |
|
|
Wedge |
80 MHz - 40 GHz |
Same as an equal-size pyramid |
Compact/tapered range, areas where pyramids backscatter |
Wedge profile cuts back scatter, transitional pieces, 200V/m |
|
|
Walkway |
200 MHz - 40 GHz |
As an encapsulated pyramid/wedge |
FAC floors with personnel/equipment access |
Load-bearing to 200 lb/ft² (90.7 kg per 0.3 m), PVC |
|
|
Filter Foam (vent) |
80 MHz - 40 GHz |
As pyramidal (NRL arch-tested) |
Airflow/air vents, high power with forced air |
Open-cellreticulated foam, ~10×power withforced air |
|
|
High Power (HP) |
200 MHz - 40 GHz |
to -45 dB (size-dependent) |
High-power & high-temperature test, vacuum |
Phenolic honeycomb lossy film, stable to 250 °C, rapid pump-down |
|
|
Dura Sorb Hybrid (DSH) |
30 MHz - 40 GHz |
≤ -20 dB above 100 MHz (perchart) |
Semi-anechoic 3–10m range, ANSI C63.25 / CISPR / MIL-STD-461 |
Ferrite tile lossy foam hybrid, low profile, 200V/m CW |
|
|
EMC Broadband (EMC) |
80 MHz - 40 GHz |
≤ 200 MHz |
MIL-STD-461D/E/F, RTCA-DO-160, reduced-profile chambers |
Blunt-tip pyramidal PU foam, double-immersion carbon, 0.5W/in² |
|
|
Ferro Sorb Hybrid (FS) |
30 MHz - 40 GHz |
See series |
MIL-STD-461/CISPR 25, floor treatment (FS-300, IEC 61000-4-3) |
Ferrite tile carbon foam, small profile(20/30 cm), 0.5 W/in² |
|
|
Convex (CX) |
2 GHz - 110 GHz |
-25 to -35 dB, wide-angle to 60° |
Microwave /mmWave, wide-angle incidence, RCS/ antenna |
Convex low-scatter geometry, rounded tips, 0.5W/in² |
|
|
Ferrite Tile (FT) |
30 MHz - 1GHz |
See series |
Low-frequency lining, ANSI C63.4 /CISPR 16-1-4, hybrid base layer |
Precision ferrite tile (±0.13 mm), ultra-thin25 mm, 1000 V/mCW |
|
|
Lossy Foam Block (LFB) |
30 MHz -100 GHz |
See series |
Corner treatment, covering positioning equipment |
Flat lossy block, cut to size |
¹ EHP material is rated to 100 GHz at series level; per-model reflectivity is tabulated to 40GHz.
Pyramidal Absorbers (EHP)
The EHP (Extra High Performance) absorber reduces reflections from 30 MHz to 100 GHz, making it the broadest-range option for EMC and antenna chambers. Each absorber is cut from high-quality urethane foam into bun configurations, then impregnated with conductive carbon to a controlled formulation.
Physical specification:
- Non-Hygroscopic Substrate
- Broadband Performance
- 1.0 W/in2 Power Handling Capability
- Reflectivity Performance Testing
- Fire Retardant
- FlexSorb Available
High-Power / High-Temperature Absorbers (HP)
ETS-Lindgren HP absorbers handle high power and high temperature. A conductive polyimide resin film, the highest-temperature polymer available, coats a pyramidal nylon honeycomb substrate that remains thermally stable up to 250 °C (482 °F).
The honeycomb structure pumps down quickly for high-vacuum work with little outgassing. HP material is far harder to ignite than conventional foam absorbers and self-extinguishes once the ignition source is removed.
Wedge Absorbers
The wedge absorber reduces reflections from 30 MHz to 100 GHz, with reflectivity equal to a pyramid absorber of the same size. Wedges suit chambers where pyramids would create excessive backscatter, typically tapered or compact-range chambers.
Transitional wedge pieces keep a continuous absorber line, for example, a 12.7 cm (5 in) wedge stepping to a 31.1 cm (12.3 in) wedge. These are usually supplied at 61 × 121.9 cm (2 ×4 ft) or 61 × 182.9 cm (2 × 6 ft).
Walkway Absorbers
Walkway absorbers give personnel a safe walking surface on a fully anechoic chamber floor while meeting reflection requirements. A pyramidal or wedge absorber is encased in low-density rigid polystyrene foam, topped with a semi-rigid PVC sheet for a smooth, dirt-resistant surface.
The walkway absorber bears up to 200 lb/ft² (90.7 kg per 0.3 m) and is durable and easy to maintain.
Absorbers by Chamber Type
Where the absorber sits matters as much as which type you pick:
- Fully anechoic chamber (FAC): line all six surfaces. Use walkway absorbers on the floor for access
- Semi-anechoic chamber (SAC): the floor is a metallic ground plane, no floor absorber. Line walls and ceiling
- Back wall: wedge or pyramid, chosen on chamber geometry and dominant field angles
- Near antennas/end walls: hybrid or ferrite-backed absorbers hold low-frequency performance in the critical region
- Corners and edges: add absorber depth or higher-performance material, where reflections accumulate
- Compact/tapered range: wedge and transitional pieces keep a continuous line
Specifications & Compliance
- Frequency: broadband types cover 30 MHz to 100 GHz (EHP, wedge)
- Reflectivity: improves with absorber height and chamber design
- Temperature: EHP service temperature 90 °C; HP stable to 250 °C
- Fire ratings: NRL 8093, UL 94, DIN 4102-B2 (EHP)
Request a Quote
APC supplies the complete ETS-Lindgren absorber range, backed by over 40+ years of EMC expertise. Our specialists help you select the optimal absorber type, size and placement, including custom and transitional solutions for tapered and compact-range chambers.