Why Use RF Absorbers?
RF absorbers prevent unwanted reflections in anechoic chambers, suppress enclosure resonances in shielded assemblies, and reduce interference between PCB sections. Choosing the right absorber depends on frequency range, required attenuation, physical thickness constraints, and operating temperature.
RF Absorber Types
| Type | Frequency Range | Mechanism | Thickness |
|---|---|---|---|
| Ferrite tile | 30 MHz – 1 GHz | Magnetic loss (μ'') | 6–12 mm |
| Carbon foam (pyramidal) | 1 GHz – 40 GHz | Resistive (ε'') | 50–300 mm |
| Hybrid (ferrite + foam) | 50 MHz – 18 GHz | Both mechanisms | 25–100 mm |
| Carbon-loaded rubber sheet | 1 GHz – 18 GHz | Resistive (ε'') | 1–3 mm |
| Magnetically lossy sheet | 0.1 – 3 GHz | Magnetic loss | 0.5–2 mm |
| Conductive foam (ESD) | <1 GHz only | Resistive | 5–25 mm |
Ferrite Tiles (Low-Frequency Champion)
Ferrite tiles exploit the magnetic loss tangent (tan δ_μ) of ferrite materials which peaks at 100–500 MHz. They are essential in any semi-anechoic chamber used for automotive or CE EMC testing below 1 GHz. Key specs:
- Reflectivity: −10 to −30 dB at 200–1000 MHz
- Permeability: μᵣ = 10–1000 (frequency dependent)
- Weight: heavy (3–5 kg/tile typical)
- Temperature stability: good to 150°C
PCB-Level Absorbers
Thin absorber sheets (0.5–3 mm) are placed inside metal enclosures to suppress cavity resonances and reduce EMI between boards:
- Reduce enclosure Q by adding loss → lower resonance peaks by 10–20 dB
- Place between PCB and metal lid for maximum effectiveness
- Must be non-conductive (will short circuits if conductive)
- Verify effectiveness by loading S11 files measured with and without absorber in RF View
Measuring Absorber Performance
The reflectivity of a flat absorber slab is measured in a free-space transmission setup or using an arch reflectivity range. Results are S11 (reflectivity in dB) vs. frequency. Load the absorber characterization .s1p file in RF View to:
- Find the frequency of minimum reflectivity (absorption peak)
- Determine the −10 dB and −20 dB bandwidth
- Compare multiple absorber samples or thicknesses in batch mode