Understanding RF attenuation charts is the only way to verify that a Faraday bag performs as advertised. Without this knowledge, buyers rely on marketing claims rather than measurable data.
What Is RF Attenuation?
RF attenuation measures the reduction in signal strength as electromagnetic energy passes through a shielding material. It is expressed in decibels (dB), a logarithmic unit that compares the power level of a signal entering the bag to the power level detected inside. A higher dB value indicates greater signal reduction. For example, a 60 dB reduction means the signal strength inside the enclosure is one-millionth of the external signal strength. This metric applies across the entire electromagnetic spectrum, from low-frequency RFID signals to millimeter-wave 5G transmissions.
Understanding dB Ratings for Faraday Bags
Manufacturers typically cite a single dB number or a range, but the protection level changes significantly across different tiers. Knowing these tiers helps you match a bag to your specific threat model.
Below 40 dB – Weak shielding
Bags in this range block casual signals but fail against determined tracking or high-power transmitters. They may stop a key fob relay attack at close range but will leak cellular or Wi-Fi signals. These are often marketed as “signal blockers” rather than true Faraday enclosures.
40-60 dB – Moderate shielding
This tier handles most consumer privacy needs. It reliably blocks cellular (4G/LTE), Wi-Fi, Bluetooth, and GPS signals in typical urban environments. It is suitable for preventing location tracking or remote wipe commands on a powered-off device.
60-80 dB – Strong shielding
Professional-grade bags start here. This attenuation level defeats high-gain antennas and amplified signals. It is the minimum standard for law enforcement evidence handling and corporate travel security where adversaries may use directional antennas.
80-90+ dB – Professional shielding
These bags approach the limits of material physics. They mitigate ultra-wideband (UWB) tracking, millimeter-wave 5G, and near-field communication (NFC) at extremely close range. Certification at this level usually requires independent lab verification across a broad frequency sweep.
What Frequencies Matter Most?
A Faraday bag must attenuate signals across the specific bands used by the threats you face. A chart showing 90 dB at 1 GHz is useless if the bag drops to 20 dB at 2.4 GHz where Wi-Fi and Bluetooth operate.
Low-Frequency Bands
- 125-134 kHz: Legacy RFID access cards and animal tracking chips.
- 13.56 MHz: NFC, contactless payment cards, and modern RFID credentials.
- Low frequencies penetrate conductive materials more easily due to skin effect physics, requiring dense, multi-layer construction or ferrite-loaded fabrics.
Mid-Frequency Bands
- 700 MHz – 2.6 GHz: Primary cellular bands (4G/LTE), GPS (1.575 GHz), and older Wi-Fi.
- 2.4 GHz: Wi-Fi 4/5/6, Bluetooth, Zigbee, and many IoT protocols.
- This range is the “sweet spot” for most metallic fabrics. A quality bag shows a flat, high-attenuation curve here.
High-Frequency Bands
- 3.5 – 6 GHz: Mid-band 5G (C-band), Wi-Fi 6E/7, and satellite communications.
- Higher frequencies have shorter wavelengths that exploit smaller seam gaps and material pores. Seam construction becomes critical.
Ultra-High Bands
- 24 – 40 GHz: Millimeter-wave 5G (mmWave).
- 6 – 10 GHz: Ultra-wideband (UWB) used for precision tracking (Apple AirTag, Samsung SmartTag).
- These signals behave more like light. They reflect off surfaces and penetrate microscopic apertures. Only bags with conductive adhesive seams or double-roll closures maintain high dB ratings here.
How to Read an RF Attenuation Chart
An attenuation chart plots frequency (X-axis, usually logarithmic) against attenuation in dB (Y-axis, linear). The resulting curve reveals the bag’s real-world consistency.
Ideal Curve
The ideal trace is a flat line across the entire measured spectrum, typically 10 MHz to 40 GHz. A flat curve at 70 dB means the bag treats a 13.56 MHz NFC signal with the same rigor as a 28 GHz 5G signal. Real-world charts show dips. A sharp dip at a specific frequency indicates a resonance in the material or a seam leak at that wavelength. Broad, gentle roll-offs at the extreme low and high ends are normal physics; sharp V-shaped notches are manufacturing defects.
Why Attenuation Ratings Vary Across Frequencies
No single material blocks all frequencies equally. Physics dictates the performance profile.
Low frequencies (RFID)
Long wavelengths require magnetic permeability for absorption or extreme conductivity for reflection. Standard copper or nickel fabrics reflect poorly at 13.56 MHz. Effective low-frequency shielding often adds a ferrite-impregnated layer or relies on multiple overlapping folds to increase the magnetic path length.
Mid frequencies (cellular, Wi-Fi)
This is the native domain of conductive textiles. Skin depth is shallow, so even thin metallized fabrics (nickel-copper ripstop) achieve 80+ dB with a single layer. Performance here is limited mostly by closure geometry.
High frequencies (5G)
As frequency rises, skin depth shrinks to microns. Surface oxidation, micro-cracks in the metallization, and weave porosity become dominant leakage paths. Multi-layer laminates with offset weaves are standard for maintaining 60+ dB above 6 GHz.
Ultra-high frequencies (UWB, mmWave)
At 28 GHz, a 1 mm gap acts as a slot antenna. The closure mechanism, Velcro, zipper, or roll-top, determines the rating more than the fabric. Conductive hook-and-loop tape often fails above 18 GHz. Double-roll Velcro closures with conductive adhesive tape seams are the current benchmark for mmWave integrity.
How Manufacturers Test Faraday Bags
Credible attenuation data comes from controlled environments, not a phone call test inside a kitchen.
- Spectrum Analyzers: Measure the power spectral density of the signal leaking into the bag across the frequency range. They provide the raw data for the attenuation chart.
- Signal Generators: Produce precise, stable RF signals at specific frequencies and power levels. They act as the “transmitter” outside the bag.
- Anechoic Chambers: Shielded rooms lined with RF absorber foam. They eliminate reflections and ambient noise (FM radio, cell towers, Wi-Fi) that would corrupt the measurement. This is the gold standard for certification.
- Independent Lab Certification: Third-party labs (often ISO 17025 accredited) run the test using standardized methods like IEEE Std 299 or MIL-DTL-83528. They issue a test report with the full curve, not a single number. You can learn more about Faraday bag categories that carry this certification.
Why Cheap Faraday Bags Often Fail Testing
Budget bags typically use a single layer of thin metallized polyester (often called “metallized Mylar”). While the fabric itself might test at 60 dB in a flat fixture, the constructed bag fails for three reasons:
- Seam leakage: Sewn seams create thousands of micro-apertures. Conductive thread helps but rarely seals above 2 GHz.
- Closure gaps: Single-fold Velcro or simple ziplock strips leave a direct line-of-sight path for RF.
- Material durability: Thin metallization cracks at fold lines after weeks of use, creating permanent leakage paths that worsen over time.
Premium bags address these with heat-sealed seams, multi-layer laminates, and engineered closure systems. For a comparison of construction methods, see our guide on how to choose a Faraday bag.
Attenuation Testing Example (Conceptual)
Consider two hypothetical bags tested in an anechoic chamber from 10 MHz to 40 GHz using IEEE Std 299 methodology.
Budget Faraday Bag
- Construction: Single-layer metallized polyester, sewn seams, single Velcro flap.
- Low Freq (13.56 MHz): 25 dB