Spec Brief · January 2025

Faraday Bag Attenuation Ratings: What dB Numbers Actually Prove

Q: How is faraday bag attenuation actually measured?

An accredited RF lab places the bag between a calibrated transmit and receive antenna inside a shielded chamber. A signal generator sweeps the target frequency band. Attenuation in dB equals the baseline power minus the shielded power at each frequency. The test should be repeated after multiple closure cycles.

Mission brief: A faraday bag's value lives in its attenuation rating. Look for tested performance of 60-100 dB across 10 MHz to 40 GHz, verified against MIL-STD-188-125 or IEEE 299. Anything else is marketing.

Published January 15, 2025 Updated January 15, 2025 Reading time 7 min Standards covered 2

The Short Answer

Decibels are the only spec that separates shielding from sewing.

A faraday bag's value lives in its attenuation rating. Demand tested performance of 60-100 dB across 10 MHz to 40 GHz, verified against MIL-STD-188-125 or IEEE 299. Anything below that is marketing copy, not shielding.

Most buyers compare faraday bags by price, fabric, and brand story. Operators compare them by one number: attenuation in decibels. This brief decodes the standards behind the spec sheets so you can separate certified shielding from sewn nylon.

Faraday bag attenuation testing inside RF shielded anechoic chamber with spectrum analyzer — RF attenuation testing inside an anechoic chamber: where claims become numbers.

What Attenuation Actually Means

Attenuation measures how much radio frequency (RF) energy a shield blocks. It is reported in decibels (dB) on a logarithmic scale. Each 10 dB step represents a 10x reduction in signal power. The math is brutal in your favor: 60 dB blocks 99.9999% of incoming RF energy.

Attenuation	Power Reduction	Operational Meaning
20 dB	99%	Consumer pouch. Will not stop modern cellular.
40 dB	99.99%	Marginal. Phone may reconnect intermittently.
60 dB	99.9999%	Operational minimum for executive protection.
80 dB	99.999999%	SCIF-grade. Defeats commercial intercept.
100 dB	99.99999999%	TEMPEST class. Government and intelligence use.

A 30 dB delta between two bags is not 30% better. It is 1,000x better. This is where cheap pouches collapse on the test bench.

The Standards That Matter

Two reference frameworks. Know which one is being cited.

MIL-STD-188-125

The U.S. Department of Defense standard for high-altitude electromagnetic pulse (HEMP) protection of fixed and mobile ground-based facilities. The original standard targets shielded enclosures, not bags, but its measurement methodology — low-level continuous wave testing from 10 kHz to 1 GHz — became the de facto reference for portable shielding. A bag claiming MIL-STD-188-125 compliance should disclose which test points were measured and which lab ran the sweep. See the official DoD specification record for the full document.

IEEE 299 (and IEEE 299.1 for small enclosures)

The civilian gold standard. IEEE 299 defines shielding effectiveness measurement for enclosures larger than 2 m on a side. IEEE 299.1 extends the protocol to enclosures as small as a faraday bag. It specifies plane wave, magnetic, and electric field tests across discrete frequency bands. If a vendor cites "IEEE 299" without distinguishing 299 vs 299.1, treat the claim as unverified.

Frequency Coverage

The bands you must see on the spec sheet.

A single dB number is meaningless without the frequency it was measured at. Shielding effectiveness varies dramatically across the spectrum. A bag that hits 90 dB at 1 GHz may collapse to 40 dB at 6 GHz where Wi-Fi 6E and modern surveillance gear operate.

10 MHz - 100 MHz: Legacy radio, FM, low-band public safety.
700 MHz - 2.7 GHz: 4G LTE, GSM, cellular IMSI catchers, GPS L1.
2.4 GHz - 5.8 GHz: Wi-Fi, Bluetooth, most consumer trackers.
6 GHz - 7.125 GHz: Wi-Fi 6E and emerging 5G mid-band.
24 GHz - 40 GHz: 5G mmWave, vehicle radar, high-band surveillance.

Demand a shielding effectiveness curve, not a single headline number. The curve is the truth. The headline is the brochure.

Faraday bag attenuation curve chart showing dB shielding effectiveness from 10 MHz to 40 GHz — Attenuation curve across the operational spectrum. The flat line above 80 dB is what you are paying for.

Lab Methodology

How independent labs actually run the test.

Reference measurement. A calibrated transmit antenna emits a known power level. A receive antenna records baseline signal strength.
Insertion. The bag is placed between transmitter and receiver, with the device of interest (or a calibrated probe) sealed inside.
Sweep. The signal generator steps through the target frequency band. Power is recorded at each point.
Calculation. Attenuation (dB) = baseline power - shielded power, expressed at each frequency.
Repeatability. The bag is opened, resealed, and retested. Closure repeatability is where most consumer bags fail.

Home methods will not replicate a calibrated chamber, but they will expose obvious failures — a useful first filter before procurement.

Procurement Diligence

Red flags on vendor spec sheets.

Single dB number, no frequency range. Marketing, not engineering.
"Up to" attenuation. The peak value is irrelevant. Demand the minimum across the band.
No test report, no lab name. Self-declared compliance is not compliance.
No closure cycle data. A bag that loses 20 dB after 100 open-close cycles is a one-mission product.
Claims above 120 dB for a portable bag. Physically implausible without a double-shielded enclosure and gasketed seam.

What REVIS-1 specifies.

Our standard product line is tested to a minimum 80 dB attenuation from 600 MHz through 40 GHz, with closure repeatability validated across 1,000 cycles. Test reports are issued by an independent A2LA-accredited RF lab and made available to procurement officers under NDA. The fabric stack and seam architecture are documented in the broader brief library.

Buyer's Decision Matrix

Pick the row that matches the threat.

Use Case	Minimum Attenuation	Reference Standard
Travel / Privacy	60 dB @ 700 MHz - 6 GHz	IEEE 299.1
Executive Protection	80 dB @ 600 MHz - 40 GHz	IEEE 299.1 + closure cycle test
Law Enforcement Evidence	80 dB across cellular + Wi-Fi bands	IEEE 299.1, NIST guidance
Government / SCIF Adjacent	100 dB, full HEMP sweep	MIL-STD-188-125 methodology

Then demand the test report. Attenuation is the only spec that distinguishes a faraday bag from a fabric pouch. Read the curve, not the headline. Verify the standard, not the slogan. Buy the bag that publishes the numbers.

FAQ

Common questions on attenuation ratings and shielding standards.

What is a good attenuation rating for a faraday bag?

For executive protection and operational use, look for a minimum of 80 dB measured across 600 MHz through 40 GHz. Sixty dB is the floor for serious privacy use. Ratings under 40 dB are consumer-grade and will not reliably block modern cellular or 5G signals.

Is MIL-STD-188-125 the same as IEEE 299?

No. MIL-STD-188-125 is a U.S. Department of Defense standard for HEMP protection of fixed facilities. IEEE 299 and IEEE 299.1 are civilian shielding effectiveness measurement standards. They share methodology but differ in scope, frequency range, and certification authority.

Why does a single dB number on a spec sheet mean nothing?

Shielding effectiveness varies sharply with frequency. A bag that achieves 90 dB at 1 GHz can collapse to 40 dB at 6 GHz. Without a tested curve across the full spectrum, a single number reveals only the best-case point and hides the operational weak band where surveillance actually happens.

How is faraday bag attenuation actually measured?

An accredited RF lab places the bag between calibrated transmit and receive antennas inside a shielded chamber. A signal generator sweeps the target frequency band. Attenuation in dB equals baseline power minus shielded power at each frequency. The test should be repeated after multiple closure cycles to verify durability.

Are attenuation claims above 120 dB realistic for a portable bag?

For a single-layer portable bag, no. Sustained attenuation above 120 dB requires double-shielded construction, gasketed seams, and laboratory conditions. Vendors advertising 120 dB or more on a soft pouch are typically citing a peak measurement at a narrow frequency, not a guaranteed minimum across the band.

Does a higher dB rating drain my phone battery faster inside the bag?

Yes, indirectly. The phone cannot find a tower so it transmits at maximum power searching for signal. This is true of any effective faraday bag, regardless of dB rating. To preserve battery, power down the device before sealing it inside the shielded enclosure.