How to Test Any Faraday Bag: Advanced Methods
Dispatch #004 gave you the five-step phone test — the quick check that tells you whether a Faraday bag blocks signal or doesn’t. This dispatch goes further. If you want to know how many decibels of attenuation you’re actually getting, at which frequencies, and whether your bag is degrading over time — these are the methods.
Why the Phone Test Isn’t Enough
The five-step test from Dispatch #004 is binary: the bag either blocks signal or it doesn’t. It tells you whether your phone can maintain a cellular or WiFi connection through the bag. That’s useful — a bag that fails the phone test is definitively worthless.
But the phone test has limitations. It doesn’t tell you the actual attenuation in decibels. It doesn’t tell you which frequency bands are blocked and which leak. It doesn’t tell you whether the bag’s performance has degraded since you bought it. And it doesn’t test frequencies above those your phone actively uses — meaning it can’t evaluate shielding at 5G mmWave, WiFi 6E, or other bands your phone may support but isn’t actively connected to at the moment of testing.
For most people, the phone test is sufficient. For people who want real numbers — engineers, security professionals, privacy researchers, or anyone who believes “trust but verify” applies to their Faraday bag — the methods below provide measurable data.
Method 1: SDR-Based Testing
What You Need
A software-defined radio (SDR) receiver. An RTL-SDR dongle ($25–35) covers 24 MHz to 1.7 GHz. For broader coverage, an Airspy or HackRF ($100–300) extends to 6 GHz. A TinySA Ultra spectrum analyzer ($100–130) covers up to 5.3 GHz with a built-in display.
A known signal source. A second phone with WiFi hotspot enabled, a Bluetooth transmitter, or a dedicated RF signal generator. The source needs to transmit at a known frequency.
Free software. SDR# (SDRSharp), GQRX, or CubicSDR for visualization.
The Test Procedure
Step 1: Place the signal source on a table. Enable its transmitter (WiFi hotspot, Bluetooth, or signal generator output).
Step 2: Position the SDR receiver at a fixed distance — 1 meter is convenient. Connect the SDR to your laptop and open the spectrum analyzer software. Tune to the frequency of your signal source.
Step 3: Record the signal strength with the source unshielded. This is your baseline. Note the peak power reading in dBm at the target frequency.
Step 4: Place the signal source inside the Faraday bag. Seal the bag completely. Do not move the SDR receiver.
Step 5: Record the signal strength with the source inside the bag. The difference between baseline and shielded readings is your measured attenuation in dB.
Step 6: Repeat at different frequencies to characterize the bag’s performance across bands.
This method gives you actual attenuation numbers rather than a binary pass/fail. A well-performing Faraday bag should show 40+ dB attenuation at cellular frequencies and 30+ dB at WiFi/Bluetooth frequencies. If the number drops below 20 dB at any frequency your phone uses, the bag is not providing reliable isolation at that band.
Method 2: Comparative Signal Strength
If you don’t want to buy an SDR, your phone itself can provide approximate attenuation data — you just need to access its signal strength readings rather than relying on bar indicators.
The Procedure
iPhone: Open the Phone app and dial *3001#12345#* then press Call. This opens Field Test mode, which shows signal strength in dBm rather than bars. Navigate to the serving cell information to find RSRP (Reference Signal Received Power) for LTE.
Android: Go to Settings → About Phone → Status → SIM Status → Signal Strength. Most Android phones display signal strength in dBm directly.
Step 1: Record the signal strength (in dBm) in your normal environment. Example: -85 dBm.
Step 2: Place the phone in the Faraday bag. Seal completely. Wait 30 seconds for the phone to attempt reconnection at maximum power.
Step 3: Remove the phone from the bag and immediately check the signal strength reading before it reconnects. Some phones display the last measured signal, which may reflect the shielded condition. Example: -130 dBm.
Step 4: The difference is approximate attenuation. In this example: 45 dB.
Caveat: This method is approximate. The phone’s signal strength readings update at different intervals depending on the device and OS. The reading you capture may not reflect the steady-state shielded condition. Use this as a rough check, not a precise measurement.
Method 3: WiFi Signal Strength Mapping
This method uses a second device to measure the WiFi signal from a phone inside the Faraday bag. It’s the easiest way to test WiFi-band shielding specifically.
The Procedure
Step 1: Enable your phone’s WiFi hotspot. Place it on a table. On a second device (laptop or phone), use a WiFi analyzer app to measure the hotspot’s signal strength in dBm. Record this as baseline.
Step 2: Place the hotspot phone in the Faraday bag. Seal completely.
Step 3: On the second device, observe whether the hotspot is still visible and its signal strength. If it’s still visible, the bag leaks at 2.4/5 GHz. Record the attenuated signal strength.
Step 4: If the phone supports WiFi 6E (6 GHz), enable a 6 GHz hotspot and repeat. This tests the highest WiFi band — the one most likely to exploit closure gaps due to its shorter wavelength.
Useful apps: WiFi Analyzer (Android), AirPort Utility (iOS with WiFi scanning enabled), or inSSIDer (Windows/Mac).
Testing for Degradation
A Faraday bag that works today may not work in six months. Dispatch #004 explained why: conductive coatings wear off at flex points, seam conductivity degrades, and closure materials lose effectiveness with repeated use. The only way to detect degradation is periodic re-testing.
Recommended Testing Schedule
Monthly: Run the basic phone test from Dispatch #004. Call the bagged phone, check for data activity, test Bluetooth discoverability. Takes 60 seconds.
Quarterly: Run one of the advanced methods above. Record the attenuation number. Compare to your initial baseline. A drop of more than 10 dB from your first test indicates significant degradation.
After any physical damage: If the bag is punctured, torn, or has been through a washing machine, re-test immediately. Any breach in the conductive layer creates a leak point that may not be visible to the eye.
Record keeping: Keep a simple log — date, method, attenuation result. Three data points over time tell you whether your bag is stable or degrading. If it’s degrading, you know before your security depends on it.
What the Numbers Mean
Quick reference for interpreting your test results. The full dB scale explanation is in Dispatch #004.
Transparency as Philosophy
We’re publishing testing methodologies that customers can use to evaluate our own products. This is either very confident or very stupid. We think it’s the former — because we designed the Signal Sleeve to pass these tests, and we’d rather you verify that than take our word for it.
Any Faraday bag manufacturer that discourages independent testing is telling you something about their product. We’re telling you something about ours: test it. Measure it. Record the results. If the numbers drop, tell us. That’s the relationship we want with the people who carry our products — one built on data, not faith.
Verify Everything
TINFOIL signal management products are designed to be tested, measured, and verified. We publish the specs. You verify the performance. That’s the deal.