Block Bluetooth Tracking (Faraday Bag Test)
A Faraday bag test checks whether a known Bluetooth Low Energy device can still send detectable radio packets while the bag is closed. To make the result meaningful, establish an open-bag baseline, keep receiver placement fixed, then repeat a timed test. No packets detected means only that your receiver did not detect them under those conditions.
A common misconception is that a phone’s Bluetooth screen can prove a bag blocks tracking. It cannot: that screen is not a reliable packet-measuring tool, and a missing device may simply be out of range or not advertising at that moment.
A useful test is a comparison, not a search for a magic pass result. You check whether an external receiver detects a known, powered device before and after you close the bag. I treat the outcome as evidence about that device, receiver, placement, closure, and test period—not proof that the bag blocks every signal or tracking method.
Planning: Define the BLE test and baseline
A baseline is a record of what your receiver detects before the device goes inside the bag. It confirms that the device is transmitting and that your receiver can observe it under the test conditions. Without this check, a quiet result from the closed bag is hard to interpret.
Bluetooth Low Energy (BLE) is a radio system used by many phones and small devices. BLE operates in the 2.400–2.4835 GHz band. Its three advertising channels are 37 at 2402 MHz, 38 at 2426 MHz, and 39 at 2480 MHz, as set out in the Bluetooth Core Specification.
Advertising packets are short messages a BLE device may send so nearby systems can detect or identify it. A known BLE transmitter could be a device you control, configured to advertise, or another device whose signal you can identify. Do not assume a tracker is advertising just because it is powered on; settings and behavior vary by device.
For a meaningful comparison, use:
- A powered BLE transmitter that is confirmed to be advertising.
- An external BLE receiver or sniffer that can report received packets.
- A bag that closes as you intend to use it.
- A fixed test plan for receiver distance, device position, and observation time.
Record the device identifier when available, receiver settings, distance between device and receiver, and the baseline packet count. Some devices use changing addresses, so the identity shown by a receiver may not stay the same. Confirm which signal belongs to your test device before relying on packet counts.
Access and clearances: Place the receiver and close the bag
Receiver placement is the position and setup used to listen for the transmitter. Keep it outside the bag and in the same place for baseline and closed-bag trials. Changing its distance or settings between checks can make the comparison unreliable, even if the bag and device have not changed.
Before testing, place the transmitter and receiver at a distance where the receiver consistently detects the transmitter with the bag open. The exact distance depends on the devices and surroundings, so there is no single distance that fits every setup. Record it, then avoid moving either device during the comparison.
Close the bag in the same way you plan to use it. Align its closure fully and check that the closure and seams are not folded or obstructed. At 2.4 GHz, the free-space wavelength is about 12.5 cm. Gaps and seams can matter, but wavelength alone cannot predict how much signal a particular bag will block.
| Test detail | What to do | Why it matters |
|---|---|---|
| Receiver | Keep it outside the bag and fixed in place | Keeps the comparison consistent |
| Baseline | Confirm repeated detections with the bag open | Shows the transmitter and receiver are working together |
| Closure | Close and align the bag as intended | Tests the real closure, not an idealized setup |
| Device position | Record it, then repeat with a new position | Helps reveal whether placement changes the result |
| Observation time | Choose and record a set period | Makes separate trials easier to compare |
A phone’s general Bluetooth settings screen is not a substitute for a BLE packet sniffer. It may show paired or nearby devices, not a complete record of advertising packets. A scanning app may also miss packets because of its scan timing, permissions, or receiver limits. Treat its “not found” message as a prompt for a better test, not proof of shielding.
Execution: Run controlled isolation trials
A controlled trial changes one factor at a time and records what happened. For this test, keep the transmitter, receiver, distance, and receiver settings consistent. First confirm open-bag detections; then close the bag and monitor the receiver for a chosen period.
- Power the transmitter and confirm that BLE is enabled and advertising.
- Set up the receiver outside the bag. Note its settings and the transmitter’s position and distance.
- With the bag open, observe the receiver and record detections. Repeat the baseline if results vary.
- Put the transmitter in the bag and close it as intended. Do not move the receiver.
- Monitor for a defined interval. Five minutes can serve as a screening check, but it is not a certification period or universal standard.
- Record whether the receiver detected packets, along with the time and any changes in setup.
- Open the bag, reposition the transmitter, reseal it, and repeat. This checks whether one placement or closure position influenced the result.
Keep the test simple enough that you can repeat it. If the transmitter stops advertising, changes its address, or enters a different power state, a lower packet count may not be caused by the bag. Confirm the baseline again if you are unsure.
Do not leave a device charging or transmitting inside a closed bag unattended. A closed, insulating bag can retain heat. Follow the device maker’s temperature and battery-safety instructions, and stop if the device becomes unusually warm or shows a warning.
Verification: Interpret packet results, limits, and retest
A packet result is what your receiver recorded during a trial. Compare the closed-bag result with the open-bag baseline, and repeat the test before drawing a conclusion. A result with no detected packets is limited to the specific receiver, device, placement, closure, and observation period you used.
If packets remain detectable, check that the closure is aligned and fully closed, then inspect how the device sits near the closure or seams. Repeat the test without changing receiver settings. If packets are not detected, state the result narrowly: “No packets were detected under these test conditions.” Do not translate that into “the device is untrackable.”
Shielding effectiveness (SE) is a measure of how much a barrier reduces transmitted signal power. It is expressed in decibels: SE = 10 log10(Pincident / Ptransmitted). A 20 dB reduction means 100 times lower power, not zero power or guaranteed silence at every receiver. This field test does not provide a certified SE rating.
IEEE 299 is a measurement reference for shielding effectiveness, not a certification procedure for consumer Faraday bags. There is no universal pass duration or pass/fail attenuation threshold for these bags. A packet-count comparison can help you check a particular use case, but it does not replace a formal lab measurement.
| Observation | What it supports | What it does not prove |
|---|---|---|
| Receiver detects packets with bag open | The device and receiver can communicate under baseline conditions | That every later detection is from the same device, unless identified |
| Fewer packets with bag closed | Detection changed in this setup | A specific shielding rating |
| No packets detected during a timed trial | Receiver did not detect packets during that interval | That no signal escaped or another receiver could not detect it |
| Packets return after opening the bag | The setup can detect the device again | That the bag blocks all radios while closed |
Practical test scenarios and comparison
A practical test scenario shows how to apply the same method to different questions. These examples are illustrative, not reported measurements: they show how to record and interpret results without inventing a pass threshold. Your own outcome depends on your equipment, bag, device, and test conditions.
In one scenario, a traveler tests a known advertising device near the middle of a closed bag. The open-bag baseline shows repeated detections; the five-minute closed-bag check shows none. The accurate report is that the receiver detected no packets in that trial. The traveler then repeats the test with the device near the closure, because position may affect the outcome.
In another scenario, the receiver still records packets after the bag is closed. The tester confirms the transmitter’s identity, checks that the closure is aligned, and repeats the trial. If detections continue, the tester has a practical reason to contact the bag maker or choose a different product, but the result still does not reveal a formal dB rating.
| Your question | Best next step | Careful conclusion |
|---|---|---|
| Does the closed bag reduce detections in my setup? | Compare repeated open and closed trials | Report packet counts and conditions |
| Could the closure or device position matter? | Reseal and repeat with a different device position | Note whether the result changes |
| Can I assign a shielding rating from this test? | No; use a suitable lab measurement method | Do not claim a certified dB value |
| Does a BLE test cover every tracking method? | No; it tests BLE only | Other radios or methods remain outside scope |
What this test cannot establish
A BLE-only test says nothing conclusive about cellular, Wi-Fi, ultra-wideband (UWB), or Global Navigation Satellite System (GNSS) signals. These are different radio systems or methods. A bag that reduces detectable BLE packets in your test has not thereby been shown to isolate a device from every network or location service.
The test also cannot show what another receiver, with different sensitivity or placement, would detect. It does not certify a product, establish a universal pass threshold, or prove that every unit of the same bag performs identically. Avoid improvised foil wrapping as a substitute for a purpose-built shielding bag; it does not provide a controlled closure or a reliable test result.
Conclusion: Report the conditions, not a guarantee
A useful bag test is repeatable and modest in what it claims. Confirm an open-bag baseline, keep the receiver fixed outside the bag, close the bag as intended, and compare results over a recorded interval. If nothing is detected, report only that the receiver found no packets in those conditions; retest if the setup changes.
Frequently asked questions
Can a Faraday bag block Bluetooth tracking?
It may reduce or prevent detectable BLE packets in a given setup. Test the specific device and bag rather than assuming all bags block all signals.
How do I test a Bluetooth-blocking bag?
Use a known BLE transmitter and an external receiver. Confirm detections with the bag open, then repeat with it closed while keeping positions and settings fixed.
Can my phone’s Bluetooth screen verify the result?
No. A general Bluetooth settings screen is not a reliable packet sniffer and may not show advertising packets.
How long should I monitor the closed bag?
There is no universal pass duration. A five-minute check can be a screening trial, but it is not certification.
What does “no packets detected” mean?
It means your receiver did not detect packets during that trial. It does not prove that no signal escaped or that another receiver would also detect nothing.
Should I test more than once?
Yes. Repeat the open-bag baseline and closed-bag trial, and try a different device position while keeping other conditions consistent.
Does a BLE test check cellular or Wi-Fi signals too?
No. It only checks BLE under the conditions tested. Cellular, Wi-Fi, UWB, and GNSS are outside its scope.
Can I claim a dB rating from packet counts?
No. Packet counts do not provide a certified shielding-effectiveness rating. A 20 dB reduction means 100 times lower power, not zero reception.
Why might packet detections change between trials?
The device may change its advertising behavior or address, or the receiver may scan differently. Keep settings steady and confirm that the transmitter is advertising.
Is it safe to leave a device inside the bag?
Do not leave it charging or transmitting inside a closed bag unattended. The bag can retain heat; follow the device maker’s safety guidance.
(This article was written by one of our staff writers, Steven Langford. Visit our Meet the Team page.)
