Shielding is physics, not a product feature. Five minutes on how it works will tell you which spec sheets mean something and which are decoration.
The physics, briefly →A faraday bag for EMP protection is the cheapest useful item in this category and the easiest one to be sold badly. The good news is that unlike almost everything else in preparedness, you can test this one yourself, today, with a phone you already own, and get a real answer in five minutes. Here is how, plus a harder test that catches the bags the phone test lets through.
What a Faraday Bag Actually Is
Strip the branding and it is a pouch made of fabric with a conductive metallised layer, usually nickel and copper deposited on ripstop nylon or polyester, sometimes with a second silver-bearing layer. That conductive skin is a Faraday shield in flexible form. The bag closes by rolling the open end over on itself several times and clamping it, which brings conductive surface against conductive surface and completes the enclosure.
Mission Darkness is the name most people encounter, and there is a long tail of unbranded bags on marketplace sites that look identical in photographs. Sometimes they are. Sometimes the conductive layer is thinner, the seams are stitched through the shield rather than bonded, or the closure is a token single fold. You cannot tell by looking, which is the entire reason to test.
The Five-Minute Phone Test
A phone is a calibrated transmitter and receiver that you already own, working across several bands at once. Use it.
- 1. Check signal first. Confirm the phone has several bars of cellular and is connected to Wi-Fi, standing where you will do the test. A test in a basement with one bar proves nothing.
- 2. Wrap it loosely in a cloth so it does not touch the inside of the bag. Direct contact with the shield can couple energy past it.
- 3. Seal properly. Roll the top over on itself at least three full turns before clamping. This is the step people skip, and it is usually the one that decides the result.
- 4. Call it from another phone. A good bag sends the call to voicemail with no ring at all. If it rings once and then cuts off, the bag is attenuating but leaking.
- 5. Test Bluetooth too. Pair the phone to a speaker, start music, then seal it. Audio should drop within a second or two. This checks 2.4 GHz specifically.
- 6. Repeat three times, resealing each time. A bag that passes twice and fails once has failed. Take the phone out promptly, because it burns battery hunting for a signal.
Do this the day the bag arrives, while you can still return it, and do it again once a year. Metallised fabric wears at the fold line, which is exactly where the seal happens.
What the Phone Test Proves, and What It Doesn’t
Between them, cellular and Wi-Fi sample a useful stretch of spectrum: roughly 600 MHz at the low end up to 5 or 6 GHz. That covers the upper part of the band where a fast pulse carries energy, and it is a genuinely relevant check.
Two limits worth being clear about. First, it is pass or fail, not a measurement. It tells you nothing about whether the bag gives 40 dB or 70 dB, only that it is doing something substantial. Second, and more important, it says almost nothing about low frequencies. Thin metallised fabric is far worse at attenuating low-frequency magnetic fields than it is at gigahertz radio, because at those frequencies the shielding mechanism a thin non-magnetic layer relies on largely stops working. A bag can silence a phone completely and still be mediocre down at a megahertz.
The Harder Test: An AM Radio
If you own a cheap AM receiver, this takes another three minutes and probes exactly the weak spot above. Tune to the strongest AM station you can find, somewhere in the 530 to 1710 kHz broadcast band, and turn the volume up. Put the running radio in the bag and seal it.
Expect a worse result than the phone gave you. A good bag will drop the station to a faint hiss; a mediocre one will merely make it quieter. This is not a reason to send the bag back, because the fast component of an EMP has most of its energy well above the AM band. It is a reason to be sceptical of any bag advertised as flatly excellent across every frequency, and a reason to consider doubling up.
Two Layers Beat One Good Layer
Shielding is roughly multiplicative when you nest enclosures, and the gain is largest when the two layers are not in electrical contact. Wrapping a radio in a plastic bag, then foil, then putting that inside a shielded pouch, gives you two independent shields with an insulating gap. Whatever leaks past the first has to defeat the second, and any small failure in one layer is unlikely to line up with a failure in the other.
It also costs nothing. This is the single highest-value habit in the whole topic, and it lets you buy the cheap bag rather than the expensive one with a good conscience.
The Closure Is the Product
Everything about shielding comes down to whether the enclosure is closed, and on a bag the closure is a strip of fabric you fold by hand. Three full turns is the usual manufacturer instruction and it exists for a reason: each fold adds conductive contact and lengthens the leakage path. One casual fold leaves a slot running the whole width of the bag, and a slot is an antenna.
Two practical points. A bag stuffed so full that the top will not fold cleanly is not sealed, so buy one size up from what you think you need. And a bag left rolled and clamped for years develops a crease that eventually cracks the metallised layer, so unroll it, inspect the fold line and retest annually.
Bag, Can, or Box?
| Option | Rough cost | Best for | Weak point |
|---|---|---|---|
| Shielded roll-top bag | $20-60 | Phones, handheld radios, travel | Fold line wears; needs closure discipline |
| Galvanised trash can | $25-40 | Bulk gear kept at home | Lid seam leaks unless taped with foil tape |
| Steel ammo can | $20-40 used | Small, heavy, stackable storage | Rubber gasket insulates the lid from the body |
| Foil-wrapped cardboard box | Under $5 | Spares and experiments | Tears easily, single use in practice |
The ammo can trap is worth spelling out, because it defeats a lot of homemade setups. The gasket that makes the can watertight is rubber, and rubber is an insulator, so the lid never makes conductive contact with the body. Either remove the gasket, or bridge it with copper tape or a short length of braid at several points around the rim, then test it.
What Goes In
Spares that are cheap now, useless in singles, and hard to buy later. A basic AM/FM/shortwave receiver so you can hear what is happening. A handheld transceiver if you are licensed. A spare solar charge controller and a small inverter board, because those are the fragile electronics in a backup power setup and the panels themselves are not going in a bag. A power bank, and an old phone loaded with offline maps and documents.
What does not go in: the equipment you are actually using, which is most of it. Nothing shielded helps if the house has no stored water and no heat, and the ordinary failures in what breaks first when the grid goes down are the ones with a real probability attached. If backup power is the plan, the sizing work in portable power station sizing and why solar panels die in a power outage is worth more of your afternoon than any bag.
What I’d Actually Buy
One mid-price bag from a maker that publishes a shielding curve rather than a single headline number, sized a step larger than your gear. Test it the day it arrives with both the phone and the radio. Inside it, wrap each item in plastic and then foil, so you always have two layers. Total outlay under fifty dollars, total effort one evening, and then it goes on a shelf and you spend your remaining attention on water, heat and carbon monoxide, where the odds are.
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Mission Darkness NeoLok faraday bag on Amazon →
Frequently Asked Questions
How do I know if my faraday bag works?
Seal a phone inside with at least three folds and call it from another phone. No ring at all and a straight drop to voicemail is a pass. Repeat three times, and add a Bluetooth audio test to check the 2.4 GHz band separately.
Does a faraday bag drain the battery?
Yes, sharply, while a live phone is inside. Cut off from the network, the radio ramps to full transmit power looking for a tower, and a phone can flatten in a few hours. Power devices off before long-term storage.
Do I need to ground a faraday bag?
No. A closed conductive envelope shields its contents whether or not it is connected to earth. There is no practical way to ground a fabric pouch anyway, and no benefit to trying.
Are cheap unbranded bags any good?
Some are, many are not, and the price tells you very little. The test costs five minutes, so buy from a seller with a straightforward return policy, test on arrival, and send back anything that lets a call through. That is a better filter than any brand name.
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