Some of our most frequently asked questions are about Faraday boxes for car keys so we thought we’d pool all of our knowledge and create a comprehensive guide to car key signal blockers.
These things are sometimes called Faraday bags, pouches, even cages or RFID bags. But no matter what we call them, the aim is to contain the signal transmitted by a car’s key. This prevents car thieves from grabbing the signal and using it to steal your car.
In this guide we look at what exactly Faraday pouches are, how they work, how you can make your own and which ones are worth buying. If you don’t want to read the whole piece, we’ve provided some helpful links, so you can skip straight to the section you’re most interested in.
All you need to know about Faraday boxes
How ‘keyless’ car keys work
In order to understand what a Faraday box does, it helps to understand how a modern car key unlocks and starts your motor.
Increasingly you can unlock and start cars without actually touching a key. All you need is to have the key – or more usually a fob – somewhere about your person.
This fob transmits an RFID (Radio Frequency Identification) signal. The receiver in the car captures this encoded signal, decodes it and if it recognises it, allows the car to be unlocked and the engine to be started when you press the start-stop button.
The signal’s encryption is unique for each car meaning one BMW owner can’t use their key to open others’ cars.
What does a Faraday pouch do?
They are something that you put your car key (or any item that transmits a signal) in when you want to contain the signal they emit. It works a bit like an RFID wallet you might have to protect your credit cards from contactless fraud.
Why do you need a Faraday bag?
There have been increasing numbers of car thefts where the thieves don’t physically have the key. They get close to where the key is, use a device to grab the signal from the key which they bounce to a colleague’s device near to the car. This fools the car into thinking the key is nearby and it unlocks and will start.
As shown in the illustration, Thief 1 uses a device which ‘pings’ the car’s entry system, which is constantly listening for the key fob. The car responds by requesting an authentication signal from the key fob. Thief 1 forwards this request to Thief 2, who is near the real key fob (inside the house) which is always on, listening for a request from the car. The key fob then responds with a valid authentication which Thief 2 sends back to Thief 1 who is then able to access and start the car.
This kind of crime is referred to as ‘keyless theft’ or ‘relay theft’ because of the way the signal is relayed. The whole process can take a team who know what they’re doing less than a minute.
How do Faraday pouches work?
The aim of a Faraday pouch is to contain the signal transmitted from a car’s key. To do this, the bag must be lined with something that doesn’t allow the signal to escape. This is usually metallic and frequently involves a number of layers to ensure the signal is well and truly contained.
Just as they don’t allow the signal to escape, neither do these bags allow any signal to enter. So once in a Faraday bag, your car key will neither transmit nor receive.
Do Faraday bags work for car keys?
They should, yes. It’s easy to tell, particularly if your car is what’s known as keyless. If the car can be unlocked remotely while the key is in the Faraday bag, then the pouch doesn’t work.
How to test a Faraday pouch
You put your key in the Faraday pouch, walk up to the car and where your doors would usually unlock automatically, they now shouldn’t.
Then get your key out of the Faraday bag, unlock the car, get in and put the key back in the Faraday bag. Press the car’s start/stop button and if the Faraday bag is working, the car shouldn’t start.
To see how good the item really is, put your mobile phone in it (assuming it fits). Then ring your phone. A fully functioning Faraday bag should prevent the signal getting through to your phone.
What makes a good Faraday pouch?
Obviously it has to fulfil its primary purpose of isolating the signal from the key. Good ones are made from several layers of copper and plastic. But if you want to make your own, you could use aluminium kitchen foil and paper.
If you do buy one and you’re going to be using it frequently, it needs to be well made. That means it should be constructed from thick wear-resistant material and have strong stitching. And any fastening should be good quality.
A poor-quality Faraday bag may stop working because it has failed and can no longer prevent the signal from escaping.
The bag must be fit for purpose too. Some people might want a Faraday bag that’s portable; some might want one that they can just leave in their house for when they’re at home. The latter might be able to get away with a less robust article.
Some people might want one to put their mobile phone in; some might want them for their car keys. It’s worth remembering that car keys can differ dramatically in size nowadays too.
Do you need a Faraday box for car keys?
Relay theft has been around for a couple of years now and car makers have got wise to crooks. Some car companies now make keys that don’t emit a signal unless they’re moving, or after they’ve been stationary for a short period of time. Other’s make keys that can be turned off if they’re not needed.
If you have a keyless car, before you go to the trouble of buying a Faraday pouch, check the key section in the car’s user manual.
When do you need a Faraday pouch?
Whenever you think your car key might be vulnerable to having its signal grabbed. Ironically, the easiest cars to steal using relay theft are those parked on people’s drives. The thief knows exactly where the keys for that car are: somewhere behind the front door.
But it’s also worth using a Faraday bag if you park on the street. There’s nothing to say that the people who might steal your car haven’t been targeting it and watching which door you come and go from.
Does aluminium foil block car key fob signal?
Yes, aluminium foil will block a car key fob signal, as long as it’s sealed around the edges and there are a couple of layers of the sheet.
Does a Faraday box drain the battery in your car key?
No. A Faraday box just blocks the signal that the key sends from reaching anything. The key doesn’t send a stronger or more regular signal because it’s being blocked.
What can you use instead of a Faraday bag?
In some cases, biscuit tins will work as a Faraday bag; in other cases not. Put your keys in a biscuit tin and if it has a tight-fitting lid, it should prevent signal getting through. If the signals are very strong, it might not.
It’s the same with a fridge. In some cases, the seal on a fridge door will be sufficient to keep the signal from the key in and any external signals out. In other cases, it won’t isolate the key sufficiently to be any use.
Some people reckon that microwave ovens can also do the job, but this will likely depend on the model, so your mileage may vary.
You could also try a cocktail shaker. Think about it: cocktail shakers are metal and have very tight seals. The perfect combination for a home-made car key jammer!
What alternatives are there to a Faraday box?
Good old-fashioned physical deterrents still work. So as well as a Faraday Bag, you might consider buying a steering wheel lock. The best ones are difficult for crooks to get off and should be sufficient to ensure they move on to a less well-protected target. Experts advise that you use a lock that covers the entire steering wheel and has been approved by Thatcham such as the Stoplock Pro. This is the insurance industry’s testing body.
How do you make your own Faraday pouch?
It’s simple, you just need aluminium foil, paper and a sandwich bag.
Put your key in the sandwich bag then wrap that in aluminium foil, carefully sealing three edges. Make sure you can still access the sandwich bag’s opening (the fourth edge that you don’t seal).
Then wrap this in paper, again keeping the opening free.
Finally, wrap your parcel in a second layer of aluminium foil. Again, fold it over carefully around the edges and keep enough so you can fold it over and seal it at the top. And that’s your home-made Faraday bag.
Why is it called a Faraday Cage?
They’re named after Michael Faraday who invented the first enclosure designed to block electromagnetic signals in 1836. He probably didn’t foresee his invention being used to prevent your car being pinched from your driveway!
What is the best key fob signal blocker?
We’ve looked around and found three Faraday bags that come highly recommended.
CAR Magazine rates the Disklabs Keyshield. This is portable, British made and approved by the police. It measures just 10 x 12cm. Auto Express testers put this second.
They preferred the Defender Signal Blocker. Good news about that is the Defender only costs £5.95 for two. Testers rated its build quality and the size (11.5 x 18.5cm) means it’ll comfortably accommodate even bigger car keys and mobile phones.
Best Adviser rates the Monojoy pouch. This is another relatively cheap one, costing £10.99 for two. But it is well built, measures 14 x 10cm and testers say it offers some of the best value for money around.
I’ve been writing about cars and motoring for more than 25 years. My career started on a long-departed classic car weekly magazine called AutoClassic. I’ve since pitched up at Autosport, Auto Express, the News of the World, Sunday Times and most recently the Daily Telegraph. When I’m not writing about cars and motoring, I’m probably doing some kind of sport or working in my garden.