One of the more popular ways of stealing cars is by what’s known as relay theft. This is where someone stands close to the door of your house/flat and (they hope) grabs the signal being emitted from the car key inside. They then use a piece of kit to bounce this to an accomplice by the car. This enables them to unlock the car and start it.
But you can protect against this kind of theft with a Faraday pouch, sometimes called a signal blocking pouch. These shield against the entry and exit of electro-magnetic fields. You can buy them, but it’s also possible to make your own Faraday pouch for car keys and you can do so for pennies with bits and bobs left around in the kitchen.
An effective Faraday cage needs a number of layers of conductive and non-conductive materials. The idea is the signals going in and out hit the non-conductive layer, flow around it and dissipate. This prevents anyone with dodgy intentions from grabbing the signal from your keys.
How to build a Faraday pouch for car keys
Step 1: For this home-made Faraday pouch to work, you need a sealable sandwich bag, some cling film or paper/flexible card and a sheet of tin foil.
Step 2: Put your car keys inside the sandwich bag. Then lie this flat and seal it.
Step 3: Wrap it in a layer of double folded tin foil. You need to seal the foil around three of its edges leaving the opening to the sandwich bag unsealed. It’s a good idea to make the foil parcel wider than the sandwich bag. This gives you plenty of foil to fold over and make a good seal around the edges.
Step 4: Wrap that layer of tin foil in paper. You can use cling film but paper is more sustainable. Again you will be folding the tin foil and cling film so you have an opening that aligns with that of the sandwich bag. If you do this, you’ll be able to re-use your Faraday pouch.
Step 5: Wrap that in a second layer of tin foil. Again, seal the edges, this time, folding over the fourth edge that aligns with the sandwich bag. This will give you a Faraday Pouch that seals your key’s signal in.
If you’re going to be using this a lot, we’d advise that you use packing tape or similar around the edges of your pouch for the key fob.
Try a cocktail shaker
If you want an off-the-shelf Faraday pouch, think about a cocktail shaker. They’re not just useful for making cocktails; they make a great Faraday pouch too. Think about it: cocktail shakers are metal and they have very tight seals. The perfect combination for a home-made shield.
So get your sandwich bag and pop the keys into it. Then put the sandwich bag into the cocktail shaker. Job done.
Are microwaves Faraday cages?
This depends on the individual item. Commercial grade microwaves may well be sufficiently insulated to block signals. According to everything we’ve read, regular domestic ones are unlikely to.
It’s a similar story with fridges and freezers. If the seal around the doors is particularly efficient, then yes, they might well work as a Faraday cage. But the seal on most domestic units isn’t sufficient to block the signals.
Alternatively, buy your own Faraday pouch.