Tuesday, July 21, 2020

Preliminary test results for Faraday bags using NESDR Mini 2+ and gqrx

In a bid to secure my car keys from relay attacks I purchased some Faraday bags and related materials (kitchen foil, copper and aluminum tape), as well as a NESDR Mini 2+ to test the strength of the signals.

Using the rtl sdr was simple: install gqrx, plug in the NESDR Mini 2+, and start capturing.

Experimental procedure was simple: Put car key inside the Faraday bag. Press button on car key. Look at readings output by gqrx.


- Plastic bags are completely useless as Faraday bags, no signal attenuation.
- Plastic bags wrapped in metal tape (both copper and aluminum) are of minimal effectiveness. Putting the copper bag inside the aluminum bag made no difference - the signal still leaked like crazy.
- One layer of kitchen foil produced significant signal attenuation, but was still detectable at point blank range.
- The Faraday bag I purchased (not going to name brands) produced significant signal attenuation, but was still detectable at point blank range.
- Two layers of kitchen foil (a single sheet wrapped around the key twice) was able to completely block the signal (drops it into noise floor).

I later shined a flashlight inside the copper bag and it showed light was leaking from the seams between pieces of copper tape that weren't tightly sealed. I've added another layer of copper tape on the aluminum bag to completely seal off any gaps but haven't retested to check if that fixes the signal leakage. It's possible that if you use the metal tape really carefully then you can avoid signal leakage - I haven't tested this.

For now, multiple layers of aluminum foil seems to be the most effective way to block signal leakage. Metal tape seems to be either ineffective or very tricky to get right. When metal tape and foil is folded over completely, it seems to create very small points of leakage at the edge of the fold. I have seen that light can leak through 2 layers of metal tape folded over the same edge. I don't know how to fix this, other than adding more layers of tape on top.

I had expected the metal tape to perform better than kitchen foil because of greater thickness (the aluminum tape was 50 microns which, according to my friend's extrapolations, is something like 10x thicker than the skin depth of aluminum for that frequency) but in fact they performed abysmally. My best guess for this is the gaps that I saw with the flashlight.

But the gaps are tiny. They are less than 1mm wide and less than 5 inches long. But the signal still leaked through.

The car keys operate at the 300-600MHz range. So the wavelength is in the 10s of cms. And yet experiment has shown that the signal can get through very very tiny gaps of smaller than 1mm. This was surprising to me.

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