Faraday Cage (Part 1)

Posted on May 5th, 2007 in Interop by jed

Faraday Cage

[Update July, 2007: I know, I know, I've been terribly delinquent about posting the details from the show. I will soon, I promise. There will be two more parts: one with pictures and notes on construction, and one with lessons learned and other info. Thanks for being patient, check back soon for the next installments of the faraday cage series.]

This year at the Interop Labs we are building a walk-in Faraday cage as part of the VoIP over Wi-Fi demonstration. Since there are usually anywhere from 250-400 wireless access points “viewable” from the show floor, we need some way to remove all the junk in the RF spectrum in order to provide a controlled environment to test in and to effectively demonstrate the differences between VoIP Wi-Fi connections with and without QoS controls and burdened and unburdened Wi-Fi networks.

Anyway, since this walk-in size Faraday cage project was my idea, somehow I got saddled with making it happen. So the first thing to do was to build a proof-of-concept before ordering the necessary materials. On the last day of hotstage (about a month before the show we get together for just over a week to do the planning and build out of the Interop Labs booth, called hotstage) I went to the hardware store, got some aluminum mesh (the kind you put on a screen door or window), some 1/4 inch hardware cloth (something you might build a rabbit cage out of) and folded them up into a couple of box shaped containers to put something in that could measure radio waves. Then I fired up Kismet on my laptop and put it in each one. It was a spectacular success. Moments after placing my laptop inside the aluminum screen “box”, the signal went away. Unfortunately, after running several successive tests, it appeared that the signal went away about 90 seconds after launching Kismet, no matter what the circumstances (I could be standing next to the access point and it didn’t seem to make a difference.

So I went on the hunt to find some other Wi-Fi measurement tools to run on my Macbook Pro. Of course the obvious choice is Macstumbler, but it hasn’t been updated in a long time and crashes at launch. Then I found iStumbler and APGrapher, and both of those seemed to work well. So I fired them up, put my laptop into the proof-of-concept device, and…the signal didn’t change. Buh.

Ok, back to the drawing board. A little more research and a refresher in high-school physics reminded me that a faraday cage is basically a big “ground” wire extended into a 3D space that you could put stuff in. Doh! I didn’t ground any of my proof-of-concept tests (well, actually, I did hold it up against a metal wall socket at the warehouse that should have been grounded, but that didn’t seem to make a bit of difference). So I set out to build a new proof-of-concept, and this time I would be more meticulous about its construction and I would make sure it was well grounded.

I decided that this time, I’d build a frame, attach the mesh to it, and secure a copper wire to the mesh that I would then connect to the ground plug (third hole) of a standard wall socket. I choose to build a box roughly 18 inches X 12 inches X 12 inches, so I could put my laptop inside of it without much trouble.

Step one: buy the necessary components. Here is the list (pics down below in the assembly instructions):

  1. Two 8′ lengths of 2″ X 2″ wood (I choose spruce “furring” strips, cause they were $1.99 at the local hardware store). This will be the main frame.
  2. One 8′ length of 2″ X 1″ wood (again, spruce furring strips, even cheaper at $1.79). This will be frame for the hinged lid.
  3. Hinge to attach lid to frame. I choose a 12″ piano hinge.
  4. Aluminum screen to cover. I made some rough calculations and bought 10′ of 36″ wide mesh.
  5. Copper wire for the group connection. I choose green 14 AWG stranded, cause it was reasonably cheap.
  6. A bolt, some wing nuts, and washers to secure the copper wire to the mesh.
  7. A plug the I could attach the copper wire to and insert into a socket.
  8. Screws and staples to secure the frame together and the mesh to the wood (I simply used what I had laying around in my garage).

Step two: Cut the wood. I cut the 2″x2″ furring strips into four 18″ lengths and eight 12″ lengths. And the 1″x2″ strip into two 18″ lengths and two 12″ lengths.

Step three: Screw the frame together. This isn’t rocket science, so I’m not gonna give you detailed instructions on this part. Just remember to drill pilot holes for your screws so you don’t split the wood. Here is the assembled frame:

Wood Frame

Step four: Build and attach the lid. I simply screwed the 1″x2″ pieces I had into a square and put them on the frame box with a piano hinge:

Frame With Lid and Hinge

Frame With Lid Open

Step five: Attach the screen mesh. I will never regret the day I bought my air compressor and pneumatic stapler. I can only imagine the terrible hand cramps had I attempted this with a standard spring loaded stapler. If you don’t have a pneumatic stapler, I highly recommend you at least look into purchasing an electric one. To make sure that everything was tight, I tried to keep staples no more than 2 inches apart (usually about 1 inch, but towards the end I got a little tired and lazy, so the space between increased a littler). Here is the before and after:

Frame With Screen and Cat

Phoenix the cat seems curious about the frame and screen.

Screened Cage Open

Screened Cage Closed

Step six: Attach the ground wire. Below is a shot of the parts mentioned above, along with Phoenix, one of our cats who was very interested in this project. Attaching the parts was pretty simple.

Ground Parts and Cat

Ground Attached to Cage

You will notice that I cut off the two prongs that get electricity. I wanted it to be perfectly clear that this plug is non-functional, and won’t create an electrified cage (cause that is a completely different project!). [NOTE: This is dangerous! Don't do it. If there are some cables crossed or your ground is wired incorrectly, you could create a lethal situation and die. Get an electrician to get you a dedicated ground wire to use for a project like this. If you kill or seriously injure yourself, don't blame me, I told you not to do this. Seriously, I'm not joking. Don't plug stuff into an outlet that connects to bare metal that you will be touching. That is just stupid.]

Step seven: Time to test! I plugged in the ground, got my laptop and started measuring Wi-Fi signals. The pictures speak for themselves:

Laptop on top of Cage while measuring signal

Laptop in cage still has signal

DOH! It still has signal. WTF?!? Well, as it turns out, the ground connection in my house is quite bad. So I brought it into the office and tried it there:

Laptop in cage with no signal

Woohoo! Just to make sure though, I opened the door, let it reacquire signal, then closed the door again:

Laptop in cage door open then closed


Check back after the show (May 20-25) for Part 2, where I will detail building the walk in cage.