Faraday Cage (Part 2)

Posted on August 10th, 2007 in Interop by jed

As I mentioned in Part 1 of this Article, this year at the InteropLabs we built a walk-in Faraday cage so that we could have an environment to test VoIP over Wi-Fi in that was a little cleaner than the show floor (which had well over 400 SSIDs viewable at any given moment!).

Completed Faraday Cage on Interop Show Floor

Completed Walk-In Faraday Cage at Interop Las Vegas 2007

I learned a lot about RF and Wi-Fi, which I’ll be reporting on in the third and final part of this article, but here I want to detail the construction of the cage.

We started with a large aluminum frame that we had the onsite construction people build. If I could have had my way, I probably would have just used two-by-fours, but because it was on the show floor and such, we had to let the pros do it, and we weren’t about to start getting picky about the materials for the frame. We also had them cut a plywood floor for us, so once we got the screen down we could set the plywood on it and not have to worry about people tearing it up.

While they were building the frame and the rest of the show floor was getting prepped, I went to the local Lowe’s Home Improvement store and went looking for the aluminum screen we had decided to use for the build. Many people have asked me why aluminum instead of something like copper or brass, and the answer is basically because it is cheaper, more available, and the reports I was able to glean off the web suggested that an additional layer of cheap stuff was going to be far more effective than a single layer of expensive stuff (although two layers of expensive stuff would be nice, but I’ll give you tips on what to build your own out of in part three of this article). I was prepared to spend two hundred dollars to buy two 48 inch by 100 foot rolls (I had calculated the surface area of the frame to be int he neighborhood of 375 sq. feet, so this should give me enough to do a double layer all around). Amazingly, Lowe’s happened to have them on clearance that week, and I got the two rolls for $25 each! What a score. In retrospect, I should have purchased four rolls.

We also purchased a rivet tool to secure the screen to itself with, because the guys on the team thought my idea to just use zip ties was bogus (and it was, so I’m pretty glad they called me on it). Along with the rivet tool and rivets, we had to purchase washers to go on each side of the rivet. Annoyingly, even though they sell the rivets in boxes of 100, the washers were all sold in boxes of 20, so we had to buy 30 boxes to go with the three boxes of rivets we purchased! [If someone from the Arrow Fastener Company ever reads this, take note: sell washers in packs that are twice as large as the rivets.]

Then we started wrapping and riveting. It was slow going, but thanks to the help of the guys on the team (and some borrowed help form the NAC team), we were able to make reasonably quick progress.

Once we got the whole cage wrapped, it was time for a smoke test. I fired up AP Grapher on my laptop and checked outside the cage to see what I could see. 389 SSIDs showed up within 20 seconds of scanning! Talk about a noisy RF environment. I zeroed in on one that I knew was in our rack about 6 feet from the cage, and stepped in to measure the signal. -10dB. Buh. That sucks.

But we didn’t let the less than stellar results get us down. We kept on riveting and sealing up the places where the screen met and would likely allow waves in, and then added a second layer on top of the first and did some more.

After many more hours, we were ready to test again. This time we were able to see about a -35 to -40 dB. Now this was at least decent. Amazingly, we could still get quite reasonable Wi-Fi signal from nearby APs, but at least we were able to filter out the vast majority of the junk on the floor.

Oh yea, remember that whole thing about grounding in the last article? Well, we had the electricians at the show bring us a nice massive ground connection from the nearest electrical box, and we hooked it to the cage with some very nicer copper wire that Wej brought. Unfortunately, when we tested it, it didn’t seem to make a bit of difference. After multiple tests with the multimeter to make sure we were getting a decent electrical connection, Jerry put forth the theory that we were simply too far from the earth for the ground to be effective, and that the impedance of our ground connection was simply too much to get the 2.4Ghz waves from where we were to the actual earth. I’ll write a little more on this topic later in Part 3. We left all the ground wires connected though, cause they looked cool.

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.