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.

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