Bad Ideas and Further Research

What Tin Can Antennas Are For

Tin can antennas do what all gain antennas do: Enable you to throw a Wi-Fi signal farther than with a simpler antenna, or sniff a signal farther. Or both. (Creating a connection at a distance requires both.) You can use tin can antennas as fixed pointto-point antennas to connect two networks in two different buildings (see the network bridge project in Chapter 16) and you can use them as directional sensing antennas for wardriving or warscanning, as I'll explain in Chapter 18.

Probably the most broadly useful application of simple tin-can antennas is as range extenders, which allow you to connect to an access point from the fringes of its range. A tin can antenna solved a problem for me that a lot of people have, and probably don't realize how easily it can be fixed.

It was like this: My office, broadband Internet connection, primary computer, and wireless access point are on the second floor of my long, rambling Arizona ranch house. My wife's office is directly below mine. Her computer connects easily to my access point right through the floor for a full-speed 11 Mbps network connection.

At the opposite corner of the house on the first floor is our living room, where the big TV and comfy couch are. Sometimes I feel like watching a movie, but I also want to keep an eye on something on the Internet, usually an eBay auction, that requires an occasional glance but not constant attention. I had hoped to do this by parking my Wi-Fi equipped laptop on the coffee table and letting the Wi-Fi machinery do its magic.

Well, it was a bridge slightly too far. If I positioned the laptop just so, it would connect-barely. When it connected, I generally got 1 Mbps. Careful tweaking of the laptop would sometimes get me 2 Mbps, depending (oddly enough) on where I was sitting on the couch, or (even more oddly) where on the coffee table I had placed my can of Diet Pepsi Twist. More than 2 Mbps, forget it. The path between the coffee table and my access point was optimal-bad, with a couple of walls (including a dense slump block wall), the microwave oven, a steel spiral stairway, and a cupboard full of tin cans in the way.

Tin cans… hmmm.

I went out in the garage, dumped the wood screws from a twenty-year-old coffee can, did some quick calculator math, drilled the can for a coaxial N connector, soldered a carefully-measured stub of wire into the connector, and bolted it together. I bent a bracket out of a scrap of aluminum sheet metal, and cobbled together a flexible gooseneck base out of pipe fittings, scrap iron, and a gooseneck I had salvaged from a long-defunct (and otherwise missing) microphone stand. It took about two hours, most of which was spent making sure I had the math right. (Figure thrice, measure twice, cut once.) I mounted the can on the base, and hauled back into the house. What I had is shown in Figure 15.1. The short length of wire is the pigtail that connects the antenna to the Orinoco Wi-Fi client adapter installed in my laptop.

Figure 15.1: The Duntemann Tin Can Bandwidth Expander, Mark I.

It looks goofy, but don't judge from appearances: When connected to my laptop via a short pigtail of coaxial cable (see Chapter 8 for more on pigtails and coaxial cable), I was able to connect to my Wi-Fi network from my coffee table at full speed, instantly, without even having to do any fussy pointing. I just twisted the gooseneck so that the can was aimed generally up and toward my second-floor office, and I was in, at 11 Mbps. I had hoped to connect at 5.5 Mbps, which would give me full advantage of my broadband connection, which sometimes gives me 2 Mbps on downloads. I wasn't expecting 11 Mbps!

The Duntemann Tin Can Bandwidth Expander, Mark I, was both ugly and larger than it needed to be. I've made several more, striving for something a little prettier and somewhat more compact. My best is shown in Figure 15.2. The Mark III uses a smaller can and a nicer base, which had once been a Fifties gooseneck desk lamp. I bought it on eBay for $6.95. I was the only bidder, and there were literally dozens of crusty old gooseneck lamps to choose from.

Figure 15.2: The Duntemann Tin Can Bandwidth Expander, Mark III.

I have since used Mark I from outside the house, to allow me to connect to the Internet from a small table beside my large Newtonian telescope out in the back of my large lot. I've used it from the garage, where my workshop is, though that's a long, metal-strewn path that won't always let me connect at full speed. Wi-Fi experts will understand that it works as well as it does in part because your typical PCMCIA card client adapter's antenna is wretched beyond hope, so almost anything would be an improvement. However, the tin can antenna is a nice compromise between effectiveness and complexity. A parabolic dish would have more reach, obviously- but if you don't need that much gain, why bother?

Admittedly, I'm a machine-tool hobbyist, amateur radio operator, and electronics technician of long standing (30 years plus now, sigh) but there really isn't much to it. If there are parts of your house that you can't reach with your laptop or other client adapter with a connector for an external antenna, consider putting one together.