The Pringles Cantenna
By Kirk Kirksey

Suppose you wanted to do a little sofa surfing with your laptop, but your tattered couch is just a little too far from your network access point. Do you blast a hole in the bedroom wall so your sofa will fit? Call the apartment manager and have your hovel rewired? Run to the U-Sack-It and buy a can of Pringles potato chips?

Any wireless aficionado worth his or her port scanner knows that the last option is the answer. Why? Because once the top has been popped, you’ve got a scrumptious snack and the makings of a world-class Wi-Fi antenna, to boot. Now that’s what I call “good eating.”

The whole Pringles Wi-Fi antenna phenomenon (these are now called Cantennas) was started by a young math whiz named Andrew S. Clapp. (You can see a picture of Clapp at his Web site, www.aeonic.com/~clapp.) One day, Andrew decided he wanted a Yagi-type directional antenna, but was hard-pressed for cash. So he did some looking around and decided to build his own.

Can Do

The Yagi antenna, which was named for Japanese electrical engineer Hidetsugu Yagi (1886–1976), is a horizontal conductor outfitted with double spines of dipoles parallel to and in the plane of the conductor. In other words, remove the innards of a Yagi antenna, and you’ll find a long, thin, horizontal strip of metal lined on both sides with long tines going up and down. The tines catch the signal from the air, and the wave energy is transferred and amplified by the center metal strip. A connection on the end of the metal transfers the signal to a TV, radio, or Wi-Fi card. The whole thing is housed in a sealed tube shaped suspiciously like—you guessed it—a Pringles potato chip can.

Our young hero made his calculations using formulae gleaned from a high school physics book and conversion tables found on a (get this) ceramics Web site. Once his design was set, Clapp assembled his materials. To mimic the Yagi design, Clapp used a 36-inch threaded metal rod, a metal tube, and a box of washers. The idea was elegant but simple. The threaded rod would be the Yagi backbone, while the washers served as the antenna’s tines. Spacers cut from the metal tube (Clapp used a hacksaw) would provide the correct distance between the individual washer tines.

You can see a picture of Clapp’s “collector” at www.netscum.com/~clapp/wireless.html. The collector was stabilized by cardboard box material rolled tightly inside the Pringles can. A standard N connector and coax cable hooked the internal collector to Clapp’s AirPort network access point.

All in all, the cost of the first Pringles Cantenna came in at less than 20 bucks—not counting the Pringles, of course. According to Clapp’s initial tests, his finished product (an 18-inch model) delivered a gain of 12–15 dBi. You can see a parts list, get a good explanation of how a Cantenna works, and see pictures of the Pringles Cantenna, not to mention other Cantennas, at www.netscum.com/~clapp/wireless.html.

Clapp’s initial work spawned a geek parade bent on making a better Cantenna. Writer and long-time FreeNetwork supporter Rob Flickenger slightly modified Clapp’s design and built a shotgun Yagi in 45 minutes. Flickenger’s design, however, emphasizes the importance of inserting the N connector between the sodium and protein measures on a Pringles salt-and-vinegar chips can. His results were similar to Clapp’s—a 12-dBi gain in Wi-Fi signal strength.

Time marches on, and so does the evolution of the Cantenna. David Marsh built his with a set of definite goals in mind: low cost, high performance, easy parts acquisition, portability, uncomplicated tools, and cookies. The result—the Cookie Cantenna, constructed from a 4-inch-diameter Pepperidge Farm Pirouette cookie can. Marsh increased performance by introducing his Funnel Cone add-on, constructed from a heating duct adapter purchased for $4.25, and gained three pounds in the process.

The “How to Build a Tin Can Waveguide Wi-Fi Antenna” Web site (www.turnpoint.net/wireless/cantennahowto.html) gives detailed construction instructions. No cookie eating here. From the looks of the pictures, you’ll need a Nalley’s brand can of Big Chunky Beef Stew, or a big ol’ can of MJB coffee. Extras on this site include a free spreadsheet for calculating optimum can-size calculations, and a link to a site describing how you can hook up your Cantenna to a surplus Primestar Satellite TV dish. Wow.

Canfounded

The Pringles Cantenna checks in as one of those ethical double-edged swords. Clapp and his colleagues are interested in the Cantenna exercise as a very practical experiment in the low-cost application of antenna technology. After all, it’s fun to build something that normally costs several hundred bucks for less than a 10 spot. Their Web sites are chock-full of technical information, parts lists, test results, and practical advice on how to put everything together. Very nice.

But on Clapp’s Web site, we catch a glimpse of the Cantenna’s not-so-pretty underbelly. In almost every photo showing stages of the Cantenna’s construction, a copy of 2600, the legendary hacker magazine, can be clearly seen close by. Coincidence, or is there a subtle message here? “Build a Cantenna, and mooch your neighbor’s hard-earned bandwidth.” Isn’t this the real crux of the matter?

FreeNetwork proponents see the Cantenna as a legitimate tool for expanding their mission of taking indoor networks outside. Their hope is to create a network of free Wi-Fi services and coverage that might be compared to the open-source movement in software. Despite FreeNetwork’s altruism, the Cantenna is viewed as a tool for hackers. Clapp’s showing 2600 with some of his projects doesn’t help dispel the notion.

The free market is even getting into the act. Sharper Concepts (www.sharperconcepts.zoovy.com) sells its do-it-yourself Cantenna kit for $14.95. As of this writing, a Cantenna search on eBay returned 37 hits ranging from fully assembled units to Cantenna components and instructions. One unit was billed as a “Wardriving super Cantenna with laser targeting for Linksys.” I would hate to see someone driving through my neighborhood with one of these.

I love the inventiveness behind the Cantenna movement. Ingenuity on this scale always impresses. But from the looks of things, the Cantenna and the dash-mounted radar detector have a lot in common. Both are used mostly to do bad things. For the Cantenna, this is a sad commentary for such a clever device.

Check out Kirk’s new book, Computer Factoids: Tales From the High-Tech Underbelly, now available at www.computerfactoid.com and Amazon.com.