802.11b Homebrew WiFi Antenna Shootout

Read the How To

Greg's obsession de' jour
In my efforts to add the words "wireless savvy" to my network admin resume, I've been reading books and web pages on radio propagation, antenna theory and design, and building wireless networks with 802.11 (WiFi). One of the first things that got me excited was the Pringles Can Antenna. Published on the internet and in a fine book by Rob Flickenger, the net admin for O'Reilly, this design for a do-it-yourself, VERY inexpensive antenna made from a recycled junkfood container is as cool as the other side of the pillow. It seems that everyone is building and using these. The various community wireless network groups all talk about them and folks are reporting that they do the job.

A friend of mine built his before me and looking at his finished antenna got me excited to understand the theory of how it works. Reviewing his plan, I came up with different spacing that he Rob did. To see if I could improve upon the design, I built mine with corrected spacing. While waiting for some wireless equipment to come in, I started looking for my next antenna project. Oddly, the more I studied, the less I understood. There seems to be quite a bit of confusion on how the Pringles antenna works and what design category it falls under. The inner lining of a Pringles can looks metallic, but my tests show it not to be. The Pringles Antenna design, and some designs that pre-date it, seem to treat it as though it were metallic. While folks are calling it a Yagi-Uda style antenna, the design of the driven element in the Pringles can antenna looks like a Waveguide style design.

Waveguide antennas don't use the director assembly (the washery bits), and therefore are much simpler to build. An old tin can of the right size, about $5 in parts and 10 minutes of time are all that are needed. The math for computing correct sizing of the components in a waveguideWiFi antenna is simple. Formulas in hand, I started searching my cupboards for tin cans that fit the spec. I found myself staring at the products on the canned food aisle at the grocery store. I even went so far as going grocery shopping with a tape measure. "No no, this spaghetti sauce looks much better. It's about three quarters of a wavelength in diameter, hon!"

What the huh?

On Feb 11th, Rob, posted an article on his newest homebrew WiFi antenna - a tin can waveguide! Rob used a large, 39oz. coffee can and placed a quarter wavelength driven element a quarter wavelength from the back of the can. He reported good results - even better than the Pringles can design used by so many. For the antennas I was building, I was using different measurements based on the antenna design material I had been reading. Now I'm a late entry into this wireless stuff and the experts are going a different way than me. It's time to benchmark.

The Shootout
My plan was to get relative performance measurements for various designs (including mine) of homebrew antennas for 802.11b (WiFi) wireless networks. To do this, I setup a wireless link and changed only the antenna- recording each antennas' performance under identical conditions. I didn't compare them to a commercial directional antenna as my only one has a male connector and I don't have the right cable to hook it up yet. The contestants were (click on each for design specifications).

Performance numbers and methodology

The Performance Summary
The results surprised me! In our test, the Flickenger Pringles can did a little better than my modified Pringles design. Both did no better than the Lucent omnidirectional. Now this is just on raw signal strength, noise rejection due to directivity still makes a directional antenna a better choice for some uses even if there is no gain benefit. The waveguides all soundly trounced the Pringles can designs. I mean they stomped them into the ground on signal strength - as much as 9 dB better. Every three dB is a doubling in power - that's three doublings (8x increase)!

Of the waveguides, the Nalley's "Big Chunk" took top marks. It was followed by the Hunts Pasta Sauce, my modified coffee can, and the Flickenger coffee can in that order. My three waveguide designs, which utilized the correct theoretical spacing, out performed the Flickenger Yuban coffee can handily. It seems that the design formulas for the waveguide design made a sizeable difference in performance. In the yagis, it didn't matter much. This could be because neither Rob's nor my designs are anywhere near right for optimum performance for a Yagi. I've decided that Yagi design is not for the timid or non-radio-expert.

With these results, I'm convinced that the waveguide design is the way to go for cheap wireless networking. The performance is good, the cost is very low and the skill required is minimal. If you can eat a big can of stew, you can make a high performance antenna.

The How To
Build your own Tin Can Waveguide WiFi Antenna (Cantenna). It's the easiest antenna design I know of.