802.11b Homebrew WiFi Antenna Shootout
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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.
What the huh?
|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,
||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.
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
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
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.
Copyright 2003-2007 Gregory Rehm - All rights reserved.
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