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Hack 72 Pringles Can Waveguide

How to make the infamous Pringles
cantenna.

At the Portland Summit
in June 2001, Andrew Clapp presented a novel yagi antenna design
(http://www.aeonic.com/~clapp/wireless/). It
used a bolt, metal tubing, washers, and PVC tubing to make an
inexpensive "shotgun" yagi, either
18" or 36" long. While his antenna shows between 12 and 15 dBi gain
(which is impressive for such a simple design), it's
also quite large. When we returned from Portland, some members of our
local group and I realized that, if we were careful, we could fit a
full wavelength inside of a Pringles can (see Figure 5-4). This would show a reduced total gain, but
would also make the entire antenna much more compact.

This now infamous hack takes about an hour to construct. Table 5-1 shows a list of the parts you need to get
started.

Of course, buying in bulk helps a lot. You probably
won't be able to find a 6" piece of all-thread; buy
the standard size (usually 1 or 2 feet) and a 10-pack of washers and
nuts while you're at it. Then
you'll have more than enough parts to make 2
antennas, all for about $10.

You also need the following tools.

Ruler

Scissors

Pipe cutter (or hacksaw or dremel tool, in a pinch)

Heavy-duty cutters (or dremel again, to cut the all-thread)

Something sharp to pierce the plastic (such as an awl or a drill bit)

Hot glue gun (unless you have a screw-down type connector)

Soldering iron

Front Collector Construction

Mark and
cut four pieces of tubing, about 1.2" (1 15/64"). Where did I get
this number? First figure out the wavelength at the bottom of the
frequency range we're using (2.412 GHz, or Channel
One). This will be the longest that the pipe should be:

W = 3.0 * 10^8 * (1 / 2.412) * 10^-9
W = (3.0 / 2.412) * 10^-1
W = 0.124 meters
W = 4.88 inches

We'll be cutting the pipe to quarter wavelength, so:

1/4 W = 4.88 / 4
1/4 W = 1.22"

Now figure out what the shortest range we'll ever
use is (2.462 GHz, or channel 11 in the United States):

W = 3.0 * 10^8 * (1 / 2.462) * 10^-9
W = (3.0 / 2.462) * 10^-1
W = 0.122 meters
W = 4.80 inches 
1/4 W = 1.20"

Practically speaking, what's the difference between
the shortest pipe and the longest pipe length? The answer is about
0.02", or less than 1/32".
That's probably about the size of the
pipe cutter blade you're using. So, just shoot for
1.2", and you'll get it close enough.

Cut the all-thread to exactly 5 5/8". The
washers we used are about 1/16" thick, so that should leave just
enough room for the pipe, washers, and nuts.

Pierce a hole in the center of the Pringles can lid big enough for
the all-thread to pass through. Now is probably a good time to start
eating Pringles. (We found it better for all concerned to just toss
the things; "Salt & Vinegar"
Pringles are almost caustic after the first 15 or so. Heed the
recommended serving size!)

Cut a 3" plastic disc, just big enough to fit
snugly inside the can. We found that another Pringles lid, with the
outer ridge trimmed off, works just fine. Poke a hole in the center
of it, and slip it over one of the lengths of pipe.

Now, assemble the pipe. You might have to use a file or dremel tool
to shave the tips of the thread, if you have trouble getting the nuts
on. The pipe is a sandwich that goes on the all-thread like this:

Nut Lid Washer Pipe Washer Pipe Washer Pipe-with-Plastic Washer Pipe Washer Nut

You can see the collector assembly clearly in Figure 5-4. Tighten down the nuts to be snug, but
don't overtighten (I bent the tubing on our first
try; aluminum bends very easily). Just get it
snug. Congratulations, you now you have the front collector.

Preparing the Can

By now you should have eaten (or tossed)
the actual chips. Wipe out the can, and measure 3 3/8"
up from the bottom of the can. Cut a hole just big enough
for the connector to pass through. We found through trial and error
that this seems to be the "sweet
spot" of the can. On our Pringles
"Salt & Vinegar" can, the N
connector sat directly between
"Sodium" and
"Protein."

Element Construction

Straighten the heavy
copper wire and solder it to the
connector. When inside the can, the wire should be just below its
midpoint (ours turned out to be about 1 1/16"). You lose a few db by
going longer, so cut it just shy of the middle of the can.

We were in a hurry, so we used hot glue to hold the connector in
place on our first antenna. If you have a connector that uses a nut
and washer, and you're really careful about cutting
the hole, these work very well (and aren't nearly as
messy as hot glue). Just remember that you're
screwing into cardboard when you connect your pigtail.
It's very easy to forget and accidentally tear the
wall of the can.

Now, insert the collector assembly into the can and close the lid.
The inside end of the pipe should not touch the
copper element; it should be just forward of it. If it touches, your
all-thread is probably too long.

How can one estimate gain without access to high end radio analysis
gear? Using the Link Test software that comes with the Orinoco silver
cards, you can see the signal and noise readings (in dB) of a
received signal, as well as your test partner's
reception of your signal. As I happen to live 0.6 mile (with clean
line of sight) from O'Reilly headquarters, we had a
fairly controlled test bed to experiment with. We shot at the omni on
the roof, and used the access point at O'Reilly as
our link test partner.

To estimate antenna
performance, we started by connecting commercial antennas of known
gain and taking readings. Then we connected our test antennas and
compared the results. We had the following at our disposal:

Two 10 dBi, 180-degree sector panel antennas

One 11 dBi, 120-degree sector panel antenna

One 24 dBi parabolic dish

The Pringles can antenna

Table 5-2 shows the average

received signal and
noise readings from each, in approximately the same physical
position.

The test partner (AP side) signal results were virtually the same.
Interestingly, even at only 0.6 mile, we saw some thermal fade
effect; as the evening turned into night, we saw about a 3 db gain
across the board. (It had been a particularly hot day: almost 100
degrees. I don't know what the relative humidity
was, but it felt fairly dry.)

Yagis and dishes are much more
directional than sectors and omnis. This bore out in the numbers, as
the perceived noise level was
consistently lower with the more directional antennas. This can help
a lot on long-distance shots, as not only will your perceived signal
be greater, but the competing noise will also seem to be less. More
directional antennas also help keep noise down for your neighbors
trying to share the spectrum as well. Be a good neighbor and use the
most directional antennas that will work for your application (yes,
noise is everybody's problem).

The Pringles can seemed to have large side lobes that extend about 45
degrees from the center of the can. Don't point the
can directly at where you're trying to go; instead,
aim slightly to the left or the right. We also found that elevating
the antenna helped a bit as well. When aiming the antenna, hold it
behind the connector, and slowly sweep from left
to right, with the Link Test program running. When you get the
maximum signal, slowly raise the end of the can to see whether it
makes a difference. Go slowly, changing only one variable at a time.

Remember that the can is polarized,
so match the phase of the antenna you're talking to
(for example, if shooting at an omni, be sure the element is on the
bottom or the top of the can, or you won't be able
to see it). See the earlier discussion on antenna polarization for
how you can use this effect to your advantage.

We were fortunate enough to have a member of our community group
bring a
return loss meter to one of
our meetings, and were able to get some actual measurements of how
much signal was returning to the radio. The results
weren't as good as I had hoped, but they showed that
the antenna was usable, particularly at lower frequencies. Most
likely, failing to take into account the thickness of the washers
made the entire front element a little too long. There
isn't nearly enough power leaving the radio to cause
damage due to high return loss, but it does point out that the
antenna isn't as well-tuned as it could be.

For a simpler, higher-gain waveguide antenna, read the next Hack. The
original article that this Hack is based on is available online at
http://www.oreillynet.com/cs/weblog/view/wlg/448.