Building the "Pringles can" 2.4 GHz directional wi-fi antenna
This is a famous design. Several people describe it but I
couldn't find clear assembly instructions. In the classic design,
the collector is hidden in a length of PVC pipe, and centered in the
can by wrapping the pipe with layers of corrugated cardboard. I
used chopsticks because they're more rigid. The supermarket near
me sells 20 bamboo chopsticks for a dollar. The classic design
uses a relatively enormous "N" connector, the standard for UHF and low
microwave. But 802.11b/g "wifi" seems to have standardized on a
modified SMA connector, much smaller. The modification is the
panel-mounted connector has a pin and external threads, and the
cable-mounted connector has the screw-cap and a socket. They call
this a "reverse polarity SMA" connector, "SMA-RP" for short.
Coaxial cable loses 1 dB per foot or more, and attaching a connector
properly requires an expensive precision crimper. So I used a USB
wi-fi transceiver, connected to the antenna with a (0.1 dB loss) barrel
adapter. Signal loss in a USB cable isn't really an issue.
I used the Ampaqs WZ-2154 802.11g wireless LAN adapter, available
unbranded from Hong Kong dealers on Ebay. They ship it with a
detachable +5 dBi rabbit ear antenna. It uses the Zydas chip
set. Recent Linux kernels recognize it, but the driver in 2.6.18
(Debian 4.0 "Etch") and 2.6.22 (Ubuntu 7.10 "Gutsy Gibbon") was
unstable. The WZ-2154 came with NDIS drivers for Windows-XP which
work perfectly under the Linux NDIS Wrapper.
The electrically active part of an antenna is called the driven
element. This one is a short length of solid copper wire soldered
to the inner conductor of the connector. The end of the wire is
soldered to the end of the connector stub. Total length above the
plane of the connector is 1.2 inches. I made a jig to hold the
wire in place for soldering. A small wood block is clamped to the
table. A popsicle stick is taped to the side of the wood
block. The wire is taped to the popsicle stick. Four 2-56
by 3/8 machine screws are glued into the flanges of the
connector. They hold it upright for soldering. Click the
picture for a larger one.
The solder joint close-up. Solder is strange stuff. Melted,
it quickly oxidizes and changes from something like mercury to a dusty,
crumbly, useless paste. So to get a good, clean solder joint, do
it fast before the solder
oxidizes. Keep a damp sponge next to the soldering iron and wipe
the oxidized solder off it, then solder within a few seconds. If
you hang around for half a minute the soldering iron will be dirty
again. If the solder doesn't go where you want right away, don't
try to cook it into place. It will just turn to dross. Tin
and clean the iron. Touch it to both the wire and the connector
stub and let them heat up for a few seconds. Then touch the
solder to the point where the wire, stub, and iron all meet. The
solder will go where you want. Remove the iron and solder
immediately.
The connector is bolted inside the Pringles can. I glued the
screws in place because there's no way to get at them inside the
can. Test the fit before trying to mount it. If the holes
are off you won't get the connector barrel and all four screws
through. The connector should drop into place.
Inserting the connector inside the can requires a special
fixture. Two chopsticks and a rubber band. Bamboo
chopsticks are better than wood or plastic. If the connector
rotates as in the picture below, wrap a piece of paper around the end
of the chopsticks for friction.
The connector inside the can. The driven element is 1.2 inches
from the rear of the can. It was barely visible in the photo so I
drew a line over it.
The holes are too big so
use washers. 
The "collector" is an 18" rod with 1 1/4" washers along it every 1.2
inches. It's not connected to anything, but one end of it is very
close to the end of the driven element. I used a length of 6-32
plated allthread and 1.15 inch lengths of copper tubing. Use
heavier allthread; 6-32 turns out to be a bit flimsy and the tubing is
loose over it. I glued chopsticks along the washers for
strength. Use hot glue. Epoxy and cyanoacrylate are too
brittle. Contact bond cement is too flexible. Casein cement
("white glue") is useless.
I suspended the collector from six chopsticks poked through holes in
the can. To get the stick exactly where you want it, cut an X
with a sharp knife, a little smaller than the cross section of the
stick. Make a small round hole where the end will come
through. The holes will stretch to fit the stick snugly.
Two of the chopsticks wedge against two washers to maintain the inside
end of the collector less than a tenth of an inch from the end of the
driven element. I tested them for fit outside the can first.
Two more chopsticks form a triangle surrounding the rod.
Insert the collector with the first two chopsticks in place. Then
wedge it in with the other four.
The complete assembly.
Creature shown for scale.
