There are many websites detailing the construction of cantennas, all of them referring to a pigtail. A pigtail connects the antenna to the WiFi adaptor. However, any length of cable induces signal loss through impedance of the electromagnetic wave.
This cantenna design does away with the pigtail by using an n-type to reverse polarity male SMA adaptor. This adaptor connects your cantenna directly to a WiFi adaptor without need for a pigtail.
The microwave signal is processed in the adaptor so you can then use a long USB cable to connect from the WiFi adaptor to your computer without signal loss.
The cantenna is directional so you will need something like a tripod to direct and hold the cantenna in place. Expect about a 3dB improvement of the signal, which is roughly a doubling in power, and hopefully many more access points being revealed.
I won't go into detail about the construction of a cantenna but point you towards http://www.saunalahti.fi/elepal/antenna2.html
for detailed measurements.
However, do look at the slide show to see the subtle difference that obviates the need for a pigtail.
High power WiFi USB adaptor with SMA connection for antenna ( I use an ALFA AWUS036H)
Can - over 76mm in diameter (Long and thin is better than short and fat)
N-type female chassis connector
N-type male to RPM SMA adaptor
Some thick unsheathed copper wire (about 1 to 1.5mm diameter mains wire is best)
Four small nuts and bolts to attach N-type female to can
Drill and bits (cone bit to make hole for n-type chassis connector)
You can see other self-reliant things I make and do at http://www.ecopunk.org.uk