[tqdm]

I've also achieved substantial improvement with simple parabolic shaped, ~40 cm wide aluminum foil constructions, but there always seemed a bit luck involved and it needed some tweaking. Not sure whether the foil mainly acted as shielding that reduces the noise floor from other directions. I've read in other forums that fiddling with aluminum foil is mostly luck because additional reflective surfaces also introduce interferrence patterns unless they are precisely aligned. Even without aluminum foil it often helps to move either receiver or sender by fractions of 12.5 cm (the wavelength of 2.4 Ghz radiation) or of 6.0 cm (in case of 5 Ghz) because the space is basically riddled with interference patterns: https://www.youtube.com/watch?v=aqqEYz38ens

[kijin]

A wok (Chinese stir-fry pan) works surprisingly well for this purpose. It's roughly parabolic, and has a smooth surface that helps you avoid irregularities in the shape of your foil.

It gets even better if you drill a hole in the center and stick an antenna vertically in there. Now you've got a wi-fi antenna with all the advantages of a satellite dish! The Chinese call it wok-fi.

[westmeal]

Huh... no kidding. I'm going to have to try that out some day. How was the signal like on the wok with the antenna through the center?

[kijin]

I haven't personally tried sticking an antenna through a wok (just putting one behind the router was enough for my purpose), but rumors say that you can achieve a range of several miles if you do it right.

[walshemj]

There are online calculators http://www.radiolabs.com/stations/wifi_calc.html note that the cable loss mentioned is the antenna feeder cable

And Cisco Press do some good books on the basics of WiFi network design.