FYI, alt.internet.wireless discusses this topic often.
I prefer the biquad antenna, which you can augment with a dish. I have
a short-cut method to build this antenna. With a combination of these
photographs and this link, you should be able to figure it out.
http://martybugs.net/wireless/biquad/
http://www.lazygranch.com/images/wifi/wifi_bq_1.jpg
http://www.lazygranch.com/images/wifi/wifi_bq_2.jpg
Note you don't need to make the loop a square. Use a circle of the
same circumference.
There is a disadvantage to using the helix. It will receive both
horizontal and vertical polarization. Most sites just send in one
polarization. In busy areas, the same channel will be used in
different sites with different polarity. Now if you use the helix to
illuminate a dish, then the circular polarization is fine and perhaps
desirable. That is, you could sniff out signals without the
attenuation associated with having the wrong polarization.