Antenna Design for Omnidirectional transmission
Ideally for a omnidirectional antenna the radiating element must be perpendicular to the ground. Wi-fi units don't kick out a lot of power usually about a 1/10th of a watt give or take. In order to have best results a high gain vertical antenna and mounted as high as feasible at your location. A good choice for antenna are colinear antennas as they direct the signal toward the horizon for maximum usable range. Gain is measured in db or decibels which is a logarithmic scale. Every 3 db is a doubling of signal. 
Here are 2 links with formulas and construction information. Enjoy... :)

http://wireless.gumph.org/articles/homemadeomni.html 

http://martybugs.net/wireless/collinear.cgi