The two types of antenna are (omni-directional, directional)
HomeBrews will tend to focus on the directional types.
<> higher gain/distance for signal, stronger signal for given distance
<> stronger signal can == higher bandwidth
<> W/ a directional (and a known WiFi HOTSPOT), you can keep the ant. inside the car.
<> Directional ant. make for poor choices when wardriving.
<> If you don't KNOW the location of the wifi ant. you 'could' be close enough to get a signal w/ a omin. but still miss it with the directional.
BUY a directional ant. :: http://www.ramseyelectronics.com/cgi...ction&key=LPY2
MAKE a directional ant. :: http://www.oreillynet.com/cs/weblog/view/wlg/448
More on homebrew wifi ant. :: http://www.freeantennas.com/.