Sorry.. had to go back to the original message to recall things...
im trying to reach the ap that 5 kilometers far from me...i have wrt54g in client mode attached to the roof with weatherproof box and i can see the ap...has anyone suggest antenna??...im planning to buy 20 dbi planet yagi...
==================== I would look into other types of antennas.... (good tutorial on the whole subject too)
Check
Parabolic or dish antennas This is where the real power is! Parabolic dish antennas put out tremendous gain but are a little hard to point and make a connection with. As the gain of an antenna increases, the antenna's radiation pattern decreases until you have a very little window to point or aim your dish correctly. Dish antennas are almost always used for a point to point system for long haul systems. The Parabolic Dish antennas work by focusing the power to a central point and beaming the radio's signal to a specific area, kind of like the adjustable reflector on a flashlight. These antennas are highly focused and are the perfect tool if you want to send your signal a very long distance. To calculate the distance of your WiFi......