Understanding "line of sight" and "near line of sight" as it applies to range
Wireless networking (WiFi) at 2.4 GHz is a two-way system. Each device must be capable of both sending and receiving a signal equal distances. Think of it as two people, a substantial distance apart, throwing a ball to each other.
A has to be strong enough to throw the ball, that substantial distance,
to person B. Person B also has to be strong enough to throw the ball
back, that substantial distance, to person A. If neither Person A nor
B can throw the ball that distance they will not have much of a throwing
Access points and bridges (as shipped from the manufacturer) have an advantage over laptop/desktop cards because they have a higher output power and therefore have the ability to send a signal further then most laptop/desktop cards. When a higher-gain antenna is installed on a desktop card the output power of that device is now increased closer to the output level of the access point or bridge therefore equaling the two devices. In some cases, the antennas of both the access point/bridge and the desktop/laptop card may need to be replaced if the distance you are attempting to achieve is greater than the capabilities of the access point/bridge when using the (factory) antennas that came with your card.
Please note that extending the range of your wireless 802.11b wireless device is only a piece of the whole puzzle. At 2.4GHz (the frequency that 802.11b WiFi operates at), Line-Of-Sight can be an important factor. Please see below regarding Line-Of-Sight.
Understanding The Importance Of Unobstructed Line-Of-Sight With WiFi
802.11b at 2.4GHz (equipped
with and using a vertically (or linear)-polarized antenna) requires unobstructed
visual Line-Of-Sight (LoS).
Unobstructed Line-Of-Sight means just that; there should not be trees, terrain,
buildings, or structures between your two (antenna) points. Basically both
antennas should physically see each other in an external outdoor bridge.
The radio waves at this low frequency will not penetrate metal, steel, concrete,
cement, stone, brick, etc. very well, if at all. For interior applications,
dry wall, sheet rock, and wood could be a problem with basic factory antennas.
Surrounding the visual Line-Of-Sight is the Fresnel zone (image 1). Any obstructions that come into the Fresnel zone, although not obstructing the visual Line-Of-Sight, may also slow down, hinder and effect your signal. The radio waves may deflect off of those obstructions.
This is called Near Line-Of-Sight (nLoS, image 2). Although you may see a slight signal with nLoS situations, your data transfer rate may decrease. You may find you are incapable of accessing the Internet.
An obstruction that cuts across the visual Line-Of-Sight and prohibits an optical visual between the two antennas in your bridge is considered Non-Line-Of-Sight (NLoS, image 3). Any signal, in this case, will be minimal or non-existent.
You may find in your bridge application, that the two antennas can visually see each other through spaces and breaks in an obstructing tree or tree line. Please note that tree branches that cross the visual Line-Of-Sight will move with the wind. This movement will disrupt and have an effect on a vertically-polarized WiFi signal. Keep in mind, if you are conducting your site survey during late Fall and Winter months, those trees will fill in come Spring and Summer. Additionally, weather, RF interferences, and other site variables can have an effect on your signal too.