With limited power condition (15-20dBm), the range of WLAN equipment is limited. Maximum range that can be achieved is that when the signal reaches the receiver sensitivity limit at the end over there. Actually, its calculation is simple, especially a rather complicated calculation process is the signal attenuation over the air (Free Space Loss) which the average size of about 100dBm depending on distance and frequency of use.
To give some idea, a WLAN connection with 15 dBi antenna and 15 dBm transmit power at both ends it will be able to reach a distance of about seven (7) taking into account the attenuation coax km 3 dBm. Quite normal for 2-11Mbps WLAN connectivity in many cities without having to use the power amplifier illegally.
Emission region is highly dependent on field conditions, location of buildings, location of obstructions, height, and shape antenna used. In the picture I try to show coverage (scope) emitted a WLAN base station in the form of spots.
For the planning / design of a Metropolitan Area Network (MAN) this beam shape we usually make an ideal into a hexagon shape as shown in the picture. This ideal form of coverage we will use later in further explanation below. Knowledge of the antenna will be very important in determining the coverage area that can form in service. In another section I describe the various forms of antenna and radiation pattern.
Example of calculation that is so can be obtained directly from me (onno@indo.net.id) in the form of Excel files for free. In Excel file I've also make some routine conversion from dBm to Watts scale to better show for us who are not familiar with the notation dBm. Alternatively, feel free to get in for free at http://www.bogor.net/idkf/ usually under the directory / physical / wireless.