Hi Bong,
It is a bit complicated and reverse engineering math , when we talk about the rage/ distance covered by a radio . Basicaly the range of an AP depends on alot of factor, TX power/interference/ obstacles / reflection/ Fresnel zone (If out door) . Theroticaly we can calculate the distance/range by considering free space path loss and the EIRP.
First you need is to determine predictive/expected the EIRP of the radio/AP and what is at your client . 2nd ,receiver sensitivity of the access point and the client . Calaculation of EIRP is as follows.
EIRP= TX power set on the AP - cable loss + Antenna gain
Lets say: 15db -3db + 11 db = 23 EIRP
Once we determine the EIRP of the access point ,we can than calculate the FSPL( Free space path loss) based on distance and the frequency . Frequency plays a very important role in calculating the distance of a transmitter . 2.4 ghz give us a good range as its wavelegth is longer than 5ghz.
Here is the formula for FSPL
FSPL(dB)} = 20log{10}(d) + 20log{10}(f) + 32.45
D5 = Distance in Miles
D9 = Frequency in Mhz
Lets say FSPL = 50db at 300ft distance
Now EIRP- Path loss would be the signal strengh i.e EIRP-Pathloss =Signal ==> 23- 50 = -27
SO lets say the client's receiving sensitivity is -70 , which means , it can have the signal as low as -70 and it will work fine. So -27 is the strong signal at 300ft.
Now if you increase the distance in the formula and try to make the signal -70 or above . As client has the receiving sensitivity of -70 , it would be the range ( Distance)for this client .
I know it is too complicated at once,however if you calculate the Free Space Path Loss and EIRP, it will give you a rough idea at what distance the client will still have a good signal,and determine the coverage area.
Hope this helps
Thanks
Munish